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.     

Methane evolution from UV-irradiated spacecraft materials under simulated martian conditions: Implications for the Mars Science Laboratory (MSL) mission

Fifteen organic and three inorganic compounds were tested for methane (CH₄) evolution under simulated martian conditions of 6.9 mbar; UVC (200-280 nm) flux of 4 W m⁻²; 20 °C; simulated optical depth of 0.1; and a Mars gas composition of CO₂ (95.3%), N₂ (2.7%), Ar (1.7%), O₂ (0.13%), and water vapor (0.03%). All three inorganic compounds (i.e., NaCl, CaCO₃, graphite) failed to evolve methane at the minimum detection level 0.5 ppm, or above. In contrast, all organic compounds evolved methane when exposed to UV irradiation under simulated martian conditions. The polycyclic aromatic hydrocarbon, pyrene, released the most methane per unit of time at 0.175 nmol CH₄ g⁻¹ h⁻¹, and a spectral reflectance target material used for the MER rovers and Phoenix lander released the least methane at 0.00065 nmol CH₄ cm⁻² h⁻¹. Methane was also released from UV-killed bacterial endospores of Bacillus subtilis. Although all organic compounds evolved methane when irradiated with UV photons under martian conditions, the concentrations of residual organics, biogenic signature molecules, and dead microbial cells should be relatively low on the exterior surfaces of the MSL rover, and, thus, not significant sources of methane contamination. In contrast, kapton tape was found to evolve methane at the rate of 0.00165 nmol CH₄ cm⁻² h⁻¹ (16.5 nmol m⁻² h⁻¹) under the UV and martian conditions tested. Although the evolution of methane from kapton tape was found to decline over time, the large amount of kapton tape used on the MSL rover (lower bound estimated at 3 m²) is likely to create a significant source of terrestrial methane contamination during the early part of the mission.     

Concatenation of HRSC colour and OMEGA data for the determination and 3D-parameterization of high-altitude CO2 clouds in the Martian atmosphere

We used Mars Express HRSC and OMEGA data to investigate mesospheric cloud features observed in the equatorial belt of Mars from December 2007 until early March 2008. This period corresponds to early northern spring of Martian year 29. The reflection peak at 4.26 μm in OMEGA data identifies the clouds as CO₂ ice clouds. HRSC observed the clouds together with OMEGA in five orbits. Cloud features are most prominent in the shortwave HRSC colour channels with wavelength centers at 440 and 530 nm, but rarely visible in all other channels. In the period of Ls 0-36°, OMEGA and HRSC together detected mesospheric CO₂ ice clouds in 40 orbits. They occur in a latitude belt of ±20° around the equator and at longitudes between 240°E (Tharsis) in the West and 30°E (Sinus Meridiani) in the East. The clouds were observed between 3 and 5 p.m. local time with mainly ripple-like to filamentary cloud forms. The viewing angles of the HRSC blue and green colour channels differ by 6.6° and the resulting parallax can be used to directly measure cloud heights by means of ray intersection. 17 HRSC data takes were found to exhibit clouds with heights from 66 to 83 km with an accuracy of 1-2 km. The pushbroom imaging technique also yields a time delay for the two observations in the order of 5-15 s close to periapsis, and therefore time-related cloud movements can be detected. A method was developed to determine the across-track cloud displacements, which can directly be translated to wind velocities. Zonal cloud movements could be measured in 13 cases and were oriented from East to West. Related wind speeds range between 60 and 93 m/s with an accuracy of 10-13 m/s.     

Wind measurements in Mars' middle atmosphere: IRAM Plateau de Bure interferometric CO observations

The IRAM Plateau de Bure Interferometer has been used to map the CO(1-0) rotational line in Mars' middle atmosphere. Absolute winds and thermal profiles were retrieved during the 1999, 2001, 2003 and 2005 planet's oppositions. The observations sampled various seasons (Ls=143, 196, 262, 317 and 322), and different dust situations (clear, global storm, regional storm). The absolute winds were derived by measuring directly the Doppler lineshifts. The main zonal circulation near 50 km is dominated by strong retrograde winds, with typical velocities of 70-170 m/s, strongly varying seasonally, latitudinally, and longitudinally (in particular between morning and evening). Comparison of the retrieved temperature with a general circulation model indicates that the model often underestimates the temperatures in the middle (20-50 km) atmosphere, and overestimates them above 50 km.     

