Icarus

Published by Elsevier
Online ISSN: 1090-2643
Print ISSN: 0019-1035
Publications
Laboratory measurements of Steffes (1986) have suggested that the intensity and shape of the microwave spectrum of Venus might be especially sensitive to the subcloud abundance of constituents such as SO2 and gaseous H2SO4. It was likewise suggested that some variations of the shape of the emission spectrum might occur between 1.5 and 3 cm (10 to 20 GHz), a wavelength range which had previously only been sparsely observed. As a result, coordinated observations of Venus emission were conducted at four wavelengths between 1.35 cm (22.2 GHz) and 3.6 cm (8.42 GHz) using the 43-m NRAO antenna at Green Bank, West Virginia, and the 64-m antenna at NASA's Deep Space Communication Complex, Goldstone, California. In this paper, we report the methodology and results of these observations, and compare the results with other observations and with calculated emission spectra. We conclude that the observed emission spectrum is consistent with an average subcloud abundance of gaseous H2SO4 in equatorial and midlatitude regions which is approximately 5 ppm. It is suggested that additional measurements of atmospheric microwave opacity be made with the Pioneer-Venus Orbiter Radio Occultation experiment to search for temporal and spatial variations in gaseous H2SO4 abundance in the Venus atmosphere. An upper limit for the subcloud abundance of SO2 is also determined.
 
On 15 August 1994 we launched the EUVS sounding rocket payload to observe the 825-1110 angstrom region of Venus's far ultraviolet airglow spectrum. The EUVS telescope/spectrograph obtained good data at five times higher spectral resolution than was previously available in the far ultraviolet. We present these data and compare our results to those obtained by the Galileo UVS and Venera 11/12 UV spectrophotometers. We identify several new spectral emission features, including both singly ionized nitrogen and molecular nitrogen in Venus's spectrum. We also see evidence for electron-impact-induced emission from CO. Finally, the EUVS data indicate that the "Ar" emissions detected in Venus's far ultraviolet spectrum by Venera 11/12 spectrophotometers are in fact not due to argon, thus eliminating the discrepancy between in situ and remote sensing measurements.
 
We discuss over 120 laboratory experiments pertaining to the identification of the new absorption band discovered by Trafton et al. (1991) at 4705.2 cm-1 (2.1253 micrometers) in the spectrum of Io. It is shown that this band is not due to overtones or combinations of the fundamental bands associated with the molecules (or their chemical complexes) already identified on Io, namely, SO2, H2S, and H2O. Thus, this band is due to a new, previously unidentified, component of Io. Experiments also demonstrate that the band is not due to molecular H2 frozen in SO2 frosts. Since the frequency of this band is very close to the first overtone of the nu 3 asymmetric stretching mode of CO2, we have investigated the spectral behavior of CO2 under a variety of conditions appropriate for Io. The profile of the Io band is not consistent with the rotational envelope expected for single, freely rotating, gaseous CO2 under Io-like conditions. It was found that pure, solid CO2 and CO2 intimately mixed in a matrix of solid SO2 and H2S produce bands with similar widths (5-10 cm-1), but that these bands consistently fall at frequencies about 10-20 cm-1 (approximately 0.007 micrometer) lower than the Io band. CO2 in SO2 : H2S ices also produces several additional bands that are not in the Io spectra. The spectral fit improves, however, as the CO2 concentration in SO2 increases, suggesting that CO2-CO2 interactions might be involved. A series of Ar : CO2 and Kr : CO2 matrix isolation experiments, as well as laboratory work done elsewhere, show that CO2 clustering shifts the band position to higher frequencies and provides a better fit to the Io band. Various laboratory experiments have shown that gaseous CO2 molecules have a propensity to cluster between 80 and 100 K, temperatures similar to those found on the colder regions of Io. We thus tentatively identify the newly discovered Io band at 4705.2 cm-1 (2.1253 micrometers) with CO2 multimers or "clusters" on Io. Whether these clusters are buried within an SO2 frost, reside on the surface, or are in a residual, steady-state "atmospheric aerosol" population over local coldtraps is not entirely clear, although we presently favor the latter possibility. The size of these clusters is not well defined, but evidence suggests groups of more than four molecules are required. The absorption strength of the 2 nu 3 CO2 cluster overtone determined in the laboratory, in conjunction with the observed strength of the Io band, suggests that the disk-integrated abundance of CO2 is less than 1% that of the SO2. Studies of the sublimation behavior of CO2 indicate that it probably resides predominantly in the cooler areas (< 100 K) of Io. The relative constancy of the Io feature over a variety of orbital phases suggests that the polar regions may contain much of the material. Some consequences of the physical properties of CO2 under conditions pertinent to Io are discussed. The presence of CO2 clusters on Io could be verified by the detection of any one of several other infrared bands associated with the CO2 molecule, of which the strongest are the nu 3 12CO2 asymmetric stretch fundamental near 2350 cm-1 (4.25 micrometers) and the nu 2 bending mode fundamental near 660 cm-1 (15.1 micrometers). Weaker bands that may also be detectable include the nu 3 13CO2 asymmetric stretch fundamental near 2280 cm-1 (4.39 micrometers), the 2 nu 2 + nu 3 combination/overtone band near 3600 cm-1 (2.78 micrometers), and the nu 1 + nu 3 combination band near 3705 cm-1 (2.70 micrometers).
 
The infrared instrument IKS flown on board the VEGA space probes was designed for the detection of emission bands of parent molecules, and for a measurement of the size and temperature of the thermal emitting nuclear region. The instrument had three channels with cooled detectors: an "imaging channel" designed to modulate the signal of the nucleus and two spectroscopic channels operating at 2.5-5 and 6-12 micrometers, respectively, equipped with circular variable filters of resolving power approximately 50. This paper presents and discusses the results from the spectral channels. On VEGA 1, usable spectra were obtained at distances D from the comet nucleus ranging from 250,000 to 40,000 km corresponding to fields of view 4000 and 700 km in diameter, respectively. The important internal background signal caused by the instrument itself, which could not be cooled, had to be eliminated. Since no sky chopping was performed, we obtain difference spectra between the current spectrum and a reference spectrum with little or no cometary signal taken at the beginning of the observing sequence (D approximately 200,000 km). Final discrimination between cometary signal and instrumental background is achieved using their different time evolution, since the instrumental background is proportional to the slow temperature drift of the instrument, and the cometary signal due to parent molecules or dust grains is expected to vary in first order as D-1. The 2.5-5 micrometers IKS spectra definitely show strong narrow signals at 2.7 and 4.25 micrometers, attributed to the nu 3 vibrational bands of H2O and CO2, respectively, and a broader signal in the region 3.2-3.5 micrometers, which may be attributed to CH-bearing molecules. All these signals present the expected D-1 intensity variation. Weaker emission features at 3.6 and 4.7 micrometers could correspond to the nu 1 and nu 5 bands of H2CO and the (1 - 0) band of CO, respectively. Molecular production rates are derived from the observed emissions, assuming that they are due to resonance fluorescence excited by the Sun's infrared radiation. For the strong bands of H2O and CO2, the rovibrational lines are optically thick, and radiative transfer is taken into account. We derive production rates, at the moment of the VEGA 1 flyby, of approximately 10(30) sec-1 for H2O, approximately 2.7 x 10(28) sec-1 for CO2, approximately 5 x 10(28) sec-1 for CO, and 4 x 10(28) sec-1 for H2CO, if attributions to CO and H2CO are correct. The production rate of carbon atoms in CH-bearing molecules is approximately 9 x 10(29) sec-1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2-3.5 micrometers emission is attributed to C-H stretch in polycyclic aromatic hydrocarbons or small organic grains. In addition, marginal features are present at 4.85 and 4.45 micrometers, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8-3.0 micrometers, as well as a narrow emission at 3.15 micrometers, which follow well the D-1 intensity variation, might be due to water ice. Emission at 2.8 micrometers is also possibly present, and might be due to OH created in vibrationally excited states after water photodissociation. The 6-12 micrometers spectrum does not show any molecular emission, nor emission in the 7.5-micrometers region. The spectrum is dominated by silicate emission showing a double structure with maxima at 9.0 and 11.2 micrometers, which suggests the presence of olivine.
 
