Meteors May Masquerade as Lightning in the Atmosphere of Venus

Abstract

Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on
how one interprets diverse observations. Quantifying when and where, or even if lightning
occurs would provide novel information about Venus’s atmospheric dynamics and chemistry.
Lightning is also a potential risk to future missions, which could float in the cloud layers (~50–
70 km above the surface) for up to an Earth-year. Over decades, spacecraft and ground-based
telescopes have searched for lightning at Venus using many instruments, including
magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and
the 61-inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the
unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning.
This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing
such energetic flashes, based in turn on the presumption that a low fraction (< 1%) of a meteor’s
optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue
experiments to show that a much higher conversion factor (~5–10%) should be expected.
Therefore, we calculate that smaller, more numerous meteors could have caused the observed
flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the
clouds of Venus. Likewise, small meteors burn up at altitudes of ~100 km, roughly twice as high
above the surface as the clouds, and also would not pose a hazard.

Full text

Posted on 25 May 2023 — CC-BY-NC-ND 4 — https://doi.org/10.22541/essoar.168500289.97929865/v1 — This a preprint and has not been peer reviewed. Data may be preliminary.
Meteors May Masquerade as Lightning in the Atmosphere of Venus
Claire H Blaske1, Joseph Ghilarducci O’Rourke1, Steven Desch1, and Madison E Borrelli1
1Arizona State University
May 25, 2023
Abstract
Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on
how one interprets diverse observations. Quantifying when and where, or even if lightning
occurs would provide novel information about Venus’s atmospheric dynamics and chemistry.
Lightning is also a potential risk to future missions, which could float in the cloud layers (˜50–
70 km above the surface) for up to an Earth-year. Over decades, spacecraft and ground-based
telescopes have searched for lightning at Venus using many instruments, including
magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and
the 61-inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the
unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning.
This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing
such energetic flashes, based in turn on the presumption that a low fraction (<1%) of a meteor’s
optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue
experiments to show that a much higher conversion factor (˜5–10%) should be expected.
Therefore, we calculate that smaller, more numerous meteors could have caused the observed
flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the
clouds of Venus. Likewise, small meteors burn up at altitudes of ˜100 km, roughly twice as high
above the surface as the clouds, and also would not pose a hazard.
1

manuscript submitted to Journal of Geophysical Research: Planets
Meteors May Masquerade as Lightning in the Atmosphere of Venus
1
C. H. Blaske1,2,†, J. G. O’Rourke2, S. J. Desch2, and M. E. Borrelli2
2
1Barrett, The Honors College, Arizona State University, Tempe, AZ, USA. 2School of Earth and
3
Space Exploration, Arizona State University, Tempe, AZ, USA.
4
Corresponding authors: C. H. Blaske (cblaske@asu.edu), J. G. O’Rourke (jgorourk@asu.edu)
5
†Current address: Earth & Planetary Sciences, Stanford University, Stanford, CA, USA.
6
Key Points:
7
• We investigate whether meteor fireballs could have produced the optical flashes that have
8
been detected at Venus and attributed to lightning
9
• We find that flashes from meteor fireballs are statistically likely to occur at the observed
10
rates and brightness
11
• There is no affirmative evidence that lightning is a hazard to missions that pass through
12
or dwell within the clouds of Venus
13

manuscript submitted to Journal of Geophysical Research: Planets
Abstract
14
Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on
15
how one interprets diverse observations. Quantifying when and where, or even if lightning
16
occurs would provide novel information about Venus’s atmospheric dynamics and chemistry.
17
Lightning is also a potential risk to future missions, which could float in the cloud layers (~50–
18
70 km above the surface) for up to an Earth-year. Over decades, spacecraft and ground-based
19
telescopes have searched for lightning at Venus using many instruments, including
20
magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and
21
the 61-inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the
22
unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning.
23
This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing
24
such energetic flashes, based in turn on the presumption that a low fraction (< 1%) of a meteor’s
25
optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue
26
experiments to show that a much higher conversion factor (~5–10%) should be expected.
27
Therefore, we calculate that smaller, more numerous meteors could have caused the observed
28
flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the
29
clouds of Venus. Likewise, small meteors burn up at altitudes of ~100 km, roughly twice as high
30
above the surface as the clouds, and also would not pose a hazard.
31
Plain Language Summary
32
Artists depicting the atmosphere of Venus love to include lightning bolts to emphasize its hellish
33
environment. Even though missions like DAVINCI would most likely be safe from strikes as
34
they descend quickly through the atmosphere, long-lived aerial platform missions supported by
35
large balloons floating in the cloud layer ~50–70 km above the surface might not be so fortunate.
36
Do engineers need to build aerial platforms with the toughness (and thus expense) required to
37
survive a lightning strike? Quantitative estimates of lightning strike frequency are inconsistent
38
based on different forms of evidence. Observations of certain electromagnetic signals,
39
interpreted as lightning in the clouds, suggest a strike rate several times that of Earth’s lightning.
40
In comparison, optical flash rates at Venus, as observed at the Mt. Bigelow observatory in
41
Arizona and by the Akatsuki mission currently orbiting Venus, suggest a far lower flash rate. In
42
this study, we argue that these optical flashes were plausibly produced by meteor fireballs ~100
43
km above the surface, not by lightning in the clouds. If so, then lightning poses no significant
44
threat to balloon missions in the clouds of Venus. Lightning may still exist at the surface,
45
produced by aeolian processes or explosive volcanism.
46
47
1 Introduction
48
Venus is a natural laboratory for studying the atmosphere of a non-Earth-like planet.
49
While Venus is unique among terrestrial planets in our Solar System, it is an analogue to a
50
common class of exoplanets (e.g., Kane et al., 2019; Way et al., 2023). For example, Venus’s
51
present-day atmosphere is far more massive than the atmospheres of Earth and Mars (e.g., Taylor
52
et al., 2018)—dominated by carbon dioxide and perpetually shrouded in clouds rich in droplets
53
of sulfuric acid at altitudes from ~47–70 km (e.g., Titov et al., 2018). Exoplanets born near their
54
parent stars may outgas thick, CO2-dominated atmospheres shortly after they accrete (e.g.,
55
Hamano et al., 2013; Gillmann et al., 2022; Miyazaki & Korenaga, 2022) or, later, if they
56
experience a runaway greenhouse (e.g., Krissansen-Totton et al., 2021; Way et al., 2022).
57

manuscript submitted to Journal of Geophysical Research: Planets
Venus’s clouds are super-rotating, moving westward much faster than the solid body (e.g., Read
58
& Lebonnois, 2018). This attribute can be exploited for exploration: a surface station would take
59
~243 Earth-days to experience a sidereal day, but an aerial platform in the clouds could
60
circumnavigate Venus at the equator every ~5–7 Earth-days (e.g., Cutts et al., 2022; O’Rourke et
61
al., 2023). Likewise, many terrestrial exoplanets may have superrotating atmospheres (e.g.,
62
Imamura et al., 2020; Lee et al., 2020). As the number of observed exoplanets grow, so does the
63
complexity of work to understand their evolution and current environments, and the need to
64
study Venus. Lightning is an important aspect of terrestrial atmospheres, in part for its ability to
65
instigate non-equilibrium chemistry (e.g., nitrogen fixation) relevant to biology and life detection
66
(e.g., Ardaseva et al., 2017). Venus offers a natural laboratory for studying lightning on a major
67
class of exoplanets.
68
On Earth, lightning flashes occur hundreds of times each second across the globe (e.g.,
69
Desch et al., 2002), illuminating areas where the atmosphere is dynamically active. Lightning
70
flashes occur after a significant charge separation has been built up in the atmosphere (e.g., Yair,
71
2012; Aplin, 2006; Dwyer & Uman, 2014). Once released, lightning dissipates an enormous
72
amount of energy, a fraction of which flashes as optical energy. In Earth’s atmosphere, lightning
73
strikes are most often facilitated via the interactions between liquid water and water ice particles,
74
where the polarity of the molecules contributes to the buildup of this charge difference. Volcanic
75
plumes (e.g., Nicoll et al., 2019; Mather & Harrison, 2006) and dust storms (e.g., Aplin, 2006)
76
can also produce lightning by triboelectric charging, in which ash and dust particles are the
77
medium of charge buildup.
78
On Venus, lightning could arise from mechanisms analogous to those seen on Earth.
79
Sulfuric acid molecules in the clouds may have sufficient polarity to produce charge separation if
80
both solid and liquid phases exist, though this is debated (e.g., McGouldrick et al., 2011). The
81
frequency of volcanic events on Venus today is uncertain. However, Venus and Earth could have
82
similar rates of volcanic activity overall (e.g., Byrne & Krishnamoorthy, 2021). Radar images
83
from Magellan revealed ~105 volcanoes on the surface larger than 5 km in diameter (Hahn &
84
Byrne, 2023). Magellan may also have observed active volcanism (e.g., Herrick & Hensley,
85
2023). At least some volcanic eruptions were explosive in the past (e.g., Ganesh, 2022; Ganesh
86
et al., 2022; Ganesh et al., 2021; Airey et al., 2015). Lightning produced by either volcanic or
87
atmospheric processes would provide a probe into Venus’s current and past evolution. Lightning
88
strikes could also excite global Schumann resonances at frequencies of tens of Hz, enabling
89
electromagnetic sounding of Venus’s lithosphere from an aerial platform (e.g., Grimm et al.,
90
2012). Finally, lightning on Venus would create unique chemical environments in the
91
atmosphere (e.g., Krasnopolsky, 2006; Delitsky & Banes, 2015). Almost anyone studying Venus,
92
especially those planning future missions, should feel motivated to find lightning, if it exists.
93
The existence of lightning in the atmosphere of Venus has been heavily debated for
94
decades based on interpretations of different types of evidence. Numerous attempts to collect
95
concrete evidence of lightning have been made, with varying results (e.g., Lorenz, 2018; Lorenz
96
et al., 2019 and references therein). Most of the claimed detections rely on observations of radio,
97
magnetic, and acoustic signals. The Venera 11–14 landers observed near-continuous signals at
98
~10–80 kHz during their descents, which resembled electrical activity (sferics) associated with
99
lightning on Earth (Ksanfomality, 1980). The Pioneer Venus Orbiter detected electric fields
100
thought to be associated with lightning flashes (e.g., Taylor et al., 1979). The Venus Express
101
magnetometer detected magnetic bursts near periapsis that were interpreted as whistler-mode
102

manuscript submitted to Journal of Geophysical Research: Planets
waves—circularly polarized electromagnetic waves with frequencies up to several hundred Hz at
103
Venus, which follow local magnetic field lines—produced by lightning below the ionosphere
104
(e.g., Russell et al. 2006). Most recently, Hart et al. (2022) claimed that Venus has a flash rate
105
several times higher than for lightning on Earth. However, there have also been non-detections of
106
lightning that are in tension with these predictions, including with the radios of Cassini (Gurnett
107
et al., 2001) and the Parker Solar Probe (Pulupa et al., 2021). All claimed detections have been
108
controversial, with spacecraft and plasma noise proposed as candidates to create some of the
109
signals (e.g., Lorenz, 2018).
110
A second approach to detect lightning at Venus is searching for optically bright flashes,
111
similar to what we associate with lightning here on Earth. Laboratory experiments simulating
112
lightning discharge conducted in a carbon dioxide-dominated atmosphere show a distinctive
113
peak at the excited atomic oxygen triplet near 777 nm (e.g., Borucki et al., 1985, 1996; Qu et al.,
114
2023). Because of the increased abundance of oxygen (in carbon dioxide) at Venus, emission
115
from the OI triplet should be relatively strong compared to at Earth (e.g., Borucki et al., 1985).
116
Numerous searches for lightning have been conducted via optical flash detection at this
117
wavelength, including with the Venera 9/10 Orbiter Spectrometer (Krasnopolsky, 1979), the
118
Pioneer Venus Star Tracker (Borucki et al., 1991), and Venus Express Visible and Infrared
119
Thermal Imaging Spectrometer (VIRTIS) (Moinelo et al., 2016), none of which returned clear
120
detections. Other endeavors, such as the observations made by the Mt. Bigelow 61-in. telescope
121
(Hansell et al., 1995) and the Akatsuki Lightning and Airglow Camera (LAC) (Takahashi et al.,
122
2021; Lorenz et al., 2022), were more successful, with several distinct light flashes recorded (see
123
Section 2.1 below). Theory and simulations predict that the clouds would only absorb ~60% of
124
the optical energy from lightning near the cloud base, although the photons would be scattered
125
horizontally to a width of ~200–300 km (e.g., Williams et al., 1982; Williams & Thomason,
126
1983). In contrast, only ~0.01% of the visible photons from near-surface lightning could escape
127
to space. Ultimately, orbiting spacecraft or ground-based telescopes could detect lightning that
128
occurs as low as the lower cloud deck.
129
However, these optical flashes may have alternative sources beyond lightning. On Earth,
130
satellites that observe lightning also observe meteor fireballs. As bolides ablate in the
131
atmosphere, some of their kinetic energy is released as optical energy, which can be observed at
132
visible wavelengths. Observations of small meteors reveal that they, like lightning, emit a
133
distinctive peak around 777 nm (e.g., Madiedo et al., 2023). While slower meteors generate a
134
Planck continuum, the OI emission line is especially strong for faster meteors (e.g., Vojáček et
135
al., 2022). At Venus, we might expect even stronger OI emission.
136
In this study, we investigate the rate of fireballs from cm-sized meteors ablating in the
137
upper atmosphere of Venus as alternative explanations for observed optical flashes. First, we
138
calculate the global rate of optical flashes inferred from the Mt. Bigelow and Akatsuki detections
139
(Section 2.1). Then, we adapt a power law for impactors at Earth (e.g., Brown et al. 2002) to
140
calculate the flux of small impactors at Venus. We derive a power law for the number of
141
impactors with a certain amount of optical energy in the OI emission line (Section 2.2). Next, we
142
compare those to the rate of observed flashes from Akatsuki and Mt. Bigelow (Section 3). While
143
these optical flashes are likely to come from the cloud layers (if produced by lightning) or even
144
higher in the atmosphere (if produced by ablating meteors or transient luminous events like
145
sprites, elves, and haloes), we also discuss the prospects for lightning elsewhere on Venus, such
146

