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Hollows on Mercury: Materials and mechanisms involved in their formation

January 2013
Icarus 229

January 2013
229

DOI:10.1016/j.icarus.2013.11.018

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CC BY 3.0

Authors:

Rebecca Thomas

David Rothery

The Open University (UK)

Susan Conway

University of Nantes

Mahesh Anand

The Open University (UK)

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking.

Characteristics distinguishing pits, hollows and spectrally red pitted ground.

…

Variation in the areal extent of hollows (A h ) by longitude bin in the region 30°S to 30°N, normalised to (a) area imaged at <180 m/px (A c ), and (b) A c Â the fraction of the surface that is not smooth plains (A np /A tot ).

…

Latitudinal variations in areal extent of hollows (A h ), normalised to (a) the area imaged at <180 m/px (A c ) and (b) A c Â the fraction of the surface that is nonplains (A np /A tot ) within each latitude bin. Hollow extent varies broadly with image quality, though observational bias does not explain the small extent of hollowing in the 30°S to 0°N bin.

…

Irregular, rimless hollows on the floor and terraced wall of an unnamed impact crater at 46.4°N, 318.7°E. Black arrows indicate individual hollows, white bracket indicates a cluster (MESSENGER image ID 2760274).

…

+13

Variation in the areal extent of hollows (Ah) by longitude bin in the region 30°S to 30°N, normalised to (a) area imaged at <180m/px (Ac), and (b) Ac×the fraction of the surface that is not smooth plains (Anp/Atot).

…

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Hollows on Mercury: Materials and mechanisms involved in their

formation

Rebecca J. Thomas

⇑

, David A. Rothery

, Susan J. Conway

, Mahesh Anand

a,b

Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK

article info

Article history:

Received 30 May 2013

Revised 17 October 2013

Accepted 13 November 2013

Available online 21 November 2013

Keywords:

Mercury, surface

Geological processes

Impact processes

abstract

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilome-

tre scale, shallow, ﬂat-ﬂoored, steep-sided rimless depressions typically surrounded by bright deposits

and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-

volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters

suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to

explain the volatile-loss have included sublimation and space weathering, and it has been suggested that

hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However,

detailed veriﬁcation of these hypotheses has hitherto been lacking.

In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the

end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location

relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes

(explosive volcanism and ﬂood lavas) on local and regional scales. We ﬁnd that there is a weak correlation

between hollow formation and insolation intensity, suggesting formation may occur by an insolation-

related process such as sublimation. The vast majority of hollow formation is in localised or regional

low-reﬂectance material within impact craters, suggesting that this low-reﬂectance material is a vola-

tile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases

hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during

crater formation, while in other cases volatiles may have accessed the surface later through re-exposure

and possibly in association with explosive volcanism. Hollows occur at the surface of thick ﬂood lavas

only where a lower-reﬂectance substrate has been exhumed from beneath them, indicating that this form

of ﬂood volcanism on Mercury lacks signiﬁcant concentrations of hollow-forming volatiles.

1. Introduction

The presence of morphologically fresh depressions on the sur-

face of Mercury has been one of the most surprising discoveries

of the MESSENGER (MErcury Surface, Space ENvironment, GEo-

chemistry, and Ranging) spacecraft. Though areas of hollows had

been imaged at low resolution by the Mariner 10 spacecraft in

the 1970s, they appeared only as high-reﬂectance, spectrally rela-

tively blue patches on the ﬂoors of impact craters (BCFDs – Bright

Crater Floor Deposits) (Dzurisin, 1977; Robinson et al., 2008; Blew-

ett et al., 2009). When MESSENGER went into orbit in 2011 and ob-

tained higher-resolution images, these were revealed to be clusters

of irregular rimless depressions with ﬂat ﬂoors and steep walls

(Fig. 1). These were dubbed ‘hollows’ to distinguish them from dee-

per ‘pits’ with sloping ﬂoors, which are proposed to form through

magmatic processes (Gillis-Davis et al., 2009; Kerber et al., 2011).

They range from individual hollows tens of meters across to clus-

ters of hollows tens of kilometres across (Blewett et al., 2011)

and shadow measurements indicate a consistent depth within a

particular host crater in the range of tens of meters (Blewett

et al., 2011; Vaughan et al., 2012). Though their consistent depths

make them ﬂat-ﬂoored overall, lumps of material do occur on hol-

low ﬂoors that may be degraded remnants of the original surface

(Blewett et al., 2011). The bright deposits that gave BCFDs their

name are revealed from orbit to occur both on hollow ﬂoors and

as surrounding haloes.

Hollows appear morphologically fresh and lack superposed im-

pact craters. This implies a young age and suggests hollow forma-

tion may be an on-going process (Blewett et al., 2011). If so, it will

be important to distinguish whether it is a gradual, continual pro-

cess or a more rapid, episodic process.

http://dx.doi.org/10.1016/j.icarus.2013.11.018

This is an open-access article distributed under the terms of the Creative

Commons Attribution License, which permits unrestricted use, distribution, and

reproduction in any medium, provided the original author and source are credited.

⇑

Corresponding author.

E-mail address: rebecca.thomas@open.ac.uk (R.J. Thomas).

Icarus 229 (2014) 221–235

Contents lists available at ScienceDirect

Icarus

journal homepage: www.elsevier.com/locate/icarus

The ﬂat-ﬂoored, closed morphology of hollows and the lack of

associated outﬂow features suggest that they form by the preferen-

tial loss of a volatile component from the surface without melting.

The nature of this material is not known: sulphides or chlorides are

possible candidates (Vaughan et al., 2012; Blewett et al., 2013;

Helbert et al., 2013) but the current resolution of surface elemental

compositional data (Goldsten et al., 2007; Hawkins et al., 2007;

Schlemm et al., 2007; Nittler et al., 2011; Peplowski et al., 2011;

Evans et al., 2012; Weider et al., 2012) is not sufﬁcient to verify this

at the scale of hollows.

Several possible release mechanisms for this volatile substance

have been suggested (Blewett et al., 2011). The feasibility of these

processes is dependent on the nature of the substance lost and the

timescale of hollow formation. In light of the high daytime surface

temperatures at Mercury and the morphological similarity be-

tween hollows and the ‘Swiss cheese terrain’ of Mars (Thomas

et al., 2000), sublimation is a strong candidate. However, various

forms of space weathering are believed to occur at Mercury, and

these may be important release mechanisms if they are relatively

intense in the material where hollows form. Along with thermal

desorption (Madey et al., 1998), photon stimulated desorption

(PSD) releases alkalis from the surface and may be the most efﬁ-

cient form of space weathering supplying these elements to the

exosphere (Cheng et al., 1987; Mura et al., 2009). On a shorter

timescale and at particular localities, physical (Killen et al., 2004)

and perhaps chemical (Potter, 1995) sputtering by the solar wind

may also be important, with the intensity of these processes

depending on the time-variable interaction between the solar wind

and the planet’s magnetic ﬁeld. Micrometeorite impact vaporisa-

tion also releases material from the surface, and unlike the pro-

cesses mentioned above, penetrates beyond the layer of atoms at

the extreme surface (Killen et al., 2007). The rate of hollow forma-

tion may however be too fast for this to apply (Blewett et al., 2011).

Hollows usually occur in material with a low reﬂectance rela-

tive to the Hermean average. In some cases this is a regional depos-

it equating to the LRM (low-reﬂectance material) spectral unit that

has been mapped over large areas of the planetary surface (Denevi

et al., 2009), in others it is a localised deposit, and in a few cases it

is a small ‘dark spot’ with even lower reﬂectance than LRM (Xiao

et al., 2013). Low-reﬂectance material may therefore be the vola-

tile-bearing unit that degrades to form hollows. The question arises

of how the volatile component in this material has been able to ac-

cess the surface recently enough to form fresh landforms despite

considerable evidence for global contraction (Strom et al., 1975;

Watters et al., 2009): this stress state would tend to hinder migra-

tion of material through the crust. The correlation of hollow forma-

tion with impact craters strongly suggests that impacts are

involved in bringing the hollow-forming volatiles to the surface.

It has been suggested that this may occur through exposure in cra-

ter walls, ﬂoors and ejecta and exhumation in peak structures

(Blewett et al., 2013), or through differentiation of impact melt

(Vaughan et al., 2012).

A deeper understanding of the distribution and mode of occur-

rence of hollows is of great interest because of the probable rela-

tionship between hollows and volatile percentage in Mercury’s

crust, now understood to be higher than previously thought

(Kerber et al., 2009, 2011; Nittler et al., 2011; Peplowski et al.,

2011). We have therefore conducted a full survey of MESSENGER

images of Mercury’s surface. This comprehensive survey has al-

lowed identiﬁcation of many areas of hollow formation not previ-

ously recognised, building on the global inventory published by

Blewett et al. (2013). We have recorded the extent, location and

associations of the observed hollow clusters, and examined latitu-

dinal and longitudinal variations in their areal extent. We consider

how their occurrence and extent may be controlled by external fac-

tors such as insolation and ion sputtering or endogenic processes

such as the formation of pyroclastic pits or surﬁcial coverage by

thick volcanic plains. On a local scale, we have examined the slope

aspects in locations where hollows occur on slopes, in order to test

whether there is a correlation with insolation intensity, and have

studied the local settings of hollow formation to evaluate possible

exposure mechanisms for hollow-forming volatiles.