Infrared spectra of Io in the region 2.5-5.0 micrometers, including new observational data, are analyzed using detailed laboratory studies of plausible surface ices. Besides the absorption bands attributable to sulfur dioxide frosts, four infrared spectral features of Io are shown to be unidentified. These unidentified features show spatial and temporal band strength variations. One pair is centered around 3.9 micrometers (3.85 and 3.91 micrometers) and the second pair is centered around 3.0 micrometers (2.97 and 3.15 micrometers). These absorptions fall close to the fundamental stretching modes in H2S and H2O, respectively. The infrared absorption spectra of an extensive set of laboratory ices ranging from pure materials, to binary mixtures of H2S and H2O (either mixed at different concentrations or layered), to H2O:H2S:SO2 mixtures are discussed. The effects of ultraviolet irradiation (120 and 160 nm) and temperature variation (from 9 to 130 K) on the infrared spectra of the ices are examined. This comparative study of Io reflectance spectra with the laboratory mixed ice transmission data shows the following: (1) Io's surface most likely contains H2S and H2O mixed with SO2. The 3.85- and 3.91-micrometers bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration (nu 1) in H2S clusters and isolated molecules in an SO2-dominated ice. The weak 2.97- and 3.15-micrometers bands which vary spatially and temporally in the Io spectra coincide with the nu 3 and nu 1 O-H stretching vibrations of clusters of H2O molecules complexed, through hydrogen bonding and charge transfer interactions, with SO2. (2) The observations are well matched qualitatively by the transmission spectra of SO2 ices containing about 3% H2S and 0.1% H2O which have been formed by the condensation of a mixture of the gases onto a 100 K surface. (3) No new features are produced in the region 2.5 to 5.0 micrometers in the spectrum of these ices under prolonged ultraviolet irradiation or temperature variation up to 120 K. (4) Comparison of the Io spectra to transmission spectra of both mixed molecular ices and layered ices indicates that only the former can explain the shifts and splitting of the absorption bands seen in the Io spectrum and additionally can account for the fact that solid H2S is observed in the surface material of Io at temperature and pressure conditions above the sublimation point of pure H2S.
 
A strong absorption band at 3590 +/- 20 cm-1 (2.790 +/- 0.015 micrometers) has been discovered in the spectrum of Io using the Kuiper Airborne Observatory (KAO). The 2 nu1 + nu3 combination mode of solid SO2 falls at this position. Since SO2 is abundant on Io it must contribute to the new band. However, a band due to H2O was predicted near this frequency in Io's spectrum based on laboratory experiments of H2O:SO2 mixed Io ice analogs which were used to assign the two weak, variable features of 3370 and 3170 cm-1 (2.97 and 3.15 micrometers) to trace amounts of H2O frozen in solid SO2 on Io. The new band probably originates from both SO2 and H2O. Unfortunately, the spectral resolution of the data is insufficient to settle the issue of whether there are two resolvable components.
 
We report on new observations of Titan with the International Ultraviolet Explorer in the mid-UV range (lambda approximately 220-335 nm). We use these data to determine upper limits for the abundances of simple organic compounds in the gas phase and to further constrain the properties of the high altitude haze on Titan. As a baseline, we adopted the parametrized microphysical model of McKay et al. (1989) which is successful at reproducing Titan's thermal structure while satisfying several other observational constraints in the visible and IR regions. However, we find that such a model--in which all particles at a given altitude are assumed to have the same size--cannot match simultaneously the IUE observations and the visible/IR data, even when allowance is made for a wide range of values in the adjustable parameters. On the other hand, a good overall agreement is obtained when considering a biomodal size distribution, with small haze particles or "polymers" (r < 0.02 micrometer) acting as strong Rayleigh absorbers below 300 nm and larger haze particles (r approximately 0.1-0.5 micrometer) being responsible for the characteristics of the albedo spectrum in the near-UV, visible, and IR regions. This approach is consistent with the results of several previous investigations of the properties of Titan's haze, although our preferred vertical structure for the haze + polymer material somewhat differs from earlier solutions. On the basis of simple dynamical considerations, we adopted a uniformly mixed layer between 150 and 600 km. The IUE data allow us to place fairly stringent constraints on the abundance of the Rayleigh absorbers, if we assume that their optical properties are similar to those of laboratory-synthesized "tholins": The column-mass density of this material--the essential observable that can be determined from our study--is of the order of 5 micrograms cm-2. This would correspond to number-densities between 10(3) and 10(7) cm-3 in the 150-600 km altitude range, if the average particle radius is between 0.001 and 0.02 micrometer. Such high number densities are a priori at odds with the estimated coagulation lifetime for particles of that size. Thus, our proposed bimodal size distribution is plausible only if inhibiting processes act to slow down considerably the coagulation of polymers in Titan's stratosphere.
 
3.5 billion years (byr) ago, when it is thought that Mars and Earth had similar climates, biological evolution on Earth had made considerable progress, such that life was abundant. It is therefore surmised that prior to this time period the advent of chemical evolution and subsequent origin of life occurred on Earth and may have occurred on Mars. Analysis for organic compounds in the soil buried beneath the Martian surface may yield useful information regarding the occurrence of chemical evolution and possibly biological evolution. Calculations based on the stability of amino acids lead to the conclusion that remnants of these compounds, if they existed on Mars 3.5 byr ago, might have been preserved buried beneath the surface oxidizing layer. For example, if phenylalanine, an amino acid of average stability, existed on Mars 3.5 byr ago, then 1.6% would remain buried today, or 25 pg-2.5 ng of C g-1 Martian soil may exist from remnants of meteoritic and cometary bombardment, assuming that 1% of the organics survived impact.
 
No known body in the Solar System has a spectrum redder than that of object 5145 Pholus. We use Hapke scattering theory and optical constants measured in this laboratory to examine the ability of mixtures of a number of organic solids and ices to reproduce the observed spectrum and phase variation. The primary materials considered are poly-HCN, kerogen, Murchison organic extract, Titan tholin, ice tholin, and water ice. In a computer grid search of over 10 million models, we find an intraparticle mixture of 15% Titan tholin, 10% poly-HCN, and 75% water ice with 10-micrometers particles to provide an excellent fit. Replacing water ice with ammonia ice improves the fits significantly while using a pure hydrocarbon tholin, Tholin alpha, instead of Titan tholin makes only modest improvements. All acceptable fits require Titan tholin or some comparable material to provide the steep slope in the visible, and poly-HCN or some comparable material to provide strong absorption in the near-infrared. A pure Titan tholin surface with 16-micrometers particles, as well as all acceptable Pholus models, fit the present spectrophotometric data for the transplutonian object 1992 QB1. The feasibility of gas-phase chemistry to generate material like Titan tholin on such small objects is examined. An irradiated transient atmosphere arising from sublimating ices may generate at most a few centimeters of tholin over the lifetime of the Solar System, but this is insignificant compared to the expected lag deposit of primordial contaminants left behind by the sublimating ice. Irradiation of subsurface N2/CH4 or NH3/CH4 ice by cosmic rays may generate approximately 20 cm of tholin in the upper 10 m of regolith in the same time scale but the identity of this tholin to its gas-phase equivalent has not been demonstrated.
 
