A search for a nonbiological explanation of the Viking Labeled Release life detection experiment

The Viking Labeled Release (LR) data obtained on Mars satisfy the criteria established for a biological response. The importance of the issue, especially when viewed against the harsh environment on Mars, requires careful consideration of possible nonbiological reactions that may have produced false positive results. A $\text{3}\text{1}\text{2}\text{-}\text{year}$ laboratory effort to investigate possible chemical, physical, and physicochemical agents or mechanisms has been concluded. Among nonbiological possibilities, hydrogen peroxide, putatively on Mars, emerged as the principal candidate. When placed on analog Mars soils prepared to match the Viking inorganic analysis of the Mars surface material, hydrogen peroxide did not duplicate the LR Mars data. When other materials were used as substrate, hydrogen peroxide could be made to evoke the type of responses obtained by the LR Mars experiment. However, essential criteria concerning the formation, accuumulation, and preservation of hydrogen peroxide to qualify it as the active agent on Mars have not been met and new data show it to be essentially absent from the Mars atmosphere. The presence of a biological agent on Mars must still be considered. This interpretation of the LR results is strengthened by a recent report that the Viking organic analysis instrument (GCMS) failed to detect organics in an Antarctic soil in which the LR instrument had demonstrated the presence of microorganisms.     

Loss of water from Mars:: Implications for the oxidation of the soil

The evolution of the martian atmosphere with regard to its H₂O inventory is influenced by thermal loss processes of H, H₂, nonthermal atmospheric loss processes of H⁺, H₂⁺, O, O⁺, CO₂, and O₂⁺ into space, as well as by chemical weathering of the surface soil. The evolution of thermal and nonthermal escape processes depend on the history of the intensity of the solar XUV radiation and the solar wind density. Thus, we use actual data from the observation of solar proxies with different ages from the Sun in Time program for reconstructing the Sun's radiation and particle environment from the present to 3.5 Gyr ago. The correlation between mass loss and X-ray surface flux of solar proxies follows a power law relationship, which indicates a solar wind density up to 1000 times higher at the beginning of the Sun's main sequence lifetime. For the study of various atmospheric escape processes we used a gas dynamic test particle model for the estimation of the pick up ion loss rates and considered pick up ion sputtering, as well as dissociative recombination. The loss of H₂O from Mars over the last 3.5 Gyr was estimated to be equivalent to a global martian H₂O ocean with a depth of about 12 m, which is smaller than the values reported by previous studies. If ion momentum transport, a process studied in detail by Mars Express is significant on Mars, the water loss may be enhanced by a factor of about 2. In our investigation we found that the sum of thermal and nonthermal atmospheric loss rates of H and all nonthermal escape processes of O to space are not compatible with a ratio of 2:1, and is currently close to about 20:1. Escape to space cannot therefore be the only sink for oxygen on Mars. Our results suggest that the missing oxygen (needed for the validation of the 2:1 ratio between H and O) can be explained by the incorporation into the martian surface by chemical weathering processes since the onset of intense oxidation about 2 Gyr ago. Based on the evolution of the atmosphere-surface-interaction on Mars, an overall global surface sink of about 2×10⁴² oxygen particles in the regolith can be expected. Because of the intense oxidation of inorganic matter, this process may have led to the formation of considerable amounts of sulfates and ferric oxides on Mars. To model this effect we consider several factors: (1) the amount of incorporated oxygen, (2) the inorganic composition of the martian soil and (3) meteoritic gardening. We show that the oxygen incorporation has also implications for the oxidant extinction depth, which is an important parameter to determine required sampling depths on Mars aimed at finding putative organic material. We found that the oxidant extinction depth is expected to lie in a range between 2 and 5 m for global mean values.     

Can rapid loss and high variability of Martian methane be explained by surface H2O2?