2. Methods

2.1. MESSENGER imagery

We examined images taken by MESSENGER’s Mercury Dual

Imaging System (MDIS) (Hawkins et al., 2007, 2009) up to the

end of MESSENGER’s fourth solar day in orbit around Mercury

(product creation times up to March 17, 2013). Monochrome

images were used to identify hollows and study them in detail,

and lower resolution colour composites were used to determine

the spectral character of their associated deposits and substrates.

2.1.1. Monochrome images

We examined all MESSENGER monochrome images with reso-

lutions of less than 180 m/px, excluding images at lower resolu-

tions because they do not reveal the irregular margins, ﬂat-ﬂoors

and rimlessness that distinguish hollows from small impact cra-

ters. These images were obtained by the 1.5°ﬁeld-of-view Narrow

Angle Camera (NAC) and the 748.7 nm ﬁlter in the 10.5°ﬁeld-of-

view Wide Angle Camera (WAC) of MDIS. The highest-resolution

images used were 7.7 m/px and the average resolution of those

available was 106 m/px.

We applied radiometric and photometric correction to all

images using the ISIS3 image processing package of the USGS.

We then overlaid these onto the 250 m/px global monochrome

mosaic version 9 produced by the MESSENGER team (released by

NASA’s Planetary Data System on 8 March 2013) and digitised fea-

tures on this global mosaic.

2.1.2. Colour images

To characterise the spectral type of local and regional sub-

strates, we examined colour composites created by combining data

from three of the twelve spectral ﬁlters in the WAC. All major sub-

strates on Mercury have red-sloped reﬂectance spectra (Denevi

et al., 2009), but the steepness of this slope varies, allowing some

Fig. 1. Irregular, rimless hollows on the ﬂoor and terraced wall of an unnamed

impact crater at 46.4°N, 318.7°E. Black arrows indicate individual hollows, white

bracket indicates a cluster (MESSENGER image ID 2760274).

222 R.J. Thomas et al. / Icarus 229 (2014) 221–235

to be classiﬁed as red or blue relative to the Hermean average. By

combining reﬂectance at 996 nm, 749 nm and 433 nm in the red,

green and blue bands, we were able to see these variations and

attribute substrates to the spectral types established by Denevi

et al. (2009), which are believed to indicate real compositional

and geological differences between surface units.

All images at a resolution of less than 1000 m/px were exam-

ined, as was the 1000 m/px global colour mosaic version 3 pro-

duced by the MESSENGER team (released by NASA’s Planetary

Data System on 8 March 2013). The highest-resolution composite

created was 64 m/px and the average resolution was 455 m/px.

2.2. Data collected

2.2.1. Hollows and pits

Data on all non-impact-related depressions visible in the

images were gathered in order to ensure that a distinction was

made between hollows and pits with a probable magmatic

(Gillis-Davis et al., 2009) or pyroclastic (Kerber et al., 2011) origin

and to make spatial comparisons between this activity and hollow

formation. Impact-related craters were distinguished from pits and

hollows on the basis of their circular shape, raised rims and the

characteristic geometry of their ejecta blankets.

The steepness of a depression’s margins and the spectral signa-

ture of its associated deposits were used to distinguish between

hollows and pits: hollows have steep margins leading to ﬂat ﬂoors

and bluer deposits while pits have gentler slopes, are deeper, and

any surrounding deposits are redder (Table 1). We identiﬁed a

third previously unidentiﬁed type of depression that is intermedi-

ate in character between pits and hollows: areas of pitted ground

ﬂoored by relatively red deposits. These either lack deﬁned mar-

gins or have steep margins that appear less crisp than those of hol-

lows. Where they have deﬁned margins, they are intermediate in

depth between hollows and pits. The presence of relatively red

deposits in these regions, the lack of relatively blue deposits and

their smoother morphology suggests that these are not hollows.

The similarity of the spectral character of their deposits to those

of pits, which are suggested to be formed by explosive volcanism

(Kerber et al., 2011), may indicate a volcanic origin.

For each depression, we gathered data on its geographical loca-

tion, area, association with tectonic structures such as thrust faults,

the spectral type of the local and regional substrate, and the type of

material hosting it. We identiﬁed the spectral type of the regional

substrate by reference to global mapping by Denevi et al. (2009,

2013) and our own observations of colour composite images, dis-

tinguishing between regional low-reﬂectance material (LRM),

intermediate terrain (IT), high-reﬂectance plains (HRP) and low-

reﬂectance blue plains (LBP). On a local scale, we noted the pres-

ence of relatively red material, bright ejecta deposits and localised

low-reﬂectance material. To record the host material we distin-

guished between the walls, peak structure, ejecta blanket and

smooth or rough ﬂoor of craters, and smooth and rough non-crater

surfaces.

We grouped hollows together on the basis of occurrence within

a particular host crater or location within 50 km of each other

where they lie outside craters. We calculated the areal extent of

hollows within each group by mapping them individually and

obtaining the spherical area (area of a polygon without the distor-

tion caused by map projection) using the Graphics and Shapes tool

for ArcGIS (Jenness, 2011).

As several of the proposed formation mechanisms for hollows

are controlled by insolation, we investigated the possibility of pref-

erential hollow formation on sun-facing slopes. We recorded the

aspect of the slope where hollows within a group occurred on

slopes of a particular orientation where that observed orientation

could not be explained by compositional differences or differences

in viewing conditions for nearby slopes at other orientations. This

aspect was taken to be the bearing of a line normal to the horizon-

tal alignment of hollows along the surface of the slope. This was

drawn by eye and rounded to the nearest 5°, as no digital terrain

model of adequate resolution was available.

In order to calculate the depths of hollows, we measured shad-

ows at the margins of hollows in cases where high resolution

(<110 m/px) images were available. A precision of half a pixel

was used to estimate the error. Where multiple images were avail-

able of the same hollow, we used the image with the highest res-

olution and lowest emission angle (angle off nadir) to minimise

error. We also avoided measuring shadows falling on steep slopes.

Because pits are generally larger-scale features than hollows

and since we wanted to investigate whether hollows occur in asso-

ciation with pits, we noted whether the resolution of the available

images of pits would allow identiﬁcation of hollows, if present.

2.2.2. Impact craters

Where hollows occur in association with an impact crater we

noted the crater diameter as a proxy for depth of excavation. This

was obtained from the Herrick et al. (2011) global crater database

or measured on a sinusoidal projection of the crater if it was not

within that database. We also noted the crater’s degree of degrada-

tion as a proxy for age using the scheme of Barnouin et al. (2012).

Degradation classes range from 1 to 5 (oldest to youngest) and are

deﬁned on the basis of characteristics such as the preservation of

the ejecta blanket, modiﬁcation of terraces and amount of super-

posed impact craters. Any crater ages mentioned in this work are

based on this scale.

3. Results

We found 445 groups of hollows, covering 57,400 km

, which

amounts to 0.08% of the surface area imaged at better than

180 m/px (locations indicated in Supplementary material). These

ranged in areal extent from 0.07 km

to 6771 km

, with a mean

of 129 km

(standard deviation = 475). 140 of these groups were

at locations previously catalogued by Blewett et al. (2013). We also

identiﬁed 173 groups of pits and 24 areas of spectrally red pitted

ground (Fig. 2).

Table 1

Characteristics distinguishing pits, hollows and spectrally red pitted ground.

Characteristic Hollows Pits Spectrally red pitted

ground

Wall slope Steep Shallow Lacking or steep

Floor slope Flat, though lumps of material may

occur

Sloping Roughly horizontal but

uneven

Surrounding deposits (when

present)

High-reﬂectance, relatively blue High-reﬂectance, relatively red High-reﬂectance,

relatively red

Depth Tens of meters Can be 1 km or more deep (Rothery et al., 2014; Gillis-Davis

et al., 2009)

Tens of meters

R.J. Thomas et al. / Icarus 229 (2014) 221–235 223

3.1. Global variations in the areal extent of hollows

Hollows occur globally, but are rare in the high reﬂectance

plains at high northern latitudes and within basins such as Caloris

(160°E, 32°N) and Rembrandt (88°E, 33°N). There is good image

coverage in these regions so this absence is not a product of obser-

vational bias. Though much of the interior of Caloris lacks hollows,

hollows do occur within younger impact craters in its ﬁll. Hollows

also occur at its rim, often in association with pits of probable

pyroclastic origin, and in two sublinear regions outside its north-

west rim.

Though hollows are not observed at high southern latitudes,

this could be largely attributable to observational bias: MESSEN-

GER’s highly elliptical orbit, with an initial periapsis altitude of

200 km at 60°N and an apoapsis altitude of 15,200 km (Hawkins

et al., 2007), means this area is imaged at much lower resolutions

than areas further north.

It has been suggested that hollow formation is controlled by

insolation and that hollows rarely occur on smooth plains sub-

strates (Blewett et al., 2013). In order to assess these hypotheses

with our global dataset, we investigated whether there is a corre-

lation between longitudinal and latitudinal variations in the areal

extent of hollows and in the intensity of insolation and the areal

extent of non-plains substrates.