Fossilization potential of iron-bearing minerals in acidic environments of Rio Tinto, Spain: Implications for Mars exploration

Acidic waters of the Rio Tinto, southwestern Spain, evaporate seasonally, precipitating a variety of iron sulfide and oxide minerals. Schwertmannite and nanophase goethite form thin laminae on biological and detrital grain surfaces, replicating, among other things, the morphologies of insect cuticle, plant tissues, fungi, algae, and bacteria. Intergrain cements also incorporate bacterial cells and filaments. Other sulfate minerals precipitated in Rio Tinto environments are transient and contribute little to short-term preservation. Because the Rio Tinto has been cutting its current valley for several million years, terrace deposits provide a window on longer term fossil preservation. Early and later diagenesis are recorded in terrace deposits formed about one thousand and two million years ago, respectively. The sedimentary structures and mineralogies of these deposits suggest that they formed under physical and chemical conditions comparable to those of modern Rio Tinto sediments. The terrace deposits show quantitative loss of sulfate minerals, increasing crystallinity of goethite and, in the older terrace, replacement of goethite by hematite. Fossils formed originally by schwertmannite and nanophase goethite replication persist through diagenesis, preserving a long term record of local biological diversity. Fossil preservation by iron oxides in the acidic environment of Rio Tinto suggests that if life was present when sedimentary rocks formed at Meridiani Planum, Mars, precipitated minerals could record their presence.     

MARCI and MOC observations of the atmosphere and surface cap in the north polar region of Mars

We used MGS-MOC and MRO-MARCI daily mapping images of the North Polar Region of Mars from 16 August 2005 (L_s = 270°) to 21 May 2009 (L_s = 270°), covering portions of three consecutive martian years (MY 27-MY 29), to observe the seasonal behavior of the polar ice cap and atmospheric phenomena. The rate of cap regression was similar in MY 28 and MY 29, but was advanced by 3.5° of L_s (∼7-8 sols) in MY 29. The spatial and temporal behaviors of dust and condensate clouds were similar in the two years and generally in accord with prior years. Dust storms (>100 km²) were observed in all seasons, with peak activity occurring at L_s = 10-20° from 50°N to 70°N and at L_s = 135-140° from 70°N to 90°N. The most active quadrant was 0-90°W in MY 28, shifting to 180-270°W in MY 29. The majority of regional storms in both years developed in longitudes from 10°W to 60°W. During late summer the larger storms obscure the North Polar Region in a cloud of dust that transitions to north polar hood condensate clouds around autumnal equinox.Changes in the distribution of perennial ice deposits, especially in Olympia Planum, were observed between the 2 years, with the MY 29 ice distribution being the most extensive observed to date. Modeling suggests that the small, bright ice patches on the residual cap are not the result of slope or elevation effects. Rather we suggest that they are the result of local meteorological effects on ice deposition. The annual darkening and brightening of peripheral areas of the residual cap around summer solstice can be explained by the sublimation of a brighter frost layer revealing an underlying darker, ice rich layer that itself either sublimes to reveal brighter material below or acts as a cold trap, attracting condensation of water vapor that brightens the surface. An alternative explanation invokes transport and deposition of dust on the surface from the cap interior, and later removal of that dust. The decrease in cap albedo and accompanying increase in near surface atmospheric stability may be related to the annual minimum of polar storm activity near northern summer solstice.     

