Travis S. Altheide

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.

The possible origin and persistence of life on Enceladus and detection of biomarkers in the plume.

The jets of icy particles and water vapor issuing from the south pole of Enceladus are evidence for activity driven by some geophysical energy source. The vapor has also been shown to contain simple organic compounds, and the south polar terrain is bathed in excess heat coming from below. The source of the ice and vapor, and the mechanisms that accelerate the material into space, remain obscure. However, it is possible that a liquid water environment exists beneath the south polar cap, which may be conducive to life. Several theories for the origin of life on Earth would apply to Enceladus. These are (1) origin in an organic-rich mixture, (2) origin in the redox gradient of a submarine vent, and (3) panspermia. There are three microbial ecosystems on Earth that do not rely on sunlight, oxygen, or organics produced at the surface and, thus, provide analogues for possible ecologies on Enceladus. Two of these ecosystems are found deep in volcanic rock, and the primary productivity is based on the consumption by methanogens of hydrogen produced by rock reactions with water. The third ecosystem is found deep below the surface in South Africa and is based on sulfur-reducing bacteria consuming hydrogen and sulfate, both of which are ultimately produced by radioactive decay. Methane has been detected in the plume of Enceladus and may be biological in origin. An indicator of biological origin may be the ratio of non-methane hydrocarbons to methane, which is very low (0.001) for biological sources but is higher (0.1-0.01) for nonbiological sources. Thus, Cassinis instruments may detect plausible evidence for life by analysis of hydrocarbons in the plume during close encounters.

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.

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.

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.