3.1.1. Longitudinal variation

The elliptical orbit of Mercury leads to a variation in mean inso-

lation along the equator: two ‘hot poles’ (0°E and 180°E) are under

Fig. 2. Global occurrence of hollows, pits and spectrally red pitted ground. Yellow: hollows; black: pits; red: spectrally red pitted ground; grey: area not imaged at <180 m/px.

(Base mosaic: MESSENGER global colour v3.) (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

-160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Longitude (°E)

-160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Longitude (°E)

tot

0.002

0.004

0.006

0.008

0.002

0.004

0.006

0.008

Fig. 3. Variation in the areal extent of hollows (A

) by longitude bin in the region 30°Sto30°N, normalised to (a) area imaged at <180 m/px (A

), and (b) A

the fraction of the

surface that is not smooth plains (A

tot

224 R.J. Thomas et al. / Icarus 229 (2014) 221–235

the Sun at perihelion and experience mean temperatures esti-

mated to be 100 K higher than those of two ‘cold poles’ (90°E

and 90°E), under the Sun at aphelion (Melosh and McKinnon,

1988). To investigate the relationship between hollow occurrence

and this longitudinal variation, we calculated the total areal extent

of hollows in 20°bins in a 30°Sto30°N equatorial strip, normalis-

ing the hollowed area to the area that is imaged at <180 m/px

(Fig. 3a). This region has the virtue of being imaged at high resolu-

tion and low to moderate solar incidence angles, which are favour-

able observation conditions for hollows. We found that the areal

extent of hollows is low near the ‘cold poles’, as expected if the

intensity of insolation controls their occurrence. The fraction of

the surface hollowed between the 40°Eto20°E bin and the

60°Eto80°E bin peaks at the ‘hot pole’ at 0°E, but a similar pattern

of increase is not seen around the other ‘hot pole’ at 180°E.

However, plains associated with the Caloris basin occupy a large

part of the equatorial strip from 150°E to 180°E. To test whether

the presence of these plains modiﬁes the pattern of hollow occur-

rence, we normalised the areal extent of hollows to the fraction of

non-plains in each bin, in effect removing the inﬂuence of this

parameter from the data (Fig. 3b). We then saw a stronger correla-

tion between the extent of hollows and the intensity of mean inso-

lation, indicating that intensity of insolation controls hollow

occurrence but is not a sufﬁcient condition for their formation on

all substrates.

The large areal extent of hollows in the 60 to 40°E region is a

clear anomaly, neither accounted for by the variation in mean inso-

lation nor by the presence or absence of smooth plains.

3.1.2. Latitudinal variation

The fraction of the surface area imaged at <180 m/px that is hol-

lowed varies widely at different latitudes (Fig. 4a). This is in major

part attributable to observational bias: this fraction is highest at

low and mid-northern latitudes where MESSENGER is closest to

the planet and lowest at high southern latitudes where it is fur-

thest away. At very high latitudes, high solar incidence angles

(Chabot et al., 2013) also preclude identiﬁcation of hollows where

they occur in craters because large parts of crater interiors are in

shadow.

The lack of hollows at high northern latitudes is likely to be fur-

ther controlled by the presence of a smooth plains substrate here.

Normalising to the fraction of non-plains does not entirely remove

the disparity between hollow occurrence at low and high northern

latitudes (Fig. 4b), but as non-plains areas near the pole are imaged

only with high solar incidence angles, it is possible that the

remaining disparity is due to observational bias.

The areal extent of hollows at low southern latitudes (30°Nto

0°N) is signiﬁcantly lower than at low northern latitudes (0–30°N),

and this contrast is not removed by normalising to the fraction of

non-plains (Fig. 4b). This suggests a further factor discouraging

hollowing at low southern latitudes or promoting it at low north-

ern latitudes.

3.2. Preferred slope aspect

It has been suggested that hollows preferentially form on sun-

facing slopes and that this is evidence that their formation is linked

to solar heating (Blewett et al., 2013). We found some evidence in

support of this phenomenon. A preferred aspect was found in only

8% of the groups of hollows observed, but in these cases there was

a good correlation to the sun-facing slope (Fig. 5). The small per-

centage of cases in which a preferred aspect was observed is partly

attributable to the fact that only the sun-facing slope is illuminated

in many of the available images of hollows at mid- to high lati-

tudes, and in such cases preferential hollow formation on that

slope was not recorded because lighting conditions were not good

enough to rule out hollow formation on the opposing slope. How-

ever, most hollows are found on ﬂat surfaces or on slopes of oppos-

ing aspects within a group so we do not ﬁnd that preferential

0 0.0005 0.001 0.0015 0.002 0.0025 0.003

-60

-30

Latitude (°N)

Anp

AhAcAtot

()

0 0.0005 0.001 0.0015 0.002 0.0025 0.003

-60

-30

Latitude (°N)

AhAc

Fig. 4. Latitudinal variations in areal extent of hollows (A

), normalised to (a) the

area imaged at <180 m/px (A

) and (b) A

the fraction of the surface that is non-

plains (A

tot

) within each latitude bin. Hollow extent varies broadly with image

quality, though observational bias does not explain the small extent of hollowing in

the 30°Sto0°N bin.

(183.9)

Fig. 5. Aspects of slopes on which hollows preferentially form in the northern

hemisphere (N= 31), showing a correlation with the sun-facing slope. Purple line

indicates the mean, red circumferential line shows one standard deviation. Radial

axis: percentage of the population of hollow groups with a preferred aspect within

the northern hemisphere. (For interpretation of the references to colour in this

ﬁgure legend, the reader is referred to the web version of this article.)

R.J. Thomas et al. / Icarus 229 (2014) 221–235 225

formation on sun-facing slopes is a common characteristic of

hollows.

3.3. Hollow depth

We calculated a mean depth for hollows of 47 m (standard devi-

ation 21) on the basis of 108 shadow measurements within 27 hol-

low clusters. Depths ranged from 5 ± 0.75 m to 98 ± 19.5 m. These

results are consistent with but more extensive and representative

than previous studies showing a hollow depth of 30 m in Kertesz

crater (Vaughan et al., 2012) and 44 m in Raditladi basin (Blewett

et al., 2011).

Because of the limited resolution of available images, there is a

large error in depth values and it is not possible to identify signif-

icant depth variations between different substrates (e.g. peak ring

vs. crater ﬂoor) or craters of different ages.

3.4. Geological settings

The majority of hollows occur in clusters within impact craters

and upon their proximal ejecta, as discussed by Blewett et al.

(2013). However, our detailed survey also revealed two large areas

of more distributed hollow formation lacking a close relationship

with speciﬁc craters, and some association with pyroclastic pits

in non-crater settings.

3.4.1. Association with impact craters

3.4.1.1. Observations. Hollows occur on a variety of crater surfaces.

In simple craters, they commonly occur in a band on the inside rim

of the crater (Fig. 6a). In complex craters, they occur on the walls,

central structures and smooth ﬂoor ﬁll (Fig. 6b) and occasionally

on the ejecta blanket. Hollows are commonly clustered, either

loosely with small (<5 km) expanses of non-hollowed surface be-

tween individual hollows (Fig. 1) or more tightly, as in Fig. 6b,

where they form a continuous hollowed area. At the rim of the Cal-

oris basin, many small groups of hollows occur on peaks in the rim

material and associated with probable pyroclastic pits.

Where hollows occur on crater ﬁll, they often cluster around the

central structures or near the walls (Fig. 6c). In old, degraded cra-

ters, they commonly occur on the high inner walls of smaller im-

pacts into the crater or in the hanging walls of crater-crossing

thrust faults (Fig. 6d).

3.4.1.2. Statistical correlation. Hollows occur within impact craters

and their proximal ejecta in 84.5% of cases, and make up 97.5% of

the total global hollowed area. Hollows are therefore strongly asso-

ciated with craters.

If hollow-forming material is brought to the surface at the time

of crater formation, then depending on the duration over which

hollows (once formed) remain visible, a correlation could be ex-

pected between the age of the crater and the areal extent of

Fig. 6. Typical locations of hollows within impact craters: (a) in a curvilinear band at inner rim of a simple crater (58.6°E, 57.4°N, EN0238696485M); (b) hollowing across a

large part of the ﬂoor and peak structure of Hopper crater (Mansurian age, 55.8°E, 12.5°N, EN0223616383M); (c) clustered in the area abutting the crater wall of Nampeyo

crater (Mansurian age, 49.9°E, 40.3°N, EN0253678867M); (d) in a young impact into and on a thrust crossing the old, degraded Duccio crater (hollowed areas outlined in

yellow) (Tolstojan age, 52.3°E, 58.2°N, MESSENGER global monochrome mosaic); (e) close-up of hollows in a younger impact crater (EN0223658124M); (f) close-up of

hollows on a thrust (EN0223614937M). North is towards the top of each image. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the

web version of this article.)