It has been reported by several groups that methane in the Martian atmosphere is both spatially and temporally variable. Gough et al. (2010) suggested that temperature dependent, reversible physical adsorption of methane onto Martian soils could explain this variability. However, it is also useful to consider if there might be chemical destruction of methane (and compensating sources) operating on seasonal time scales. The lifetime of Martian methane due to known chemical loss processes is long (on the order of hundreds of years). However, observations constrain the lifetime to be 4 years or less, and general circulation models suggest methane destruction must occur even faster (<1 year) to cause the reported variability and rapid disappearance. The Martian surface is known to be highly oxidizing based on the Viking Labeled Release experiments in which organic compounds were quickly oxidized by samples of the regolith. Here we test if simulated Martian soil is also oxidizing towards methane to determine if this is a relevant loss pathway for Martian methane. We find that although two of the analog surfaces studied, TiO₂·H₂O₂ and JSC-Mars-1 with H₂O₂, were able to oxidize the complex organic compounds (sugars and amino acids) used in the Viking Labeled Release experiments, these analogs were unable to oxidize methane to carbon dioxide within a 72 h experiment. Sodium and magnesium perchlorate, salts that were recently discovered at the Phoenix landing site and are potential strong oxidants, were not observed to directly oxidize either the organic solution or methane. The upper limit reaction coefficient, α, was found to be <4×10^?17 for methane loss on TiO₂·H₂O₂ and <2×10^?17 for methane loss on JSC-Mars-1 with H₂O₂. Unless the depth of soil on Mars that contains H₂O₂ is very deep (thicker than 500 m), the lifetime of methane with respect to heterogeneous oxidation by H₂O₂ is probably greater than 4 years. Therefore, reaction of methane with H₂O₂ on Martian soils does not appear to be a significant methane sink, and would not destroy methane rapidly enough to cause the reported atmospheric methane variability.     

Injection of dust into the martian atmosphere: Evidence from the viking gas exchange experiment

It is proposed that dust storms on Mars that develop during predawn hours may be triggered by a freeze/thaw dust injection process. The model is based on a phenomenon that was observed during the Viking Gas Exchange experiments on Mars, in which adsorbed gas was catastrophically desorbed from soil samples when exposed to humidification at ∼5°C. Similar conditions may develop at midlatitude locations on Mars near perihelion, and a similar humidification-driven desorption process might occur in the soil column. If soils are dampened during humikification, desorbed gases in confined pore spaces could possibly reach 8.6 bar. Diurnal freezing may possibly cause H₂O to crystallize within the pores, possibly producing cohesive soil failure, release of the trapped gas, and explosive injection of freeze-dried powdery overburden dust into the atmospheric column. The process could potentially occur at 5–20 cm depth, and the freeze/thaw dust injection event may initiate after 10:00 PM local time (20°S lat). Dust would be injected at velocities approaching 450 m sec⁻¹ and it would remain in the atmosphere for several hours before settling out. The plumes could potentially regenerate diurnally until the growing atmospheric dust load produced sufficient dampening of the diurnal thermal wave to prevent freeze/thaw. Seasonal replenishment of H₂O could potentially occur by upward migration from depth during the period between 150 and 475 sols after perihelion. The model was experimentally tested and the results were in good agreement with predictions, although a factor of 14 times more gas evolved from the laboratory samples than from the Viking samples. Most of the characteristics of the predawn storms could possibly be adequately explained by the freeze/thaw injection model, including (1) predawn onsets, (2) postperihelion seasonal occurence, (3) daily recurrence during the initial phases of the storms, and (4) generation of blue clouds (H₂O ice) at the storm sites. The process may possibly occur over widespread locations at midlatitudes during seasonal retreat of “tempofrost” from these latitude belts. Permanent low albedo features in these latitude belts may possibly be regions of preferential humidification-induced dust entrainment and net dust removal. The H₂O injected into the atmosphere may potentially be a major source of H₂SO₄ and HCl aerosols, which may possibly chemically react with the regolith to form soluble sulfate and chloride salts. Mg²⁺ may be preferentially depleted from the dust.     

Iron(VI) Seems an Unlikely Explanation for Viking Labeled Release Results

Tsapin et al. (2000, Icarus147, 68-78) propose the strong oxidant ferrate(VI) to explain the Viking Labeled Release Mars life detection results. However, their data do not support that theory. Further, sensitive IR searches for oxidants on Mars found none, and Viking produced physical evidence against an oxidizing surface. Finally, Tsapin et al. (2000, Icarus147, 68-78) report no precautions to prevent microbial contamination from confounding their results.     

Why exobiology on Mars?