We report our discovery of an absorption feature in the infrared spectrum of Io centered at 2.1253 μm (4705.2 cm-2). This contrasts with the 30- to 50-cm-1 widths of the board absorption features previously detected at longer wavelengths which arise from mixtures of SO2 with H2S and H2O. This newly discovered feature is relatively weak, having a core only 5% below the continuum at this resolving power. Our survey from 1.98 to 2.46 μm (5050-4065 cm-1) at this same resolving power revealed no other feature greater than 1% of the continuum level shortward of 2.35 μm, and 3% elsewhere. The feature does not correspond to any gas-or solid-phase absorption that might be expected from previously identified constituents of Io's surface. No temporal of longitudinal variational has been detected in the course of 18 nights of observation over the past year and no significant variation in the strength of the feature was seen during an emergence from eclipse. These observations indicate that the source material of the feature is reasonably stable, and is more uniformly distibuted in longitude than Io's hot spots. These characteristics all indicate that the feature belongs to a class different from those characterizing other known absorption feature in Io's spectrum. Consequently, it should reveal important new information about Io's atmosphere-surface composition and interation. A series of laboratory experiments of plausible surface ices indicates that (i) the band does not arise from overtones or combinations of any of the molecular variations associated with species already identified on Io (SO2, H2S, H2O) or from chemical complexes of these molecules, (ii) the band does not arise from H2 trapped in SO2, and (iii) the band may arise from the 2v3 mode of CO2. If the band arises from CO2, it is clear from its detailed shape and position that the molecdules are not embedded in an SO2 matrix, as are H2S and H2O, but may be present as multimers or "clusters.".
 
The observed ratio of C2H6 to C2H2 in the Jovian stratosphere increases from approximately 55 at 2 mbar to approximately 277 at 12 mbar. In current photochemical models this ratio typically increases between 2 and 12 mbar by a factor of < or = 3. Recent laboratory kinetics studies on the reaction between C2H3 and H2 to form C2H4 suggest an efficient chemical mechanism for hydrogenation of C2H2 to C2H6. Inclusion of this scheme as part of a comprehensive updated model for hydrocarbon photochemistry in the atmosphere of Jupiter provides an explanation of the altitude variation of the C2H6/C2H2 ratio. The sensitivity of these results to uncertainties in the key rate constants at low temperatures is illustrated, identifying needs for additional laboratory measurements. Since the key reaction rate constants decrease with decreasing temperature, the hydrogenation of C2H2 as proposed predicts a qualitatively decreasing trend in the C2H6/C2H2 value with decreasing distance from the Sun. The observed variation between Jupiter and Saturn is consistent with this prediction.
 
Analysis of high-spatial-resolution (approximately 0.8 arcsec) methane band and continuum imagery of Neptune's relatively homogeneous Equatorial Region yields significant constraints on (1) the stratospheric gaseous methane mixing ratio (fCH4,s), (2) the column abundances and optical properties of stratospheric and tropospheric hydrocarbon hazes, and (3) the wavelength-dependent single-scattering albedo of the 3-bar opaque cloud. From the center-to-limb behavior of the 7270-angstroms and 8900-angstrom sCH4 bands, the stratospheric methane mixing ratio is limited to fCH4,s < 1.7 x 10(-3), with a nominal value of fCH4,s = 3.5 x 10(-4), one to two orders of magnitude less than pre-Voyager estimates, but in agreement with a number of recent ultraviolet and thermal infrared measurements, and largely in agreement with the tropopause mixing ratio implied by Voyager temperature measurements. Upper limits to the stratospheric haze mass column abundance and 6190-angstroms and 8900-angstroms haze opacities are 0.61 microgram cm-2 and 0.075 and 0.042, respectively, with nominal values of 0.20 microgram cm-2 and 0.025 and 0.014 for the 0.2-micrometer radius particles preferred by the recent Voyager PPS analysis of Pryor et al. (1992, Icarus 99, 302-316). The tropospheric CH4 haze opacities are comparable to that found in the stratosphere, upper limits of 0.104 and 0.065 at 6190 angstroms and 8900 angstroms, respectively, with nominal values of 0.085 and 0.058. This indicates a column abundance less than 11.0 micrograms cm-2, corresponding to the methane gas content within a well-mixed 3% methane tropospheric layer only 0.1 cm thick near the 1.5-bar CH4 condensation level. Constraints on the single-scattering albedos of these hazes include (1) for the stratospheric component, 6190-angstroms and 8900-angstroms imaginary indices of refraction less than 0.047 and 0.099, respectively, with 0.000 (conservative scattering) being the nominal value at both wavelengths, and (2) CH4 haze single-scattering albedos greater than 0.85 and 0.50 at these two wavelengths, with conservative scattering again begin the preferred value. However, conservative scattering is ruled out for the opaque cloud near 3 bars marking the bottom of the visible atmosphere. Specifically, we find cloud single-scattering albedos of 0.915 +/- 0.006 at 6340 angstroms, 0.775 +/- 0.012 at 7490 angstroms, and 0.803 +/- 0.010 at 8260 angstrom. Global models utilizing a complete global spectrum confirm the red-absorbing character of the 3-bar cloud. The global-mean model has approximately 7.7 times greater stratospheric aerosol content then the Equatorial Region. An analysis of stratospheric haze precipitation rates indicates a steady-state haze production rate of 0.185-1.5 x 10(-14) g cm-2 sec-1, in agreement with recent theoretical photochemical estimates. Finally, reanalysis of the Voyager PPS 7500-angstroms phase angle data utilizing the fCH4,s value derived here confirms the Pryor et al. result of a tropospheric CH4 haze opacity of a few tenths in the 22-30 degrees S latitude region, several times that of the Equatorial Region or of the globe. The factor-of-10 reduction in fCH4,s below that assumed by Pryor et al. implies decreased gas absorption and consequently a decrease in the forward-scattering component of tropospheric aerosols.
 
The physical and chemical processes that lead to the preferential escape of hydrogen over deuterium in the Martian atmosphere are studied in detail using a one-dimensional photochemical model. Comparison of our theory with recent observations of HDO suggests that, averaged over the planet, Mars contains 0.2 m of crustal water that is exchangeable with the atmosphere. Our estimate is considerably lower than recent estimates of subsurface water on Mars based on geomorphological analysis of Viking images. The estimate can be reconciled if only a small fraction of crustal water can exchange with the atmosphere.
 
The Sun may well have formed in the type of interstellar cloud currently referred to as a cold, dark cloud. We present current tabulations of the totality of known interstellar molecules and of the subset which have been identified in cold clouds. Molecular abundances are given for two such clouds which show interesting chemical differences in spite of strong physical similarities, Taurus Molecular Cloud 1 (TMC-1) and Lynd's 134N (L134N, also referred to as L183). These regions may be at different evolutionary stages.
 