manuscript submitted to Journal of Geophysical Research: Planets
as volcanic or aeolian lightning near the surface (Section 4.1). Finally, we assess whether cloud-
147
based lightning would threaten future probes or aerial platforms (Section 4.2).
148
2 Methods
149
To begin unravelling the mystery of lightning at Venus, we predict the rate of optical
150
flashes produced by meteor fireballs. We compare those rates to those inferred from the Akatsuki
151
(Takahashi et al., 2021; Lorenz et al., 2022) and Mt. Bigelow (Hansell et al., 1995) surveys. If
152
the observed flashes occur at a rate consistent with that predicted for meteor fireballs, then one
153
could conclude that meteor fireballs are a plausible explanation for all the flashes. However, if
154
the observed flashes are much more frequent, then some of them probably originated from
155
lightning.
156
2.1 Searches for optical flashes at Venus
157
Table 1 summarizes five searches for optical flashes at Venus, following Table 1 in
158
Lorenz et al. (2019). Here, our first goal is to use these observations to place statistical
159
constraints on the global, yearly rate of optical flashes at Venus.
160
Two surveys yielded the most believable detections of optical flashes at Venus. The
161
Akatsuki mission entered orbit at Venus in December 2015 (e.g., Nakamura et al., 2016) and
162
initiated a search for optical transients with its Lightning and Airglow Camera (LAC) in 2016
163
(e.g., Takahashi et al., 2018). LAC uses a filter centered at 780.6 nm with a bandwidth (full
164
width at half maximum) of 9.0 nm, which is sufficient to capture the emission from the OI triplet
165
at 777 nm. The sampling rate is 31.25 kHz with a spatial resolution of ~175 km at a distance of
166
~5000 km (Takahashi et al., 2018). LAC is only operated during the spacecraft’s orbit when
167
Venus blocks sunlight from directly hitting its sensor. In its first years of operation, the LAC
168
team confirmed that the triggering system functioned correctly and detected several cosmic rays
169
(Takahashi et al., 2018). After three years, LAC operated for a total of 16.8 hours, covering an
170
area-time product of ~82 × 106 km2-hr, without any detections of a flash attributable to lightning
171
or meteors (Lorenz et al. 2019). By late 2020, LAC accumulated an area-time product of >100 ×
172
106 km2-hr and detected a single optical flash that lasted ~100 ms with a total optical energy near
173
777 nm of EOI ~ 1.1 × 107 J (Takahashi et al., 2021), which is several times brighter than the
174
detection limit of EOI ~ 5 × 105 J to 2 × 106 J (Takahashi et al., 2018). As of late 2022, LAC
175
reached an area-time product of at least ~200 × 106 km2-hr without any additional detections of
176
lightning-like flashes, as shown in Table 1 (Lorenz et al., 2022).
177
In the early 1990s, a ground-based telescope observed several candidate flashes at Venus.
178
That search used a 1.5-m telescope on Mt. Bigelow, Arizona to image the night side of Venus
179
with a sampling rate of 18.8 Hz over several nights. The instrument used a narrowband filter
180
centered at 777.4 nm, with a bandwidth of 0.7 nm (Hansell et al., 1995). Overall, this survey
181
detected seven flashes, each in a single frame of the imaging sequence collected in ~53 ms.
182
Takahashi et al. (2018) argued that using only one frame per flash admits the possibility that
183
cosmic rays or unknown electrical noise could have produced some flashes. Table 2 of Hansell et
184
al. (1995) reports the “associated optical energy” for each flash, which equals 2.5 times the
185
optical energy measured near 777 nm. This factor of 2.5 (=1/0.4) was inserted based on the
186
assumption that Venusian lightning would emit ~40% of its total optical energy near 777 nm
187
(e.g., Borucki et al., 1981). Therefore, associated optical energies of ~0.1–2.1 × 109 J are
188
equivalent to EOI ~4–84 × 107 J near 777 nm. Hansell et al. (1995) inferred a detection limit of
189

manuscript submitted to Journal of Geophysical Research: Planets
EOI ~0.6–2.5 × 106 J for 50–95% detections. Overall, the area-time product from this ground-
190
based survey reached at least ~800 × 106 km2-hr (Hansell et al., 1995), counting additional nights
191
of non-detections by that group that were not published (Lorenz et al., 2019). Other groups may
192
have conducted similar searches using ground-based telescopes with no detections (Yair et al.,
193
2012), but those efforts have not been published or included in Table 1.
194
Three other attempts to observe optical flashes at Venus were less successful. The
195
Pioneer Venus Star Tracker (Borucki et al., 1991) and Venus Express VIRTIS (Moinelo et al.,
196
2016) accumulated area-time products of ~105 and 14 × 106 km2-hr, respectively, with no
197
detections. The Venera 9/10 Orbiter Spectrometer recorded a burst of light lasting ~70 s, almost
198
immediately after it began observations (Krasnopolsky, 1979; 1983). Some authors attribute this
199
observation to lightning (e.g., Hart et al., 2022), while others argue that instrument anomalies or
200
even spacecraft debris are a more likely explanation (e.g., Lorenz, 2018; Lorenz et al., 2019).
201
The total area-time product associated with Venera 9/10 was relatively tiny, only ~2.5 × 103
202
km2-hr (Lorenz et al., 2019). Ultimately, we might not be surprised that the two surveys with by
203
far the largest coverage are the ones that yielded the most reliable detections.
204
205
Table 1. Two surveys produced relatively reliable detections of optical flashes in the
atmosphere of Venus. Three other surveys, which had much lower coverage, did not return
any uncontested detections.
Search
Coverage
(106
km2-hr)
Number of
lightning-
esque
flashes
Equivalent global,
yearly rate (95%
confidence
intervals)
Detection
threshold (J)
Energy of
the dimmest
flash near
777 nm (J)
Mt. Bigelow
800
7
35423
(17476, 66808)
2.5 × 106
(95%)
0.6 × 106
(50%)
2.8 × 107
Akatsuki
200
1
20241 (4902,
112770)
0.5–2 × 106
1.1 × 107
Venera 9/10
Orbiter
Spectrometer
0.0025
0
0 (< 5 × 109)
3 × 107
-
Venus
Express
VIRTIS
14
0
0 (< 9 × 105)
Unknown
-
Pioneer
Venus Star
Tracker
0.1
0
0 (< 108)
2 × 108
-
To compare with the power laws derived in the next subsection, we need to translate
206
these observations into statistical constraints on the global rate of flashes at Venus. Say that a
207
survey observes a given area (AS) for a given time (TS). Its area-time product is ASTS. If N is the
208

manuscript submitted to Journal of Geophysical Research: Planets
total number of bright flashes (i.e., with optical energies above a certain detection limit) across
209
Venus during one Earth-year, then the expected number of flashes observed by that survey is
210
!!" #
$
%!
%"
&$
'!
'#
&
(
)
*
+
211
where AV = 4.6 × 108 km2 is the surface area of Venus and TY = 8.8 × 103 hr is the duration of an
212
Earth-year. We assume that Poisson statistics govern optical flashes (from lightning or meteors),
213
meaning that the probability of observing an integer number of flashes (k) during a survey is
214
,
)
-.
/
.!!
+
"!!
$0%&!
-1 2
)
3
+
215
Here, we know that k = 1 and 7 for the Akatsuki and Mt. Bigelow surveys, respectively. We want
216
to calculate the probability density function for lS and thus N according to both surveys. By
217
Bayes’ theorem, P(λS | k) 4 P(k | λS). We then convert lS to N using Eq. 1 and renormalize the
218
probability density function so it integrates to 1 from N equals zero to infinity.
219
Figure 1 shows our statistical constraints on the global, yearly rate of optical flashes at
220
Venus. For surveys with k = 0, we can say that lS ≤ 3 with 95% confidence. The upper limits on
221
lS are thus ~9 × 105, ~108, and ~5 × 109, respectively, from the non-detections by Venus Express
222
VIRTIS, the Pioneer Venus Star Tracker, and the Venera 9/10 Orbiter Spectrometer (assuming
223
Venera 9/10 saw only spacecraft debris or an instrument anomaly). For surveys with more
224
reliable detections of optical flashes (k ≥ 1), the most likely value of lS is simply k. Using Eq. 1,
225
the most likely values of N are 20241 and 35423 for Akatsuki and Mt. Bigelow, respectively.
226
With Eq. 2, we calculated the 95% (“two-sigma”) confidence intervals associated with each
227
survey: (4902, 112770) for Akatsuki and (17476, 66808) for Mt. Bigelow. The 68.3% (“one-
228
sigma”) confidence intervals are (14325, 66808) and (14325, 54509), respectively. These
229
surveys are both consistent with a global flash rate of N ~ 104–105 per Earth-year. However, we
230
report these estimates separately because each survey may have a different (albeit similar)
231
detection limit.
232
233
Figure 1. Probability density functions for the global, yearly rate of bright flashes at Venus (i.e.,
234
the expected number across the entire planet in one Earth-year with optical energies above the
235
Akatsuki
Mt. Bigelow

manuscript submitted to Journal of Geophysical Research: Planets
detection limit of each instrument), estimated from the coverage and detections in surveys from
236
Akatsuki and Mt. Bigelow.
237
2.2 Impactor production functions and luminous efficiency
238
To quantify the number of bolides that enter Venus’s atmosphere in one Earth-year, we
239
turned to existing studies of meteors at Earth (Figure 2). Le Feuvre & Wieczorek (2011)
240
developed a 10th-order power law to describe impactor flux at Venus over a wide range of bolide
241
diameters (Figure 2a). However, our study requires only a log-linear power law, for simplicity, to
242
describe the flux of small impactors at Venus. We thus start with power laws developed by
243
Brown et al. (2002), which were the basis for the low-mass end of the expression derived in Le
244
Feuvre & Wieczorek (2011). Based on the observations described in Section 2.1, we are
245
interested in meteors that could produce flashes with optical energies ~107 J. As discussed
246
below, the kinetic energy of those meteors should be at least an order of magnitude higher (~108
247
J). Depending on the impact velocity, the masses of meteors potentially relevant to the detected
248
optical flashes are thus roughly ~0.1–1 kg.
249
We start with an established relationship between the frequency and kinetic energy of
250
small bolides that collide with Earth (Brown et al., 2002):
251
567'( #)"**289:;28;567'( <*(
)
9
+
252
where NE is the cumulative number of bolides that strike Earth in one Earth-year with a kinetic
253
energy of at least EK (in Joules). This equation is the same as Eq. 2 by Brown et al. (2002), who
254
expressed EK in units of kiloton-TNT equivalent.
255
Next, we write the equivalent equation for Venus. On average, bolides strike Venus with
256
faster velocities and thus higher kinetic energies. Le Feuvre & Wieczorek (2011) found that the
257
average impact velocity at Venus is 25.0 km/s, whereas the average velocity for Earth is 20.3
258
km/s (Brown et al., 2002). From Table 3 in Le Feuvre & Wieczorek (2011), the impact rate at
259
Venus per unit area is higher by a factor of 1.75/1.58 = 1.11 than the impact rate at Earth.
260
However, Venus also has less surface area than Earth, by a factor of (6052/6371)2 = 0.902.
261
Overall, we can combine these factors to write the power law for the bolide flux at Venus:
262
567'( # " *32;8 : ;28;567'( <*(
)
=
+
263
where N is now the cumulative number of bolides that strike Venus in one Earth-year with
264
kinetic energies of at least EK (in Joules).
265
When a meteor ablates in a planet’s atmosphere, only a fraction of the total kinetic energy
266
is converted to optical energy. We can relate kinetic and optical energy with EO = t EK, where t
267
is the luminous efficiency of the bolide. For meteors with mass m up to a few kg and entry
268
velocities v < 25.372 km/s, t may obey the following equation (e.g., Popova et al. 2005):
269
5>? " ;2@AB:*;29;B5>C D 82BE*
)
5>C
+
+:92;=*=.
)
5>C
+
,D;293*9
)
5>C
+
-D270
;29=BFG>H
)
;29E5>I
+
2)@+
271
We defined v as the average entry velocity (25 km/s) for meteors ranging in mass from 10-1 to
272
102 kg. We then found the best-fit power law that relates t (dimensionless) and EO (J), following
273
Brown et al. (2002):
274
? " ;2;*==3.<.
(/(0-1(
)
A
+
275

