Special Regions on Mars are defined as environments able to host liquid water that simultaneously meets certain temperature and water activity requirements that allow known terrestrial organisms to replicate1,2 and therefore could be habitable. Such regions would be of concern for planetary protection policies owing to the potential for forward contamination (biological contamination from Earth). Pure liquid water is unstable on the Martian surface3,4 but brines may be present3,5. Experimental work has shown that brines persist beyond their predicted stability region, leading to metastable liquids6,7,8. 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 thought9,10. 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 its shallow subsurface (a few centimetres 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 Region requirements, reducing the risk of forward contamination and easing threats related to the exploration of the Martian surface.
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The data that support the figures within this paper are available at https://doi.org/10.6084/m9.figshare.11907984 or from the corresponding author upon reasonable request. The newly recalibrated environmental data from the Thermal and Electrical Conductivity Probe on the Phoenix lander are available on the NASA Planetary Data System Geosciences Node (https://pds-geosciences.wustl.edu/missions/phoenix/martinez.htm).
The MarsWRF (Mars Weather Research and Forecasting) GCM is available from A.S. upon reasonable request. Other software used in this work is available from E.G.R.-V. upon reasonable request.
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This material is based on work funded by NASA under grant number 80NSSC17K0742 issued through the Habitable Worlds program and partially under grant number NNX15AM42G issued through the Mars Data Analysis program. We thank C. Lee for help with the microphysics scheme in MarsWRF and J. Hanley for feedback. The Lunar and Planetary Institute (LPI) is operated by Universities Space Research Association (USRA) under a cooperative agreement with the Science Mission Directorate of NASA. This work is LPI Contribution No. 2322.
The authors declare that they have no competing financial interests.
Peer review information Nature Astronomy thanks Erik Fischer, John Hallsworth and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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In Fig. 1, the Mars-relevant pairings of surface temperature and atmospheric water vapour pressure are contained between the cyan lines. Here we show the frequency (in colour) of each combination of surface temperature and water vapour pressure predicted by MarsWRF in support of the presented analysis in Figure 1. Frequency here is the log base 10 of the number of hours across the surface of Mars at a 5∘ × 5∘ latitude/longitude grid and one-hour temporal resolution. For the purposes of counting, temperature and water vapour pressure were binned in increments of 1 K and 1 Pa. Here the cyan lines from Fig. 1 are in black.
As per Fig. 1, the maximum water activity of a brine that is thermodynamically stable on Mars is 0.66. This would imply that a brine with a eutectic water activity higher than this value would not readily form on present-day Mars. Another way of seeing this is in the phase diagram. Following the typical phase diagram (relative humidity vs temperature), here we show the ice line in blue (that is, RHice = 100%), the temperature-dependent deliquescence relative humidity (DRH) for calcium and magnesium perchlorate, as well as the sodium chlorate hydrate line in shades of green, from light to dark respectively. In dashed black lines, we plot two isobars for water vapour pressure, showing the typical maximum water vapour pressure measured on Mars by the Mars Science Laboratory rover and Phoenix lander, as well as the maximum surface water vapour pressure predicted by the MarsWRF model. The hyperarid conditions on Mars would not permit a salt with a eutectic water activity higher than 0.66 to form (that is, eutectic temperature > 230 K). For example, at a eutectic temperature of 236 K, sodium chlorate would not form a brine because there is insufficient water vapour in the Martian atmosphere.
In Figures 2 and 3 we showed results for the (meta)stability of brines formed by the deliquescence of calcium and magnesium perchlorate. Here we show the thermodynamically stable (area shaded in dark grey) and metastable (area shaded in light grey) regions of a calcium perchlorate brine in the typical phase diagram. A brine can form between the ice line (that is, RHice = 100%), here the blue solid line, and the temperature-dependent deliquescence relative humidity (DRH), here shown as the green solid line. However, experiments have shown that metastable solutions exist beyond RHice = 100%, up to RHice = 145%, here shown as the orange solid line. Furthermore, although thermodynamically a solution should efflorescence once conditions fall below the DRH, experimental work has shown that solutions persist until much lower relative humidities are reached (that is, the efflorescence relative humidity, here the red solid line). Non-shaded regions on this plot are conditions that would not permit for stable or metastable solutions of calcium perchlorate. The eutectic temperature (~ 198 K) is the black dashed line.
Extended Data Fig. 4 Maximum achievable brine temperature and corresponding water activity of calcium perchlorate brines formed through deliquescence.
To better resolve the habitability of (meta)stable calcium perchlorate brines, here we plot (a) the maximum temperature and (b) corresponding water activity of resulting brines. The latitude range is restricted to non-polar regions. The background is the grey-scaled shaded relief map of Mars based on MOLA data.
Extended Data Fig. 5 Measured surface and modeled subsurface conditions at the Phoenix landing site.
Using newly recalibrated environmental data from the Thermal and Electrical Conductivity Probe on the Phoenix lander along with a model of the subsurface, we studied the potential to form liquids at the Phoenix landing site. In a, we plot the Phoenix measured temperature and relative humidity with respect to liquid (purple points) on the phase diagram of calcium perchlorate (following the colour code in Supplementary Fig. 3). As can be seen, several measured conditions are within the liquid stability region of calcium perchlorate brines. Furthermore, in b we plot the simulated subsurface conditions during which a brine is (meta)stable, assuming an ice table depth of 10 cm. The model’s surface predictions for both temperature and relative humidity were validated against the Thermal and Electrical Conductivity Probemeasurements. In purple scale, from light to dark, we show results for 2, 5 and 8 cm.
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Rivera-Valentín, E.G., Chevrier, V.F., Soto, A. et al. Distribution and habitability of (meta)stable brines on present-day Mars. Nat Astron 4, 756–761 (2020). https://doi.org/10.1038/s41550-020-1080-9
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