Analysis of the most primitive meteorites can yield detailed information about environmental conditions and physical/chemical processes in the earliest Solar System, including the nebular stage during which planetesimals were accreted. Such information pertains to time scales, thermal and chemical evolution, inhomogeneity and mixing, magnetic fields, and grain growth in the solar nebula. Nebular processes identified include evaporation, condensation, localized melting, and fractionation both of solids from gas and among different solids. Little direct evidence remains even in primitive meteorites of the actual accretion process. The absence of intrinsic factors capable of enhancing accretion among meteoritic constituents suggests that gravitational instabilities might have been important in promoting planetesimal accretion.
 
The structure and morphological properties of polymers produced photochemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution to the haze layers found on Titan. A structural analysis of these polymers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass through the atmosphere of Titan in the year 2004. Infrared analysis, elemental analysis, and thermal methods (thermogravimetric analysis, thermolysis, pyrolysis) were used to examine structures of polycyanoacetylenes produced by irradiation of the gas phase HC3N at 185 and 254 nm. The resulting brown to black polymer, which exists as small particles, is believed to be a branched chain of conjugated carbon-carbon double bonds, which, on exposure to heat, cyclizes to form a graphitic structure. Similar methods of analysis were used to show that when HC3N is photolyzed in the presence of Titan's other atmospheric constituents (CH4, C2H6, C2H2, and CO), a copolymer is formed in which the added gases are incorporated as substituents on the polymer chain. Of special significance is the copolymer of HC3N and acetylene (C2H2). Even in experiments where C2H2 was absorbing nearly all of the incident photons, the ratio of C2H2 to HC3N found in the resulting polymer was only 2:1. Scanning electron microscopy was used to visually examine the polymer particles. While pure polyacetylene particles are amorphous spheres roughly 1 micrometer in diameter, polycyanoacetylenes appear to be strands of rough, solid particles slightly smaller in size. The copolymer of HC3N and C2H2 exhibits characteristics of both pure polymers. This is particularly important as pure polyacetylenes do not match the optical constants measured for Titan's atmospheric hazes. The copolymers produced by the incorporation of other minor atmospheric constituents, like HC3N, into the polyacetylenes are expected to have optical constants more comparable to those of the Titan haze.
 
Photolysis of NH3 in the presence of propyne yields dimethylketazine (4) as the main product along with dimethylketimine, isopropylamine, and propioazine (7). Dimethylketazine and isopropylamine are the principal reaction products when the photolysis is performed at 198 K. The conversion to dimethylketazine is about 35 times greater at 198 K because it is not volatile and condenses on the wall of the photolysis cell out of the UV flux. Photolysis of dimethylketazine at 185 nm yields acetonitrile and small amounts of N-methyldimethylketimine (8). Photolysis of 8 gives acetonitrile. Photolysis of NH3 in the presence of 2-butyne gives the cis and trans isomers of 2-butene as the principal products along with the corresponding azine (9). Photolysis of azine 9 yields acetonitrile and propionitrile. Photolysis of hydrazine in the presence of propyne yields acetonitrile and isopropylamine but no azines were detected as reaction products. Quantum yields and percentage conversion to products are reported. These studies show that acetylenic hydrocarbons formed by the photolysis of methane in the stratosphere of Jupiter may react with radicals formed by NH3 photolysis to give nonvolatile yellow-brown polymers, dialkylazines, alkylnitriles, and eventually HCN. This scenario accounts for the observation of both HCN and chromophores on Jupiter.
 
An organic heteropolymer (Titan tholin) was produced by continuous dc discharge through a 0.9 N2/0.1 CH4 gas mixture at 0.2 mbar pressure, roughly simulating the cloudtop atmosphere of Titan. Treatment of this tholin with 6N HCl yielded 16 amino acids by gas chromatography after derivatization of N-trifluroacetyl isopropyl esters on two different capillary columns. Identifications were confirmed by GC/MS. Glycine, aspartic acid, and alpha- and beta-alanine were produced in greatest abundance; the total yield of amino acids was approximately 10(-2), approximately equal to the yield of urea. The presence of "nonbiological" amino acids, the absence of serine, and the fact that the amino acids are racemic within experimental error together indicate that these molecules are not due to microbial or other contamination, but are derived from the tholin. In addition to the HCN, HC2CN, and (CN)2 found by Voyager, nitriles and aminonitriles should be sought in the Titanian atmosphere and, eventually, amino acids on the surface. These results suggest that episodes of liquid water in the past or future of Titan might lead to major further steps in prebiological organic chemistry on that body.
 
In order to simulate prebiotic synthetic processes on Europa and other ice-covered planets and satellites, we have investigated the prebiotic synthesis of organic compounds from dilute solutions of NH4CN frozen for 25 years at -20 and -78 degrees C. In addition, the aqueous products of spark discharge reactions from a reducing atmosphere were frozen for 5 years at -20 degrees C. We find that both adenine and guanine, as well as a simple set of amino acids dominated by glycine, are produced in substantial yields under these conditions. These results indicate that some of the key components necessary for the origin of life may have been available on Europa throughout its history and suggest that the circumstellar zone where life might arise may be wider than previously thought.
 
This paper presents results from a high spatial resolution survey of 33 main-belt asteroids with diameters >40 km using the Keck II Adaptive Optics (AO) facility. Five of these (45 Eugenia, 87 Sylvia, 107 Camilla, 121 Hermione, 130 Elektra) were confirmed to have satellite. Assuming the same albedo as the primary, these moonlets are relatively small (∼5% of the primary size) suggesting that they are fragments captured after a disruptive collision of a parent body or captured ejecta due to an impact. For each asteroid, we have estimated the minimum size of a moonlet that can positively detected within the Hill sphere of the system by estimating and modeling a 2-σ detection profile: in average on the data set, a moonlet located at 2/100 × R(Hill) (1/4 × R(Hill)) with a diameter larger than 6 km (4 km) would have been unambiguously seen. The apparent size and shape of each asteroid was estimated after deconvolution using a new algorithm called AIDA. The mean diameter for the majority of asteroids is in good agreement with IRAS radiometric measurements, though for asteroids with a D < 200 km, it is underestimated on average by 6-8%. Most asteroids had a size ratio that was very close to those determined by lightcurve measurements. One observation of 104 Klymene suggests it has a bifurcated shape. The bi-lobed shape of 121 Hermione described in Marchis et al. [Marchis, F., Hestroffer, D., Descamps, P., Berthier, J., Laver, C., de Pater, I., 2005c. Icarus 178, 450-464] was confirmed after deconvolution. The ratio of contact binaries in our survey, which is limited to asteroids larger than 40 km, is surprisingly high (∼6%), suggesting that a non-single configuration is common in the main-belt. Several asteroids have been analyzed with lightcurve inversions. We compared lightcurve inversion models for plane-of-sky predictions with the observed images (9 Metis, 52 Europa, 87 Sylvia, 130 Elektra, 192 Nausikaa, and 423 Diotima, 511 Davida). The AO images allowed us to determine a unique photometric mirror pole solution, which is normally ambiguous for asteroids moving close to the plane of the ecliptic (e.g., 192 Nausikaa and 52 Europa). The photometric inversion models agree well with the AO images, thus confirming the validity of both the lightcurve inversion method and the AO image reduction technique.
 