manuscript submitted to Journal of Geophysical Research: Planets
which predicts luminous efficiencies from ~0.05–0.08 over this size range (Figure 2c). By
276
incorporating Eqn. (6) into Eqn. (4), we then find that
277
JKL'(# " *;2=9:;2E3.JKL'(<.2
)
B
+
278
However, not all the optical energy is emitted near 777 nm, to which Akatsuki’s LAC filter and
279
the search for flashes at Mt. Bigelow were restricted. Only a small fraction of the total optical
280
energy is due to the excited oxygen triplet:
281
<.2 " M.2<.(
)
E
+
282
where 0 < fOI < 1. In terms of EOI, the power law for the rate of meteor fireballs at Venus is:
283
JKL'(# : *;2=9:;2E3JKL'( <.2
M.2 2
)
8
+
284
The factor fOI is uncertain and variable for meteor fireballs at Earth (e.g., Vojáček et al., 2022).
285
The uncertainty is even greater for meteors at Venus, but we expect relatively high values of fOI
286
because of the relative abundance of oxygen atoms in carbon dioxide. If we assume a certain
287
value of fOI , then we can estimate the rate of meteor fireballs with different brightnesses at
288
Venus—and thus if the observed flashes plausibly originated from meteors, not lightning.
289
290
Figure 2. The number, velocity, and energy of bolides colliding with Venus can be modeled
291
from previous work. In (a) we plot the number of bolides above a certain diameter colliding with
292
Venus per Earth-year (e.g., Le Feuvre & Wieczorak, 2011). In (b) is the cumulative distribution
293
function for bolide entry velocity. The average entry velocity for meteors at Venus is 25 km/s. In
294
(c) we plot the best-fit power law for the luminous efficiency of meteors that we found using
295
equations from Popova et al. (2005). Luminous efficiency—the amount of kinetic energy
296
converted to optical energy—increases as entry mass and entry velocity increase. In (d) we plot
297

manuscript submitted to Journal of Geophysical Research: Planets
the number of bolides with a certain energy colliding with Earth and Venus per Earth-year.
298
Impactors at Venus have higher kinetic energy due to their higher average entry velocity. EO is
299
the optical energy emitted per bolide; only a fraction of kinetic energy is converted to optical
300
energy during ablation, as defined by the luminous efficiency in (c).
301
2.3 Experimental simulations of meteor plasma
302
No one has yet measured the emission spectrum of a meteor fireball at Venus. In the
303
absence of direct observations, we can turn to laboratory experiments for hints about what
304
fraction of the total optical energy might be emitted near 777 nm. Many groups have used laser-
305
induced breakdown spectroscopy (LIBS) to simulate a meteor fireball (e.g., Krivkova et al.,
306
2021; Ferus et al., 2018; Dell’Aglio et al., 2010). A high-power laser can ablate a meteorite in a
307
similar fashion to meteor ablation during high-speed collisions with atmospheric molecules.
308
Experimental studies relevant to terrestrial meteors typically conduct LIBS experiments on
309
meteorites in a vacuum or under ambient atmospheric conditions. However, for application to
310
Venus meteors, we would prefer to invoke LIBS experiments conducted on rock and mineral
311
samples surrounded by (predominantly) carbon dioxide at a pressure of a few mbar.
312
Coincidentally, air at the surface of Mars has approximately the same composition and pressure
313
as air at an altitude of ~100 km above the surface of Venus.
314
Because of this similarity, we utilized results from the LIBS experiments that were
315
conducted to calibrate the ChemCam instrument package on the Mars rover Curiosity (Wiens et
316
al., 2013). We used data from the Los Alamos National Laboratory ChemCam experiments,
317
where samples were measured in 5 locations. For each sample, 50 laser pulses were taken at each
318
location and averaged together. After the data were collected, the spectra were cleaned and
319
calibrated. We used the cleaned and calibrated dataset to estimate the fraction of optical energy
320
near 777 nm. We analyzed 5 different samples to determine an average fraction of optical
321
energy. Olivine ([Fe,Mg]2SiO4) and pyroxene ([Fe,Mg]SiO3) are common in stony meteorities as
322
chondrules. We also analyzed spectra from samples of diopside (CaMgSi2O6), llanite (a
323
rhyolite), and basalt. Diopside and llanite provided comparative results despite not being as
324
common in meteorites as the other materials. The calibration samples did not include water ice,
325
but we expect that a comet’s fireball would produce even more OI emission than a rocky meteor.
326
To calculate fOI for each mineral or rock, we calculated the area under the spectral curve for the
327
entire spectrum from 350–800 nm, as well as the area within the OI peak from 771–800 nm. We
328
then divided the area under the OI peak by the total area of the spectrum, producing fOI.
329
3 Results
330
3.1 Meteor fireballs are not (always) blackbodies
331
Many studies assume that ablating meteors in Venus’s atmosphere would emit as
332
blackbodies (e.g., Takahashi et al. 2021). If a meteor ablating at ~6000 K emitted as a blackbody,
333
then only a very small amount of the total optical energy (<1%) would be contained in the
334
excited oxygen line at ~777 nm or in the bandpass of the instruments designed to detect this line.
335
The small amount of observed energy would require a very large, and thus very infrequent,
336
meteor to cause the observed amount of optical energy. However, recent studies have shown that
337
small meteors, such as the one observed by Madiedo et al. (2013) in the Geminid meteor shower
338
on Earth, do not always emit as blackbodies (Figure 3). By calculating the area under the
339

manuscript submitted to Journal of Geophysical Research: Planets
spectral curve, we determine that ~7% of the total optical energy produced by this meteor was
340
contained in the excited atomic oxygen line.
341
342
Figure 3. Two spectra with the same amount of optical energy near 777 nm, but very different
343
amounts of total optical energy. A Geminid meteor produced a spectrum (black and purple)
344
distinctly different than a blackbody curve (gray) for an effective ablation temperature of 6000
345
K. At Venus, due to the large relative abundance of oxygen, ~5–30% of a small meteor’s optical
346
emission may be contained in the OI triplet (purple).
347
348
Figure 4. Fraction of total optical energy emitted in the 777 nm bandpass, based on inspection of
349
LIBS spectra from the ChemCam calibration database, which provide a potential analogue for
350
the emission spectra of meteors. Near the surface of Mars, the atmosphere is CO2-dominated
351
with pressures of a few millibars—exactly the conditions at altitudes near ~100 km in the
352
atmosphere of Venus where small meteors burn up. We find that ~4–7% of the total optical
353
energy contained in these spectra is typically emitted near 777 nm. For minerals and rock types
354
that are common in meteors, as well as the more silicic llanite, this plot shows the mean (gold)
355
and standard deviation (maroon) of that fraction for 5 samples in the database.
356
Blackbody
SPMN151209 Geminid bolide
(Madiedo et al., 2013)
OI Emission

manuscript submitted to Journal of Geophysical Research: Planets
We used LIBS data from the calibration of Curiosity’s ChemCam instrument to estimate
357
meteoritic emission in the OI emission line for ablating bolides at Venus. To better constrain the
358
amount of energy in the excited oxygen triplet for different materials, we analyzed common
359
meteoritic materials for the fraction of optical energy contained around 777 nm (Figure 4). We
360
found that, for common meteoritic materials such as olivine, pyroxene, and basalt, the value of
361
fOI ranges from ~0.04–0.07 (Figure 4). Based on these laboratory experiments and the
362
observations of some terrestrial meteors, we thus expect that fireballs from small meteors at
363
Venus are an order-of-magnitude brighter than blackbodies near 777 nm. However, the emission
364
spectrum of an individual meteor will depend on many factors, including its composition, mass,
365
entry velocity, entry angle, and irregularities that cause it to fragment and/or spin as it ablates.
366
3.2 Meteor fireballs in surveys from Akatsuki and the Mt. Bigelow 61-in. telescope
367
Figure 5 plots the number of flashes per Earth-year expected for a certain amount of
368
optical energy produced by the ablating bolide, based on the amount of optical energy contained
369
in the OI filter centered near 777 nm (fOI). For higher values of fOI, we expect to see more flashes
370
of a given brightness in one Earth-year. As detailed above, a meteor fireball that emits as a
371
blackbody should have fOI ~0.007. Observations of terrestrial fireballs and LIBS experiments
372
suggest that fOI ~0.05–0.10 is more realistic for small meteors. We estimate that an upper limit
373
for fOI is ~0.3, perhaps for a comet that hit Venus at high velocities. We compare these
374
expectations to the flash rates inferred from the two surveys with relatively reliable detections.
375
The one optical flash detected by Akatsuki’s LAC (so far) had a brightness that is
376
consistent with a meteor fireball. That is, if fOI ~0.07, then we expect to witness the same number
377
of flashes in one Earth-year from these meteors as the expected number from Akatsuki’s
378
estimated global flash rate. However, this result is somewhat conditional on the true detection
379
limit for Akatsuki’s LAC. Even if LAC’s detection limit is higher than claimed by Takahashi et
380
al. (2018) (i.e., equal to the flash brightness in the worst case), a meteor is still statistically
381
probable within the 95% confidence intervals for fOI > 0.10. However, Takahashi et al. (2018)
382
also claimed that the detection limit for Akatsuki’s LAC is perhaps as low as EOI ~5 × 105 J, or
383
even lower. In that case, observing at least one flash from a meteor fireball is not surprising.
384
Meteor fireballs are perhaps also bright and frequent enough to explain the observations
385
from the Mt. Bigelow 61-in. telescope. If the dimmest observed flash were observed at exactly
386
the detection limit of that survey, then we would only predict the observation of seven meteor
387
fireballs if ~30% of the total optical energy were concentrated in the OI filter. However, Hansell
388
et al. (1995) estimated that their detection limit was much lower, which is also consistent with fOI
389
~0.05–0.10, exactly what we expect for fireballs at Venus. Yair et al. (2012) also conveyed a
390
personal communication about “repeated attempts by large-mirror ground telescopes to repeat
391
the Hansell et al. (1995) observations,” which apparently have not yielded any additional
392
detections. If the effective area-time product for ground-based surveys is higher than Table 1
393
indicates, then the extrapolated number of flashes per Earth-year would decrease—and thus
394
agree even better with the predicted rate of meteor fireballs at Venus. Finally, recent papers
395
noted that the observations at Mt. Bigelow were not conducted at a high enough sampling rate to
396
take more than one image per flash, leaving some ambiguity about whether a cosmic ray or
397
electrical noise produced one or more of the flashes (e.g., Takahashi et al. 2018). If one or two of
398
the claimed flashes did not originate from Venus, then the observed rate would agree perfectly
399
with what we predict for meteor fireballs and with the current results from Akatsuki. Using Eq. 1
400

manuscript submitted to Journal of Geophysical Research: Planets
and 2, we calculate that the upper limit on the global, yearly rate of lightning in the clouds is N ≤
401
11975 (< 4 × 10-4 Hz), with 95% confidence—if none of the observed flashes originate from
402
lightning.
403
Our study focuses on reproducing the hypothesized lightning flash rates at Venus with an
404
alternative source of optical energy in the form of ablating meteors tens of km above the cloud
405
layer. However, the shapes of any observed light curves would provide additional constraints.
406
The shape of the light curve observed by Akatsuki was positively skewed (e.g., Takahashi et al.
407
2021), which the team argued as most consistent with lightning. Models predict a negatively
408
skewed light curve from the ablation of a spherical meteor that does not fragment. However,
409
ablating meteors probably are non-spherical and also fragment and spin as they descend through
410
the atmosphere, making it difficult to predict the shape of an individual bolide-produced light
411
curve. We did not construct any models for the light curves of meteors at Venus—but such
412
efforts will only become more important as the number of time-resolved observations increases.
413
Preliminary work suggests that Venusian meteors are indeed brighter than terrestrial meteors
414
(e.g., McAuliffe & Christou, 2006; Christou & Gritsevich, 2023).
415
416
Figure 5. Estimate of the expected number of optical flashes at Venus in one Earth-year (
N
) that
417
would release at least a certain amount of energy near 777 nm (EOI). The black and grey circles
418
show the global rates inferred from Akatsuki and Mt. Bigelow, respectively. The vertical bars
419
denote the 95% confidence intervals on the global rate. The horizontal, dashed bars reflect
420
uncertainty about the detection limits of both surveys. On the right, the dashed bars extend to the
421
optical energy near 777 nm from the dimest flash that each survey detected, which is the highest
422
possible detection limit. The circles are centered at the claimed detection limits for Akatsuki
423
(e.g., Takahashi et al., 2018) and the Mt. Bigelow survey (Hansell et al., 1995). On the left, the
424
dashed bars extend to the lowest plausible values of the detection limit claimed for each survey.
425
fOI = 0.07
fOI= 0.10
fOI = 0.30
fOI = 0.05
fOI = 0.007

