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An investigation of Pluto's troposphere using stellar occultation light curves and an atmospheric radiative-conductive-convective model
Abstract
We use a radiative-conductive-convective model to assess the height of Pluto's troposphere, as well as surface pressure and surface radius, from stellar occultation data from the years 1988, 2002, and 2006. The height of the troposphere, if it exists, is less than 1 km for all years analyzed. Pluto has at most a planetary boundary layer and not a troposphere. As in previous analyses of Pluto occultation light curves, we find that the surface pressure is increasing with time, assuming that latitude and longitude variations in Pluto's atmosphere are negligible. The surface pressure is found to be slightly higher ( 12.5-2.4+1.9 μbar in 1988, 18.0-1.7+11 μbar in 2002, and 18.5 ± 4.7 μbar in 2006) than in our previous analyses with the troposphere excluded. The surface radius is determined to be 1173-10+20km. Comparison of the minimum reduced chi-squared values between the best-fit radiative-conductive-convective (i.e., troposphere-included) model and best-fit radiative-conductive (i.e., troposphere-excluded) shows that the troposphere-included model is only a slightly better fit to the data for all 3 years. Uncertainties in the small-scale physical processes of Pluto's lower atmosphere and consequently the functional form of the model troposphere lend more confidence to the troposphere-excluded results.
It began with Aristotle. After sunset on May 4th in 357 BCE one of antiquity’s most influential philosophers watched the planet Mars disappear behind the Moon. His view is simulated in Fig. 20.1. From this simple observation he concluded that Mars must be the farther of the two bodies (Stephenson 2000). In his own words:
Aims: The goal is to determine the composition of Pluto's atmosphere and to constrain the nature of surface-atmosphere interactions. Methods: We perform high-resolution spectroscopic observations in the 2.33-2.36 mum range, using CRIRES at the VLT. Results: We obtain (i) the first detection of gaseous methane in this spectral range, through lines of the nu3 + nu4 and nu1 + nu4 bands (ii) strong evidence (6-sigma confidence) for gaseous CO in Pluto. For an isothermal atmosphere at 90 K, the CH4 and CO column densities are 0.75 and 0.07 cm-am, within factors of 2 and 3, respectively. Using a physically-based thermal structure model of Pluto's atmosphere also satisfying constraints from stellar occultations, we infer CH4 and CO mixing ratios qCH_4= 0.6+0.6_{-0.3}% (consistent with results from the 1.66 mum range) and qCO = 0.5+1_{-0.25 × 10-3}. The CO atmospheric abundance is consistent with its surface abundance. As for Triton, it is probably controlled by a thin, CO-rich, detailed balancing layer resulting from seasonal transport and/or atmospheric escape.
We present observations of 15 Pluto-Charon mutual events which were
obtained with the 60 in. telescope at Palomar Mountain Observatory. A
CCD camera and Johnson V filter were used for the observations, except
for one event that was observed with a Johnson B filter, and another
event that was observed with a Gunn R filter. We observed two events in
their entirety, and three pairs of complementary mutual
occultation-transit events.
Observations of the 2007 March 18 occultation of the star P445.3 (2UCAC 25823784; R = 15.3) by Pluto were obtained at high time resolution at five sites across the western United States and reduced to produce light curves for each station using standard aperture photometry. Global models of Pluto's upper atmosphere are fitted simultaneously to all resulting light curves. The results of these model fits indicate that the structure of Pluto's upper atmosphere is essentially unchanged since the previous occultation observed in 2006, leading to a well-constrained measurement of the atmospheric half-light radius at 1291 ± 5 km. These results also confirm that the significant increase in atmospheric pressure detected between 1988 and 2002 has ceased. Inversion of the Multiple Mirror Telescope Observatory light curves with unprecedented signal-to-noise ratios reveals significant oscillations in the number density, pressure, and temperature profiles of Pluto's atmosphere. Detailed analysis of this highest resolution light curve indicates that these variations in Pluto's upper atmospheric structure exhibit a previously unseen oscillatory structure with strong correlations of features among locations separated by almost 1200 km in Pluto's atmosphere. Thus, we conclude that these variations are caused by some form of large-scale atmospheric waves. Interpreting these oscillations as Rossby (planetary) waves allows us to establish an upper limit of less than 3 m s–1 for horizontal wind speeds in the sampled region (radius 1340-1460 km) of Pluto's upper atmosphere.