Photodissociation of methane at high altitude levels in Neptune's atmosphere leads to the production of complex hydrocarbon species such as acetylene (C2H2), ethane (C2H6), methylacetylene (CH3C2H), propane (C3H8), diacetylene (C4H2), and butane (C4H8). These gases diffuse to the lower stratosphere where temperatures are low enough to initiate condensation. Particle formation may not occur readily, however, as the vapor species become supersaturated. We present a theoretical analysis of particle formation mechanisms at conditions relevant to Neptune's troposphere and stratosphere and show that hydrocarbon nucleation is very inefficient under Neptunian conditions: saturation ratios much greater than unity are required for aerosol formation by either homogeneous, heterogeneous, or ion-induced nucleation. Homogeneous nucleation will not be important for any of the hydrocarbon species considered; however, both heterogeneous and ion-induced nucleation should be possible on Neptune for most of the above species. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the particular species, the abundance of the condensable species, the temperature at which the vapor becomes supersaturated, and the number and type of condensation nuclei or ions available. Typical saturation ratios required for observable particle formation rates on Neptune range from approximately 3 for heterogeneous nucleation of methane in the upper troposphere to greater than 1000 for heterogeneous nucleation of methylacetylene, diacetylene, and butane in the lower stratosphere. Thus, methane clouds may form slightly above, and stratospheric hazes far below, their saturation levels. When used in conjunction with the results of detailed models of atmospheric photochemistry, our nucleation models place realistic constraints on the altitude levels at which we expect hydrocarbon hazes or clouds to form on Neptune.
 
Microphysical simulations of Titan's stratospheric haze show that aerosol microphysics is linked to organized dynamical processes. The detached haze layer may be a manifestation of 1 cm sec-1 vertical velocities at altitudes above 300 km. The hemispherical asymmetry in the visible albedo may be caused by 0.05 cm sec-1 vertical velocities at altitudes of 150 to 200 km, we predict contrast reversal beyond 0.6 micrometer. Tomasko and Smith's (1982, Icarus 51, 65-95) model, in which a layer of large particles above 220 km altitude is responsible for the high forward scattering observed by Rages and Pollack (1983, Icarus 55, 50-62), is a natural outcome of the detached haze layer being produced by rising motions if aerosol mass production occurs primarily below the detached haze layer. The aerosol's electrical charge is critical for the particle size and optical depth of the haze. The geometric albedo, particularly in the ultraviolet and near infrared, requires that the particle size be near 0.15 micrometer down to altitudes below 100 km, which is consistent with polarization observations (Tomasko and Smith 1982, West and Smith 1991, Icarus 90, 330-333). Above about 400 km and below about 150 km Yung et al.'s (1984, Astrophys. J. Suppl. Ser. 55, 465-506) diffusion coefficients are too small. Dynamical processes control the haze particles below about 150 km. The relatively large eddy diffusion coefficients in the lower stratosphere result in a vertically extensive region with nonuniform mixing ratios of condensable gases, so that most hydrocarbons may condense very near the tropopause rather than tens of kilometers above it. The optical depths of hydrocarbon clouds are probably less than one, requiring that abundant gases such as ethane condense on a subset of the haze particles to create relatively large, rapidly removed particles. The wavelength dependence of the optical radius is calculated for use in analyzing observations of the geometric albedo. The lower atmosphere and surface should be visible outside of regions of methane absorption in the near infrared. Limb scans at 2.0 micrometers wavelength should be possible down to about 75 km altitude.
 
We have measured the spectrum of Titan near 5 micrometers and have found it to be dominated by absorption from the carbon monoxide 1-0 vibration-rotation band. The position of the band edge allows us to constrain the abundance of CO in the atmosphere and/or the location of the reflecting layer in the atmosphere. In the most likely case, 5 micrometers radiation is reflected from the surface and the mole fraction of CO in the atmosphere is qCO=10(+10/-5) ppm, significantly lower than previous estimates for tropospheric CO. The albedo of the reflecting layer is approximately 0.07(+0.02/-0.01) in the 5 micrometers continuum outside the CO band. The 5 micrometers albedo is consistent with a surface of mixed ice and silicates similar to the icy Galilean satellites. Organic solids formed in simulated Titan conditions can also produce similar albedos at 5 micrometers.
 
Using kinetic data, we have estimated the racemization half-lives and times for total racemization of amino acids under conditions relevant to the surface of Mars. Amino acids from an extinct martian biota maintained in a dry, cold (<250 K) environment would not have racemized significantly over the lifetime of the planet. Racemization would have taken place in environments where liquid water was present even for time periods of only a few million years following biotic extinction. The best preservation of both amino acid homochirality and nucleic acid genetic information associated with extinct martian life would be in the polar regions.
 
Spectroscopic analyses have shown that smecthites enhanced in the laboratory with additionalferric species exhibitimportant similarities to those of the soils on MArs. Ferrihydrite in these chemically treated smectites has features in the visible to near- infrared region that resemble the energies and band strengths of features in reflectance spectra observed for several bright regions on Mars. New samples have been prepared with sulfate as well, becauseS was found by Viking to be a major component in the surface materialon MArs. A suite of ferrihydrite-bearing and ferric sulfate-bearing montmorillonites, prepared with variable Fe3+ and S concentrations and variable pH conditions, has been analyzed using reflectance spectroscopy in the visible and infrared regions, Moessbauer spectroscopy at room temperature and 4 K, differential thermal analysis, and X-ray diffraction. These analyses support the formationof ferrihydrite of variable crystallinity in the ferrihydrite-bearingmontmorillonites and a combination of schwertmannite and ferrihydritein the ferric sulfate-bearing montmorillonites. Small quantities of poorly chrystalline and nanophase forms of other ferric materials mayalso be present in these samples. The chemical formation conditions of the ferrihydrite-bearing and ferric sulfate-bearing montmorillonites influence the character of the low temperature moessbauer sextets and the visible reflectance spectra. An absorptionminimum is observed at 0.88-0.89 muem in spectra of the ferric sulfate-bearing samples, and at 0.89-0.92 muem in spectra of the ferrihydrite-bearing montmorillonites. Moessbauer spectra of the ferric sulfate-bearing montmorillonites indicate variable concentrations of ferrihydrite and schwertmannite in the interlaminarspaces and along grain surfaces.
 
Thermally promoted formaldehyde (H2CO) reactions in very low temperature ices have been studied to test their importance as a source of organic molecules in astrophysical environments such as comets and interstellar ices. The infrared absorption strengths of a number of the H2CO bands were measured in 10 K ices of pure H2CO and H20:H2CO = 100:3. Infrared spectroscopy was used to monitor the formaldehyde chemistry during warm-up of ices containing H2CO and one or more of the molecules H2O, CH3OH, CO, CO2, O2, and NH3. Formaldehyde reactions do not proceed at low temperatures in the absence of NH3. However, even small traces of NH3 (NH3/H2CO > or = 0.005) are sufficient to induce conversion of a considerable fraction (> 40%) of the H2CO into organic residues. Formaldehyde reactions were observed to start at temperatures as low as 40 K for NH3:H2CO binary ices and at approximately 80 K in astrophysically relevant (i.e., H2O-dominated) ices. A total of five different organic products of these reactions can be distinguished by infrared spectroscopy. One of them is polyoxymethylene (POM), a well-known H2CO polymerization product, whereas the others are reaction products of H2CO with H20, CH3OH and NH3. These all seem to be derivatives of polyoxymethylene. The nature of the components and their relative abundances depend strongly on the initial composition of the ice mixture as well as on the ice's irradiation history. We estimate that about 1% of the organics found in the coma of Comet Halley could have been produced by thermal formaldehyde reactions taking place in the nucleus.
 