Interactive effects of hypobaria, low temperature, and CO2 atmospheres inhibit the growth of mesophilic Bacillus spp. under simulated martian conditions

Robotic spacecraft are launched with finite levels of terrestrial microorganisms that are similar to the microbial communities within facilities in which spacecraft are assembled. In particular, spores of mesophilic aerobic Bacillus species are common spacecraft contaminants considered most likely to survive interplanetary transfer to Mars. During the cruise phase to Mars, and then again during surface operations, microbial bioloads are exposed to a diversity of biocidal factors that are likely to render the microbial species either dead or significantly inhibited from active metabolic activity and replication. We report here, for the first time, that interactive effects of low pressure, low temperature, and high CO₂ atmospheres approaching conditions likely to be encountered on the martian surface strongly inhibit the growth and replication of seven common Bacillus spp. isolated from spacecraft. Tests were conducted within a small glass bell-jar system maintained in a low-temperature microbial incubator. Atmospheric pressures were controlled at 1013 (Earth-normal), 100, 50, 35, 25, or 15 mb, and temperatures were maintained at 30, 20, 15, 10, or 5 °C. Experiments were carried out for 48 h or 7 days under either Earth-normal O₂/N₂ or pure CO₂ atmospheres. Results indicated that low pressure, low temperature, and high CO₂ atmospheres, applied separately or in combination, were capable of inhibiting the growth and replication of B. pumilus SAFR-032, B. pumilus FO-36B, B. subtilis HA-101, B. subtilis 42HS-1, B. megaterium KL-197, B. licheniformis KL-196, and B. nealsonii FO-092 under simulated martian conditions. Endospores of all seven Bacillus spp. strains failed to germinate and grow at 25 mb at 30 °C. Although, vegetative cells of these strains exhibited a slightly greater ability to replicate at lower pressures than did endospores, vegetative cells of these species failed to grow at pressures below 25 mb. Interactive effects of these environmental parameters acted to generally increase the inhibitory nature of the low-pressure conditions on growth and replication of the seven Bacillus spp. tested.     

Variability of the methane trapping in martian subsurface clathrate hydrates

Recent observations have evidenced traces of methane (CH₄) heterogeneously distributed in the martian atmosphere. However, because the lifetime of CH₄ in the atmosphere of Mars is estimated to be around 300-600 years on the basis of photochemistry, its release from a subsurface reservoir or an active primary source of methane have been invoked in the recent literature. Among the existing scenarios, it has been proposed that clathrate hydrates located in the near subsurface of Mars could be at the origin of the small quantities of the detected CH₄. Here, we accurately determine the composition of these clathrate hydrates, as a function of temperature and gas phase composition, by using a hybrid statistical thermodynamic model based on experimental data. Compared to the other recent works, our model allows us to calculate the composition of clathrate hydrates formed from a more plausible composition of the martian atmosphere by considering its main compounds, i.e. carbon dioxide, nitrogen and argon, together with methane. Besides, because there is no low temperature restriction in our model, we are able to determine the composition of clathrate hydrates formed at temperatures corresponding to the extreme ones measured in the polar caps. Our results show that methane enriched clathrate hydrates could be stable in the subsurface of Mars only if a primitive CH₄-rich atmosphere has existed or if a subsurface source of CH₄ has been (or is still) present.     

An ensemble Kalman filter data assimilation system for the martian atmosphere: Implementation and simulation experiments

The local ensemble transform Kalman filter (LETKF) is applied to the GFDL Mars general circulation model (MGCM) to demonstrate the potential benefit of an advanced data assimilation method. In perfect model (aka identical twin) experiments, simulated observations are used to assess the performance of the LETKF-MGCM system and to determine the dependence of the assimilation on observational data coverage. Temperature retrievals are simulated at locations that mirror the spatial distribution of the Thermal Emission Spectrometer (TES) retrievals from the Mars Global Surveyor (MGS). The LETKF converges quickly and substantially reduces the analysis and subsequent forecast errors in both temperature and velocity fields, even though only temperature observations are assimilated. The LETKF is also found to accurately estimate the magnitude of forecast uncertainties, notably those associated with the phase and amplitude of baroclinic waves along the boundary of the polar ice cap during Northern Hemisphere winter.     