Processing of organic molecules by liquid water was probably an essential requirement towards the emergence of terrestrial primitive life. According to Oparin's hypothesis, organic building blocks required for early life were produced from simple organic molecules formed in a primitive reducing atmosphere. Geochemists favour now a less reducing atmosphere dominated by carbon dioxide. In such an atmosphere, very few building blocks are formed. Import of extraterrestrial organic molecules may represent an alternative supply. Experimental support for such an alternative scenario is examined in comets, meteorites and micrometeorites. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars attesting the presence of an atmosphere capable of decelerating C-rich micro-meteorites. Therefore, primitive life may have developed on Mars, as well. Liquid water disappeared from the surface of Mars very early, about 3.8 Ga ago. The Viking missions did not find, at the surface of the Martian soil, any organic molecules or clear-cut evidence for microbial activities such as photosynthesis, respiration or nutrition. The results can be explained referring to an active photochemistry of Martian soil driven by the high influx of solar UV. These experiments do not exclude the existence of organic molecules and fossils of micro-organisms which developed on early Mars until liquid water disappeared. Mars may store below its surface some well preserved clues of a still hypothetical primitive life.     

Methane emissions from Earth’s degassing: Implications for Mars

The presence of methane on Mars is of great interest, since one possibility for its origin is that it derives from living microbes. However, CH₄ in the martian atmosphere also could be attributable to geologic emissions released through pathways similar to those occurring on Earth. Using recent data on methane degassing of the Earth, we have estimated the relative terrestrial contributions of fossil geologic methane vs. modern methane from living methanogens, and have examined the significance that various geologic sources might have for Mars.Geologic degassing includes microbial methane (produced by ancient methanogens), thermogenic methane (from maturation of sedimentary organic matter), and subordinately geothermal and volcanic methane (mainly produced abiogenically). Our analysis suggests that ～80% of the “natural” emission to the terrestrial atmosphere originates from modern microbial activity and ～20% originates from geologic degassing, for a total CH₄ emission of ～28.0×10⁷ tonnes year^?1.Estimates of methane emission on Mars range from 12.6×10¹ to 57.0×10⁴ tonnes year^?1 and are 3–6 orders of magnitude lower than that estimated for Earth. Nevertheless, the recently detected martian, Northern-Summer-2003 CH₄ plume could be compared with methane expulsion from large mud volcanoes or from the integrated emission of a few hundred gas seeps, such as many of those located in Europe, USA, Mid-East or Asia. Methane could also be released by diffuse microseepage from martian soil, even if macro-seeps or mud volcanoes were lacking or inactive. We calculated that a weak microseepage spread over a few tens of km², as frequently occurs on Earth, may be sufficient to generate the lower estimate of methane emission in the martian atmosphere.At least 65% of Earth’s degassing is provided by kerogen thermogenesis. A similar process may exist on Mars, where kerogen might include abiogenic organics (delivered by meteorites and comets) and remnants of possible, past martian life. The remainder of terrestrial degassed methane is attributed to fossil microbial gas (～25%) and geothermal-volcanic emissions (～10%). Global abiogenic emissions from serpentinization are negligible on Earth, but, on Mars, individual seeps from serpentinization could be significant. Gas discharge from clathrate-permafrost destabilization should also be considered.Finally, we have shown examples of potential degassing pathways on Mars, including mud volcano-like structures, fault and fracture systems, and major volcanic edifices. All these types of structures could provide avenues for extensive gas expulsion, as on Earth. Future investigations of martian methane should be focused on such potential pathways.     

The Mars oxidant experiment (MOx) for Mars '96

The MOx instrument was developed to characterize the reactive nature of the martian soil. The objectives of MOx were: (1) to measure the rate of degradation of organics in the martian environment; (2) to determine if the reactions seen by the Viking biology experiments were caused by a soil oxidant and measure the reactivity of the soil and atmosphere: (3) to monitor the degradation, when exposed to the martian environment, of materials of potential use in future missions; and, finally, (4) to develop technologies and approaches that can be part of future soil analysis instrumentation. The basic approach taken in the MOx instrument was to place a variety of materials composed as thin films in contact with the soil and monitor the physical and chemical changes that result. The optical reflectance of the thin films was the primary sensing-mode. Thin films of organic materials, metals, and semiconductors were prepared. Laboratory simulations demonstrated the response of thin films to active oxidants.     