226 R.J. Thomas et al. / Icarus 229 (2014) 221–235

hollowing. We therefore plotted the average extent of hollowing as

a percentage of the crater’s ﬂoor area (

where r= crater radius)

in craters of each degradation state (Fig. 7), using degradation state

as a proxy for age (Barnouin et al., 2012). We divided the data into

crater diameter bins to allow for the possibility that a similar deg-

radation state may occur in a shorter period for smaller craters

than larger craters. We found no clear increase in the percentage

of the crater area hollowed in older craters. In fact the average per-

centage hollowed is somewhat lower in older craters than younger

craters (note the logarithmic vertical scale), though it can range up

to 5.5% even in very old craters (degradation class 1, signifying a

Pre-Tolstojan age). Because slope processes and burial by regolith

can potentially obscure the characteristic morphology of hollows

(in particular their steep, sharp margins), hollows seen clearly

now can be assumed either to be still forming or to have ceased

forming in the relatively recent past.

3.4.2. Hollows outside craters

Hollows outside craters make up loose groupings rather than

clusters, and in most cases (excluding those discussed below) the

extent of hollowing within each group is small, averaging

15.2 km

(standard deviation = 38.0). This compares to a mean area

of 148.6 km

(standard deviation = 514.0) for hollow groups within

and in the proximal ejecta of craters. Hollows outside craters usu-

ally occur on hummocky surfaces or in a linear pattern cross-cut-

ting geological units, suggesting they formed in distal ejecta

(Fig. 8). Some occur around pits with surrounding relatively spec-

trally red deposits, as reported below (Section 3.5).

The detailed examination undertaken by this study has revealed

for the ﬁrst time two dispersed groupings of hollows covering large

areas (136,000 km

and 52,000 km

) roughly radial to the Caloris

basin rim and extending to the west and northwest (Fig. 9a). In

the western grouping, hollows with an areal extent totalling

150 km

occur on the partially-preserved, heavily-cratered raised

rims of old craters that are ﬂoored by smooth plains (Fig. 9b). The

hollows do not appear to be particularly associated with any one

crater but occur wherever this rougher, higher elevation substrate

occurs. This substrate can be classiﬁed as low-reﬂectance material

(LRM), whereas the crater ﬁlls are high reﬂectance plains (HRP).

Hollows in the northwest grouping have an areal extent totalling

498 km

. The eastern part of this grouping lies in and around a

wide graben outside the northwest rim of the Caloris basin, which

may have been carved by ejecta during the Caloris impact event

(Fassett et al., 2009). From this area towards the west, hollows oc-

cur on the circum-Caloris low-reﬂectance blue plains (LBP) at the

margins of a curvilinear unit of high reﬂectance material that ap-

pears contiguous with a region featuring several broad channels,

possibly lava channels (Byrne et al., 2013) near the margins of

0.1

10.0

12345

Degradation class (oldest to youngest)

% of crater floor area hollowed (log km2)

Crater diameter bin

16 − 64 km

64 − 256 km

Fig. 7. Spread in area of hollows as a percentage of the calculated crater ﬂoor area against the degradation state of the host crater. Bottom and top of boxes indicate the ﬁrst

and third quartiles, band inside each box indicates the median, numbers indicate number of observations and are vertically centred to the mean; lines above and below boxes

extend to the most extreme data point where it is no more than 1.5 times the interquartile range; dots indicate outliers.

Fig. 8. Hollows of small areal extent (indicated by white arrows) occur approximately radial to a Mansurian-aged unnamed complex crater with hollows on its peak ring,

ﬂoor and terraces (all hollows outlined in yellow). This association suggests they formed in distal ejecta from the crater. (Figure centred at 65.0°E, 44.8°N, image: excerpt

from global monochrome mosaic.) (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

R.J. Thomas et al. / Icarus 229 (2014) 221–235 227

the northern smooth plains (Fig. 9c). In the far northwest, hollows

preferentially form in and around localised very low reﬂectance

deposits (dubbed ‘dark spots’ by Xiao et al. (2013)), which occur

on a non-plains substrate adjacent to the smooth-ﬂoored channels

and to pits that are possible sources of ﬂuid lava that carved the

channels (Byrne et al., 2013). The ‘dark spots’ occur on a rough, cra-

tered substrate that appears superﬁcially smoothed. This smooth-

ing is particularly pronounced in lower elevation areas (Fig. 9d).

3.5. Association with pyroclastic features

Non-impact-related pits have hollows within 50 km of them in

74% of the cases where the resolution of the available images al-

lows hollow identiﬁcation, showing a strong association. 71% of

these pits have surrounding spectrally-red deposits, proposed to

be pyroclastic in origin (e.g. Kerber et al., 2011). The association be-

tween hollows and pits within craters is stronger than the associ-

ation of hollows and pits outside craters. 77% of pits within craters

have nearby hollows, while only 47% of pits outside craters do. All

of the areas of spectrally red pitted ground that were imaged at a

high-enough resolution to identify hollows did have hollows with-

in 50 km of them.

The reverse relationship is not as strong: only 22% of hollow

groups lie within 50 km of a pit or pitted spectrally red area, 93%

of which have associated bright red deposits indicating pyroclastic

activity. The areal extent of these groups of hollows is higher than

average: they total 52.5% of the total hollowed area and have a

mean extent per group of 307 km

(standard deviation = 855)

(compared to a mean of 129 km

(standard deviation = 475) for

the total population). Formation of hollows in the vicinity of pits

is more frequent in longitude bins crossing the ‘cold poles’ than

near the ‘hot poles’ (Fig. 10a), but variations in percentage of the

hollowed area within those bins do not follow this pattern

(Fig. 10b).

3.6. Association with regional substrates

Previous studies have indicated an association between hollow

formation and low-reﬂectance material, both the regional LRM unit

(Blewett et al., 2013) and localised, possibly lower reﬂectance (Xiao

et al., 2013) deposits. The results of our survey support this: 96% of

the total hollowed area occurs associated with either regional or

localised low-reﬂectance material. The hollows incise the low-

reﬂectance material and are ﬂoored and/or haloed by bright rela-

tively blue material.

We found that hollows are considerably rarer on plains sub-

strates: only 7% of the hollowed area occurs on high-reﬂectance

plains (HRP) and 8% on low-reﬂectance blue plains (LBP). Where

hollows occur on regional HRP, the low-reﬂectance material is al-

most always present at the surface locally (37 out of 38 cases). Lo-

cal low-reﬂectance material is also present in 25 out of 33 cases

where hollows occur on regional LBP.

The preference for hollow formation in low-reﬂectance material

rather than high-reﬂectance plains is particularly clear where an

impact crater straddles a contact between these two regional sub-

strates: Fig. 11 shows an 80 km diameter crater that intersects the

Fig. 9. (a) Two dispersed groupings of hollows (hollows outlined in yellow) occur to the northwest of the Caloris basin (dashed white line = basin rim). The northwest

grouping extends to a region featuring several named valles, thought to be lava channels (Byrne et al., 2013), several pits (outlined in blue) and a group of areas of spectrally

red pitted ground (outlined in red). Hollows associated with speciﬁc impact craters have been omitted from this diagram. Extent of insets indicated by white boxes (excerpt

from the global colour mosaic); (b) hollow formation in the southern grouping occurs on LRM forming degraded crater rims (image ID EW0264188888G); (c) hollows in the

mid-part of the northern grouping form at the margins and on the margin-proximal ﬂoor of a smooth, curvilinear unit of HRP (excerpt from the global monochrome mosaic);

(d) hollows (outlined in yellow) occur in ‘dark spots’ on regions of the non-plains surface that are adjacent to smooth channel ﬂoors and appear superﬁcially smoothed

(mosaic of image ID EW0231135561G, EW0231135600G and EW0231135586G). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to

the web version of this article.)

228 R.J. Thomas et al. / Icarus 229 (2014) 221–235

LRM rim of Rembrandt basin and its HRP ﬁll. The north and south

halves of the younger crater reﬂect the spectral properties of these

substrates, and hollow formation is only seen in the low-reﬂec-

tance substrate. This association supports the theory that the hol-

low-forming volatiles are derived from a constituent of low-

reﬂectance material.

4. Discussion

The results presented in this study have implications for the

mechanisms that form hollows and bring hollow-forming volatiles

to the surface, and provide clues to the origin of these volatiles.

4.1. Hollow formation mechanisms

4.1.1. Exogenic processes

Previous studies have suggested that hollows may form by sub-

limation or by space weathering processes (Blewett et al., 2011),

such as photon-stimulated desorption or sputtering by the solar

wind.

Sublimation could form hollows if a moderately-volatile sub-

stance that is unstable at the temperatures and pressures at the

surface of Mercury becomes exposed. It would then transition from

solid to gas and be lost to space via the exosphere, leaving a

depression. One reason why this is thought to be a viable mecha-

nism is the morphological similarity between hollows and ‘Swiss

cheese’ terrain in the south polar region of Mars. ‘Swiss cheese’ ter-

rain is believed to form by scarp retreat as CO

ice sublimes (Byrne

and Ingersoll, 2003a). The depth of sublimation may be limited by

the thickness of the subliming layer: the CO

ice overlies a water

ice layer that is more stable. This may also apply for hollow forma-

-150 -120 -90 -60 -30 0 30 60 90 120 150 180

% groups of hollows associated

with pits or red pitted ground

Longitude ( E)

100

-150 -120 -90 -60 -30 0 30 60 90 120 150 180

% hollowed area associated

with pits or red pitted ground

Longitude ( E)

Fig. 10. Latitudinal variation in the association of hollows with pits and spectrally red pitted ground. (a) Hollows more commonly occur near these features at ‘cold pole’-

crossing latitudes (90°E and 90°E) than at ‘hot pole’-crossing latitudes (0°E and 180°E) but (b) the percentage of the surface area that is hollowed shows no regular variation.

