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Special Regions on Mars are defined as environments able to host liquid water that meets certain temperature and water activity requirements that allow known terrestrial organisms to replicate, and therefore could be habitable. Such regions would be a concern for planetary protection policies owing to the potential for forward contamination (biological contamination from Earth). Pure liquid water is unstable on the Martian surface, but brines may be present. Experimental work has shown that brines persist beyond their predicted stability region, leading to metastable liquids. Here we show that (meta)stable brines can form and persist from the equator to high latitudes on the surface of Mars for a few percent of the year for up to six consecutive hours, a broader range than previously thought. However, only the lowest eutectic solutions can form, leading to brines with temperatures of less than 225 K. Our results indicate that (meta)stable brines on the Martian surface and shallow subsurface (a few centimeters deep) are not habitable because their water activities and temperatures fall outside the known tolerances for terrestrial life. Furthermore, (meta)stable brines do not meet the Special Regions requirements, reducing the risk for forward contamination and easing threats related to the exploration of the Martian surface.

Saturn's moon Titan is the only extraterrestrial body known to host stable lakes and a hydrological cycle. Titan's lakes predominantly contain liquid methane, ethane, and nitrogen, with methane evaporation driving its hydrological cycle. Molecular interactions between these three species lead to non-ideal behavior that causes Titan's lakes to behave differently than Earth's lakes. Here, we numerically investigate how methane evaporation and non-ideal interactions affect the physical properties, structure, dynamics, and evolution of shallow lakes on Titan. We find that, under certain temperature regimes, methane-rich mixtures are denser than relatively ethane-rich mixtures. This allows methane evaporation to stratify Titan's lakes into ethane-rich upper layers and methane-rich lower layers, separated by a strong compositional gradient. At temperatures above 86K, lakes remain well-mixed and unstratified. Between 84 and 86K, lakes can stratify episodically. Below 84K, lakes permanently stratify, and develop very methane-depleted epilimnia. Despite small seasonal and diurnal deviations (<5K) from typical surface temperatures, Titan's rain-filled ephemeral lakes and "phantom lakes" may nevertheless experience significantly larger temperature fluctuations, resulting in polymictic or even meromictic stratification, which may trigger ethane ice precipitation.

We have studied the pressure and temperature dependence of solubility of nitrogen in methane and ethane using vapor-liquid equilibrium simulations of binary mixtures of nitrogen in methane and ethane for a range of pressures between 1.5 atm and 3.5 atm and temperatures between 90 K and 110 K, thermodynamic conditions that may exist on the Saturn's moon, Titan. We find that the solubility of nitrogen in methane increases linearly with pressure while the solubility of nitrogen in ethane increases exponentially with pressure at temperature 90 K. Solubility of nitrogen in both methane and ethane exhibits an exponential decrease with temperature at a pressure of 3 atm. The solubility of nitrogen in methane is much larger compared to that in ethane in the range of pressure and temperature studied here. Our results are in quantitative agreement with the available experimental measurements of the solubility of nitrogen in methane and ethane. Furthermore, we find that the surface adsorption of nitrogen increases with increasing pressure at temperature 90 K, while the adsorption free energy increases with increasing pressure. Moreover, we find that the surface tension decreases linearly with pressure for both nitrogen-methane and nitrogen-ethane systems. The rate of decrease of surface tension with pressure for nitrogen-ethane system is much larger as compared to the nitrogen-methane system. Finally, we find that the absorption of a nitrogen molecule into the liquid-phase from the interface is diffusive and does not involve any appreciable energy barrier. Our results suggest that homogeneous nucleation of bubbles is unlikely on Titan and the bubble formation in the lakes on Titan must arise from heterogeneous nucleation of bubbles., Comment: 9 pages, 8 figures. arXiv admin note: text overlap with arXiv:1910.13343

Titan has an abundance of lakes and seas, as confirmed by Cassini. Major components of these liquid bodies include methane ($CH_4$) and ethane ($C_2H_6$); however, evidence indicates that minor components such as ethylene ($C_2H_4$) may also exist in the lakes. As the lake levels drop, 5 $\mu m$-bright deposits, resembling evaporite deposits on earth, are left behind. Here, we provide saturation values, evaporation rates, and constraints on ethylene evaporite formation by using a Titan simulation chamber capable of reproducing Titan surface conditions (89-94 K, 1.5 bar $N_2$). Experimental samples were analyzed using Fourier transform infrared spectroscopy, mass, and temperature readings. Ethylene evaporites form more quickly in a methane solvent than in an ethane solvent or in a mixture of methane/ethane. We measured an average evaporation rate of $(2.8 \pm 0.3) \times 10^{-4} kg \; m^{-2} \; s^{-1}$ for methane and an average upper limit evaporation rate of less than $5.5 \times 10^{-6} kg \; m^{-2} \; s^{-1}$ for ethane. Additionally, we observed red shifts in ethylene absorption bands at 1.630 and 2.121 $\mu m$ and the persistence of a methane band at 1.666 $\mu m$., Comment: 33 pages, 12 figures, 2 tables, published in ACS Earth and Space Chemistry

We have studied the temperature dependence of the solubility of nitrogen in methane, ethane, and mixtures of methane and ethane using vapor-liquid equilibrium simulations of binary and ternary mixtures of nitrogen, methane and ethane for a range of temperatures between 90K and 110K at a pressure of 1.5 atm, thermodynamic conditions that may exist on the Saturn's giant moon, Titan. We find that --(i) the solubility of nitrogen in both methane and ethane decreases with increasing temperature; (ii) the solubility of nitrogen in methane is much larger compared to that in ethane at low temperatures, (iii) solubility of nitrogen in a ternary mixture of methane, ethane, and nitrogen increases upon increasing mole-fraction of methane. Our results are in quantitative agreement with the recent experimental measurement of the solubility of nitrogen in methane, ethane, and a mixture of methane and ethane. Furthermore, we find a strong temperature-dependent surface adsorption of nitrogen at the nitrogen-hydrocarbon interface, previously unknown. We have also investigated surface tension of the gas-liquid interface and find that it decreases upon decreasing temperature. Moreover, we find that the interfacial layer of adsorbed nitrogen and ethane show a preferential orientational ordering at the interface., Comment: 11 pages, 7 figures

The Mars Science Laboratory (MSL) Rover Environmental Monitoring Station (REMS) has now made continuous in-situ meteorological measurements for several martian years at Gale crater, Mars. Of importance in the search for liquid formation are REMS' measurements of ground temperature and in-air measurements of temperature and relative humidity, which is with respect to ice. Such data can constrain the surface and subsurface stability of brines. Here we use updated calibrations to REMS data and consistent relative humidity comparisons (i.e., w.r.t. liquid vs w.r.t. ice) to investigate the potential formation of surface and subsurface liquids throughout MSL's traverse. We specifically study the potential for the deliquescence of calcium perchlorate. Our data analysis suggests that surface brine formation is not favored within the first 1648 sols as there are only two times (sols 1232 and 1311) when humidity-temperature conditions were within error consistent with a liquid phase. On the other hand, modeling of the subsurface environment would support brine production in the shallow subsurface. Indeed, we find that the shallow subsurface for terrains with low thermal inertia ($\Gamma\lesssim 300$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) may be occasionally favorable to brine formation through deliquescence. Terrains with $\Gamma\lesssim175$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ and albedos of $\gtrsim0.25$ are the most apt to subsurface brine formation. Should brines form, they would occur around Ls 100$^{\circ}$. Their predicted properties would not meet the Special nor Uncertain Region requirements, as such they would not be potential habitable environments to life as we know it., Comment: 25 pages, 4 figures