manuscript submitted to Journal of Geophysical Research: Planets
4 Discussion
426
4.1 Sub-cloud lightning is possible
427
Regardless of whether lightning exists high in Venus’s atmosphere, lightning could occur
428
close to the surface from either volcanic or aeolian processes (Figure 6). On Earth, volcanic
429
lightning often occurs in the ash plume associated with an explosive eruption. The particles in
430
the plume can become charged through several mechanisms, but fracto-electrification and tribo-
431
electrification are considered the most important because they are closely related to explosive
432
eruption dynamics (e.g., Cimarelli & Genareau, 2022). Material is fractured into ash-sized
433
particles during an explosive eruption, which can release electrons and positive ions and charge
434
the fragmented particles themselves (fracto-electrification). Ash particles of various
435
compositions within the plume will then collide with each other, charging the particles through
436
friction (tribo-electrification) (e.g., Cimarelli & Genareau, 2022). At the surface, winds carrying
437
small particles can also create charges through tribo-electrification. This process could be a
438
common phenomenon on Venus because wind speeds are close to the transport threshold (e.g.,
439
Lorenz, 2018). The Venera landers observed the movement of surface material, which implies
440
that wind may be capable of charging particles on Venus’s surface (e.g., Lorenz, 2018).
441
Previous studies disregarded volcanic lightning on Venus as impossible due to the
442
supposed lack of explosive volcanism. Borucki (1982) argued that if volcanism caused the then-
443
claimed observation of 30 lightning flashes in 3 years by the Pioneer Venus Orbiter, there would
444
have been 10 eruptions per year. If explosive volcanism were occurring at this rate, then it would
445
release particles into the atmosphere that could be detected. However, the cloud-particle-size
446
spectrometer on Pioneer Venus did not detect particles of the size expected to result from
447
explosive volcanism. Borucki (1982) therefore concluded that, even if Venus were volcanically
448
active, explosive volcanism was not common and thus not a probable source of lightning. They
449
also argued that lightning would have to occur in the clouds because the lower atmosphere would
450
absorb energy at the wavelengths produced by lightning (e.g., Borucki, 1982). However, those
451
specific detections are now attributed to cosmic rays (e.g., Lorenz, 2018). The electromagnetic
452
observations that yield the highest inferred rates of lightning only constrain the source of those
453
signals to below the ionosphere—compatible with a near-surface origin.
454
New evidence of explosive volcanism on Venus has recently emerged. For example,
455
Ganesh et al. (2021) modeled the formation of several proposed pyroclastic deposits on Venus.
456
These pyroclastic flows would have formed through the collapse of ash plumes created during
457
explosive eruptions. Their models of collapsing plumes provided good matches to deposits at
458
several locations on Venus hypothesized to be associated with explosive volcanism. Recently, a
459
volcano that changed shape over the course of eight months during the Magellan mission was
460
identified (e.g., Herrick & Hensley, 2023). This is evidence of active volcanism on Venus in
461
recent years, which further supports the position that the possibility of volcanic lightning should
462
not be disregarded. Of course, new observations from future geophysical orbiters such as
463
VERITAS and EnVision are needed to unveil the volcanic history of Venus.
464

manuscript submitted to Journal of Geophysical Research: Planets
465
Figure 6. A cartoon of possible phenomena in Venus’s atmosphere. Small meteors may burn up
466
far above the clouds, while near-surface lightning could generate the putative whistler-mode
467
waves from far below the clouds. Adapted from Figure 6 in O’Rourke et al. (2023).
468
4.2 Lightning is not a hazard to missions in the clouds
469
Many missions to Venus passed through its clouds. No mission has, to our knowledge,
470
been struck by lightning, but lightning nonetheless poses a potential risk to any mission. Starting
471
with Venera 7, ~14 probes have delivered data from below the clouds (e.g., Taylor et al., 2018).
472
Two balloons floated at an equilibrium altitude of ~53 km as part of the Soviet VeGa mission,
473
reporting data for ~47 hours before running out of battery power (e.g., Sagdeev et al., 1986;
474
Moroz, 1987; Crisp et al., 1990). Given these past experiences, lightning is not an obvious
475
hazard to atmospheric probes or short-lived balloons. However, future missions may include
476
extended stays in the clouds to answer high-priority scientific questions (e.g., O’Rourke et al.,
477
2021; Arredondo et al., 2022; Cutts et al., 2022). For example, Phantom is an exciting, well-
478
developed concept that features an aerial platform that dwells in the clouds for at least ~30 Earth-
479
200
100
0
150
50
Altitude (km)
Ionosphere
(~120–300 km)
Meteor
fireballs?
Clouds
(~40–70 km)
Volcanism &
aeolian activity?

manuscript submitted to Journal of Geophysical Research: Planets
days, and plausibly ~100 Earth-days or longer (e.g., Byrne et al., 2023). Scientists have also
480
proposed sending flotillas of long-lived balloons to search for active biology (Hein et al., 2020)
481
or seismic and volcanic activity (Krishnamoorthy & Bowman, 2023; Rossi et al., 2023). Is
482
lightning a threat to long-duration missions in the clouds of Venus?
483
We can use simple statistics to estimate the hazard from lightning to various types of
484
missions. Say that TM is the duration of the mission (in seconds); GL is the overall rate of
485
lightning in the clouds (in strikes per second); and RH is the radius within which a lightning
486
strike is potentially hazardous (in meters). The estimated number of lightning strikes within the
487
hazardous radius during the mission is then
488
!3" '4N3
$
O5
3O"
&
+(
)
*;
+
489
where RV = 6052 km is the radius of Venus, assuming that RH << RV. If lightning strikes happen
490
without spatial bias but with timing that obeys Poisson statistics, then we can calculate the
491
probability that a hazardous strike will occur near the mission:
492
,5" * :0%6"2
)
**
+
493
Mission designers often consider a hazard with a probability of PH < 10-6 to be negligible. We
494
considered three possible rates of lightning in the clouds. Overall, our study is compatible with
495
the hypothesis that cloud-based lightning is vanishingly rare (GL ≤ 10-9 Hz). If the optical flashes
496
observed by Akatsuki and ground-based telescopes originated from cloud-based lightning, then
497
GL ~ 1.2 × 10-4 Hz (Lorenz et al., 2022). If the claimed whistler-mode waves were attributed to
498
cloud-based lightning, then we fear that GL ~ 320 Hz (Hart et al., 2022). Of course, as on Earth,
499
we would expect to find lightning on Venus concentrated in particular regions at any given time.
500
However, we can use the assumption of global homogeneity to estimate relative hazards. If
501
lightning were ubiquitous in the clouds, then requiring a mission to survive a strike could seem
502
prudent. However, no caution is necessary if there is no (or very rare) lightning.
503
Figure 7 shows the probability of a hazardous lightning encounter for four classes of
504
missions. First, probes can pass quickly through the clouds. For example, the DAVINCI mission
505
notionally plans to descend from ~70–40 km in ~34 minutes between the deployment and release
506
of its parachute (Garvin et al., 2022). If lightning were indeed ≥ 7 times as common on Venus as
507
Earth (Hart et al., 2022), then we might expect ~40 strikes within ~100 km of DAVINCI (and all
508
past probes). However, the expected number of strikes near a probe is < 1 using the flash rate
509
inferred from Akatsuki’s search. Second, the VeGa mission was the archetype of a short-lived
510
balloon, operating for ~47 hours. Again, the flash rate inferred from Akatsuki is compatible with
511
the non-detection of lightning (i.e., lL ~ 10-3 for RH ~ 100 km). Third, we assume that a long-
512
lived balloon has a lifetime of ~30 Earth-days. The chances of a nearby strike (RH < 100 km) are
513
then only 1-in-50, according to the optical flash rate from Akatsuki. This contrasts with the
514
analysis in Hart et al. (2022), which implies that such a mission could operate in the vicinity of
515
>50,000 strikes. Finally, we imagined a flotilla of 10 balloons that each have lifetimes of ~100
516
Earth-days. Using the flash rate from Akatsuki, it is more likely than not that at least one of those
517
balloons encountering a strike within ~90 km. However, perhaps such a moderately distant strike
518
would seem more exciting than dangerous. Ultimately, especially given the possibility that
519
meteor fireballs produced all the optical flashes observed at a Venus, there is as of yet no
520

manuscript submitted to Journal of Geophysical Research: Planets
affirmative evidence that lightning is common enough in the clouds to pose a hazard to even
521
fleets of long-lived aerial platforms.
522
Figure 7. Relative risk of lightning to various mission architectures. Lightning is only a hazard
523
to missions in the clouds if lightning exists in the clouds. We have calculated the probability that
524
a lightning strike would occur within a certain horizontal distance (vertical axes) as a function of
525
the total time that a mission spends within the clouds (horizontal axes). Although that time varies
526
by five orders of magnitude for different types of missions, estimates for the rate of lightning in
527
the clouds span ~10 orders of magnitude. In (a) we use the rate of lightning inferred from studies
528
of putative whistler-mode waves (Hart et al., 2022). In (b) we use the global rate derived from
529
Akatsuki’s observation of a single flash so far, assuming that flash originated from lightning
530
(Lorenz et al., 2022). In (c) we use the highest rate that implies that even a balloon flotilla
531
experiences a negligible risk (PH < 10-6) from lightning, which agrees with the hypothesis that no
532
flashes from lightning have ever been seen at Venus.
533
Probability of a “hazardous” lightning strike, log10(PH)
Time in the clouds, log10(TM[s])
Radius (km)
GL= 320 Hz
Radius (km)
GL= 1.2 ×10-4 Hz
Radius (km)
GL= 10-9 Hz
Probe
(~34 min.) Short-lived
balloon
(~47 hours)
Long-lived
balloon
(~30 Earth-
days)
Balloon
flotilla
(~10 ×100
Earth-days)
a
b
c
10
100
10
100
10
100

manuscript submitted to Journal of Geophysical Research: Planets
4.3 The search must go on
534
In our work we have developed a production function for meteor fireballs in the
535
atmosphere of Venus, which should be revisited as flash rates become better determined by
536
future observations. While we found that small meteors ablating high (~100 km) in the
537
atmosphere are plausible explanations for the observed optical flashes, more optical flash
538
observations would serve to sharpen our statistics and provide tighter quantifications of flash
539
rates. Spectrally resolving optical flashes at Venus could verify our study’s estimate that meteor
540
fireballs at Venus have strong emission near 777 nm. Additionally, determining the altitude of
541
recorded optical flashes would allow us to conclude whether they originated above or within the
542
cloud layer, providing more evidence of their source. Meteor fireballs and sprites would occur
543
tens of kilometers above the clouds. Only a very rare, huge meteor would reach the clouds.
544
The “gold standard” approach to lightning detection would be simultaneous optical and
545
radio observations (e.g., Aplin & Fischer, 2017; Cartier, 2020). Lightning above the lower cloud
546
deck could produce an observable optical flash, whistlers at kHz frequencies, and sferics at MHz
547
frequencies. In contrast, meteors would not produce strong radio emissions. Meteors themselves
548
make important contributions to atmospheric chemistry (e.g., Pätzold, et al., 2009; Carrillo-
549
Sánchez et al., 2020) and produce infrasonic signatures that aerial platforms could observe (e.g.,
550
Silber et al., 2018; 2023). Lightning below the clouds may produce radio emissions but not
551
optical flashes visible from space. If the diagram of transient phenomena on Venus shown in
552
Figure 6 were correct, then we would expect optical flashes consistent with the power laws
553
derived in this study, plus non-simultaneous radio emission in the form of whistlers and sferics.
554
5 Conclusions
555
For decades, searches have been conducted for concrete proof of lightning in the
556
atmosphere of Venus. Proving or disproving its existence would have vast implications for
557
scientists’ understanding of Venus’s atmospheric chemistry, weather patterns, and even the
558
potential for life in the clouds. Though multiple pieces of evidence such as whistler-mode waves
559
and optical flashes have been put forward as proof of lightning, the presence of cloud-based
560
lightning remains an open question. In this study, we have investigated whether small meteors
561
may have produced optical flashes in the atmosphere of Venus that were interpreted as lightning.
562
We calculated the rates of expected optical flashes from ablating bolides, and compared those to
563
the rates inferred from optical surveys. We also calculated the risk posed to cloud-based missions
564
considering the estimated lightning rates from these optical surveys. We find that based on
565
observations of meteor fireballs at Earth, ablating fireballs at ~100 km altitude may be
566
responsible for most, or even possibly all of the observed flashes. Lightning thus does not seem
567
like a threat to missions that pass through (e.g., probes) or even linger within (e.g., aerial
568
platforms) the clouds. Future optical surveys should find more meteor fireballs at rates and
569
brightnesses that match our power laws. Simultaneous optical and radio measurements would
570
help in the hunt for definitive evidence of lightning.
571
Acknowledgments
572
C. H. Blaske acknowledges the ASU/NASA Space Grant Internship program for providing
573
funding for the LIBS/ChemCam portion of this research project.
574