Comparison of PFS and TES observations of temperature and water vapor in the martian atmosphere

The interval from L_s = 330° in Mars Year (MY) 26 until L_s = 84° in MY 27 has been used to compare and validate measurements from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and the Mars Express Planetary Fourier Spectrometer (PFS). We studied differences between atmospheric temperatures observed by the two instruments. The best agreement between atmospheric temperatures was found at 50 Pa between 40°S and 40°N latitude, where differences were within ±5 K. For other atmospheric levels, differences as large as ∼25 K were observed between the two instruments at some locations. The largest temperature differences occurred mainly over the Hellas Planitia, Argyre Planitia, Tharsis and Valles Marineris regions.On this basis we report on the variability of the martian atmosphere during the 5.5 martian years of Mars climatology obtained by combining the two data sets from TES and PFS. Atmospheric temperatures at 50 Pa responded to the global-scale dust storms of MY 25 and in MY 28 raising temperatures from ∼220 K to ∼250 K during the daytime. An atmospheric temperature of ∼140 K at 50 Pa was observed poleward of 70°N during northern winter and poleward of 60°S during southern winter each year in both the PFS and TES results. Water vapor observed by the two spectrometers showed consistent seasonal and latitudinal variations.     

Photochemistry of the martian atmosphere: Seasonal, latitudinal, and diurnal variations

There is a significant progress in the observational data relevant to Mars photochemistry in the current decade. These data are not covered by and sometimes disagree with the published models. Therefore we consider three types of models for Mars photochemistry. A steady-state model for global-mean conditions is currently the only way to calculate the abundances of long living species (H₂, O₂, and CO). However, our model does not fit the observed CO abundance using gas-phase chemistry and reasonable values of heterogeneous loss of odd hydrogen on the water ice aerosol. The second type of the calculated models is steady-state models for local conditions. The MGS/TES data on temperature profiles, H₂O, and dust are input parameters for these models. The calculations have been made for nine seasonal points spread over the martian year and for twelve latitudes with a step of 10° for each season. The only adopted heterogeneous reaction is a weak loss of H₂O₂ on water ice with probability of 5×¹⁰⁻⁴. The results are in good agreement with the recent observations of the O₂ dayglow at 1.27 μm and the O₃ and H₂O₂ abundances. Global maps of the seasonal and latitudinal behavior of these species have been made. The third type of models is a time-dependent model for local conditions. These models show that odd hydrogen quickly converts to H₂O₂ at the nighttime and the chemistry is switched off while the association of O, the heterogeneous loss of H₂O₂, and eddy diffusion continue. This requires significant changes in the global-mean and local steady-state models discussed above, and these changes have been properly done. The calculated diurnal variations of Mars photochemistry are discussed. The martian photochemistry at low and middle latitudes is significantly different in the aphelion period at LS=10°-130° from that in the remaining part of the year.     

MGS TES observations of the water vapor above the seasonal and perennial ice caps during northern spring and summer

We report on new retrievals of water vapor column abundances from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data. The new retrievals are from the TES nadir data taken above the ‘cold’ surface areas in the North polar region (T_surf < 220 K, including seasonal frost and permanent ice cap) during spring and summer seasons, where retrievals were not performed initially. Retrievals are possible (with some modifications to the original algorithm) over cold surfaces overlaid by sufficiently warm atmosphere. The retrieved water vapor column abundances are compared to the column abundances observed by other spacecrafts in the Northern polar region during spring and summer and good agreement is found. We detect an annulus of water vapor growing above the edge of the retreating seasonal cap during spring. The formation of the vapor annulus is consistent with the previously proposed mechanism for water cycling in the polar region, according to which vapor released by frost sublimation during spring re-condenses on the retreating seasonal CO₂ cap. The source of the vapor in the vapor annulus, according to this model, is the water frost on the surface of the CO₂ at the retreating edge of the cap and the frost on the ground that is exposed by the retreating cap. Small contribution from regolith sources is possible too, but cannot be quantified based on the TES vapor data alone. Water vapor annulus exhibits interannual variability, which we attribute to variations in the atmospheric temperature. We propose that during spring and summer the water ice sublimation is retarded by high relative humidity of the local atmosphere, and that higher atmospheric temperatures lead to higher vapor column abundances by increasing the water holding capacity of the atmosphere. Since the atmospheric temperatures are strongly influenced by the atmospheric dust content, local dust storms may be controlling the release of vapor into the polar atmosphere. Water vapor abundances above the residual polar cap also exhibit noticeable interannual variability. In some years abundances above the cap are lower than the abundances outside of the cap, consistent with previous observations, while in the other years the abundances above the cap are higher or similar to abundances outside of the cap. We speculate that the differences may be due to weaker off-cap transport in the latter case, keeping more vapor closer to the source at the surface of the residual cap. Despite the large observed variability in water vapor column abundances in the Northern polar region during spring and summer, the latitudinal distribution of the vapor mass in the atmosphere is very similar during the summer season. If the variability in vapor abundances is caused by the variability of vapor sources across the residual cap then this would mean that they annually contribute relatively little vapor mass to significantly affect the vapor mass budget. Alternatively this may suggest that the vapor variability is caused by the variability of the polar atmospheric circulation. The new water vapor retrievals should be useful in tuning the Global Circulation Models of the martian water cycle.     