Monte-Carlo simulations of atmospheric muon production: Implication of the past martian environment

In order to evaluate the obliquity-driven atmospheric-density path length effect on nuclide production rate on Mars, we performed a Monte-Carlo simulation to produce the number of secondary particles such as muons, neutrons and protons in the martian atmosphere and to simulate that production of ¹⁰Be and ³⁶Cl in the martian regolith by muons and neutrons depends on how much atmosphere had been present for the past 10 million years. The vertical profile of the present martian atmosphere to generate secondary particles has been determined based on the data provided by the Viking missions. For other thickness profiles, we scaled Linsley's atmospheric model. Atmospheric shower has been generated with the SIBYLL 2.1 for high-energy hadronic interactions and EHSA for low energy photonuclear interactions. With increasing atmospheric thickness, more primary interactions occur in the atmosphere. Consequently the proton flux is reduced and the secondary cosmic ray flux, such as muons or energetic neutrons increases at surface. The result indicates that the muon production is more sensitive to obliquity-driven atmospheric variations than proton reduction. A thicker atmosphere would result in enhanced nuclide production at a place deeper than 5 m below the surface and the nuclides present in detectable concentrations. Application to the polar deposit is described.     

Detection of methane in the martian atmosphere: evidence for life?

Using the Fourier Transform Spectrometer at the Canada-France-Hawaii Telescope, we observed a spectrum of Mars at the P-branch of the strongest CH₄ band at 3.3 μm with resolving power of 180,000 for the apodized spectrum. Summing up the spectral intervals at the expected positions of the 15 strongest Doppler-shifted martian lines, we detected the absorption by martian methane at a 3.7 sigma level which is exactly centered in the summed spectrum. The observed CH₄ mixing ratio is 10±3 ppb. Total photochemical loss of CH₄ in the martian atmosphere is equal to , the CH₄ lifetime is 340 years and methane should be uniformly mixed in the atmosphere. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH₄ during its diffusion through the regolith may be significant. There are no processes of CH₄ formation in the atmosphere, so the photochemical loss must therefore be balanced by abiogenic and biogenic sources. Outgassing from Mars is weak, the latest volcanism is at least 10 million years old, and thermal emission imaging from the Mars Odyssey orbiter does not reveal any hot spots on Mars. Hydrothermal systems can hardly be warmer than the room temperature at which production of methane is very low in terrestrial waters. Therefore a significant production of hydrothermal and magmatic methane is not very likely on Mars. The calculated average production of CH₄ by cometary impacts is 2% of the methane loss. Production of methane by meteorites and interplanetary dust does not exceed 4% of the methane loss. Methane cannot originate from an extinct biosphere, as in the case of “natural gas” on Earth, given the exceedingly low limits on organic matter set by the Viking landers and the dry recent history which has been extremely hostile to the macroscopic life needed to generate the gas. Therefore, methanogenesis by living subterranean organisms is a plausible explanation for this discovery. Our estimates of the biomass and its production using the measured CH₄ abundance show that the martian biota may be extremely scarce and Mars may be generally sterile except for some oases.     

The gas exchange experiment for life detection: The Viking Mars Lander

The Gas Exchange Experiment of the Viking mission accepts a sample of Martian soil, incubates this soil with nutrient medium, and periodically samples the enclosed atmosphere over this soil for the gases H₂, N₂, O₂, CH₄, Kr, and CO₂. These gases are analyzed by an automated gas chromatograph, and the data are transmitted to Earth. The design of the experiment and the qualitative and quantitative changes, if any, of gas composition should allow conclusions to be made on the presence of life on Mars. Data and theory substantiating this approach are presented.     

Laboratory simulations of Mars aqueous geochemistry

We report on laboratory experiments in which we allowed an SNC-derived mineral mix to react with pure water under a simulated Mars atmosphere for 7 months. These experiments were performed at one bar and at three different temperatures in order to simulate the subsurface conditions that most likely exist where liquid water and rock interact on Mars today. The dominant cations dissolved in the solutions we produced, which may be characterized as dilute brines, are Ca²⁺, Mg²⁺, Al³⁺, and Na⁺, while the major anions are dissolved C, F⁻, SO²⁻₄ and Cl⁻. Typical solution pH was in the range of 4.2-6.0. Abundance patterns of elements in our synthetic sulfate-chloride brines are distinctly unlike those of terrestrial ocean water or continental waters, however, they are quite similar to those measured in the martian fines at the Mars Pathfinder and Viking 1 and 2 Landing sites. This suggests that salts present in the martian regolith may have formed over time as a result of the interaction of surface or subsurface liquid water with basalts in the presence of a martian atmosphere similar in composition to that of today. If most of the mobile surface layer was formed during the Noachian when erosion rates were much higher than at present, and if this layer is homogeneous in salt composition, the total amount of salt in the martian fines is approximately the same as in the Earth's oceans. The minimum quantity of circulating water necessary to deposit this amount of salt is approximately equivalent to a global layer 625 m deep.     