manuscript submitted to Journal of Geophysical Research: Planets
Open Research
575
Software (other than for typesetting) was not used for this research. Datasets for this research are
576
available in these in-text data citation references: Wiens et al. (2013).
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manuscript submitted to Journal of Geophysical Research: Planets
Meteors May Masquerade as Lightning in the Atmosphere of Venus
1
C. H. Blaske1,2,†, J. G. O’Rourke2, S. J. Desch2, and M. E. Borrelli2
2
1Barrett, The Honors College, Arizona State University, Tempe, AZ, USA. 2School of Earth and
3
Space Exploration, Arizona State University, Tempe, AZ, USA.
4
Corresponding authors: C. H. Blaske (cblaske@asu.edu), J. G. O’Rourke (jgorourk@asu.edu)
5
†Current address: Earth & Planetary Sciences, Stanford University, Stanford, CA, USA.
6
Key Points:
7
• We investigate whether meteor fireballs could have produced the optical flashes that have
8
been detected at Venus and attributed to lightning
9
• We find that flashes from meteor fireballs are statistically likely to occur at the observed
10
rates and brightness
11
• There is no affirmative evidence that lightning is a hazard to missions that pass through
12
or dwell within the clouds of Venus
13

manuscript submitted to Journal of Geophysical Research: Planets
Abstract
14
Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on
15
how one interprets diverse observations. Quantifying when and where, or even if lightning
16
occurs would provide novel information about Venus’s atmospheric dynamics and chemistry.
17
Lightning is also a potential risk to future missions, which could float in the cloud layers (~50–
18
70 km above the surface) for up to an Earth-year. Over decades, spacecraft and ground-based
19
telescopes have searched for lightning at Venus using many instruments, including
20
magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and
21
the 61-inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the
22
unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning.
23
This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing
24
such energetic flashes, based in turn on the presumption that a low fraction (< 1%) of a meteor’s
25
optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue
26
experiments to show that a much higher conversion factor (~5–10%) should be expected.
27
Therefore, we calculate that smaller, more numerous meteors could have caused the observed
28
flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the
29
clouds of Venus. Likewise, small meteors burn up at altitudes of ~100 km, roughly twice as high
30
above the surface as the clouds, and also would not pose a hazard.
31
Plain Language Summary
32
Artists depicting the atmosphere of Venus love to include lightning bolts to emphasize its hellish
33
environment. Even though missions like DAVINCI would most likely be safe from strikes as
34
they descend quickly through the atmosphere, long-lived aerial platform missions supported by
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large balloons floating in the cloud layer ~50–70 km above the surface might not be so fortunate.
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Do engineers need to build aerial platforms with the toughness (and thus expense) required to
37
survive a lightning strike? Quantitative estimates of lightning strike frequency are inconsistent
38
based on different forms of evidence. Observations of certain electromagnetic signals,
39
interpreted as lightning in the clouds, suggest a strike rate several times that of Earth’s lightning.
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In comparison, optical flash rates at Venus, as observed at the Mt. Bigelow observatory in
41
Arizona and by the Akatsuki mission currently orbiting Venus, suggest a far lower flash rate. In
42
this study, we argue that these optical flashes were plausibly produced by meteor fireballs ~100
43
km above the surface, not by lightning in the clouds. If so, then lightning poses no significant
44
threat to balloon missions in the clouds of Venus. Lightning may still exist at the surface,
45
produced by aeolian processes or explosive volcanism.
46
47
1 Introduction
48
Venus is a natural laboratory for studying the atmosphere of a non-Earth-like planet.
49
While Venus is unique among terrestrial planets in our Solar System, it is an analogue to a
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common class of exoplanets (e.g., Kane et al., 2019; Way et al., 2023). For example, Venus’s
51
present-day atmosphere is far more massive than the atmospheres of Earth and Mars (e.g., Taylor
52
et al., 2018)—dominated by carbon dioxide and perpetually shrouded in clouds rich in droplets
53
of sulfuric acid at altitudes from ~47–70 km (e.g., Titov et al., 2018). Exoplanets born near their
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parent stars may outgas thick, CO2-dominated atmospheres shortly after they accrete (e.g.,
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Hamano et al., 2013; Gillmann et al., 2022; Miyazaki & Korenaga, 2022) or, later, if they
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experience a runaway greenhouse (e.g., Krissansen-Totton et al., 2021; Way et al., 2022).
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Venus’s clouds are super-rotating, moving westward much faster than the solid body (e.g., Read
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& Lebonnois, 2018). This attribute can be exploited for exploration: a surface station would take
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~243 Earth-days to experience a sidereal day, but an aerial platform in the clouds could
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circumnavigate Venus at the equator every ~5–7 Earth-days (e.g., Cutts et al., 2022; O’Rourke et
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al., 2023). Likewise, many terrestrial exoplanets may have superrotating atmospheres (e.g.,
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Imamura et al., 2020; Lee et al., 2020). As the number of observed exoplanets grow, so does the
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complexity of work to understand their evolution and current environments, and the need to
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study Venus. Lightning is an important aspect of terrestrial atmospheres, in part for its ability to
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instigate non-equilibrium chemistry (e.g., nitrogen fixation) relevant to biology and life detection
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(e.g., Ardaseva et al., 2017). Venus offers a natural laboratory for studying lightning on a major
67
class of exoplanets.
68
On Earth, lightning flashes occur hundreds of times each second across the globe (e.g.,
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Desch et al., 2002), illuminating areas where the atmosphere is dynamically active. Lightning
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flashes occur after a significant charge separation has been built up in the atmosphere (e.g., Yair,
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2012; Aplin, 2006; Dwyer & Uman, 2014). Once released, lightning dissipates an enormous
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amount of energy, a fraction of which flashes as optical energy. In Earth’s atmosphere, lightning
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strikes are most often facilitated via the interactions between liquid water and water ice particles,
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where the polarity of the molecules contributes to the buildup of this charge difference. Volcanic
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plumes (e.g., Nicoll et al., 2019; Mather & Harrison, 2006) and dust storms (e.g., Aplin, 2006)
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can also produce lightning by triboelectric charging, in which ash and dust particles are the
77
medium of charge buildup.
78
On Venus, lightning could arise from mechanisms analogous to those seen on Earth.
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Sulfuric acid molecules in the clouds may have sufficient polarity to produce charge separation if
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both solid and liquid phases exist, though this is debated (e.g., McGouldrick et al., 2011). The
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frequency of volcanic events on Venus today is uncertain. However, Venus and Earth could have
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similar rates of volcanic activity overall (e.g., Byrne & Krishnamoorthy, 2021). Radar images
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from Magellan revealed ~105 volcanoes on the surface larger than 5 km in diameter (Hahn &
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Byrne, 2023). Magellan may also have observed active volcanism (e.g., Herrick & Hensley,
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2023). At least some volcanic eruptions were explosive in the past (e.g., Ganesh, 2022; Ganesh
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et al., 2022; Ganesh et al., 2021; Airey et al., 2015). Lightning produced by either volcanic or
87
atmospheric processes would provide a probe into Venus’s current and past evolution. Lightning
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strikes could also excite global Schumann resonances at frequencies of tens of Hz, enabling
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electromagnetic sounding of Venus’s lithosphere from an aerial platform (e.g., Grimm et al.,
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2012). Finally, lightning on Venus would create unique chemical environments in the
91
atmosphere (e.g., Krasnopolsky, 2006; Delitsky & Banes, 2015). Almost anyone studying Venus,
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especially those planning future missions, should feel motivated to find lightning, if it exists.
93
The existence of lightning in the atmosphere of Venus has been heavily debated for
94
decades based on interpretations of different types of evidence. Numerous attempts to collect
95
concrete evidence of lightning have been made, with varying results (e.g., Lorenz, 2018; Lorenz
96
et al., 2019 and references therein). Most of the claimed detections rely on observations of radio,
97
magnetic, and acoustic signals. The Venera 11–14 landers observed near-continuous signals at
98
~10–80 kHz during their descents, which resembled electrical activity (sferics) associated with
99
lightning on Earth (Ksanfomality, 1980). The Pioneer Venus Orbiter detected electric fields
100
thought to be associated with lightning flashes (e.g., Taylor et al., 1979). The Venus Express
101
magnetometer detected magnetic bursts near periapsis that were interpreted as whistler-mode
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waves—circularly polarized electromagnetic waves with frequencies up to several hundred Hz at
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Venus, which follow local magnetic field lines—produced by lightning below the ionosphere
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(e.g., Russell et al. 2006). Most recently, Hart et al. (2022) claimed that Venus has a flash rate
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several times higher than for lightning on Earth. However, there have also been non-detections of
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lightning that are in tension with these predictions, including with the radios of Cassini (Gurnett
107
et al., 2001) and the Parker Solar Probe (Pulupa et al., 2021). All claimed detections have been
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controversial, with spacecraft and plasma noise proposed as candidates to create some of the
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signals (e.g., Lorenz, 2018).
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A second approach to detect lightning at Venus is searching for optically bright flashes,
111
similar to what we associate with lightning here on Earth. Laboratory experiments simulating
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lightning discharge conducted in a carbon dioxide-dominated atmosphere show a distinctive
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peak at the excited atomic oxygen triplet near 777 nm (e.g., Borucki et al., 1985, 1996; Qu et al.,
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2023). Because of the increased abundance of oxygen (in carbon dioxide) at Venus, emission
115
from the OI triplet should be relatively strong compared to at Earth (e.g., Borucki et al., 1985).
116
Numerous searches for lightning have been conducted via optical flash detection at this
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wavelength, including with the Venera 9/10 Orbiter Spectrometer (Krasnopolsky, 1979), the
118
Pioneer Venus Star Tracker (Borucki et al., 1991), and Venus Express Visible and Infrared
119
Thermal Imaging Spectrometer (VIRTIS) (Moinelo et al., 2016), none of which returned clear
120
detections. Other endeavors, such as the observations made by the Mt. Bigelow 61-in. telescope
121
(Hansell et al., 1995) and the Akatsuki Lightning and Airglow Camera (LAC) (Takahashi et al.,
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2021; Lorenz et al., 2022), were more successful, with several distinct light flashes recorded (see
123
Section 2.1 below). Theory and simulations predict that the clouds would only absorb ~60% of
124
the optical energy from lightning near the cloud base, although the photons would be scattered
125
horizontally to a width of ~200–300 km (e.g., Williams et al., 1982; Williams & Thomason,
126
1983). In contrast, only ~0.01% of the visible photons from near-surface lightning could escape
127
to space. Ultimately, orbiting spacecraft or ground-based telescopes could detect lightning that
128
occurs as low as the lower cloud deck.
129
However, these optical flashes may have alternative sources beyond lightning. On Earth,
130
satellites that observe lightning also observe meteor fireballs. As bolides ablate in the
131
atmosphere, some of their kinetic energy is released as optical energy, which can be observed at
132
visible wavelengths. Observations of small meteors reveal that they, like lightning, emit a
133
distinctive peak around 777 nm (e.g., Madiedo et al., 2023). While slower meteors generate a
134
Planck continuum, the OI emission line is especially strong for faster meteors (e.g., Vojáček et
135
al., 2022). At Venus, we might expect even stronger OI emission.
136
In this study, we investigate the rate of fireballs from cm-sized meteors ablating in the
137
upper atmosphere of Venus as alternative explanations for observed optical flashes. First, we
138
calculate the global rate of optical flashes inferred from the Mt. Bigelow and Akatsuki detections
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(Section 2.1). Then, we adapt a power law for impactors at Earth (e.g., Brown et al. 2002) to
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calculate the flux of small impactors at Venus. We derive a power law for the number of
141
impactors with a certain amount of optical energy in the OI emission line (Section 2.2). Next, we
142
compare those to the rate of observed flashes from Akatsuki and Mt. Bigelow (Section 3). While
143
these optical flashes are likely to come from the cloud layers (if produced by lightning) or even
144
higher in the atmosphere (if produced by ablating meteors or transient luminous events like
145
sprites, elves, and haloes), we also discuss the prospects for lightning elsewhere on Venus, such
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as volcanic or aeolian lightning near the surface (Section 4.1). Finally, we assess whether cloud-
147
based lightning would threaten future probes or aerial platforms (Section 4.2).
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2 Methods
149
To begin unravelling the mystery of lightning at Venus, we predict the rate of optical
150
flashes produced by meteor fireballs. We compare those rates to those inferred from the Akatsuki
151
(Takahashi et al., 2021; Lorenz et al., 2022) and Mt. Bigelow (Hansell et al., 1995) surveys. If
152
the observed flashes occur at a rate consistent with that predicted for meteor fireballs, then one
153
could conclude that meteor fireballs are a plausible explanation for all the flashes. However, if
154
the observed flashes are much more frequent, then some of them probably originated from
155
lightning.
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2.1 Searches for optical flashes at Venus
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Table 1 summarizes five searches for optical flashes at Venus, following Table 1 in
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Lorenz et al. (2019). Here, our first goal is to use these observations to place statistical
159
constraints on the global, yearly rate of optical flashes at Venus.
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Two surveys yielded the most believable detections of optical flashes at Venus. The
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Akatsuki mission entered orbit at Venus in December 2015 (e.g., Nakamura et al., 2016) and
162
initiated a search for optical transients with its Lightning and Airglow Camera (LAC) in 2016
163
(e.g., Takahashi et al., 2018). LAC uses a filter centered at 780.6 nm with a bandwidth (full
164
width at half maximum) of 9.0 nm, which is sufficient to capture the emission from the OI triplet
165
at 777 nm. The sampling rate is 31.25 kHz with a spatial resolution of ~175 km at a distance of
166
~5000 km (Takahashi et al., 2018). LAC is only operated during the spacecraft’s orbit when
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Venus blocks sunlight from directly hitting its sensor. In its first years of operation, the LAC
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team confirmed that the triggering system functioned correctly and detected several cosmic rays
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(Takahashi et al., 2018). After three years, LAC operated for a total of 16.8 hours, covering an
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area-time product of ~82 × 106 km2-hr, without any detections of a flash attributable to lightning
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or meteors (Lorenz et al. 2019). By late 2020, LAC accumulated an area-time product of >100 ×
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106 km2-hr and detected a single optical flash that lasted ~100 ms with a total optical energy near
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777 nm of EOI ~ 1.1 × 107 J (Takahashi et al., 2021), which is several times brighter than the
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detection limit of EOI ~ 5 × 105 J to 2 × 106 J (Takahashi et al., 2018). As of late 2022, LAC
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reached an area-time product of at least ~200 × 106 km2-hr without any additional detections of
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lightning-like flashes, as shown in Table 1 (Lorenz et al., 2022).
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In the early 1990s, a ground-based telescope observed several candidate flashes at Venus.
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That search used a 1.5-m telescope on Mt. Bigelow, Arizona to image the night side of Venus
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with a sampling rate of 18.8 Hz over several nights. The instrument used a narrowband filter
180
centered at 777.4 nm, with a bandwidth of 0.7 nm (Hansell et al., 1995). Overall, this survey
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detected seven flashes, each in a single frame of the imaging sequence collected in ~53 ms.
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Takahashi et al. (2018) argued that using only one frame per flash admits the possibility that
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cosmic rays or unknown electrical noise could have produced some flashes. Table 2 of Hansell et
184
al. (1995) reports the “associated optical energy” for each flash, which equals 2.5 times the
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optical energy measured near 777 nm. This factor of 2.5 (=1/0.4) was inserted based on the
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assumption that Venusian lightning would emit ~40% of its total optical energy near 777 nm
187
(e.g., Borucki et al., 1981). Therefore, associated optical energies of ~0.1–2.1 × 109 J are
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equivalent to EOI ~4–84 × 107 J near 777 nm. Hansell et al. (1995) inferred a detection limit of
189