Fig. 11. Mansurian age crater straddling the southern rim of Rembrandt basin.

Hollows (outlined in yellow) occur in the southern part of the crater, which has a

low-reﬂectance substrate, and not in the high-reﬂectance northern part. (a)

Location of the crater at the southern margin of Rembrandt basin (excerpt from

global colour composite, centred at 88.1°E, 37.3°N); (b) the relation of hollows, a

small pit and an pitted red area to the two substrates (colour composite based on

EW0221673142G); (c) sketch map of the area in (b), black outlines: crater terraces

and peak structures; dark grey: hollowed area; light grey: pit; dotted ﬁll: pitted red

area; hatched: LRM surfaces. (For interpretation of the references to colour in this

ﬁgure legend, the reader is referred to the web version of this article.)

R.J. Thomas et al. / Icarus 229 (2014) 221–235 229

tion, though unless the volatile component of hollow-forming

material is 100%, accumulation of a residual lag is likely to limit

hollow depth (Blewett et al., 2013).

‘Swiss cheese’ terrain is not a perfect analogue for hollows. The

ﬂoors of the depressions in ‘Swiss cheese’ terrain are smooth and

their outlines are more regular and cuspate. The cuspate outlines

are believed to result from the consistency of the solar incidence

angle through the day in polar regions (Thomas et al., 2000; Byrne

and Ingersoll, 2003b). As most of the hollows we have observed are

closer to the equator, it is perhaps not surprising that hollows have

more irregular margins if sublimation is responsible for their for-

mation. The uneven ﬂoors of hollows compared to Swiss cheese

terrain suggest that unlike sublimation of CO

ice, the formation

of hollows leaves an appreciable lag fraction, and is perhaps

brought to a halt when the surface lag has reached a critical

thickness.

If hollows form by sublimation, it is probable that their occur-

rence would be correlated with local and regional variations in

insolation. Our results are consistent with this. Though few hollow

groups show preferential formation on slopes of a particular orien-

tation, when they do so there is a strong preference for the sun-fac-

ing slope (Fig. 5). The longitudinal variation in hollowing also

supports a correlation with insolation intensity: when the effect

of substrate is removed from a plot of the variation in the extent

of hollowing at equatorial latitudes (Fig. 3b), it varies broadly with

variations in the intensity of insolation. If surface temperature con-

trols hollowing, one would expect to see a greater reduction in the

extent of hollowing with increased latitude than at different longi-

tudes because the difference in maximum surface temperature be-

tween the equator and poles is larger than between points on the

equator (Peplowski et al., 2013). Our data show such a pattern, but

observational biases and differences in substrate mean this is not a

robust result.

Thus we ﬁnd that hollow formation appears to be correlated

with insolation intensity, but not strongly. In most cases hollows

form on ﬂat surfaces, or else hollows within a group occur on

slopes with a variety of aspects. This may suggest that the thresh-

old above which insolation causes hollow formation is commonly

met on the surface of Mercury.

A correlation of hollow location with insolation intensity does

not uniquely point to sublimation as the formation mechanism.

High insolation also means a higher photon ﬂux, promoting pho-

ton-stimulated desorption (PSD). In this process, UV photons strike

the planetary surface and excite atoms, which are then desorbed

and can be lost to the exosphere. This process is intensiﬁed by high

temperatures, possibly due to enhanced diffusion to the topmost

surface (Yakshinskiy and Madey, 2004), and PSD ﬂuxes contribut-

ing to the exosphere are up to three times higher from equatorial

surfaces at perihelion than at aphelion (Lammer et al., 2003). This

is consistent with the disparity of our results at different equatorial

longitudes. However, PSD is a phenomenon of the extreme surface,

affecting the topmost layer of atoms. Unless the volatile elements

that are susceptible to loss by this mechanism can be very efﬁ-

ciently delivered to the surface through any lag components, or un-

less churning of the regolith by impact gardening exposes fresh

materials very efﬁciently, it is not probable that it plays a major

part in hollow formation.

Another possibility is that hollow formation is enhanced by,

rather than caused by, high daytime insolation, because this leads

to greater diurnal temperature variation. The varying temperature

in the top few tens of cm of the surface (Vasavada et al., 1999)

could set up a circulation system that could concentrate volatiles

sufﬁciently to allow hollow formation by one or more of the pro-

cesses suggested here.

In light of the extreme conditions at Mercury, at an average of

0.387 AU from the Sun, the solar wind must be considered as a pos-

sible agent to produce hollows. When solar ions strike the planet’s

surface, they may remove material through momentum transfer in

a process known as physical sputtering. The importance of this

process is potentially testable by looking at latitudinal variations

in hollow formation: under normal solar wind conditions, the max-

imum precipitation ﬂux of solar wind ions onto Mercury’s surface

is expected at high latitudes due to their direction along open mag-

netic ﬁeld lines (Sarantos et al., 2007), while other areas of the pla-

net’s surface are subject to ion bombardment only during relatively

short-lived conditions of higher dynamic pressure (Siscoe and

Christopher, 1975; Kabin et al., 2000; Slavin et al., 2010). This

may mean the effects of physical sputtering are more pronounced

at high latitudes than low latitudes. One would also expect a stron-

ger effect on areas under the Sun at perihelion than at aphelion, as

the ﬂux through open ﬁeld lines is modelled to vary by a factor of

four and the area open to the solar wind by a factor of two between

these orbital points (Sarantos et al., 2007). However, given the

observational difﬁculties that hamper identiﬁcation of hollows at

polar and high southern latitudes and the presence of a plains sub-

strate at high northern latitudes that appears to preclude hollow

formation on compositional grounds, our current data does not al-

low us to conﬁdently compare the extent of hollow formation at

high vs. low latitudes. We do not rule out solar wind sputtering

playing a part in hollow formation, though the stronger evidence

for a correlation with insolation intensity at lower latitudes sug-

gests that surface temperature plays a strong role and that subli-

mation is probably the dominant mechanism.

4.1.2. Endogenic processes

The strong correlation of pyroclastic pits and areas of spectrally

red pitted ground with hollows (Section 3.5) suggests that endo-

genic heat sources may contribute to the heat necessary to release

the volatiles within hollow-forming substrates. If magmatic activ-

ity was contemporaneous with hollow formation at these sites, the

heat of subsurface magma may have mobilised the volatile compo-

nent of the host rock. This component may either ascend to the

surface, condense and later be removed by sublimation, possibly

aided by heat from below, or ascend as a gas and cause hollows

to form by collapse of surface material due to volume loss in the

underlying substrate.

The ﬁrst hypothesis best ﬁts the evidence, as hollows around

pits have the same morphology as those on crater surfaces. Though

it is possible that hollows and pits are found together only because

they both occur in the same substrates for independent reasons, it

is revealing that the areal extent of hollowing is on average higher

around pyroclastic pits than elsewhere. Additionally, at locations

where there is less insolation and so initiation of hollows would

be relatively more strongly affected by any endogenic component

to volatile mobilisation, a higher proportion of hollow groups occur

near pits. This suggests that proximity to pyroclastic pits leads to

more hollow formation. Also, the percentage of the total hollowed

area within each longitudinal band that is near pits is not strongly

correlated to variations in the intensity of insolation (Fig. 10b),

suggesting that conditions in the vicinity of particular pits exert

a stronger control on the extent of hollowing than do variations

in mean insolation.

The second hypothesis, that hollows form by surface collapse

when hollow-forming volatiles are lost, may be a more suitable

explanation for the shallow areas of pitted ground with red depos-

its. These areas look similar to hollows but are deeper, more un-

even, have a less morphologically-crisp appearance and in all

cases have hollows in their vicinity. They are also all found on

smooth substrates, in most cases crater ﬂoors. This juxtaposition

is seen clearly at Rachmaninoff basin (Fig. 12). Here, hollows form

in the low-reﬂectance material of the crater’s peak ring and walls,

and on the younger volcanic crater ﬁll (Marchi et al., 2011) around

230 R.J. Thomas et al. / Icarus 229 (2014) 221–235

areas of spectrally red pitted ground. The presence of low-reﬂec-

tance material on the crater’s peak ring and walls suggests this

material also forms the substrate to the volcanic inﬁll. The lava

may have heated the substrate and released its volatile component.

Before the lava fully solidiﬁed, disruption of its surface by escaping

volatiles and collapse due to volume loss in the substrate may have

given it a pitted morphology. This can be seen as broadly analogous

to the process by which pitted terrain is suggested to form by the

release of volatiles through impact melt on Mars (Boyce et al.,

2012) and Vesta (Denevi et al., 2012). The spectrally-red deposits

may indicate that magmatic volatiles and entrained juvenile mate-

rial also escaped to the surface along the same pathways as the

hollow-forming volatiles.