Testing the survival of microfossils in artificial martian sedimentary meteorites during entry into Earth’s atmosphere: The STONE 6 experiment

If life ever appeared on Mars, could we find traces of primitive life embedded in sedimentary meteorites? To answer this question, a 3.5-byr-old volcanic sediment containing microfossils was embedded in the heat shield of a space capsule in order to test survival of the rock and the microfossils during entry into the Earth’s atmosphere (the STONE 6 experiment). The silicified volcanic sediment from the Kitty’s Gap Chert (Pilbara, Australia) is considered to be an excellent analogue for Noachian-age volcanic sediments. The microfossils in the chert are also analogues for potential martian life. An additional goal was to investigate the survival of living microorganisms (Chroococcidiopsis) protected by a 2-cm thick layer of rock in order to test whether living endolithic organisms could survive atmospheric entry when protected by a rocky coating.Mineralogical alteration of the sediment due to shock heating was manifested by the formation of a fusion crust, cracks in the chert due to prograde and retrograde changes of α quartz to β quartz, increase in the size of the fluid inclusions, and dewatering of the hydromuscovite-replaced volcanic protoliths. The carbonaceous microfossils embedded in the chert matrix survived in the rock away from the fusion crust but there was an increase in the maturity index of the kerogen towards the crust. We conclude that this kind of sediment can survive atmospheric entry and, if it contains microfossils, they could also survive. The living microorganisms were, however, completely carbonised by flame leakage to the back of the sample and therefore non-viable. However, using an analytical model to estimate the temperature reached within the sample thickness, we conclude that, even without flame leakage, the living organisms probably need to be protected by at least 5 cm of rock in order to be shielded from the intense heat of entry.     

Interannual and seasonal behavior of Martian residual ice-cap albedo

The Mars Orbiter Laser Altimeter (MOLA), functioning as a high-resolution radiometer, has observed several appearances of the Martian residual ice caps. We examine these data to quantify both seasonal behavior and interannual differences. The northern residual cap (NRC) was found to be mostly stable with the exception of one, previously identified, region of strong variability. Interannual change in the extent of the NRC appears to be small and reversible on timescales of 1 or 2 years. The NRC has an elaborate seasonal evolution of albedo. Annuli of fine-grained CO₂ and water frost, which track the inner and outer edges of the seasonal CO₂ cap, cause large temporary brightenings. The NRC albedo is stable from just after solstice to L_s 150°, after which albedo decreases steadily. This late-summer darkening can be explained by shadowing within the rough topography of the NRC, leading to a lower limit on topographic relief of 80 cm. The southern residual cap (SRC) appears stable in extent. As has been previously discovered, its seasonal frost albedo behavior appears to be correlated with insolation. However, residual CO₂ appears not to share this characteristic; we use this behavioral difference to infer net deposition of CO₂ ice on the SRC during 1 out of 3 years. Uncharacteristically, the SRC abruptly darkens at L_s 320° in 1 Martian year (year beginning April 2002). Circumstantial evidence suggests atmospheric scattering by dust is responsible. The 2001 global dust-storm appears, either, to have had no effect on the polar cap albedos, or, resulted in slightly brighter ice deposits.     