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EOI ~0.6–2.5 × 106 J for 50–95% detections. Overall, the area-time product from this ground-
190
based survey reached at least ~800 × 106 km2-hr (Hansell et al., 1995), counting additional nights
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of non-detections by that group that were not published (Lorenz et al., 2019). Other groups may
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have conducted similar searches using ground-based telescopes with no detections (Yair et al.,
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2012), but those efforts have not been published or included in Table 1.
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Three other attempts to observe optical flashes at Venus were less successful. The
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Pioneer Venus Star Tracker (Borucki et al., 1991) and Venus Express VIRTIS (Moinelo et al.,
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2016) accumulated area-time products of ~105 and 14 × 106 km2-hr, respectively, with no
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detections. The Venera 9/10 Orbiter Spectrometer recorded a burst of light lasting ~70 s, almost
198
immediately after it began observations (Krasnopolsky, 1979; 1983). Some authors attribute this
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observation to lightning (e.g., Hart et al., 2022), while others argue that instrument anomalies or
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even spacecraft debris are a more likely explanation (e.g., Lorenz, 2018; Lorenz et al., 2019).
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The total area-time product associated with Venera 9/10 was relatively tiny, only ~2.5 × 103
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km2-hr (Lorenz et al., 2019). Ultimately, we might not be surprised that the two surveys with by
203
far the largest coverage are the ones that yielded the most reliable detections.
204
205
Table 1. Two surveys produced relatively reliable detections of optical flashes in the
atmosphere of Venus. Three other surveys, which had much lower coverage, did not return
any uncontested detections.
Search
Coverage
(106
km2-hr)
Number of
lightning-
esque
flashes
Equivalent global,
yearly rate (95%
confidence
intervals)
Detection
threshold (J)
Energy of
the dimmest
flash near
777 nm (J)
Mt. Bigelow
800
7
35423
(17476, 66808)
2.5 × 106
(95%)
0.6 × 106
(50%)
2.8 × 107
Akatsuki
200
1
20241 (4902,
112770)
0.5–2 × 106
1.1 × 107
Venera 9/10
Orbiter
Spectrometer
0.0025
0
0 (< 5 × 109)
3 × 107
-
Venus
Express
VIRTIS
14
0
0 (< 9 × 105)
Unknown
-
Pioneer
Venus Star
Tracker
0.1
0
0 (< 108)
2 × 108
-
To compare with the power laws derived in the next subsection, we need to translate
206
these observations into statistical constraints on the global rate of flashes at Venus. Say that a
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survey observes a given area (AS) for a given time (TS). Its area-time product is ASTS. If N is the
208

manuscript submitted to Journal of Geophysical Research: Planets
total number of bright flashes (i.e., with optical energies above a certain detection limit) across
209
Venus during one Earth-year, then the expected number of flashes observed by that survey is
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!!" #
$
%!
%"
&$
'!
'#
&
(
)
*
+
211
where AV = 4.6 × 108 km2 is the surface area of Venus and TY = 8.8 × 103 hr is the duration of an
212
Earth-year. We assume that Poisson statistics govern optical flashes (from lightning or meteors),
213
meaning that the probability of observing an integer number of flashes (k) during a survey is
214
,
)
-.
/
.!!
+
"!!
$0%&!
-1 2
)
3
+
215
Here, we know that k = 1 and 7 for the Akatsuki and Mt. Bigelow surveys, respectively. We want
216
to calculate the probability density function for lS and thus N according to both surveys. By
217
Bayes’ theorem, P(λS | k) 4 P(k | λS). We then convert lS to N using Eq. 1 and renormalize the
218
probability density function so it integrates to 1 from N equals zero to infinity.
219
Figure 1 shows our statistical constraints on the global, yearly rate of optical flashes at
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Venus. For surveys with k = 0, we can say that lS ≤ 3 with 95% confidence. The upper limits on
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lS are thus ~9 × 105, ~108, and ~5 × 109, respectively, from the non-detections by Venus Express
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VIRTIS, the Pioneer Venus Star Tracker, and the Venera 9/10 Orbiter Spectrometer (assuming
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Venera 9/10 saw only spacecraft debris or an instrument anomaly). For surveys with more
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reliable detections of optical flashes (k ≥ 1), the most likely value of lS is simply k. Using Eq. 1,
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the most likely values of N are 20241 and 35423 for Akatsuki and Mt. Bigelow, respectively.
226
With Eq. 2, we calculated the 95% (“two-sigma”) confidence intervals associated with each
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survey: (4902, 112770) for Akatsuki and (17476, 66808) for Mt. Bigelow. The 68.3% (“one-
228
sigma”) confidence intervals are (14325, 66808) and (14325, 54509), respectively. These
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surveys are both consistent with a global flash rate of N ~ 104–105 per Earth-year. However, we
230
report these estimates separately because each survey may have a different (albeit similar)
231
detection limit.
232
233
Figure 1. Probability density functions for the global, yearly rate of bright flashes at Venus (i.e.,
234
the expected number across the entire planet in one Earth-year with optical energies above the
235
Akatsuki
Mt. Bigelow

manuscript submitted to Journal of Geophysical Research: Planets
detection limit of each instrument), estimated from the coverage and detections in surveys from
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Akatsuki and Mt. Bigelow.
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2.2 Impactor production functions and luminous efficiency
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To quantify the number of bolides that enter Venus’s atmosphere in one Earth-year, we
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turned to existing studies of meteors at Earth (Figure 2). Le Feuvre & Wieczorek (2011)
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developed a 10th-order power law to describe impactor flux at Venus over a wide range of bolide
241
diameters (Figure 2a). However, our study requires only a log-linear power law, for simplicity, to
242
describe the flux of small impactors at Venus. We thus start with power laws developed by
243
Brown et al. (2002), which were the basis for the low-mass end of the expression derived in Le
244
Feuvre & Wieczorek (2011). Based on the observations described in Section 2.1, we are
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interested in meteors that could produce flashes with optical energies ~107 J. As discussed
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below, the kinetic energy of those meteors should be at least an order of magnitude higher (~108
247
J). Depending on the impact velocity, the masses of meteors potentially relevant to the detected
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optical flashes are thus roughly ~0.1–1 kg.
249
We start with an established relationship between the frequency and kinetic energy of
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small bolides that collide with Earth (Brown et al., 2002):
251
567'( #)"**289:;28;567'( <*(
)
9
+
252
where NE is the cumulative number of bolides that strike Earth in one Earth-year with a kinetic
253
energy of at least EK (in Joules). This equation is the same as Eq. 2 by Brown et al. (2002), who
254
expressed EK in units of kiloton-TNT equivalent.
255
Next, we write the equivalent equation for Venus. On average, bolides strike Venus with
256
faster velocities and thus higher kinetic energies. Le Feuvre & Wieczorek (2011) found that the
257
average impact velocity at Venus is 25.0 km/s, whereas the average velocity for Earth is 20.3
258
km/s (Brown et al., 2002). From Table 3 in Le Feuvre & Wieczorek (2011), the impact rate at
259
Venus per unit area is higher by a factor of 1.75/1.58 = 1.11 than the impact rate at Earth.
260
However, Venus also has less surface area than Earth, by a factor of (6052/6371)2 = 0.902.
261
Overall, we can combine these factors to write the power law for the bolide flux at Venus:
262
567'( # " *32;8 : ;28;567'( <*(
)
=
+
263
where N is now the cumulative number of bolides that strike Venus in one Earth-year with
264
kinetic energies of at least EK (in Joules).
265
When a meteor ablates in a planet’s atmosphere, only a fraction of the total kinetic energy
266
is converted to optical energy. We can relate kinetic and optical energy with EO = t EK, where t
267
is the luminous efficiency of the bolide. For meteors with mass m up to a few kg and entry
268
velocities v < 25.372 km/s, t may obey the following equation (e.g., Popova et al. 2005):
269
5>? " ;2@AB:*;29;B5>C D 82BE*
)
5>C
+
+:92;=*=.
)
5>C
+
,D;293*9
)
5>C
+
-D270
;29=BFG>H
)
;29E5>I
+
2)@+
271
We defined v as the average entry velocity (25 km/s) for meteors ranging in mass from 10-1 to
272
102 kg. We then found the best-fit power law that relates t (dimensionless) and EO (J), following
273
Brown et al. (2002):
274
? " ;2;*==3.<.
(/(0-1(
)
A
+
275

