Present-day thermal and water activity environment of the Mars Sample Return collection

Abstract

The Mars Sample Return mission intends to retrieve a sealed collection of rocks, regolith, and atmosphere sampled from Jezero Crater, Mars, by the NASA Perseverance rover mission. For all life-related research, it is necessary to evaluate water availability in the samples and on Mars. Within the first Martian year, Perseverance has acquired an estimated total mass of 355 g of rocks and regolith, and 38 μmoles of Martian atmospheric gas. Using in-situ observations acquired by the Perseverance rover, we show that the present-day environmental conditions at Jezero allow for the hydration of sulfates, chlorides, and perchlorates and the occasional formation of frost as well as a diurnal atmospheric-surface water exchange of 0.5-10 g water per m² (assuming a well-mixed atmosphere). At night, when the temperature drops below 190 K, the surface water activity can exceed 0.5, the lowest limit for cell reproduction. During the day, when the temperature is above the cell replication limit of 245 K, water activity is less than 0.02. The environmental conditions at the surface of Jezero Crater, where these samples were acquired, are incompatible with the cell replication limits currently known on Earth.

Keywords: Environment; Habitability; Jezero; Mars sample return; Temperature; Water activity.

Figure 1

(Left) Perseverance’s traverse during the first 766 sols, from the landing site, through the Crater Floor and Delta Front campaign, and towards the western delta of Jezero crater, Mars. The white line indicates the rover traverse, green dots mark the deployment sites of the First Cache, and red crosses mark the sampling sites (including the sample sealed on sol 749, acquired above the delta after the construction of the sample depot). Credit: CAMP and MRO HiRISE, The University of Arizona. (Right) Annotated landscape of the Sample Depot at Three Forks, as seen by Perseverance, with the different sealed tubes. Credits: NASA/JPL-Caltech/ASU/MSSS.

Figure 2

(Top) Annual (sol number and Ls) and night-time (LMST) variation of the Water Volume Mixing Ratio (VMR), with error bars, at Jezero crater during the first Martian year provided by the MEDA instrument at 1.45 m above the surface. Daytime relative humidity measurements (marked in gray) fall below the 2% accuracy of the MEDA relative humidity sensor and the VMR cannot be estimated. The spring equinox starts at L_s = 0°, the summer solstice at L_s = 90°, the autumnal equinox at L_s = 180°, and the winter solstice at L_s = 270°. (Bottom) Total column of H₂O abundance (in precipitable microns): TES zonally-averaged orbiter data for MY24 to MY27 (daytime, ~ 14 LMST) compared with MEDA (pre-dawn) in-situ surface measurements (lower data set) at Jezero Crater. For orbital data, the error bars are the 1-sigma standard deviation on the average that is plotted. MEDA error bars are derived from the MEDA reported uncertainty value in the relative humidity (RH) measurements and in the humidity sensor board temperature.

Figure 3

Near-surface diurnal cycle of water Volume Mixing Ratio (VMR) and air temperature (T) as a function of LMST during the sols around the sampling time of Robine. Single-column model (SCM) VMR results—dark and light blue lines—at 1.45 m and 0.84 m, respectively, are compared to MEDA values (including the uncertainty in H₂O VMR retrieval) at 1.45 m for sols 285 to 305 (Ls = 139°–149°). The SCM air temperature estimate—black line—for the same period compared with the Air Temperature Sensor (ATS) observations at 0.84 m (with 300 s moving average). The time of sealing is marked with a vertical dashed black line, whereas sunset and sunrise times are marked with a blue and orange line, respectively.

Figure 4

Diurnal variation, as a function of LMST, of the derived surface water activity concerning liquid (with a_w error bars) and measured ground temperature provided by MEDA during one full Martian year. For illustration, the environmental data are overlayed with the hydration lines of calcium and magnesium sulfates, and calcium perchlorate deliquescence and efflorescence lines. The water activity a_w is derived assuming equilibrium, from the relative humidity (RH), with respect to liquid, as a_w = RH/100, All data points to the left of the ice saturation line (RH_ice = 100%) are saturated with respect to ice and may allow frost formation. The Deliquescence RH (DRH) and hydration state lines of some perchlorates and sulfate salts are included for reference^,.

Figure 5

Modelled thermal-water activity curves experienced by the samples within the sealed tubes. The H₂O partial pressure isobars (i.e., constant water vapor pressure) for the higher and lower partial pressure reported in Table 1 are compared with the eutectic points of different salts of relevance to Mars, which may be within the sampled rocks (colored symbols), the temperature-dependent deliquescence relative humidity (DRH) for calcium perchlorate (red line), and the ice liquidus line (i.e., equilibrium between water ice and liquid brine; light yellow)^,,. For comparison, the isobar for the H₂O partial pressure values that are expected at polar regions, i.e. 0.4 Pa and 1.4 Pa, is also included.