The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2 ices, appropriate for applications to Triton and Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450 cm-1 (2.25 micrometers) and extends to lower frequencies, may be due to alkanes (C(n)H2n+2) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemistry of N2:CH4 and N(2):CH4:CO ices was explored demonstrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intrinsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have also been determined. These can be used to aid in their search and to place constraints on their abundances. For example, using these A values the abundance ratios CH4/N2 approximately 1.3 x 10(-3), C2H4/N2 < or = 9.5 x 10(-7) and H2CO/N2 < or = 7.8 x 10(-7) are deduced for Triton and CH4/N2 approximately 3.1 x 10(-3), C2H4/N2 < or = 4.1 x 10(-6), and H2CO/N2 < or = 5.2 x 10(-6) deduced for Pluto. The small amounts of C2H4 and H2CO in the surface ices of these bodies are in disagreement with the large abundances expected from many theoretical models.
 
Endoevaporites studied by Rothschild et al. (submitted for publication). (a) An area of the evaporite deposits located in evaporation pond 9, at the salt company Exportadora de Sal, S.A. The crusts in these deposits range from submerged in the brine to completely exposed at the shore. (b) In the interior of the crusts are l-2 stripes. These stripes contain a microbial community which includes the unicellular cyanobacterium Synechococcus sp. and a halophilic bacterium of the Halobacteriaceae. The cyanobacterium may be the same organism described by some authors as Aphunothec~ hulophyrica (e.g.. Brock 1976, Holser et al. 1981, Javor 1989).
The prospect of life on Mars today is daunting. Especially problematic for a potential life form is a lack of water, particularly in a liquid state; extremely cold temperatures; ultraviolet and ionizing radiation; and soil oxidants. Yet, "oases" where life might persist have been suggested to occur in rocks (in analogy with endolithic microorganisms described from deserts around the world), in polar ice caps (in analogy with snow and ice algae) and in possible volcanic regions (in analogy with chemoautotrophs living in deep sea hydrothermal vents); all are critically examined. Microorganisms are known to be able to survive in salt crystals, and recently it has been shown that organisms can metabolize while encrusted in evaporites. Because evaporites are thought to occur on Mars and can attenuate light in the UV range while being far more transparent to radiation useful for photosynthesis (400-700 nm), and because of the properties of these "endoevaporitic" organisms, I propose that such communities provide a new model system for studying potential life on Mars. On the basis of this model, I suggest possibilities for site selection for future exobiological experiments on Mars.
 
The organic solid (tholin) produced by spark discharge in a CH4 + NH3 + H2O atmosphere is investigated, along with the separable components of its water-soluble fraction. The chemistry of this material serves as a provisional model for the interaction of Jovian organic heteropolymers with the deep aqueous clouds of Jupiter. Intact (unhydrolyzed) tholin is resolved into four chemically distinct fractions by high-pressure liquid chromatography (HPLC). Gel filtration chromatography reveals abundant components at molecular weights approximately or equal to 600-700 and 200-300 Da. Gas chromatography/mass spectrometry of derivatized hydrolysis products of unfractionated tholin shows about 10% by mass protein and nonprotein amino acids, chiefly glycine, alanine, aspartic acid, beta-alanine, and beta-aminobutyric acid, and 12% by mass other organic acids and hydroxy acids. The stereospecificity of alanine is investigated and shown to be racemic. The four principal HPLC fractions yield distinctly different proportions of amino acids. Chemical tests show that small peptides or organic molecules containing multiple amino acid precursors are a possibility in the intact tholins, but substantial quantities of large peptides are not indicated. Candidate 700-Da molecules have a central unsaturated, hydrocarbon- and nitrile-rich core, linked by acid-labile (ester or amide) bonds to amino acid and carboxylic acid side groups. The core is probably not HCN "polymer." The concentration of amino acids from tholin hydrolysis in the lower aqueous clouds of Jupiter, about 0.1 micromole, is enough to maintain small populations of terrestrial microorganisms even if the amino acids must serve as the sole carbon source.
 
The isotopic enrichment of argon, krypton, and xenon, when trapped in water ice, was studied experimentally. The isotopes were found to be enriched according to their (m1/m2)1/2 ratio. These enrichment factors could be useful for comparison among the uncertain cosmic or solar isotopic ratios, the hopeful in situ cometary ratio, and those in Earth's atmosphere, in the context of cometary delivery of volatiles to Earth.
 
A mechanism for stabilizing climate on the earth and other earthlike planets is described, and the physical processes that define the inner and outer boundaries of the habitable zone (HZ) around the sun and main sequence stars are discussed. Physical constraints on the HZ obtained from Venus and Mars are taken into account. A 1D climate model is used to estimate the width of the HZ and the continuously habitable zone around the sun, and the analysis is extended to other main sequence stars. Whether other stars have planets and where such planets might be located with respect to the HZ is addressed. The implications of the findings for NASA's SETI project are considered.
 
We investigate the orbital evolution of 1013- to 1025-g planetesimals near 1 AU and in the asteroid belt (near 2.6 AU) prior to the stage of evolution when the mutual perturbations between the planetesimals become important. We include nebular gas drag and the effects of Jupiter and Saturn at their present masses and in their present orbits. Gas drag introduces a size-dependent phasing of the secular perturbations, which leads to a pronounced dip in encounter velocities (Venc) between bodies of similar mass. Planetesimals of identical mass have Venc ∼1 and ∼10 m s−1 (near 1 and 2.6 AU, respectively) while bodies differing by ∼10 in mass have Venc ∼10 and ∼100 m s−1 (near 1 and 2.6 AU, respectively). Under these conditions, growth, rather than erosion, will occur only by collisions of bodies of nearly the same mass. There will be essentially no gravitational focusing between bodies less than 1022 to 1025 g, allowing growth of planetary embryos in the terrestrial planet region to proceed in a slower nonrunaway fashion. The environment in the asteroid belt will be even more forbidding and it is uncertain whether even the severely depleted present asteroid belt could form under these conditions.
 
We present results from a number of 2D high-resolution hydrodynamical simulations of asteroids striking the atmosphere of Venus. These cover a wide range of impact parameters (velocity, size, and incidence angle), but the focus is on 2-3 km diameter asteroids, as these are responsible for most of the impact craters on Venus. Asteroids in this size range are disintegrated, ablated, and significantly decelerated by the atmosphere, yet they retain enough impetus to make large craters when they meet the surface. We find that smaller impactors (diameter <1-2 km) are better described by a "pancaking" model in which the impactor is compressed and distorted, while for larger impactors (>2-3 km) fragmentation by mechanical ablation is preferred. The pancaking model has been modified to take into account effects of hydrodynamical instabilities. The general observation that most larger impactors disintegrate by shedding fragments generated from hydrodynamic instabilities spurs us to develop a simple heuristic model of the mechanical ablation of fragments based on the growth rates of Rayleigh-Taylor instabilities. Although in principle the model has many free parameters, most of these have little effect provided that they are chosen reasonably. In practice the range of model behavior can be described with one free parameter. The resulting model reproduces the mass and momentum fluxes rather well, doing so with reasonable values of all physical parameters.
 
Near-infrared (J, H and K bands) spectra of nine dark asteroids (chosen among a sample of supposed primitive objects between C and D classes) have been obtained at the Mauna Kea Observatory (Hawaii) with the 2.2-m telescope using KSPEC as spectrograph. The aim of this work was to search for evidence of the presence of organic materials in these objects as found in other planetary bodies as 5145 Pholus, and in some cometary nuclei. A careful analysis of the data has revealed flat or slightly redder spectra than the solar one for all observed asteroids. No evidence of distinct absorption features was found.
 