Such a process may also explain the morphology of hollowed

areas on crater ﬂoors over buried peaks rings, such as Sousa crater

(Fig. 13). The morphology of these areas is similar to that of those

areas of spectrally red pitted ground which lack margins, but they

have no associated red deposits. Here the crater ﬁll (either impact

melt or a later volcanic inﬁll) may have volatilised a component of

the buried low-reﬂectance peak ring, and released this material to

the surface at the point where the ﬁll is thinnest. This led to col-

lapse of the surface and some formation of crisper hollows where

escaped material condensed on the surface before losing its vola-

tile component via sublimation. This hypothesis is an extension

of that of Blewett et al. (2013) that hollows may form through con-

centration of hollow-forming volatiles by contact heating.

The global distribution of hollows does not simply mirror vari-

ations in insolation, and many hollows occur at a distance from pits

and potential pyroclastic activity and without contact with crater

ﬁlls. A further factor plays a stronger controlling role on the forma-

tion of hollows: substrate. This determines the quantity of hollow-

forming volatiles at and near the surface, and the ability of volatiles

to ascend to the surface. We explore this aspect below.

4.2. Means of transfer of hollow-forming volatiles to the surface

4.2.1. Exhumation and exposure by impacts

The strong correlation of hollows with craters suggests a genet-

ic link. The vast majority of hollows lie within impact craters and

their proximal ejecta. Hollows do not form on volcanic plains ex-

cept where these have been breached by later impacts, and most

of the small hollow clusters that occur outside impact craters ap-

Fig. 12. (a) Hollows (outlined in yellow) form in low-reﬂectance material in the peak ring and walls of Rachmaninoff basin (3.6 Ga old (Marchi et al., 2011), 57.4°E, 27.6°N)

and around bright, relatively-red areas (outlined in red) south of a breach in its peak ring (excerpt from global colour mosaic); (b) the area south of the breach in the peak ring,

where the bright spectrally-red areas are seen to be areas of pitted ground with hollows near their margins (composite of EW0254942264G, EW0254942268F and

EW0254942272I); (c) closeup of an area of pitted ground with steep margins at some points (black arrows) and hollow formation near its margins (white arrows)

(EN0219350311M). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 13. Hollows (outlined in yellow) on the LRM peak ring and the crater ﬁll where this overlies the peak ring in Sousa crater (Mansurian age, 0.5°E, 46.8°N). (a) Locations of

hollows within the crater (outlined in yellow) (excerpt from global colour mosaic) and (b) close-up showing the majority of the hollowed region in the crater ﬁll is pitted

ground, with some small crisp hollows (white arrows) (mosaic of EN0251054159M and EN0251054171M). (For interpretation of the references to colour in this ﬁgure legend,

the reader is referred to the web version of this article.)

R.J. Thomas et al. / Icarus 229 (2014) 221–235 231

pear to form in impact ejecta. For these reasons, it has previously

been suggested that hollows form in material exposed and ex-

humed by large impacts (Blewett et al., 2011). Larger impacts sam-

ple the crust to great depths compared to smaller impacts,

exposing the strata underlying the impacted surface in their walls

and ejecta, and uplifting material from depth in their central struc-

tures. The evidence supports this as a mechanism for exposure of

hollow-forming material. Hollows occur in low-reﬂectance mate-

rial within impact craters, particularly on peak rings, which are

the part of a crater that is exhumed from the greatest depth

(Fig. 14). The surface distribution of this low-reﬂectance material

is consistent with the substrate into which the crater incised

(Fig. 11) as expected if it is exhumed material.

The lack of correlation between increasing crater age and extent

of hollows (Fig. 7) could be seen as weakening this hypothesis: if

hollowing begins at the time of crater formation, older craters

should have a larger extent of hollows. However, ﬁrstly, the small

scale of hollows means they may become obscured by later overly-

ing ejecta over time and the areal extent of hollows visible in older

craters may not be indicative of the true cumulative amount of hol-

low formation there. Secondly, if the quantity of material that can

form hollows is limited, hollow formation would eventually cease.

This explains the drop-off in observed hollowed extent in craters of

Calorian age (degradation state 3) and older (Fig. 7).

Estimates of potential burial rates at Mercury’s surface vary:

regolith formation has been estimated at 5–10 m in the last 3–

4Ga(Langevin, 1997), while burial of polar ice deposits has been

modelled to occur at a rate of 0.43 cm/Myr (Crider and Killen,

2005). The former rate would be sufﬁcient to obscure the morphol-

ogy of hollows and the latter to completely bury them in the esti-

mated 3.9 Ga (Neukum et al., 2001)) since the Calorian period.

It has alternatively been suggested (Vaughan et al., 2012) that

hollows form in material differentiated out of impact melt during

crater formation. We ﬁnd this to be an unconvincing explanation:

hollows are found in small ejecta deposits distal from their host

craters (Fig. 8) and on a variety of steep surfaces. While differenti-

ation is viable in pooled melt, it is difﬁcult to envision in settings

such as these.

4.2.2. Post-impact exposure

The presence of hollows in even very old craters (Fig. 7) indi-

cates that in these cases a process is operating that replenishes hol-

low-forming material at the surface of craters long after crater

formation. Our observation that hollows in older craters are found

in the walls of younger craters and on thrust faults (Fig. 6d) sug-

gests that these are the agents of this late exposure. If hollow-

forming material was exposed to surface conditions during the for-

mation of the original crater, hollow formation may have ceased

prior to its depletion in the near-surface due to deposition of a

lag or burial by ejecta. Small new craters, crater-crossing thrust

faults and fractures at fault-bend folds may have later exposed it

to surface conditions, at which time fresh hollows formed.

Such processes may operate on a regional scale to produce the

western broad area of distributed hollowing outside the Caloris ba-

sin (Fig. 9a). The low-reﬂectance deposits here are close enough to

Caloris to have been deposited as ejecta from that impact, and may

have originally been as volatile-rich as the extensively-hollowed,

very dark LRM deposits that are exhumed by younger impacts into

the Caloris ﬁll. The heavily-cratered appearance of the hollowed

surfaces suggests they are old and that any initial hollow formation

in them would have long ceased, but smaller impacts and possibly

mass wasting may continue to expose new volatile-bearing mate-

rial to surface conditions and initiate new hollows.

The association of hollows with pyroclastic pits within craters

could indicate that the structures associated with this volcanism

aided the release of hollow-forming volatiles from depth. Pyroclas-

tic pits occur primarily in parts of impact craters that are underlain

by planes of weakness such as wall terraces and central structures.

This suggests that crater-related faults act as conduits for the re-

lease of volatiles or volatile-bearing magma towards the surface,

possibly aided by on-going fault movement in response to global

contraction (Klimczak et al., 2013). The same may be true for hol-

low-forming volatiles. The evidence does not, however, allow a

deﬁnite identiﬁcation of this phenomenon. For example, Fig. 15

shows hollows around a pyroclastic pit in the north-west rim of

a younger crater that intersects the wall of an older crater, and also

hollows in small areas to its south. This could be explained by the

migration of hollow-forming volatiles up the same crater-wall

faults as were exploited by the probable pyroclastic volcanism.

However, exhumation is a viable alternative explanation: hollow-

forming volatiles may have been present in the deeply-excavated

wall material of the older crater and then been exposed by the

younger crater. The small cluster of distal hollows may be located

in the ejecta from this impact. In this scenario, the association of

hollows with pyroclastic volcanism may be partly due to the spa-

tial coincidence of deep fractures that are conducive to magma as-

cent with volatile-bearing wall rocks, and possibly, if hollow

formation and volcanism were contemporaneous, partly due to in-

creased heat ﬂow in this area enhancing upward diffusion of vola-

tiles or hollow formation by sublimation (Section 4.1).

Fig. 14. Localisation of hollows (outlined in yellow) and low-reﬂectance deposits in the peak ring of Renoir basin (Tolstojan age, 51.8°E, 18.3°N). Dashed white line shows

the outer rim of the basin, where no low-reﬂectance material or hollows are observed. Left: excerpt from global monochrome mosaic; right: mosaic of colour composites

based on EW0253851174G, EW0253851412G and EW0241374406G. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web

version of this article.)

232 R.J. Thomas et al. / Icarus 229 (2014) 221–235

4.2.3. Exposure outside craters

A non-impact-related process is necessary to explain the expo-

sure of hollow-forming material to produce the broad area of dis-

tributed hollowing along a curvilinear unit of HRP to the northwest

of Caloris. These hollows form in LBP and on LRM outcrops stand-

ing above the plains. We propose that the HRP unit is a lava ﬂow,

and that contact heating by this material caused concentration of

volatiles from within the LBP and LRM substrates at the surface

(in a process similar to that suggested for other incidences of hol-

lowing by Blewett et al. (2013)), after which hollows formed by

sublimation.