manuscript submitted to Journal of Geophysical Research: Planets
which predicts luminous efficiencies from ~0.05–0.08 over this size range (Figure 2c). By
276
incorporating Eqn. (6) into Eqn. (4), we then find that
277
JKL'(# " *;2=9:;2E3.JKL'(<.2
)
B
+
278
However, not all the optical energy is emitted near 777 nm, to which Akatsuki’s LAC filter and
279
the search for flashes at Mt. Bigelow were restricted. Only a small fraction of the total optical
280
energy is due to the excited oxygen triplet:
281
<.2 " M.2<.(
)
E
+
282
where 0 < fOI < 1. In terms of EOI, the power law for the rate of meteor fireballs at Venus is:
283
JKL'(# : *;2=9:;2E3JKL'( <.2
M.2 2
)
8
+
284
The factor fOI is uncertain and variable for meteor fireballs at Earth (e.g., Vojáček et al., 2022).
285
The uncertainty is even greater for meteors at Venus, but we expect relatively high values of fOI
286
because of the relative abundance of oxygen atoms in carbon dioxide. If we assume a certain
287
value of fOI , then we can estimate the rate of meteor fireballs with different brightnesses at
288
Venus—and thus if the observed flashes plausibly originated from meteors, not lightning.
289
290
Figure 2. The number, velocity, and energy of bolides colliding with Venus can be modeled
291
from previous work. In (a) we plot the number of bolides above a certain diameter colliding with
292
Venus per Earth-year (e.g., Le Feuvre & Wieczorak, 2011). In (b) is the cumulative distribution
293
function for bolide entry velocity. The average entry velocity for meteors at Venus is 25 km/s. In
294
(c) we plot the best-fit power law for the luminous efficiency of meteors that we found using
295
equations from Popova et al. (2005). Luminous efficiency—the amount of kinetic energy
296
converted to optical energy—increases as entry mass and entry velocity increase. In (d) we plot
297

manuscript submitted to Journal of Geophysical Research: Planets
the number of bolides with a certain energy colliding with Earth and Venus per Earth-year.
298
Impactors at Venus have higher kinetic energy due to their higher average entry velocity. EO is
299
the optical energy emitted per bolide; only a fraction of kinetic energy is converted to optical
300
energy during ablation, as defined by the luminous efficiency in (c).
301
2.3 Experimental simulations of meteor plasma
302
No one has yet measured the emission spectrum of a meteor fireball at Venus. In the
303
absence of direct observations, we can turn to laboratory experiments for hints about what
304
fraction of the total optical energy might be emitted near 777 nm. Many groups have used laser-
305
induced breakdown spectroscopy (LIBS) to simulate a meteor fireball (e.g., Krivkova et al.,
306
2021; Ferus et al., 2018; Dell’Aglio et al., 2010). A high-power laser can ablate a meteorite in a
307
similar fashion to meteor ablation during high-speed collisions with atmospheric molecules.
308
Experimental studies relevant to terrestrial meteors typically conduct LIBS experiments on
309
meteorites in a vacuum or under ambient atmospheric conditions. However, for application to
310
Venus meteors, we would prefer to invoke LIBS experiments conducted on rock and mineral
311
samples surrounded by (predominantly) carbon dioxide at a pressure of a few mbar.
312
Coincidentally, air at the surface of Mars has approximately the same composition and pressure
313
as air at an altitude of ~100 km above the surface of Venus.
314
Because of this similarity, we utilized results from the LIBS experiments that were
315
conducted to calibrate the ChemCam instrument package on the Mars rover Curiosity (Wiens et
316
al., 2013). We used data from the Los Alamos National Laboratory ChemCam experiments,
317
where samples were measured in 5 locations. For each sample, 50 laser pulses were taken at each
318
location and averaged together. After the data were collected, the spectra were cleaned and
319
calibrated. We used the cleaned and calibrated dataset to estimate the fraction of optical energy
320
near 777 nm. We analyzed 5 different samples to determine an average fraction of optical
321
energy. Olivine ([Fe,Mg]2SiO4) and pyroxene ([Fe,Mg]SiO3) are common in stony meteorities as
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chondrules. We also analyzed spectra from samples of diopside (CaMgSi2O6), llanite (a
323
rhyolite), and basalt. Diopside and llanite provided comparative results despite not being as
324
common in meteorites as the other materials. The calibration samples did not include water ice,
325
but we expect that a comet’s fireball would produce even more OI emission than a rocky meteor.
326
To calculate fOI for each mineral or rock, we calculated the area under the spectral curve for the
327
entire spectrum from 350–800 nm, as well as the area within the OI peak from 771–800 nm. We
328
then divided the area under the OI peak by the total area of the spectrum, producing fOI.
329
3 Results
330
3.1 Meteor fireballs are not (always) blackbodies
331
Many studies assume that ablating meteors in Venus’s atmosphere would emit as
332
blackbodies (e.g., Takahashi et al. 2021). If a meteor ablating at ~6000 K emitted as a blackbody,
333
then only a very small amount of the total optical energy (<1%) would be contained in the
334
excited oxygen line at ~777 nm or in the bandpass of the instruments designed to detect this line.
335
The small amount of observed energy would require a very large, and thus very infrequent,
336
meteor to cause the observed amount of optical energy. However, recent studies have shown that
337
small meteors, such as the one observed by Madiedo et al. (2013) in the Geminid meteor shower
338
on Earth, do not always emit as blackbodies (Figure 3). By calculating the area under the
339

manuscript submitted to Journal of Geophysical Research: Planets
spectral curve, we determine that ~7% of the total optical energy produced by this meteor was
340
contained in the excited atomic oxygen line.
341
342
Figure 3. Two spectra with the same amount of optical energy near 777 nm, but very different
343
amounts of total optical energy. A Geminid meteor produced a spectrum (black and purple)
344
distinctly different than a blackbody curve (gray) for an effective ablation temperature of 6000
345
K. At Venus, due to the large relative abundance of oxygen, ~5–30% of a small meteor’s optical
346
emission may be contained in the OI triplet (purple).
347
348
Figure 4. Fraction of total optical energy emitted in the 777 nm bandpass, based on inspection of
349
LIBS spectra from the ChemCam calibration database, which provide a potential analogue for
350
the emission spectra of meteors. Near the surface of Mars, the atmosphere is CO2-dominated
351
with pressures of a few millibars—exactly the conditions at altitudes near ~100 km in the
352
atmosphere of Venus where small meteors burn up. We find that ~4–7% of the total optical
353
energy contained in these spectra is typically emitted near 777 nm. For minerals and rock types
354
that are common in meteors, as well as the more silicic llanite, this plot shows the mean (gold)
355
and standard deviation (maroon) of that fraction for 5 samples in the database.
356
Blackbody
SPMN151209 Geminid bolide
(Madiedo et al., 2013)
OI Emission

manuscript submitted to Journal of Geophysical Research: Planets
We used LIBS data from the calibration of Curiosity’s ChemCam instrument to estimate
357
meteoritic emission in the OI emission line for ablating bolides at Venus. To better constrain the
358
amount of energy in the excited oxygen triplet for different materials, we analyzed common
359
meteoritic materials for the fraction of optical energy contained around 777 nm (Figure 4). We
360
found that, for common meteoritic materials such as olivine, pyroxene, and basalt, the value of
361
fOI ranges from ~0.04–0.07 (Figure 4). Based on these laboratory experiments and the
362
observations of some terrestrial meteors, we thus expect that fireballs from small meteors at
363
Venus are an order-of-magnitude brighter than blackbodies near 777 nm. However, the emission
364
spectrum of an individual meteor will depend on many factors, including its composition, mass,
365
entry velocity, entry angle, and irregularities that cause it to fragment and/or spin as it ablates.
366
3.2 Meteor fireballs in surveys from Akatsuki and the Mt. Bigelow 61-in. telescope
367
Figure 5 plots the number of flashes per Earth-year expected for a certain amount of
368
optical energy produced by the ablating bolide, based on the amount of optical energy contained
369
in the OI filter centered near 777 nm (fOI). For higher values of fOI, we expect to see more flashes
370
of a given brightness in one Earth-year. As detailed above, a meteor fireball that emits as a
371
blackbody should have fOI ~0.007. Observations of terrestrial fireballs and LIBS experiments
372
suggest that fOI ~0.05–0.10 is more realistic for small meteors. We estimate that an upper limit
373
for fOI is ~0.3, perhaps for a comet that hit Venus at high velocities. We compare these
374
expectations to the flash rates inferred from the two surveys with relatively reliable detections.
375
The one optical flash detected by Akatsuki’s LAC (so far) had a brightness that is
376
consistent with a meteor fireball. That is, if fOI ~0.07, then we expect to witness the same number
377
of flashes in one Earth-year from these meteors as the expected number from Akatsuki’s
378
estimated global flash rate. However, this result is somewhat conditional on the true detection
379
limit for Akatsuki’s LAC. Even if LAC’s detection limit is higher than claimed by Takahashi et
380
al. (2018) (i.e., equal to the flash brightness in the worst case), a meteor is still statistically
381
probable within the 95% confidence intervals for fOI > 0.10. However, Takahashi et al. (2018)
382
also claimed that the detection limit for Akatsuki’s LAC is perhaps as low as EOI ~5 × 105 J, or
383
even lower. In that case, observing at least one flash from a meteor fireball is not surprising.
384
Meteor fireballs are perhaps also bright and frequent enough to explain the observations
385
from the Mt. Bigelow 61-in. telescope. If the dimmest observed flash were observed at exactly
386
the detection limit of that survey, then we would only predict the observation of seven meteor
387
fireballs if ~30% of the total optical energy were concentrated in the OI filter. However, Hansell
388
et al. (1995) estimated that their detection limit was much lower, which is also consistent with fOI
389
~0.05–0.10, exactly what we expect for fireballs at Venus. Yair et al. (2012) also conveyed a
390
personal communication about “repeated attempts by large-mirror ground telescopes to repeat
391
the Hansell et al. (1995) observations,” which apparently have not yielded any additional
392
detections. If the effective area-time product for ground-based surveys is higher than Table 1
393
indicates, then the extrapolated number of flashes per Earth-year would decrease—and thus
394
agree even better with the predicted rate of meteor fireballs at Venus. Finally, recent papers
395
noted that the observations at Mt. Bigelow were not conducted at a high enough sampling rate to
396
take more than one image per flash, leaving some ambiguity about whether a cosmic ray or
397
electrical noise produced one or more of the flashes (e.g., Takahashi et al. 2018). If one or two of
398
the claimed flashes did not originate from Venus, then the observed rate would agree perfectly
399
with what we predict for meteor fireballs and with the current results from Akatsuki. Using Eq. 1
400

manuscript submitted to Journal of Geophysical Research: Planets
and 2, we calculate that the upper limit on the global, yearly rate of lightning in the clouds is N ≤
401
11975 (< 4 × 10-4 Hz), with 95% confidence—if none of the observed flashes originate from
402
lightning.
403
Our study focuses on reproducing the hypothesized lightning flash rates at Venus with an
404
alternative source of optical energy in the form of ablating meteors tens of km above the cloud
405
layer. However, the shapes of any observed light curves would provide additional constraints.
406
The shape of the light curve observed by Akatsuki was positively skewed (e.g., Takahashi et al.
407
2021), which the team argued as most consistent with lightning. Models predict a negatively
408
skewed light curve from the ablation of a spherical meteor that does not fragment. However,
409
ablating meteors probably are non-spherical and also fragment and spin as they descend through
410
the atmosphere, making it difficult to predict the shape of an individual bolide-produced light
411
curve. We did not construct any models for the light curves of meteors at Venus—but such
412
efforts will only become more important as the number of time-resolved observations increases.
413
Preliminary work suggests that Venusian meteors are indeed brighter than terrestrial meteors
414
(e.g., McAuliffe & Christou, 2006; Christou & Gritsevich, 2023).
415
416
Figure 5. Estimate of the expected number of optical flashes at Venus in one Earth-year (
N
) that
417
would release at least a certain amount of energy near 777 nm (EOI). The black and grey circles
418
show the global rates inferred from Akatsuki and Mt. Bigelow, respectively. The vertical bars
419
denote the 95% confidence intervals on the global rate. The horizontal, dashed bars reflect
420
uncertainty about the detection limits of both surveys. On the right, the dashed bars extend to the
421
optical energy near 777 nm from the dimest flash that each survey detected, which is the highest
422
possible detection limit. The circles are centered at the claimed detection limits for Akatsuki
423
(e.g., Takahashi et al., 2018) and the Mt. Bigelow survey (Hansell et al., 1995). On the left, the
424
dashed bars extend to the lowest plausible values of the detection limit claimed for each survey.
425
fOI = 0.07
fOI= 0.10
fOI = 0.30
fOI = 0.05
fOI = 0.007