Methane clathrate (CH4 nH2O)--expected in cometary nuclei, in outer Solar System satellites, and perhaps in interstellar grains--as well as ices prepared from other combinations of CH4, C2H6, or C2H2 with H2O (and sometimes with NH3) were irradiated at 77 degrees K by plasma discharge. CH4 clathrate and other H2O/hydrocarbon ices color and darken noticeably after a dose approximately 10(8) to approximately 10(9) erg cm-2 over a period of 1-10 hr. Upon evaporation of the now yellowish to tan irradiated ices, a colored solid film adheres to the walls of the reaction vessel at room temperature. Transmission measurements of these organic films were made from 2.5 to 50 micrometers wavelength. The residue left after CH4 nH2O irradiation exhibits IR bands which we tabulate and identify with alkane, aldehyde, alcohol, and perhaps alkene and substituted aromatic functional groups. Aldehydes are especially well indicated, and may be related to recent claims of polyoxymethylene (H2CO)n in the coma of Comet Halley. Spectra presented here are compared with previous studies of UV or proton-irradiated, nonenclathrated hydrocarbon-containing ices may be useful for interpreting infrared features found in the spectra of comets and interstellar grains.
 
During the 1981 Voyager encounter, Titan's stratosphere exhibited a large thermal asymmetry, with high northern latitudes being colder than comparable southern latitudes. Given the short radiative time constant, this asymmetry would not be expected at the season of the Voyager observations (spring equinox), if the infrared and solar opacity sources were distributed symmetrically. We have investigated the radiative budget of Titan's stratosphere, using two selections of Voyager IRIS spectra recorded at symmetric northern and southern latitudes. In the region 0.1-1 mbar, temperatures are 7 K colder at 50 degrees N than at 53 degrees S and the difference reaches approximately 13 K at 5 mbar. On the other hand, the northern region is strongly enriched in nitriles and hydrocarbons, and the haze optical depth derived from the continuum emission between 8 and 15 micrometers is twice as large as in the south. Cooling rate profiles have been computed at the two locations, using the gas and haze abundances derived from the IRIS measurements. We find that, despite lower temperatures, the cooling rate profiles in the pressure range 0.15-5 mbar are 20 to 40% larger in the north than in the south, because of the enhanced concentrations of infrared radiators. Because the northern hemisphere appears darker than the southern one in the Voyager images, enhanced solar heating is also expected to take place at 50 degrees N. Solar heating rate profiles have been calculated, with two different assumptions on the origin of the hemispheric asymmetry. In the most likely case where it results from a variation in the absorbance of the haze material, the heating rates are found to be 12-15% larger at the northern location than at the southern one, a smaller increase than that in the cooling rates. If the lower albedo in the north results from an increase in the particle number density, a 55 to 75% difference is found for the pressure range 0.15-5 mbar, thus larger than that calculated for the cooling rates. Considering the uncertainties in the haze model, dynamical heat transport may significantly contribute to the meridional temperature gradients observed in the stratosphere. On the other hand, the latitudinal variation in gas and haze composition may be sufficient to explain the entire temperature asymmetry observed, without invoking a lag in the thermal response of the atmosphere due to dynamical inertia.
 
Application of a radiative-convective equilibrium model to the thermal structure of Uranus' atmosphere evaluates the role of hazes in the planet's stratospheric energy budget and places a lower limit on the internal energy flux. The model is constrained by Voyager and post-Voyager observations of the vertical aerosol and radiative active gas profiles. Our baseline model generally reproduces the observed tropospheric and stratospheric temperature profile. However, as in past studies, the model stratosphere from about 10(-3) to 10(-1) bar is too cold. We find that the observed stratospheric hazes do not warm this region appreciably and that any postulated hazes capable of warming the stratosphere sufficiently are inconsistent with Voyager and ground-based constraints. We explore the roles played by the stratospheric methane abundance, the H2 pressure-induced opacity, photochemical hazes, and C2H2, and C2H6 in controlling the temperature structure in this region. Assuming a vertical methane abundance profile consistent with that found by the Voyager UVS occultation observations, the model upper stratosphere, from 10 to 100 microbar, is also too cold. Radiation in the 7.8-micrometers band from a small abundance of hot methane in the lower thermosphere absorbed in this region can warm the atmosphere and bring models into closer agreement with observations. Finally, we find that internal heat fluxes < or approximately 60 erg cm-2 sec-1 are inconsistent with the observed tropospheric temperature profile.
 
We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984, Astrophys. J. Suppl. 55, 465-506), the most significant recent change in the reactions used is the updated methane photodissociation scheme (Mordaunt et al. 1993, J. Chem. Phys. 98(3), 2054-2065). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990, Icarus, 85, 43-57) using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992, Icarus 100, 512-526). We have run the model in both steady-state and diurnal modes, with 62 speices involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.
 
The Viking Gas Chromatograph Mass Spectrometer failed to detect organic compounds on Mars, and both the Viking Labeled Release and the Viking Gas Exchange experiments indicated a reactive soil surface. These results have led to the widespread belief that there are oxidants in the martian soil. Since H2O2 is produced by photochemical processes in the atmosphere of Mars, and has been shown in the laboratory to reproduce closely the Viking LR results, it is a likely candidate for a martian soil oxidant. Here, we report on the results of a coupled soil/atmosphere transport model for H2O2 on Mars. Upon diffusing into the soil, its concentration is determined by the extent to which it is adsorbed and by the rate at which it is catalytically destroyed. An analytical model for calculating the distribution of H2O2 in the martian atmosphere and soil is developed. The concentration of H2O2 in the soil is shown to go to zero at a finite depth, a consequence of the nonlinear soil diffusion equation. The model is parameterized in terms of an unknown quantity, the lifetime of H2O2 against heterogeneous catalytic destruction in the soil. Calculated concentrations are compared with a H2O2 concentration of 30 nmoles/cm3, inferred from the Viking Labeled Release experiment. A significant result of this model is that for a wide range of H2O2 lifetimes (up to 10(5) years), the extinction depth was found to be less than 3 m. The maximum possible concentration in the top 4 cm is calculated to be approximately 240 nmoles/cm3, achieved with lifetimes of greater than 1000 years. Concentrations higher than 30 nmoles/cm3 require lifetimes of greater than 4.3 terrestrial years. For a wide range of H2O2 lifetimes, it was found that the atmospheric concentration is only weakly coupled with soil loss processes. Losses to the soil become significant only when lifetimes are less than a few hours. If there are depths below which H2O2 is not transported, it is plausible that organic compounds, protected from an oxidizing environment, may still exist. They would have been deposited by meteors, or be the organic remains of past life.
 
We have developed a coupled atmosphere and ocean model of Titan's surface. The atmospheric model is a 1-D spectrally-resolved radiative-convective model. The ocean thermodynamics are based upon solution theory. The ocean, initially composed of CH4, becomes progressively enriched in ethane over time. The partial pressures of N2 and CH4 in the atmosphere are dependent on the ocean temperature and composition. We find that the resulting system is stable against a runaway greenhouse. Accounting for the decreased solar luminosity, we find that Titan's surface temperature was about 20 K colder 4 Gyr ago. Without an ocean, but only small CH4 lakes, the temperature change is 12 K. In both cases we find that the surface of Titan may have been ice covered about 3 Gyr ago. In the lakes case condensation of N2 provides the ice, whereas in the ocean case the ocean freezes. The dominant factor influencing the evolution of Titan's surface temperature is the change in the solar constant--amplified, if an ocean is present, by the temperature dependence of the solubility of N2. Accretional heating can dramatically alter the surface temperature; a surface thermal flux of 500 erg cm-2 sec-1, representative of small levels of accretional heating, results in a approximately 20 K change in surface temperatures.
 