The formation of hollows in small ‘dark spots’ at the far north-

west of this grouping (Fig. 9d) is more enigmatic. These ‘dark

spots’ occur on a non-plains substrate in which lower elevation

areas appear anomalously smooth, adjacent to broad, smooth-

ﬂoored channels and to the pits that have been suggested as

the source of voluminous lavas that carved those channels (Byrne

et al., 2012; Hurwitz et al., 2013). It is possible that the surface

here appears somewhat smoothed because it has been draped

by a thin layer of lava. This may have covered a volatile-bearing

substrate to a shallow depth. Subsequently either a high regional

heatﬂow connected to magmatic activity volatilised the underly-

ing substrate or burial was sufﬁciently shallow (<1 m (Vasavada

et al., 1999)) for penetration of solar heating do so. Pressure built

up and ﬁnally volatiles were released through fractures in the

overlying material. This is consistent with the hypothesis that

‘dark spots’ form during intense outgassing during hollow forma-

tion (Xiao et al., 2013). This process may have directly produced

hollows in the manner of fumerolic vents, or through deposition

of the volatile-rich dark material on the surface followed by its

sublimation.

4.3. Nature of the hollow-forming material

Hollows are commonly found in low-reﬂectance material. Con-

versely, they are found in high-reﬂectance smooth plains only

where low-reﬂectance material is locally present such as where

younger impact craters exhume it from beneath the volcanic ﬁll

of the Caloris basin.

This strongly suggests that the volatile material responsible for

hollow formation is not present in high-reﬂectance ﬂood lavas, but

is a component of low-reﬂectance material. The nature of low-

reﬂectance deposits on Mercury is not yet established, although

it has been suggested that the regional LRM unit could be primary

crust (Rothery et al., 2010) or a cumulate darkened by Fe- or Ti-

bearing or other opaque oxides (Denevi et al., 2009; Riner et al.,

2009). Space weathering complicates the determination of the

composition of LRM on the basis of reﬂectance: the creation of

nanophase iron during space weathering leads to more darkening

of rock types initially richer in iron, so the albedo of any rock is

the product of its composition and mineralogy, its exposure time

and its susceptibility to space weathering (Riner and Lucey, 2012).

The presence of low-reﬂectance material in some craters of a

particular diameter (and thus excavation depth) and absence in

others shows that low-reﬂectance material is not present globally

at a speciﬁc depth, so variations in the igneous and/or tectonic his-

tory of different parts of Mercury’s crust appear to play a role in its

occurrence. For example, the formation of the Caloris impact basin

may have exhumed large amounts of this material, providing the

substrate for the broad regions of dispersed hollow formation to

its northwest.

The relatively lower reﬂectance of some localised deposits

around hollows (Xiao et al., 2013) may suggest that before LRM

has been hollowed it contains an additional darkening agent, and

that this is the hollow-forming volatile. This may explain the pres-

ence of bright material in haloes around hollows and on their

ﬂoors, which could be a brighter residue formed by the removal

of a spectrally dark component (Blewett et al., 2013). Alternatively,

the high reﬂectivity of these deposits may be due to an unusual

texture or small grain size (Blewett et al., 2013). The diffused mar-

gins of the haloes suggest two possible emplacement mechanisms

for the bright material: (a) ballistic ejection of a bright component

as a result of high-energy escape of the hollow-forming volatiles,

or (b) diffusive alteration of hollow wall rock as a result of chemical

reactions during hollow formation. In both cases, either the com-

positional or physical characteristics of these bright deposits could

potentially be the cause of their high reﬂectance.

The composition of the darker, volatile component is as yet un-

known, but will perhaps be resolved when BepiColombo with its

higher-resolution visible-NIR, thermal infrared and X-ray spec-

trometers (Rothery et al., 2010) arrives at Mercury in the coming

decade.

Fig. 15. (a) Hollows (outlined in yellow) occur at the north-west edge of a Calorian age crater at 3.6°E, 25.6°N, around a pyroclastic pit (outlined in orange) and scattered

towards the south. This may be due to explosive escape of hollow-forming volatiles up the same conduits as used by the pyroclastic volcanism, or, if the two phenomena were

contemporaneous, hollow formation in crater deposits and ejecta intensiﬁed in the region of volcanism by endogenic heat ﬂow (MESSENGER global monochrome mosaic); (b)

colour composite of the superposed crater in the north (based on EW0225312562G); (c) sketch map of (b), hatched area: low-reﬂectance material; dotted area: bright ejecta;

pink area: bright ‘red’ deposits; orange area: pit; yellow area: hollowed area; black outline: crater walls. (For interpretation of the references to colour in this ﬁgure legend,

the reader is referred to the web version of this article.)

R.J. Thomas et al. / Icarus 229 (2014) 221–235 233

5. Conclusions

1. In a global survey of the surface of Mercury, we found that the

shallow rimless depressions known as hollows cover

57,400 km

, which is 0.08% of the total surface imaged at bet-

ter than 180 m/px.

2. A weak overall correlation was found between hollow occur-

rence and insolation, as well as a possible correlation with sub-

surface heat sources. Both suggest a thermal control on hollow

formation, thus supporting sublimation as the primary hollow-

forming mechanism.

3. In most cases it appears probable that material containing hol-

low-forming volatiles was exposed and exhumed from depth by

large impacts.

4. Some small impact craters and thrust faults within older craters

also have hollows, hence these structures may expose subsur-

face hollow-forming material or facilitate the migration of vol-

atiles to the surface. This suggests that some volatiles remain in

the near-subsurface even after hollow formation has ceased at

the surface.

5. Hollows do not occur in volcanic plains but are found mostly in

low-reﬂectance material. This suggests that this low-reﬂectance

material has a volatile component, and that hollows are formed

by loss of that component. The widespread occurrence of hol-

lows suggests that this material is similarly widespread within

the crust of Mercury.

Acknowledgments

Rebecca Thomas acknowledges support via a PhD grant from

the Science and Technology Facilities Council (UK) and David Roth-

ery acknowledges support from the UK Space Agency in prepara-

tion for the BepiColombo mission. Excerpts from global mosaics

used in ﬁgures in this paper are credited to NASA/Johns Hopkins

University Applied Physics Laboratory/Carnegie Institution of

Washington, and the NAC and WAC images are credited to NASA/

JPL-Caltech. We thank David Blewett and Laura Kerber for their

constructive and insightful reviews that helped improve the qual-

ity of the manuscript. We also thank Oded Aharonson for his edito-

rial handling.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in

the online version, at http://dx.doi.org/10.1016/j.icarus.2013.11.

018.

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Full-text available

Jul 2023

Comets are active geological worlds with primitive surfaces that have been shaped to varying degrees by sublimation‐driven sediment transport and mass wasting processes. Rosetta's rendezvous with comet 67P/Churyumov‐Gerasimenko (67P) in 2014 provided data with the necessary spatial and temporal resolutions to observe many evolutionary processes on micro‐gravity worlds. Rosetta's observations have thus far revealed that many changes to the surface occurred within 67P's smooth terrains, vast sedimentary deposits that blanket a significant fraction of the nucleus. Understanding the global context of these changes, and therefore the sediment transport pathways that govern the evolution of 67P's surface, requires a thorough description of their changing morphologies and an evaluation of existing global‐scale spatial and temporal trends. Accordingly, we present a time‐resolved synthesis of erosion and deposition activity on comet 67P as it passed through its 13 August 2015 perihelion from September 2014 to August 2016. Our mapping results indicate that, around perihelion, sediment is globally redistributed inter‐regionally from 67P's more active south to the north. Equally important, however, are local, topographically influenced sediment transport processes, with large volumes of sediment moving intra‐regionally over sub‐kilometer distances. We also show evidence for regions of near‐zero net erosion/deposition between approximately 30°N–60°N latitude, which may act as terminal sedimentary sinks, with the remobilization of these materials hindered by multiple factors. Our work therefore provides the most complete mapping of sediment transport processes and pathways across 67P, a critical step toward understanding the global landscape evolution of both 67P and other comets.

Mercury

Chapter

Jan 2024

Geology of the Neruda quadrangle (H13), Mercury

Article

Sep 2023
J MAPS

Evidence for a carbon-rich Mercury from the distribution of low-reflectance material (LRM) associated with large impact basins

Article

Jul 2023
EARTH PLANET SC LETT

Principal Component Analysis applied on MASCS/MESSENGER data for the spectral investigation of Mercury's surface

Article

May 2023
ICARUS

On the formation of thrust fault-related landforms on Mercury: The key parameters controlling the mechanical structure of the lithosphere

Article

Apr 2023
ICARUS

A hybrid geological map of Sibelius Crater on Mercury, and its associated ejecta and impact melt deposits

Article

Apr 2023

Using MESSENGER Mercury Dual Imaging System data, we produced 3 new maps of Sibelius Crater, Mercury. Geomorphological and spectral maps were combined into a single hybrid map containing units associated with ejecta deposits, crater floor landforms and impact melt ponding. Spatial measurement of these units show that ∼50% of the mapped melt pond area lies within a large, degraded impact crater (crater B), beyond the significantly lower northern Sibelius rim, with a potential melt flow to a smaller, degraded impact structure further north (crater C). Freshly processed spectral data from the 8-color Map Projected Multispectral Reduced Data Record (MDR) data highlight the emplacement of multiple uplifted ejecta units with distinct spectral properties. A new, high resolution digital elevation model was created to help define and analyse crater floor uplift features, disrupted crater rims and to create detailed cross sections. These illustrate a proposed location of B's central uplift structure exposed in the northern wall slopes of Sibelius. Small features at the limit of visibility, such as a groove possibly associated with a rolling or sliding mega-boulder, and lobate melt flow on the crater floor with accompanying channel opening, are highlighted for future investigations by BepiColombo's instruments once it reaches orbit.