manuscript submitted to Journal of Geophysical Research: Planets
4 Discussion
426
4.1 Sub-cloud lightning is possible
427
Regardless of whether lightning exists high in Venus’s atmosphere, lightning could occur
428
close to the surface from either volcanic or aeolian processes (Figure 6). On Earth, volcanic
429
lightning often occurs in the ash plume associated with an explosive eruption. The particles in
430
the plume can become charged through several mechanisms, but fracto-electrification and tribo-
431
electrification are considered the most important because they are closely related to explosive
432
eruption dynamics (e.g., Cimarelli & Genareau, 2022). Material is fractured into ash-sized
433
particles during an explosive eruption, which can release electrons and positive ions and charge
434
the fragmented particles themselves (fracto-electrification). Ash particles of various
435
compositions within the plume will then collide with each other, charging the particles through
436
friction (tribo-electrification) (e.g., Cimarelli & Genareau, 2022). At the surface, winds carrying
437
small particles can also create charges through tribo-electrification. This process could be a
438
common phenomenon on Venus because wind speeds are close to the transport threshold (e.g.,
439
Lorenz, 2018). The Venera landers observed the movement of surface material, which implies
440
that wind may be capable of charging particles on Venus’s surface (e.g., Lorenz, 2018).
441
Previous studies disregarded volcanic lightning on Venus as impossible due to the
442
supposed lack of explosive volcanism. Borucki (1982) argued that if volcanism caused the then-
443
claimed observation of 30 lightning flashes in 3 years by the Pioneer Venus Orbiter, there would
444
have been 10 eruptions per year. If explosive volcanism were occurring at this rate, then it would
445
release particles into the atmosphere that could be detected. However, the cloud-particle-size
446
spectrometer on Pioneer Venus did not detect particles of the size expected to result from
447
explosive volcanism. Borucki (1982) therefore concluded that, even if Venus were volcanically
448
active, explosive volcanism was not common and thus not a probable source of lightning. They
449
also argued that lightning would have to occur in the clouds because the lower atmosphere would
450
absorb energy at the wavelengths produced by lightning (e.g., Borucki, 1982). However, those
451
specific detections are now attributed to cosmic rays (e.g., Lorenz, 2018). The electromagnetic
452
observations that yield the highest inferred rates of lightning only constrain the source of those
453
signals to below the ionosphere—compatible with a near-surface origin.
454
New evidence of explosive volcanism on Venus has recently emerged. For example,
455
Ganesh et al. (2021) modeled the formation of several proposed pyroclastic deposits on Venus.
456
These pyroclastic flows would have formed through the collapse of ash plumes created during
457
explosive eruptions. Their models of collapsing plumes provided good matches to deposits at
458
several locations on Venus hypothesized to be associated with explosive volcanism. Recently, a
459
volcano that changed shape over the course of eight months during the Magellan mission was
460
identified (e.g., Herrick & Hensley, 2023). This is evidence of active volcanism on Venus in
461
recent years, which further supports the position that the possibility of volcanic lightning should
462
not be disregarded. Of course, new observations from future geophysical orbiters such as
463
VERITAS and EnVision are needed to unveil the volcanic history of Venus.
464

manuscript submitted to Journal of Geophysical Research: Planets
465
Figure 6. A cartoon of possible phenomena in Venus’s atmosphere. Small meteors may burn up
466
far above the clouds, while near-surface lightning could generate the putative whistler-mode
467
waves from far below the clouds. Adapted from Figure 6 in O’Rourke et al. (2023).
468
4.2 Lightning is not a hazard to missions in the clouds
469
Many missions to Venus passed through its clouds. No mission has, to our knowledge,
470
been struck by lightning, but lightning nonetheless poses a potential risk to any mission. Starting
471
with Venera 7, ~14 probes have delivered data from below the clouds (e.g., Taylor et al., 2018).
472
Two balloons floated at an equilibrium altitude of ~53 km as part of the Soviet VeGa mission,
473
reporting data for ~47 hours before running out of battery power (e.g., Sagdeev et al., 1986;
474
Moroz, 1987; Crisp et al., 1990). Given these past experiences, lightning is not an obvious
475
hazard to atmospheric probes or short-lived balloons. However, future missions may include
476
extended stays in the clouds to answer high-priority scientific questions (e.g., O’Rourke et al.,
477
2021; Arredondo et al., 2022; Cutts et al., 2022). For example, Phantom is an exciting, well-
478
developed concept that features an aerial platform that dwells in the clouds for at least ~30 Earth-
479
200
100
0
150
50
Altitude (km)
Ionosphere
(~120–300 km)
Meteor
fireballs?
Clouds
(~40–70 km)
Volcanism &
aeolian activity?

manuscript submitted to Journal of Geophysical Research: Planets
days, and plausibly ~100 Earth-days or longer (e.g., Byrne et al., 2023). Scientists have also
480
proposed sending flotillas of long-lived balloons to search for active biology (Hein et al., 2020)
481
or seismic and volcanic activity (Krishnamoorthy & Bowman, 2023; Rossi et al., 2023). Is
482
lightning a threat to long-duration missions in the clouds of Venus?
483
We can use simple statistics to estimate the hazard from lightning to various types of
484
missions. Say that TM is the duration of the mission (in seconds); GL is the overall rate of
485
lightning in the clouds (in strikes per second); and RH is the radius within which a lightning
486
strike is potentially hazardous (in meters). The estimated number of lightning strikes within the
487
hazardous radius during the mission is then
488
!3" '4N3
$
O5
3O"
&
+(
)
*;
+
489
where RV = 6052 km is the radius of Venus, assuming that RH << RV. If lightning strikes happen
490
without spatial bias but with timing that obeys Poisson statistics, then we can calculate the
491
probability that a hazardous strike will occur near the mission:
492
,5" * :0%6"2
)
**
+
493
Mission designers often consider a hazard with a probability of PH < 10-6 to be negligible. We
494
considered three possible rates of lightning in the clouds. Overall, our study is compatible with
495
the hypothesis that cloud-based lightning is vanishingly rare (GL ≤ 10-9 Hz). If the optical flashes
496
observed by Akatsuki and ground-based telescopes originated from cloud-based lightning, then
497
GL ~ 1.2 × 10-4 Hz (Lorenz et al., 2022). If the claimed whistler-mode waves were attributed to
498
cloud-based lightning, then we fear that GL ~ 320 Hz (Hart et al., 2022). Of course, as on Earth,
499
we would expect to find lightning on Venus concentrated in particular regions at any given time.
500
However, we can use the assumption of global homogeneity to estimate relative hazards. If
501
lightning were ubiquitous in the clouds, then requiring a mission to survive a strike could seem
502
prudent. However, no caution is necessary if there is no (or very rare) lightning.
503
Figure 7 shows the probability of a hazardous lightning encounter for four classes of
504
missions. First, probes can pass quickly through the clouds. For example, the DAVINCI mission
505
notionally plans to descend from ~70–40 km in ~34 minutes between the deployment and release
506
of its parachute (Garvin et al., 2022). If lightning were indeed ≥ 7 times as common on Venus as
507
Earth (Hart et al., 2022), then we might expect ~40 strikes within ~100 km of DAVINCI (and all
508
past probes). However, the expected number of strikes near a probe is < 1 using the flash rate
509
inferred from Akatsuki’s search. Second, the VeGa mission was the archetype of a short-lived
510
balloon, operating for ~47 hours. Again, the flash rate inferred from Akatsuki is compatible with
511
the non-detection of lightning (i.e., lL ~ 10-3 for RH ~ 100 km). Third, we assume that a long-
512
lived balloon has a lifetime of ~30 Earth-days. The chances of a nearby strike (RH < 100 km) are
513
then only 1-in-50, according to the optical flash rate from Akatsuki. This contrasts with the
514
analysis in Hart et al. (2022), which implies that such a mission could operate in the vicinity of
515
>50,000 strikes. Finally, we imagined a flotilla of 10 balloons that each have lifetimes of ~100
516
Earth-days. Using the flash rate from Akatsuki, it is more likely than not that at least one of those
517
balloons encountering a strike within ~90 km. However, perhaps such a moderately distant strike
518
would seem more exciting than dangerous. Ultimately, especially given the possibility that
519
meteor fireballs produced all the optical flashes observed at a Venus, there is as of yet no
520

manuscript submitted to Journal of Geophysical Research: Planets
affirmative evidence that lightning is common enough in the clouds to pose a hazard to even
521
fleets of long-lived aerial platforms.
522
Figure 7. Relative risk of lightning to various mission architectures. Lightning is only a hazard
523
to missions in the clouds if lightning exists in the clouds. We have calculated the probability that
524
a lightning strike would occur within a certain horizontal distance (vertical axes) as a function of
525
the total time that a mission spends within the clouds (horizontal axes). Although that time varies
526
by five orders of magnitude for different types of missions, estimates for the rate of lightning in
527
the clouds span ~10 orders of magnitude. In (a) we use the rate of lightning inferred from studies
528
of putative whistler-mode waves (Hart et al., 2022). In (b) we use the global rate derived from
529
Akatsuki’s observation of a single flash so far, assuming that flash originated from lightning
530
(Lorenz et al., 2022). In (c) we use the highest rate that implies that even a balloon flotilla
531
experiences a negligible risk (PH < 10-6) from lightning, which agrees with the hypothesis that no
532
flashes from lightning have ever been seen at Venus.
533
Probability of a “hazardous” lightning strike, log10(PH)
Time in the clouds, log10(TM[s])
Radius (km)
GL= 320 Hz
Radius (km)
GL= 1.2 ×10-4 Hz
Radius (km)
GL= 10-9 Hz
Probe
(~34 min.) Short-lived
balloon
(~47 hours)
Long-lived
balloon
(~30 Earth-
days)
Balloon
flotilla
(~10 ×100
Earth-days)
a
b
c
10
100
10
100
10
100

manuscript submitted to Journal of Geophysical Research: Planets
4.3 The search must go on
534
In our work we have developed a production function for meteor fireballs in the
535
atmosphere of Venus, which should be revisited as flash rates become better determined by
536
future observations. While we found that small meteors ablating high (~100 km) in the
537
atmosphere are plausible explanations for the observed optical flashes, more optical flash
538
observations would serve to sharpen our statistics and provide tighter quantifications of flash
539
rates. Spectrally resolving optical flashes at Venus could verify our study’s estimate that meteor
540
fireballs at Venus have strong emission near 777 nm. Additionally, determining the altitude of
541
recorded optical flashes would allow us to conclude whether they originated above or within the
542
cloud layer, providing more evidence of their source. Meteor fireballs and sprites would occur
543
tens of kilometers above the clouds. Only a very rare, huge meteor would reach the clouds.
544
The “gold standard” approach to lightning detection would be simultaneous optical and
545
radio observations (e.g., Aplin & Fischer, 2017; Cartier, 2020). Lightning above the lower cloud
546
deck could produce an observable optical flash, whistlers at kHz frequencies, and sferics at MHz
547
frequencies. In contrast, meteors would not produce strong radio emissions. Meteors themselves
548
make important contributions to atmospheric chemistry (e.g., Pätzold, et al., 2009; Carrillo-
549
Sánchez et al., 2020) and produce infrasonic signatures that aerial platforms could observe (e.g.,
550
Silber et al., 2018; 2023). Lightning below the clouds may produce radio emissions but not
551
optical flashes visible from space. If the diagram of transient phenomena on Venus shown in
552
Figure 6 were correct, then we would expect optical flashes consistent with the power laws
553
derived in this study, plus non-simultaneous radio emission in the form of whistlers and sferics.
554
5 Conclusions
555
For decades, searches have been conducted for concrete proof of lightning in the
556
atmosphere of Venus. Proving or disproving its existence would have vast implications for
557
scientists’ understanding of Venus’s atmospheric chemistry, weather patterns, and even the
558
potential for life in the clouds. Though multiple pieces of evidence such as whistler-mode waves
559
and optical flashes have been put forward as proof of lightning, the presence of cloud-based
560
lightning remains an open question. In this study, we have investigated whether small meteors
561
may have produced optical flashes in the atmosphere of Venus that were interpreted as lightning.
562
We calculated the rates of expected optical flashes from ablating bolides, and compared those to
563
the rates inferred from optical surveys. We also calculated the risk posed to cloud-based missions
564
considering the estimated lightning rates from these optical surveys. We find that based on
565
observations of meteor fireballs at Earth, ablating fireballs at ~100 km altitude may be
566
responsible for most, or even possibly all of the observed flashes. Lightning thus does not seem
567
like a threat to missions that pass through (e.g., probes) or even linger within (e.g., aerial
568
platforms) the clouds. Future optical surveys should find more meteor fireballs at rates and
569
brightnesses that match our power laws. Simultaneous optical and radio measurements would
570
help in the hunt for definitive evidence of lightning.
571
Acknowledgments
572
C. H. Blaske acknowledges the ASU/NASA Space Grant Internship program for providing
573
funding for the LIBS/ChemCam portion of this research project.
574

manuscript submitted to Journal of Geophysical Research: Planets
Open Research
575
Software (other than for typesetting) was not used for this research. Datasets for this research are
576
available in these in-text data citation references: Wiens et al. (2013).
577
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