Comment on “Methane on Mars: A product of H2O photolysis in the presence of CO” by A. Bar-Nun and V. Dimitrov

Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320-322] suggested a sequence of reactions to form methane on Mars. These reactions are based on the study of products in the N₂-CO-H₂O mixture irradiated at 185 nm. The suggested scheme was not quantitatively justified by chemical kinetics. One of the key reactions is effectively blocked by O₂ in the martian atmosphere, and another key reaction does not exist. There are no pathways for effective formation of methane in the martian atmosphere.     

Determination of the wind induced detachment threshold for granular material on Mars using wind tunnel simulations

Experimental results are presented of wind induced grain detachment under Mars simulation conditions. A simple force balance equation is applied to quantify the wind shear stress required for removal of glass spheres from a sand bed. The transport of fine grained martian dust is simulated by the detachment of hollow glass spheres which resemble low mass density dust aggregates observed to form during simulations when using Mars analogue material. The results agree well with observations of dust removal and wind speed measurements made by the NASA Viking landers at the martian surface. This work supports the suggestion that dust aggregate fragmentation allows wind induced dust entrainment at substantially lower wind shear than that of solid sand grains and has direct application to the study of global dust transport and martian climatology.     

Possibilities for the detection of hydrogen peroxide-water-based life on Mars by the Phoenix Lander

The Phoenix Lander landed on Mars on 25 May 2008. It has instruments on board to explore the geology and climate of subpolar Mars and to explore if life ever arose on Mars. Although the Phoenix mission is not a life detection mission per se, it will look for the presence of organic compounds and other evidence to support or discredit the notion of past or present life.The possibility of extant life on Mars has been raised by a reinterpretation of the Viking biology experiments [Houtkooper, J. M., Schulze-Makuch, D., 2007. A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted. International Journal of Astrobiology 6, 147-152]. The results of these experiments are in accordance with life based on a mixture of water and hydrogen peroxide instead of water. The near-surface conditions on Mars would give an evolutionary advantage to organisms employing a mixture of H₂O₂ and H₂O in their intracellular fluid: the mixture has a low freezing point, is hygroscopic and provides a source of oxygen. The H₂O₂-H₂O hypothesis also explains the Viking results in a logically consistent way. With regard to its compatibility with cellular contents, H₂O₂ is used for a variety of purposes in terran biochemistry. The ability of the anticipated organisms to withstand low temperatures and the relatively high water vapor content of the atmosphere in the Martian arctic, means that Phoenix will land in an area not inimical to H₂O₂-H₂O-based life. Phoenix has a suite of instruments which may be able to detect the signatures of such putative organisms.     

A photochemical model of the martian atmosphere

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 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, O2, O3, 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 + HO2 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 HOx 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.     

Alteration of five organic compounds by glow discharge plasma and UV light under simulated Mars conditions

The Viking missions to Mars failed to detect any organic material in regolith samples. Since then, several removal mechanisms of organic material have been proposed. Two of these proposed methods are removal due to exposure to plasmas created in dust devils and exposure to UV irradiation. The experiments presented here were performed to identify similarities between the two potential removal mechanisms and to identify any compounds produced from these mechanisms that would have been difficult for the Viking instruments to detect. Five organic compounds, phenanthrene, octadecane, octadecanoic acid, decanophenone and benzoic acid, were exposed to a glow discharge plasma created in simulated martian atmospheres as might be present in dust devils, and to UV irradiation similar to that found at the surface of Mars. Glow discharge exposure was carried out in a chamber with 6.9 mbar pressure of a Mars like gas composed mostly of carbon dioxide. The plasma was characterized using emission spectroscopy and found to contain cations and excited neutral species including carbon dioxide, carbon monoxide, and nitrogen. UV irradiation experiments were performed in a Mars chamber which simulates the temperature, pressure, atmospheric composition, and UV fluence rates of equatorial Mars. The non-volatile residues left after each exposure were characterized by mass loss, infrared spectroscopy and high resolution mass spectrometry. Oxidized, higher molecular weight versions of the parent compounds containing carbonyl, hydroxyl and alkenyl functional groups were identified. The presence of these oxidized compounds suggests that searches for organic material in soils on Mars use instrumentation suitable for detection of compounds which contain the above functional groups. Discussions of possible reaction mechanisms are given.     