Mercury's hollows: Constraints on formation and composition from analysis of geological setting and spectral reflectance

Article

Jan 2012

D.T. Blewett

Dark spots on Mercury: A distinctive low-reflectance material and its relation to hollows

Article

Aug 2013

Enhanced sodium abundance in Mercury’s north polar region revealed by the MESSENGER Gamma-Ray Spectrometer

Article

Jan 2014

MESSENGER Gamma-Ray Spectrometer measurements demonstrate that the abundance of Na varies across the surface of Mercury. The maximum Na/Si abundance ratio of 0.20 ± 0.03 by weight (∼5 wt% Na) is observed at high northern latitudes and is significantly larger than the equatorial Na/Si ratio of 0.11 ± 0.01 (∼2.6 wt% Na). Comparisons of forward-modeled surface distributions with the gamma-ray measurements suggest that the observed distribution of Na can be explained by differences in elemental composition between the volcanic smooth plains units and heavily cratered terrain. The comparison improves when thermally driven depletion of Na from areas near Mercury’s hot poles is included. When combined with other MESSENGER data sets, these results indicate that the smooth plains units include substantial abundances of alkali feldspars. Thermal depletion of Na from the hot poles without an assumed underlying compositional variability can also reproduce the measured Na/Si distribution, but that mechanism fails to account for other MESSENGER observations that support the presence of higher abundances of feldspars in the smooth plains units.

Prolonged eruptive history of a compound volcano on Mercury: Volcanic and tectonic implications

Article

Jan 2014
EARTH PLANET SC LETT

A 27×13 km27×13 km ‘rimless depression’ 100 km inside the southwest rim of the Caloris basin is revealed by high resolution orbital imaging under a variety of illuminations to consist of at least nine overlapping volcanic vents, each individually up to 8 km in diameter. It is thus a ‘compound’ volcano, indicative of localised migration of the site of the active vent. The vent floors are at a least 1 km below their brinks, but lack the flat shape characteristically produced by piston-like subsidence of a caldera floor or by flooding of a crater bottom by a lava lake. They bear a closer resemblance to volcanic craters sculpted by explosive eruptions and/or modified by collapse into void spaces created by magma withdrawal back down into a conduit. This complex of overlapping vents is at the summit of a subtle edifice at least 100 km across, with flank slopes of about only 0.2 degrees, after correction for the regional slope. This is consistent with previous interpretation as a locus of pyroclastic eruptions. Construction of the edifice could have been contributed to by effusion of very low viscosity lava, but high resolution images show that the vent-facing rim of a nearby impact crater is not heavily embayed as previously supposed on the basis of lower resolution flyby imaging. Contrasts in morphology (sharpness versus blurredness of the texture) and different densities of superposed sub-km impact craters inside each vent are consistent with (but do not prove) substantial differences in the age of the most recent activity at each vent. This suggests a long duration of episodic magmagenesis at a restricted locus. The age range cannot be quantified, but could be of the order of a billion years. If each vent was fed from the same point source, geometric considerations suggest a source depth of at least 50 km. However, the migration of the active vent may be partly controlled by a deep-seated fault that is radial to the Caloris basin. Other rimless depressions in this part of the Caloris basin fall on or close to radial lines, suggesting that elements of the Pantheon Fossae radial fracture system that dominates the surface of the central portion of the Caloris basin may continue at depth almost as far as the basin rim.

Dark Spots on Mercury: A Distinctive Low-Reflectance Material and its Relation to Hollows

Article

Sep 2013

Orbital images acquired by the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft reveal a distinctive low-reflectance material on the surface of Mercury. Such material occurs in small, isolated, and thin surficial units. We term these features dark spots. Dark spots have the lowest average reflectance yet documented on the planet. In every case observed at sufficiently high resolution, dark spots feature hollows at their centers. Not all hollows, however, are surrounded by a dark spot. Dark spots have been found on low-reflectance smooth plains, intercrater plains, heavily cratered terrain, and impact craters at almost all longitudes on Mercury, but they have not been documented on high-reflectance smooth plains material. Dark spots are one of the youngest endogenic features on Mercury, and some postdate craters with distinctive rays. Sulfides may be the phase responsible for the low albedo of dark spot material. We propose that dark spots form during the initial stages of hollow formation, perhaps in a manner similar to intense outgassing events that feature exit velocities in excess of 100 m/s. Such outgassing could contemporaneously produce a depression that constitutes an embryonic hollow. Under this scenario, dark spot material is subsequently removed or modified by regolith gardening or other surface processes on time scales shorter than the lifetime of the central hollow.

The Role of Thrust Faults as Conduits for Volatiles on Mercury

Article

Mar 2013

The frequent spatial association of volcanic pits with thrust faults on Mercury is motivation to study if faults functioned as conduits for volatile-rich magmas.

Craters hosting radar-bright deposits in Mercury's north polar region: Areas of persistent shadow determined from MESSENGER images

Article

Jan 2013

Radar-bright features near Mercury's poles were discovered in Earth-based radar images and proposed to be water ice present in permanently shadowed areas. Images from MESSENGER's one-year primary orbital mission provide the first nearly complete view of Mercury's north polar region, as well as multiple images of the surface under a range of illumination conditions. We find that radar-bright features near Mercury's north pole are associated with locations persistently shadowed in MESSENGER images. Within 10° of the pole, almost all craters larger than 10 km in diameter host radar-bright deposits. There are several craters located near Mercury's north pole with sufficiently large diameters to enable long-lived water ice to be thermally stable at the surface within regions of permanent shadow. Craters located farther south also host radar-bright deposits and show a preference for cold-pole longitudes; thermal models suggest that a thin insulating layer is required to cover these deposits if the radar-bright material consists predominantly of long-lived water ice. Many small (<10 km diameter) and low-latitude (extending southward to 66°N) craters host radar-bright material, and water ice may not be thermally stable in these craters for ~1 Gy, even beneath an insulating layer. The correlation of radar-bright features with persistently shadowed areas is consistent with the deposits being composed of water ice, and future thermal modeling of small and low-latitude craters has the potential to further constrain the nature, source, and timing of emplacement of the radar-bright material.

The distribution and origin of smooth plains on Mercury

Article

May 2013

Orbital images from the MESSENGER spacecraft show that ~27% of Mercury's surface is covered by smooth plains, the majority (>65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali-basalt-like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin-type plains, occupy an additional 2% of Mercury's surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size-frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large-scale volcanic deposits rather than differences in crustal formational process.

Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations

Article

Mar 2013

Volcanic plains identified on Mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies. We analyzed images acquired by the MESSENGER spacecraft to identify features on Mercury that may have formed by lava erosion. We used analytical models to estimate eruption flux, erosion rate, and eruption duration to characterize the formation of candidate erosional features, and we compared results with analyses of similar features observed on Earth, the Moon, and Mars. Results suggest that lava erupting at high effusion rates similar to those required to form the Teepee Butte Member of the Columbia River flood basalts (0.1-1.2 × 106 m3 s-1) would have been necessary to form wide valleys (>15 km wide) observed in Mercury's northern hemisphere, first by mechanical erosion to remove an upper regolith layer, then by thermal erosion once a lower rigid layer was encountered. Alternatively, results suggest that lava erupting at lower effusion rates similar to those predicted to have formed Rima Prinz on the Moon (4400 m3 s-1) would have been required to form, via thermal erosion, narrower channels (<7 km wide) observed on Mercury. Although these results indicate how erosion might have occurred on Mercury, the observed features may have formed by other processes, including lava flooding terrain sculpted during the formation of the Caloris basin in the case of the wide valleys, or impact melt carving channels into impact ejecta in the case of the narrower channels.

An assemblage of lava flow features on Mercury

Article

Jun 2013

contrast to other terrestrial planets, Mercury does not possess a great variety of volcanic features, its history of volcanism instead largely manifest by expansive smooth plains. However, a set of landforms at high northern latitudes on Mercury resembles surface flow features documented on Earth, the Moon, Mars, and Venus. The most striking of such landforms are broad channels that host streamlined islands and that cut through the surrounding intercrater plains. Together with narrower, more sinuous channels, coalesced depressions, evidence for local flooding of intercrater plains by lavas, and a first-order analysis of lava flow rates, the broad channels define an assemblage of flow features formed by the overland flow of, and erosion by, voluminous, high-temperature, low-viscosity lavas. This interpretation is consistent with compositional data suggesting that substantial portions of Mercury's crust are composed of magnesian, iron-poor lithologies. Moreover, the proximity of this partially flooded assemblage to extensive volcanic plains raises the possibility that the formation of these flow features may preface total inundation of an area by lavas emplaced in a flood mode and that they escaped complete burial only due to a waning magmatic supply. Finally, that these broad channels on Mercury are volcanic in nature yet resemble outflow channels on Mars, which are commonly attributed to catastrophic water floods, implies that aqueous activity is not a prerequisite for the formation of such distinctive landforms on any planetary body.

Hollows on Mercury: Materials and mechanisms involved in their formation

Abstract and Figures

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