Evidence for pervasive mud volcanism in Acidalia Planitia, Mars

We have mapped 18,000+ circular mounds in a portion of southern Acidalia Planitia using their sizes, shapes, and responses in Nighttime IR. We estimate that 40,000+ of these features could occur in the area, with a distribution generally corresponding to the southern half of the proposed Acidalia impact basin. The mounds have average diameters of about 1 km and relief up to 180 m and most overlie units mapped as Early Amazonian.High resolution images of mound surfaces show relatively smooth veneers, apron-like extensions onto the plains, moats, and concentric circular crestal structures. Some images show lobate and flow-like features associated with the mounds. Albedo of the mounds is generally higher than that of the surrounding plains. Visible and near-infrared spectra suggest that the mounds and plains have subtle mineralogical differences, with the mounds having enhanced coatings or possibly greater quantities of crystalline ferric oxides.Multiple analogs for these structures were assessed in light of new orbital data and regional mapping. Mud volcanism is the closest terrestrial analogy, though the process in Acidalia would have had distinctly martian attributes. This interpretation is supported by the geologic setting of the Acidalia which sits at the distal end of the Chryse-Acidalia embayment into which large quantities of sediments were deposited through the Hesperian outflow channels. In its distal position, Acidalia would have been a depocenter for accumulation of mud and fluids from outflow sedimentation.Thus, the profusion of mounds in Acidalia is likely to be a consequence of this basin’s unique geologic setting. Basinwide mud eruption may be attributable to overpressure (developed in response to rapid outflow deposition) perhaps aided by regional triggers for fluid expulsion related to events such as tectonic or hydrothermal pulses, destabilization of clathrates, or sublimation of a frozen body of water. Significant release of gas may have been involved, and the extensive mud volcanism could have created long-lived conduits for upwelling groundwaters, providing potential habitats for an in situ microbiota.Mud volcanism transports minimally-altered materials from depth to the surface, and mud volcanoes in Acidalia, therefore, could provide access to samples from deep zones that would otherwise be inaccessible. Since the distal setting of Acidalia also would favor concentration and preservation of potentially-present organic materials, samples brought to the surface by mud volcanism could include biosignatures of possible past or even present life. Accordingly, the mounds of Acidalia may offer a new class of exploration target.     

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.     

Hydrogen peroxide on Mars: evidence for spatial and seasonal variations

Hydrogen peroxide (H₂O₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10¹⁵-10¹⁶ cm⁻². However, a stringent upper limit of the H₂O₂ abundance on Mars (9×10¹⁴ cm⁻²) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H₂O₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring (Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H₂O₂ shows a maximum in the morning around the sub-solar latitude. The mean H₂O₂ column density (6×10¹⁵ cm⁻²) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 (Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).     

Residual south polar cap of Mars: Stratigraphy, history, and implications of recent changes

The residual south polar cap (RSPC) of Mars includes a group of different depositional units of CO₂ ice undergoing a variety of erosional processes. Complete summer coverage of the RSPC by ∼6-m/pixel data of the Context Imager (CTX) on Mars Reconnaissance Orbiter (MRO) has allowed mapping and inventory of the units in the RSPC. Unit maps and estimated thicknesses indicate the total volume of the RSPC is currently <380 km³, and represents less than 3% of the total mass of the current Mars atmosphere. Scarp retreat rates in the CO₂ ice derived from comparison of High Resolution Imaging Science Experiment (HiRISE) data with earlier images are comparable to those obtained for periods up to 3 Mars years earlier. These rates, combined with sizes of depressions suggest that the oldest materials were deposited more than 125 Mars years ago. Most current erosion is by backwasting of scarps 1-12 m in height. This backwasting is initiated by a series of scarp-parallel fractures. In the older, thicker unit these fractures form about every Mars year; in thinner, younger materials they form less frequently. Some areas of the older, thicker unit are lost by downwasting rather than by the scarp retreat. A surprising finding from the HiRISE data is the scarcity of visible layering of RSPC materials, a result quite distinct from previous interpretations of layers in lower resolution images. Layers ∼0.1 m thick are exposed on the upper surfaces of some areas, but their timescale of deposition is not known. Late summer albedo changes mapped by the CTX images indicate local recycling of ice, although the amounts may be morphologically insignificant. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data show that the primary material of all the different forms of the RSPC is CO₂ ice with only small admixtures of water ice and dust.     

Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales

Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be “diurnal” on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO₂ sublimation/condensation. The corresponding LOD variations (ΔLOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe.