Vincent F. Chevrier

Early geochemical environment of Mars as determined from thermodynamics of phyllosilicates

Images of geomorphological features that seem to have been produced by the action of liquid water have been considered evidence for wet surface conditions on early Mars. Moreover, the recent identification of large deposits of phyllosilicates, associated with the ancient Noachian terrains suggests long-timescale weathering of the primary basaltic crust by liquid water. It has been proposed that a greenhouse effect resulting from a carbon-dioxide-rich atmosphere sustained the temperate climate required to maintain liquid water on the martian surface during the Noachian. The apparent absence of carbonates and the low escape rates of carbon dioxide, however, are indicative of an early martian atmosphere with low levels of carbon dioxide. Here we investigate the geochemical conditions prevailing on the surface of Mars during the Noachian period using calculations of the aqueous equilibria of phyllosilicates. Our results show that Fe3+-rich phyllosilicates probably precipitated under weakly acidic to alkaline pH, an environment different from that of the following period, which was dominated by strongly acid weathering that led to the sulphate deposits identified on Mars. Thermodynamic calculations demonstrate that the oxidation state of the martian surface was already high, supporting early escape of hydrogen. Finally, equilibrium with carbonates implies that phyllosilicate precipitation occurs preferentially at a very low partial pressure of carbon dioxide. We suggest that the possible absence of Noachian carbonates more probably resulted from low levels of atmospheric carbon dioxide, rather than primary acidic conditions. Other greenhouse gases may therefore have played a part in sustaining a warm and wet climate on the early Mars.

Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars

[1] We studied the low-temperature properties of sodium and magnesium perchlorate solutions as potential liquid brines at the Phoenix landing site. We determined their theoretical eutectic values to be 236 ± 1 K for 52 wt% sodium perchlorate and 206 ± 1 K for 44.0 wt% magnesium perchlorate. Evaporation rates of solutions at various concentrations were measured under martian conditions, and range from 0.07 to 0.49 mm h ―1 for NaClO 4 and from 0.06 to 0.29 mm h ―1 for Mg(ClO 4 ) 2 . The extrapolation to Phoenix landing site conditions using our theoretical treatment shows that perchlorates are liquid during the summer for at least part of the day, and exhibit very low evaporation rates. Moreover, magnesium perchlorate eutectic solutions are thermodynamically stable over vapour and ice during a few hours a day. We conclude that liquid brines may be present and even stable for short periods of time at the Phoenix landing site.

Low temperature aqueous ferric sulfate solutions on the surface of Mars

[1] We have studied the low-temperature properties of ferric sulfate Fe 2 (SO 4 ) 3 solutions as a model for potential liquid brines on the surface of Mars. Geochemical modeling demonstrates that concentrated ferric sulfate brines form through sulphur-rich acidic evaporation processes in cold oxidizing environments. Experiments and thermodynamic calculations show that the Fe 2 (SO 4 ) 3 eutectic temperature is 205 ± 1 K for 48 ± 2 wt% concentration. As a result of low water activity, these solutions exhibit evaporation rates ranging from 0.42 mm h -1 (29.1 wt%) to 0.03 mm h -1 (58.2 wt%), thus down to 20 times lower than pure water. The combination of extremely low eutectic temperature and evaporation rates allow subsurface liquids to be stable at high latitudes, where the majority of gullies and viscous flow features are located. Therefore, we conclude that episodic releases of highly concentrated ferric sulfate brines are a potential agent for the formation of recent and present-day gullies on Mars.

Noachian and more recent phyllosilicates in impact craters on Mars

Hundreds of impact craters on Mars contain diverse phyllosilicates, interpreted as excavation products of preexisting subsurface deposits following impact and crater formation. This has been used to argue that the conditions conducive to phyllosilicate synthesis, which require the presence of abundant and long-lasting liquid water, were only met early in the history of the planet, during the Noachian period (> 3.6 Gy ago), and that aqueous environments were widespread then. Here we test this hypothesis by examining the excavation process of hydrated minerals by impact events on Mars and analyzing the stability of phyllosilicates against the impact-induced thermal shock. To do so, we first compare the infrared spectra of thermally altered phyllosilicates with those of hydrated minerals known to occur in craters on Mars and then analyze the postshock temperatures reached during impact crater excavation. Our results show that phyllosilicates can resist the postshock temperatures almost everywhere in the crater, except under particular conditions in a central area in and near the point of impact. We conclude that most phyllosilicates detected inside impact craters on Mars are consistent with excavated preexisting sediments, supporting the hypothesis of a primeval and long-lasting global aqueous environment. When our analyses are applied to specific impact craters on Mars, we are able to identify both pre- and postimpact phyllosilicates, therefore extending the time of local phyllosilicate synthesis to post-Noachian times.

Experimental investigation into the effects of meteoritic impacts on the spectral properties of phyllosilicates on Mars

[1] Phyllosilicates have been identified in some of the most highly cratered Noachian terrains on Mars. To study the effects of such impacts on the properties of phyllosilicates, we experimentally shocked six phyllosilicate minerals relevant to the Martian surface: montmorillonite, nontronite, kaolinite, prehnite, chlorite, and serpentine. The shock-treated samples were analyzed with X-ray diffraction (XRD), near- and mid-infrared (NIR and MIR) spectroscopy, Raman spectroscopy, cathodoluminescence (CL), and the shock pressures and temperatures in some were modeled using Autodyn modeling software. XRD data show that the structure of each mineral, except prehnite, underwent partial structural deformation or amorphization. We also found that while the NIR spectra of shocked samples were very similar to that of the original sample, the MIR spectra changed significantly. This may explain some of the discrepancies between CRISM/OMEGA data (NIR) and TES/THEMIS (MIR) observations of phyllosilicates on Mars. Quartz was identified as a secondary phase in the XRD of shocked chlorite.

Near‐ and mid‐infrared reflectance spectra of hydrated oxychlorine salts with implications for Mars

The presence and distribution of oxychlorine salts (e.g., chlorates and perchlorates) on Mars have implications for the stability of water, most notably, that they lower the freezing temperature. To date, elemental chlorine has been measured by all lander missions, with the perchlorate ion identified at both the Phoenix and Curiosity landing sites, but detection by near-infrared (NIR) and mid-infrared (MIR) remote sensing has been limited to deposits of anhydrous chlorides. Given that oxychlorine salts can form numerous hydrated phases, we have measured their NIR and MIR reflectance spectra from 1 to 25 µm for comparison to data collected from orbiting spectrometers. Anhydrous oxychlorine salts show almost no features in the NIR, except for small bands of residual adsorbed water. However, hydrated oxychlorine salts show numerous features due to water in the NIR, specifically at ~1.4 and ~1.9 µm. Increasing the hydration state increases the depth and width of the water bands. All oxychlorine salts exhibit an additional feature at ~2.2 µm due to a Cl-O combination or overtone feature, although it is less prominent in the hydrated perchlorate salts, likely overwhelmed by the ClO4-H2O feature at 2.14 µm. All oxychlorine salts show features in the MIR due to the fundamental vibrations of Cl-O longward of ~8 µm. The NIR spectral features of hydrated oxychlorine salts are similar to other hydrated salts, especially hydrated sulfates; thus, identification from orbit may be ambiguous. However by utilizing the NIR and MIR laboratory data presented here for comparison, oxychlorine salts may be detectable by orbiting spectrometers.

Dynamic Temperature Fields under Mars Landing Sites and Implications for Supporting Microbial Life

While average temperatures on Mars may be too low to support terrestrial life-forms or aqueous liquids, diurnal peak temperatures over most of the planet can be high enough to provide for both, down to a few centimeters beneath the surface for some fraction of the time. A thermal model was applied to the Viking 1, Viking 2, Pathfinder, Spirit, and Opportunity landing sites to demonstrate the dynamic temperature fields under the surface at these well-characterized locations. A benchmark temperature of 253 K was used as a lower limit for possible metabolic activity, which corresponds to the minimum found for specific terrestrial microorganisms. Aqueous solutions of salts known to exist on Mars can provide liquid solutions well below this temperature. Thermal modeling has shown that 253 K is reached beneath the surface at diurnal peak heating for at least some parts of the year at each of these landing sites. Within 40 degrees of the equator, 253 K beneath the surface should occur for at least some fraction of the year; and, within 20 degrees , it will be seen for most of the year. However, any life-form that requires this temperature to thrive must also endure daily excursions to far colder temperatures as well as periods of the year where 253 K is never reached at all.

Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry

[1] Jarosite occurs within altered tephra from the saline‐alkaline paleolake deposits of Pliocene‐Pleistocene Olduvai Gorge, Tanzania. Zeolites (mainly phillipsite), authigenic K‐feldspar, and Mg/Fe‐smectites dominate the mineral assemblage, indicating saline‐ alkaline diagenetic conditions (pH > 9). As jarosite is ordinarily an indicator of acidic conditions on Earth and Mars, its association with such undisputed high‐pH indicators is unexpected. Of 55 altered tephra samples collected from the paleolake basin and margin deposits, eleven contained jarosite detectable by X‐ray Diffraction (XRD) (>0.15%). Mossbauer spectroscopy, Fourier Transform Infrared Reflectance (FTIR), Electron Probe Microanalysis (EPMA), X‐ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM) analyses confirm the presence and nature of the jarosite. This paper documents this occurrence and presents mechanisms that could produce this unusual and contradictory mineral assemblage. We favor a mechanism by which jarosite formed recently, perhaps as modern ground and meteoric water interacted with and oxidized paleolacustrine pyrite, providing local and temporary acidic conditions. However, local groundwater (at modern springs) has a pH > 9. In recent studies of Mars, the presence of jarosite or other Fe or Mg sulfates is often used to indicate dominantly acidic conditions. Regardless, the current study shows that jarosite can form in sediments dominated by alkaline minerals and solutions. Its coexistence with Mg/Fe smectites in particular makes it relevant to recent observations of Martian paleolakes.

Correction to “Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars”

[1] In the paper ‘‘Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars’’ by Vincent F. Chevrier et al. (Geophysical Research Letters, 36, L10202, doi:10.1029/2009GL037497, 2009) there is an error in Figure 3b of the published paper. The curve describing the GCM (Global Circulation Model) results for humidity at the Phoenix landing site is wrong. The new figure gives the right humidity variations from the GCM (the legend is the same as for the original Figure 3b). This error does not affect subsequent calculations and all the conclusions of the paper remain valid, since they were based on the measured humidity data by the Phoenix Thermal and Electrical Conductivity Prove TECP instrument rather than on the GCM model results. [2] The description of the GCM results in the paper (page 5, end of the first paragraph) should be modified accordingly. The GCM results on water vapor pressure show a similar trend than the TECP data, with high daytime values up to 7 Pa and low nighttime value, down to 3.5 10 3 Pa. However, the amplitude of pressure variations calculated form the GCM is higher than the TECP data, which range from 2 10 3 to 2 Pa. This suggests some form of interaction of water vapor with the regolith, compared to pure atmospheric processes.

ACETYLENE ON TITAN’S SURFACE

Le Mouélic R. N. Clark, L. Maltaglia, V. F. Chevrier 1 Bear Fight Institute, 22 Fiddler’s Rd, Winthrop, WA 98862 (ssingh@bearfightinstitute.com), Arkansas Center for Space and Planetary Science, University of Arkansas, Fayetteville, AR, 72701, 3 Laboratoire Astrophysique, Instrumentation et Modélisation (AIM),CNRS-UMR 7158, Université Paris-Diderot, CEA-SACLAY, 91191 Gif sur Yvette, France, 4 European Space Agency (ESA), European Space Astronomy Centre (ESAC), PO BOX 78, 28691 Villanueva de la Cañada (Madrid), Spain, 5 Laboratoire de Planétologie et Géophysique de Nantes, Université de Nantes, UMR 6112 CNRS, 2 rue de la Houssinière BP92208, Nantes Cedex 3, France, U.S. Geological Survey, Denver Federal Center, Denver, Colorado, USA.