The global martian volcanic evolutionary history

Viking mission image data revealed the total spatial extent of preserved volcanic surface on Mars. One of the dominating surface expressions is Olympus Mons and the surrounding volcanic province Tharsis. Earlier studies of the global volcanic sequence of events based on stratigraphic relationships and crater count statistics were limited to the image resolution of the Viking orbiter camera. Here, a global investigation based on high-resolution image data gathered by the High-Resolution Stereo Camera (HRSC) during the first years of Mars Express orbiting around Mars is presented. Additionally, Mars Orbiter Camera (MOC) and Thermal Emission Imaging System (THEMIS) images were used for more detailed and complementary information. The results reveal global volcanism during the Noachian period (>3.7 Ga) followed by more focused vent volcanism in three (Tharsis, Elysium, and Circum-Hellas) and later two (Tharsis and Elysium) volcanic provinces. Finally, the volcanic activity became localized to the Tharsis region (about 1.6 Ga ago), where volcanism was active until very recently (200-100 Ma). These age results were expected from radiometric dating of martian meteorites but now verified for extended geological units, mainly found in the Tharsis Montes surroundings, showing prolonged volcanism for more than 3.5 billions years. The volcanic activity on Mars appears episodic, but decaying in intensity and localizing in space. The spatial and temporal extent of martian volcanism based on crater count statistics now provides a much better database for modelling the thermodynamic evolution of Mars.     

Methane adsorption on a martian soil analog: An abiogenic explanation for methane variability in the martian atmosphere

Recent observations suggest methane in the martian atmosphere is variable on short spatial and temporal scales. However, to explain the variability by loss reactions requires production rates much larger than expected. Here, we report results of laboratory studies of methane adsorption onto JSC-Mars-1, a martian soil simulant, and suggest that this process could explain the observations. Uptake coefficient (γ) values were measured as a function of temperature using a high-vacuum Knudsen cell able to simulate martian temperature and pressure conditions. Values of γ were measured from 115 to 135 K, and the data were extrapolated to higher temperatures with more relevance to Mars. Adsorptive uptake was found to increase at lower temperatures and larger methane partial pressures. Although only sub-monolayer methane surface coverage is likely to exist under martian conditions, a very large mineral surface area is available for adsorption as atmospheric methane can diffuse meters into the regolith. As a result, significant methane may be temporarily lost to the regolith on a seasonal time scale. As this weak adsorption is fully reversible, methane will be re-released into the atmosphere when surface and subsurface temperatures rise and so no net loss of methane occurs. Heterogeneous interaction of methane with martian soil grains is the only process proposed thus far which contains both rapid methane loss and rapid methane production mechanisms and is thus fully consistent with the reported variability of methane on Mars.     

Characterization of rock populations on planetary surfaces: Techniques and a preliminary analysis of Mars and Venus

Characteristics of rock populations on the surfaces of Mars and Venus can be derived from analyses of rock morphology and morphometry data. We present measurements of rock sizes and sphericities made from Viking lander images using an interactive digital image display system. The rocks considered are in the gravel size range (16–256 mm in diameter). Mean sphericities, form ratios, and roundness factors are found to be very similar for both Viking lander sites. Size distributions, however, demonstrate differences between the sites; there are significantly more cobble size fragments at VL-2 than at VL-1. A model calling for aphanitic basalts emplaced as ejecta or lava flows at the Viking sites is supported by the rock shape, size, and roundness data.Morphologic features pertaining to the modification history of a rock are considered for Mars and Venus. A multi-parameter clustering algorithm is utilized to objectively categorize martian and venusian rocks in terms of various criteria. Erosional markings such as flutes are demonstrated to be most important in separating VL-1 rock morphologic groups, while rock form (i.e., shape) represents the primary separator of subpopulations at VL-2 and the Venera landing sites. Fillets are common around VL-1 and Venera 10 fragments. Obstacle scours occur frequently only at VL-1. Cavities in rocks are ubiquitous at all lander sites except Venera 9. Eolian processes, possibly assisted by local solution weathering, are a strong candidate for the origin of cavities and flutes in martian rocks.