We have developed a radiative-convective model of the thermal structure of Titan's atmosphere. The model computes the solar and infrared radiation in a series of spectral intervals with vertical resolution. Sources of opacity in the visible and near infrared include stratospheric haze particles, methane cloud particles, and gaseous methane; sources of opacity in the thermal infrared include the pressure-induced opacity of N2, CH4, and H2, the permitted transitions of C2H2 and C2H6, and particulate opacity. The haze properties are determined with a simple microphysics model. The model contains a minimum of free parameters and we try to determine these by fits to independent data sets. We find that gas and haze opacity alone, with the temperatures fixed by Voyager observations, produces a model that is within a few percent of radiative convective balance everywhere in the atmosphere. In a self-consistent computation of temperatures, we find that our model calculation for the surface temperature is, in general, colder than the observed value by 5-10 degrees K. The presence or absence of methane condensation clouds only slightly alters the results. Good agreement can be obtained by adjusting the parameters in the model. The model parameters in these optimized cases are typically within 15% of the baseline values and within the limits allowed by observations. We conclude that the most important factors controlling Titan's thermal structure are absorption of sunlight by the stratospheric haze and the pressure-induced gas opacity in the infrared. Within the uncertainties of the model, these effects can explain the observed temperature profile. Condensation clouds play a minor role, if any.
 
The factors governing the amounts of CO, O_2, and O_3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO_2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O_2, O_3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO_2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HO_x radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.
 
Global Martian atmospheric results derived from the infrared imaging spectrometer ISM flown aboard the Phobos 2 Soviet spacecraft are presented. Over low altitude regions the expected CO mixing ratio of (8 +/- 3) x 10(-4) is measured. Variations of the 2.35-micrometers feature are inconsistent with this value over the Great Martian Volcanoes. If the 2.35-micrometers band is entirely attributable to carbon monoxide, the CO mixing ratio is typically depleted by a factor of 3 over these high altitude areas. Orography should play a major role in the existence of this CO "hole." If, however, these spectral variations at 2.35 micrometers are due to the surface composition, the fraction of the surface covered by the responsible mineral must smoothly decrease as the surface elevation decreases. This phenomenon implies a strong interaction between the surface and the atmosphere for the Great Martian Volcanoes. Diurnal behavior and latitudinal variations of water vapor are globally consistent with Viking measurements. During the Phobos observations, the water vapor amounts over the bright equatorial regions range around 11 pr-micrometers during the day. These amounts are slightly larger than those inferred from 1976 to 1979. The lack of global dust storms during 1988-1989 could explain the enhancement of H2O in the atmosphere.
 
The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.
 
The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Météorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.
 
Many investigators of the early martian climate have suggested that a dense carbon dioxide atmosphere was present and warmed the surface above the melting point of water (J. B. Pollack, J. F. Kasting, S. M. Richardson, and K. Poliakoff 1987.Icarus71,203–224). However, J. F. Kasting (1991.Icarus94,1–13) pointed out that previous thermal models of the primitive martian atmosphere had not considered the condensation of CO_2. When this effect was incorporated, Kasting found that CO_2 by itself is inadequate to warm the surface. SO_2 absorbs strongly in the near UV region of the solar spectrum. While a small amount of SO_2 may have a negligible effect by itself on the surface temperature, it may have significantly warmed the middle atmosphere of early Mars, much as ozone warms the terrestrial stratosphere today. If this region is kept warm enough to inhibit the condensation of CO_2, then CO_2 remains a viable greenhouse gas. Our preliminary radiative modeling shows that the addition of 0.1 ppmv of SO_2 in a 2 bar CO_2 atmosphere raises the temperature of the middle atmosphere by approximately 10 degrees, so that the upper atmosphere in a 1D model remains above the condensation temperature of CO_2. In addition, this amount of SO_2 in the atmosphere provides an effective UV shield for a hypothetical biosphere on the martian surface.
 
The competition between impact erosion and impact supply of volatiles to planetary atmospheres can determine whether a planet or satellite accumulates an atmosphere. In the absence of other processes (e.g., outgassing), we find either that a planetary atmosphere should be thick, or that there should be no atmosphere at all. The boundary between the two extreme cases is set by the mass and velocity distributions and intrinsic volatile content of the impactors. We apply our model specifically to Titan, Callisto, and Ganymede. The impacting population is identified with comets, either in the form of stray Uranus-Neptune planetesimals or as dislodged Kuiper belt comets. Systematically lower impact velocities on Titan allow it to retain a thick atmosphere, while Callisto and Ganymede get nothing. Titan's atmosphere may therefore be an expression of a late-accreting, volatile-rich veneer. An impact origin for Titan's atmosphere naturally accounts for the high D/H ratio it shares with Earth, the carbonaceous meteorites, and Halley. It also accounts for the general similarity of Titan's atmosphere to those of Triton and Pluto, which is otherwise puzzling in view of the radically different histories and bulk compositions of these objects.
 
A one-dimensional climate model is used to study the response of an Earth-like atmosphere to large increases in solar flux. For fully saturated, cloud-free conditions, the critical solar flux at which a runaway greenhouse occurs, that is, the oceans evaporate entirely, is found to be 1.4 times the present flux at Earth's orbit (S0). This value is close to the flux expected at Venus' orbit early in solar system history. Is is nearly independent of the amount of CO2 present in the atmosphere, but is sensitive to the H2O absorption coefficient in the 8- to 12-micrometers window region. Clouds should tend to depress the surface temperature on a warm, moist planet; thus, Venus may originally have had oceans if its initial water endowment was close to that of Earth. It lost them early in its history, however, because of rapid photodissociation of water vapor followed by escape of hydrogen to space. The critical solar flux above which water is rapidly lost could be as low as 1.1S0. The surface temperature of a runaway greenhouse atmosphere containing a full ocean's worth of water would have been in excess of 1500 degrees K--above the solidus for silicate rocks. The presence of such a steam atmosphere during accretion may have significantly influenced the early thermal evolution of both Earth and Venus.
 
The refractory meteorite inclusions known as CAIs (calcium-aluminum rich inclusions) display melted rims that were produced by thermal events of only a few seconds duration. We show that gas dynamic deceleration in a temporary atmosphere around an accreting parent body, produced by gas release during accretion, could provide a regime of sufficiently high gas density and small scale height to achieve partial melting of the CAIs. In addition, the presence of dust in the atmosphere would increase the gradient of pressure with height (i.e., effectively reduce the scale height), lower the rate of blowoff (thus keeping more gas around the body), as well as allow dust particles to become trapped in the partially melted material as is observed in some cases. Thus, CAIs may be regarded as probes of a primitive atmosphere by virtue of the thermal and mineralogical alteration that occurred upon their passage through the atmosphere.
 
Top-cited authors
James Head
  • Brown University
Ralph Detlef Lorenz
  • Johns Hopkins University
Edward Cloutis
  • The University of Winnipeg
Paul Helfenstein
  • Cornell University
Bernard Schmitt
  • Université Grenoble Alpes