We have observed the 2002 August 21 occultation by Pluto of the R = 15.7 mag star P131.1, using 0.5 s cadence observations in integrated white light with the Williams College frame-transfer, rapid-readout CCD at the 2.24 m University of Hawaii telescope. We detected an occultation that lasted 5 minutes, 9.1 ± 0.7 s between half-light points. The "kinks" in the ingress and egress parts of the curve that were apparent in 1988 had become much less pronounced by the time of the two 2002 occultations that were observed, indicating a major change in the structure of Pluto's atmosphere. Analysis of our light curves shows that the pressure in Pluto's atmosphere has increased at all the altitudes that we probed. Essentially, the entire pressure scale has moved up in altitude, increasing by a factor of 2 since 1988. Spikes in our light curve reveal vertical structure in Pluto's atmosphere at unprecedentedly high resolution. We have confirmation of our spikes at lower time resolution as part of observations of the emersion made at 1.4 s and 2.4 s cadence with the 3.67 m AEOS telescope on Maui.
We present a method for obtaining atmospheric temperature, pressure, and number density profiles for small bodies through inversion of light curves recorded during stellar occultations. This method avoids the assumption that the atmospheric scale height is small compared with the radius of the body, and it includes the variation of gravitational acceleration with radius. First we derive the integral equations for temperature, scale height, pressure, number density, refractivity, and radius in terms of the light-curve flux. These are then cast into summation form suitable for numerical evaluation. Equations for the errors in these quantities caused by Gaussian noise in the occultation light curve are also derived. The method allows for an arbitrary atmospheric boundary condition above the inversion region, and one particular boundary condition is implemented through least-squares fitting. When the inversion equations are applied to noiseless test data for a simulated isothermal atmosphere, numerical errors in the calculated temperature profile are less than 5 parts in 104. Nonisothermal test cases are also presented. We explore the effects of (1) the boundary condition, (2) data averaging (in the time, observer-plane, and body-plane domains), (3) systematic errors in the zero stellar flux level, and (4) light-curve noise on the accuracy of the inversion results. A criterion is presented for deciding whether inversion would be an appropriate analysis for a given stellar occultation light curve, and limitations to the radial resolution of the inversion results are discussed. The inversion method is then employed on the light curves for the 1988 June 9 occultation by Pluto observed with the Kuiper Airborne Observatory and the 1997 November 4 occultation by Triton observed with the Hubble Space Telescope. Under the (possibly incorrect) assumption that no extinction effects are present in the occultation light curve, the Pluto inversion yields a 110 K isothermal profile down to approximately 1215 km radius, at which point a strong thermal gradient, 3.9 ± 0.6 K km-1, abruptly appears, reaching 93 K at the end of the inversion. The Triton inversion yields a differently shaped profile, which has an upper level thermal gradient, ~0.4 K km-1, followed by a ~51 K isothermal profile at lower altitudes. The Triton inversion shows wavelike temperature variations in the lower atmosphere, with amplitudes of ~1 K and wavelengths of ~20 km, that could be caused by horizontal or vertical atmospheric waves.
We obtained Pluto's spectrum using the CSHELL echelle spectrograph at NASA's IRTF on Mauna Kea, on 25–26 May 1992, with a spectral resolution of 13,300. The spectral range (5998–6018 cm−1, or 1661.8–1666.9 nm) includes theR(0) and theQ(1)–Q(9) lines of the 2ν3band of methane. The resulting spectrum shows the first detection of gaseous methane on Pluto, with a column height of 1.20+3.15−0.87cm-A (3.22+8.46−2.34× 1019molecule cm−2).
Context. Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported. Aims. The goal is to constrain essential but so far unknown parameters of Pluto's atmosphere such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio. Methods. We use high-resolution spectroscopic observations of gaseous methane, and a novel analysis of occultation light-curves. Results. We show that (i) Pluto's surface pressure is currently in the 6.5-24 μbar range (ii) the methane mixing ratio is 0.5±0.1 %, adequate to explain Pluto's inverted thermal structure and ∼100 K upper atmosphere temperature (iii) a troposphere is not required by our data, but if present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case methane is supersaturated in most of it. The atmospheric and bulk surface abundance of methane are strikingly similar, a possible consequence of the presence of a CH4-rich top surface layer.
Pluto's tenuous nitrogen atmosphere was first detected by the imprint left on the light curve of a star that was occulted by the planet in 1985 (ref. 1), and studied more extensively during a second occultation event in 1988 (refs 2-6). These events are, however, quite rare and Pluto's atmosphere remains poorly understood, as in particular the planet has not yet been visited by a spacecraft. Here we report data from the first occultations by Pluto since 1988. We find that, during the intervening 14 years, there seems to have been a doubling of the atmospheric pressure, a probable seasonal effect on Pluto.
Stellar occultations--the passing of a relatively nearby body in front of a background star--can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Pluto's atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of approximately 1,215 km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Pluto's atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Pluto's atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.
Over the past several years Pluto-Charon mutual events have yielded progressively more accurate estimates of Charon's orbital elements and the radii of Pluto and Charon (e.g., Buie, Tholen, and Horne, 1992). Analysis of the 1988 stellar occultation by Pluto indicates a radius for Pluto that is about 4%, or 50 km, larger than the mutual event radius of 1151 km. One possible explanation for the discrepancy is that the mutual event modeling treats Pluto and Charon as uniformly bright disks. If they are limb-darkened, the mutual event fits could underestimate their radii. In this paper we use an independent mutual event data set (Young and Binzel, 1992) to fit for Pluto and Charon's radii in a manner independent of either object's limb profile or albedo distribution. Our least-squares solution indicates that Pluto's radius is 1164 +/- 22.9 km and Charon's radius is 621 +/- 20.6 km.
A radiative-thermal conduction model for the vertical thermal structure of Io's atmosphere is developed with solar heating by SO2 absorption in UV and near-IR bands and non-local thermodynamic equilibrium (LTE) cooling by SO2 nu(sub 1), nu(sub 2), nu(sub 3), vibrational bands and rotational lines. The model predicts the existence of a mesopause in Io's atmosphere when the surface pressure exceeds approximately 10 nbar. The radiative time constant for establishing a mesosphere/mesopause on Io is only approximately 20 min, whereas the thermospheric radiative time constant is about 1 hr. These time constants are significantly shorter than the diurnal time scale and competitive with dynamic time scales. In the thermosphere when solar UV heating dominates, the asymptotic thermospheric temperature is approximately 270 K, only 140 K greater than the surface temperature because at high altiudes non-LTE cooling by SO2 rotation lines exceeds cooling in the nu(sub 2) virbrational band. Solar-heating-only modles are incapable of generating warm enough atmospheres to satisfy the observational inferences from UV and especially millimeter-wave meausrements. Joule heating driven by the penetration of Jupiter's corotational electric field into Io's conducting ionosphere is demonstrated to be the dominant heating mechanism in the subnanobar regions of Io's atmosphere with tempertures ranging from 150 to 1000 K as a function of decreasing pressure from 1 to 0.1 nbar, The asymoptotic thermospheric temperature can attain a value as high as 1800 K.
The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25-2.50 micron spectral images for studying surface compositions, and measurements of Pluto's atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to acheive the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth-moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N2:CH4 atmospheres (such as Titan, Triton, and the early Earth).
We use a radiative-conductive model to least-squares fit Pluto stellar occultation light curve data. This model predicts atmospheric temperature based on surface temperature, surface pressure, surface radius, and CH 4 and CO mixing ratios, from which model light curves are to be calculated. The model improves upon previous techniques for deriving Pluto's atmospheric thermal structure from stellar occultation light curves by calculating temperature (as a function of height) caused by heating and cooling by species in Pluto's atmosphere, instead of a general assumption that temperature follows a power law with height or some other idealized function. We are able to fit for model surface radius, surface pressure, and CH 4 mixing ratio with one of the 2006 datasets and for surface pressure and CH 4 mixing ratio for other datasets from the years 1988, 2002, 2006, and 2008. It was not possible to fit for CO mixing ratio and surface temperature because the light curves are not sensitive to these parameters. We determine that the model surface radius, under the assumption of a stratosphere only (i.e. no troposphere) model in radiative-conductive balance, is 1180-10+20km. The CH 4 mixing ratio results are more scattered with time and are in the range of 1.8-9.4 × 10 -3. The surface pressure results show an increasing trend from 1988 to 2002, although it is not as dramatic as the factor of 2 from previous studies.
Pluto’s tenuous atmosphere was probed in 1988 with a stellar occultation observed from the Kuiper Airborne Observatory (KAO, Elliot et al. 1989) and a variety of ground-based sites (Millis et al. 1993). These data, subsequent theoretical modeling (e.g. Strobel et al. 1996), and spectroscopic observations (Owen et al. 1993; Young et al. 1997), gave us the following post-occultation picture of Pluto’s atmosphere (Yelle & Elliot 1997; Elliot, Person,
&
Qu 2003b): N
2
is the dominant constituent of the atmosphere, which also contains small amounts of CH
4
and CO. These molecules are in vapor-pressure equilibrium with their surface ices, and this process acts as a thermostat to keep the N
2
ice at ~38 K around the body. The temperature of the 1-3 μbar pressure region probed by the occultation was ~100 K. The KAO light curve dropped abruptly, however, just below half-light. This abrupt drop could be due to one of two potential properties of Pluto’s atmosphere: extinction, or a steep thermal gradient. Each of these explanations has strengths and weaknesses (Yelle & Elliot 1997; Elliot, et al. 2003b).
The refractivity of nitrogen gas has been measured, with high relative precision, at vacuum wavelengths from 4679 Å to 20 586 Å. The absolute value of refractivity for 5462 Å, reduced to one atmosphere at 0°C., is 2.991×10-4. A dispersion formula fitting the data well at 15°C. is 108 (n-1) = 6497.378+3073 864.9/ (144-σ2), where σ is wave number in reciprocal microns. This is compared to Svensson’s dispersion formula for air. Alternative dispersion formulas are discussed. Quantities related to dispersion are deduced: mean absorption frequency, effective total oscillator strength., dielectric constant, Verdet constant, and Rayleigh scattering coefficient.
We have analyzed all photometric observations of the 9 June 1988 occultation of the star P8 by Pluto in order to derive the radius of Pluto and certain parameters of its atmosphere. Our analysis is based on a "haze" model; but where relevant, we also discuss the thermal gradient model. With either model, the occultation observations yield a diameter for the limb of Pluto that is significantly larger than that found from mutual event observations by D. J. Tholen and M. W. Buie (1989, Bull. Am. Astron. Soc. 21, 981-982), but is in good agreement with the value from E. F. Young (1992, Ph.D. thesis, Massachusetts Institute of Technology). For a clear atmosphere (i.e., the thermal gradient model), we find the radius of the solid surface of Pluto and, obviously, also of the limb, to be 1195 ± 5 km. If the haze model is correct, the solid surface of the planet falls below 1180 ± 5 km, but the radius of the visible limb would be in the haze layer between 1185 and 1200 km. The degree to which the structure of Pluto's atmosphere is globally homogeneous is also discussed.
A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH43.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH47.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4molecule. Partial thermalization of absorbed solar radiation in the CH43.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km−1forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2individually as the major constituent. If a steep temperature gradient ∼20 K km−1is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4mixing ratio to attain a calculated ratio ofT/(= 3.5 K amu−1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2as the dominant constituent. One possibility is a 3 μbar N2atmosphere with 0.3% CH4that has 106 K isothermal region (T/= 3.8 K amu−1) and ∼8 K km−1surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH4density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.).
We apply scintillation theory to stellar signal fluctuations in the
high-resolution, high signal/noise, dual-wavelength data from the MMT
observation of the 2007 March 18 occultation of P445.3 by Pluto. A well-defined
high wavenumber cutoff in the fluctuations is consistent with viscous-thermal
dissipation of buoyancy waves (internal gravity waves) in Pluto's high
atmosphere, and provides strong evidence that the underlying density
fluctuations are governed by the gravity-wave dispersion relation.
We present model occultation lightcurves demonstrating that a strong thermal inversion layer at the base of Pluto's stratosphere can reproduce the minimum flux measured by the Kuiper Airborne Observatory (KAO) during the 1988 occultation of a star by Pluto. The inversion layer also forms the occultation equivalent of a mirage at a radius of 1198 km, which is capable of hiding tropospheres of significant depth. Pluto's surface lies below 1198 km, its radius depending on the depth of the troposphere. We begin by computing plausible temperature structures for Pluto's lower atmosphere, constrained by a calculation of the temperature of the atmosphere near the surface. We then trace rays from the occulted star through the model atmosphere, computing the resultant bending of the ray. Model light curves are obtained by summing the contribution of individual rays within the shadow of Pluto on Earth. We find that we can reproduce the KAO lightcurve using model atmospheres with a temperature inversion and no haze. We have explored models with tropospheres as deep as 40 km (implying a Pluto radius of 1158 km) that reproduce the suite of occultation data. Deeper tropospheres can be fitted to the data, but the mutual event radius of 1150 km probably provides a lower bound. If Pluto has a shallow or nonexistent troposphere, its density is consistent with formation in the solar nebula with modest water loss due to impact ejection. If the troposphere is relatively deep, implying a smaller radius and larger density, signficant amounts of water loss are required.
The seasonal effects on Pluto's atmosphere of a simplified system of CH4 and N2 saturated over a solid solution are investigated, and the results are compared with previous CH4 models. It is found that bulk escape occurs for CH4 mole fractions less than 0.7 of Pluto's volatile reservoir. Greater CH4 abundance leads to diffusive separation during the escape of both species and an atmospheric mixing ratio of about Xc(0). If Xc(0) is in the range 0.02-0.10, Pluto's atmosphere remains largely intact at aphelion rather than virtually freezing out as it does for Xc(0) greater than 0.3 or less than 0.001, or form an atmosphere with only a single volatile gas. An upper limit for the CH4 mixing ratio is about 0.07 if N2 is the second gas. On the other hand, CH4 is the dominant surface constituent of the volatile deposit if Xc(0) is greater than 0.0001.
The horizontal flow of SO2 gas from the day side to the night side of IO is calculated on the basis of a hydrodynamic model. The flow speed is found to be supersonic for all realistic values of the parameters. The surface pressure follows the frost vapor pressure within a factor of 2 in spite of day-night pressure ratios of 10,000 or more. Atmospheric temperature is generally below the surface temperature due to decompression in the expanding flow. The greatest sensitivity of the solution is connected with the frost temperature at the subsolar point. The quantities that involve the mass of the atmosphere (density, pressure, mass transport, and condensation rate) all vary as the vapor pressure of the frost, which is a sensitive function of frost temperature.
Consideration is given to an analytic model for a stellar-occultation light curve developed for a small, spherically symmetric planetary atmosphere that includes thermal and molecular weight gradients in a region that overlies an extinction layer. The model incorporates two equivalent sets of parameters. One set specifies the occultation light curve in terms of signal levels, times, and time intervals. The other set specifies physical parameters of the planetary atmosphere. Equations are given for the transforming between the sets of parameters, including their errors and correlation coefficients. Detailed numerical calculations are presented for a benchmark case. The results obtained are consistent with the isothermal prediction of the 'methane-thermostat' model of Pluto's atmosphere.
The present thermal profile calculation for a Pluto atmosphere model characterized by a high number fraction of CH4 molecules encompasses atmospheric heating by solar UV flux absorption and conductive transport cooling to the surface of Pluto. The stellar occultation curve predicted for an atmosphere of several-microbar surface pressures (which entail the existence of a substantial temperature gradient close to the surface) agrees with observations and implies that the normal and tangential optical depth of the atmosphere is almost negligible. The minimum period for atmospheric methane depletion is calculated to be 30 years.
Previous work by K.A. Tryka et al. (Science 261, 751-754, 1993) has shown that the profile of the 2.148-micrometers band of solid nitrogen can be used as a "thermometer" and determined the temperature of nitrogen ice on Triton to be 38(+2)-1 K. Here we reevaluate that data and refine the temperature value to 38 +/- 1 K. Applying the same technique to Pluto we determine that the temperature of the N2 ice on that body is 40 +/- 2 K. Using this result we have created a nonisothermal flux model of the Pluto-Charon system. The model treats Pluto as a body with symmetric N2 polar caps and an equatorial region devoid of N2. Comparison with the infrared and millimeter flux measurements shows that the published fluxes are consistent with models incorporating extensive N2 polar caps (down to +/- 15 degrees or +/- 20 degrees latitude) and an equatorial region with a bolometric albedo < or = 0.2.
We present a method for speeding up numerical calculations of a light curve for a stellar occultation by a planetary atmosphere with an arbitrary atmospheric model that has spherical symmetry. This improved speed makes least-squares fitting for model parameters practical. Our method takes as input several sets of values for the first two radial derivatives of the refractivity at different values of model parameters, and interpolates to obtain the light curve at intermediate values of one or more model parameters. It was developed for small occulting bodies such as Pluto and Triton, but is applicable to planets of all sizes. We also present the results of a series of tests showing that our method calculates light curves that are correct to an accuracy of 10(exp -4) of the unocculted stellar flux. The test benchmarks are (i) an atmosphere with a l/r dependence of temperature, which yields an analytic solution for the light curve, (ii) an atmosphere that produces an exponential refraction angle, and (iii) a small-planet isothermal model. With our method, least-squares fits to noiseless data also converge to values of parameters with fractional errors of no more than 10(exp -4), with the largest errors occurring in small planets. These errors are well below the precision of the best stellar occultation data available. Fits to noisy data had formal errors consistent with the level of synthetic noise added to the light curve. We conclude: (i) one should interpolate refractivity derivatives and then form light curves from the interpolated values, rather than interpolating the light curves themselves; (ii) for the most accuracy, one must specify the atmospheric model for radii many scale heights above half light; and (iii) for atmospheres with smoothly varying refractivity with altitude, light curves can be sampled as coarsely as two points per scale height.
We present single-scattering albedo maps of the surfaces of Pluto and Charon based primarily on mutual event observations. The dataset contains 3374 photometric observations that cover 15 different satellite transit events, 14 satellite eclipse events, and other out-of-eclipse photometry spanning 1954 to 1986. The maps consist of a 59 × 29 grid of tiles for each body. We applied the technique of maximum entropy image reconstruction to invert the lightcurves, thus revealing surface maps of single-scattering albedo. The surface of Pluto is seen to have albedo features similar to our previous spot model maps (Buie and Tholen 1989). In particular, a south polar cap is evident in the map of Pluto. The north polar region is brighter than the equatorial regions but is not as bright as the south pole. Single-scattering albedos range from 0.98 in the south polar cap to a low near 0.2 at longitudes corresponding to the lightcurve minimum. The map of Charon is somewhat darker with single-scattering albedos as low as 0.03.
A thermal model, developed to predict seasonal nitrogen cycles on Triton, has been modified and applied to Pluto. The model used to calculate the partitioning of nitrogen between surface frost deposits and the atmosphere, as a function of time for various sets of input parameters. Pluto's high obliquity is found to have a significant on the distribution of frost on its surface. Conditions that would lead to permanent polar caps on Triton are found to lead to permanent frost bands on Pluto. In some instances, frost sublimes from a seasonal cap outwards, resulting in a bald Dark frost does not satisfy observable on Pluto , in contrast to our findings for Triton. Bright frost comes closer to matching observable, but is not completely satisfactory. Atmospheric pressure variations exist seasonally, but the amplitudes, and to a lesser extent the phase, of the variations depends significantly on frost and substrate properties. In most cases two peaks in atmospheric pressure are obse...
Extinction layer detected by the 2003 star occultation on Pluto Albedo maps of Pluto and improved physical parameters of the Pluto–Charon system
- P Rannou
- G Durry
Rannou, P., Durry, G., 2009. Extinction layer detected by the 2003 star occultation
on Pluto. J. Geophys. Res. 114 (E13), 11013.
Reinsch, K., Burwitz, V., Festou, M.C., 1994. Albedo maps of Pluto and
improved physical parameters of the Pluto–Charon system. Icarus 108,
209–218.
Parameterization and convection in global climate models Observation, Theory, and Modeling of Atmospheric Variability
- G.-J Zhang
Zhang, G.-J., 2004. Parameterization and convection in global climate models. In:
Zhu, X., Li, X., Cai, M., Zhou, S., Zhu, Y., Jin, F.-F., Zou, X., Zhang, M. (Eds.),
Observation, Theory, and Modeling of Atmospheric Variability. World Scientific
Pub. Co., Singapore, pp. 184–295.