 THERMODYNAMIC ANALYSIS OF MSL'S REMS DATA: SUPPORT FOR DELIQUESCENCE DURING THE MARTIAN NIGHT.  C. A. Estévez-Galarza1,2 and E. G. Rivera-Valentín1; 1Arecibo Observatory (USRA), Arecibo, PR; 2Physics Department, University of Puerto Rico at Mayagüez, Mayagüez, PR.     Introduction: The Mars Science Laboratory (MSL) Curiosity rover successfully landed at Gale crater, Mars (4.6° S, 137.4° E) in early August 2012 and has operated for over a martian year. Among its mission prerogatives are to assess Gale crater's biological potential and investigate processes that influence habitability [1]. To further these goals, MSL included the Rover Environmental Monitoring Station (REMS) to study the local environment by measuring air and ground temperature, wind speed and direction, relative humidity, and UV flux [2].     MSL's detection of perchlorates (ClO4-) in Gale crater's regolith [3, 4], possibly in the form of hydrated calcium perchlorate (Ca(ClO4)2!nH2O) [5], allows for deliquescence, the transition from a solid crystalline salt into an aqueous brine solution [6, 7, 8], a process that may impact Gale crater's habitability. Indeed, meteorological measurements by REMS indicate surface conditions are appropriate for the formation of nighttime transient brines by deliquescence of calcium perchlorate [9]. Such transient processes may be the trigger mechanism for the observed nearby fluvial features [10], as recently hinted at by the discovery of hydrated minerals in recurring slope lineae [11].     Here we search for further evidence supporting small-scale transient liquid water processes at Gale crater by conducting a thermodynamic analysis of MSL's REMS observations. Near-surface water vapor and temperature data are examined in order to search for enthalpic evidence of liquid formation, following the methods previously applied for the Phoenix Lander's environmental measurements [12]. Results are compared with the inferred behavior of deliquescence of calcium perchlorate [9]. We find exothermically dominated reactions are occurring during the same periods when transient brines are suggested to occur, supporting the existence of present-day liquid water processes at Gale crater, Mars.     Methods:  Active near-surface processes, such as sublimation, condensation, hydration state changes, or deliquescence, result in enthalpy changes (ΔH) that can be inferred from the near-surface water vapor pressure and temperature. The inferred change in enthalpy will be the sum of all active processes and so at minimum this procedure can inform on what are the dominant ongoing near-surface processes.    We analyzed data from sols 10 - 602, which corresponds to solar longitude of Ls 156° through 115° of the next martian year. These sols were chosen to agree with [9]. For the purposes of this project, REMS measures three relevant variables: air relative humidity (RHa), air temperature (Ta), and ground temperature (Tg). We infer the water vapor pressure in the air (PH2O) from RHa by calculating the saturation vapor pressure with the measured air temperature (Psat(Ta)), using the formulation from [13]. For simplicity, we assume a well-mixed near-surface air column such that PH2O measured 1.5 m in the air is equivalent to surface water vapor pressure.    Data used is publicly available on the PDS Planetary Atmospheres Node. On the standard cadence, the REMS sensors sample daily data during the first five minutes of every hour at a 1 Hz rate. The instrument can also operate in extended mode, sampling continuously during one hour, up to 5 hours per sol. The REMS data package, including engineering and telemetry information, is delivered regularly to the PDS in Reduced Data Records (RDR). Each RDR provides a FMT file describing the name of each column, their types, size, and description. In the first level, named TELRDR, electrical values and temperatures are estimated in combination with calibration information. The ENVRDR second level processes environmental physical magnitudes, and finally in the MODRDR level some corrections are applied to eliminate the rover's influence as well as other factors that may be altering the measurement accuracy.    Enthalpy is derived from vapor pressure curves using the REMS measured surface and air temperatures along with the inferred water vapor pressure. The slope of these curves in ln(PH2O) vs 1/Tg space is related to the change in enthalpy by ΔH = -βR, where R is the ideal gas constant, and β is the least squares fit slope, which is found to 90% confidence.    Fig. 1: Water vapor pressure curve using all of the studied data with the least squares fit (black line), and prediction limits (dashed line). Colors indicate time periods where, red=0000-0300, orange=0400-0700, yellow=0800-1100, green=1200-1500, blue = 1600-1900, and violet=2000-2300.                         Fig. 2:  Enthalpy changes every five sols over the solar longitude [Ls = 90° (winter), 180° (spring), 270° (summer) and 360° (autumn)]. Colored squares represent statistically significant negative (red) and positive (black) values, while open squares are statistically neutral points, where (A) corresponds to 2100 - 0000 LST time span, and (B) corresponds to 0300 - 0600 LST time span, when deliquescence of calcium perchlorate is likely [9].     Results: In Figure 1 we show all of the analyzed data in a vapor pressure curve, which can be generally fit by a least squares fit of slope β = -1866.6 ± 12.9, implying ΔH = 15.5 ± 0.1 kJ/mol. Though the data can be fit well, several points lie outside of the prediction limits. We therefore study the enthalpic changes using data every five sols focusing on 2100 - 0000 LST and 0300 - 0600 LST, where deliquescence of calcium perchlorate is likely [9]. We find no statistically significant negative enthalpy changes (Fig. 2A) during the late evening, implying that liquid water processes are not dominant. On the other hand, at 0300 - 0600 LST we find negative enthalpy changes (Fig. 2B) occurring throughout the martian year, but mostly when the Curiosity rover moved through the sand dominated terrains such as Rocknest and Dingo Gap.     Conclusions: A thermodynamic analysis of MSL's REMS data was conducted focusing on the enthalpic changes during the martian night, as this was the time environmental conditions permitted the deliquescence of calcium perchlorate [9]. Our results suggest negative enthalpy changes dominate between 0300 until 0600 LST, which is evidence of exothermic reactions occurring during the early morning. We find that the seasonal behavior of these processes agree well with the inferred Ls ranges when deliquescence is possible on the surface of Gale crater during the early morning hours [9].    However, though REMS measurements suggest deliquescence is possible during the late evening, we do not find supporting enthalpic evidence. This does not preclude the possibility of ongoing liquid water processes during the evening, but rather provides constraints on its abundance.     We conclude that our results, in addition to MSL's detection of perchlorates in Gale crater's regolith [3, 4] and previous analysis of the REMS surface data [9], support liquid formation via deliquescence during the martian night.   Acknowledgements: This material is based upon work supported by the National Aeronautics and Space Administration under Grant No. NNX15AM42G issued through the Mars Data Analysis Program.     References: [1] J. Grotzinger et al. (2012) SSR 170 (14), 5-56.  [2] J. Gómez-Elvira et al. (2012) SSR 170 (1-4), 583-640.  [3] L. A. Leshin et al. (2013) Science, 341, 1238937.  [4] D. W. Ming et al. (2014) Science, 343 (6169), 1245267.  [5] D. P. Glavin et al. (2013) JGRP, 118, 1-19.  [6] R. V. Gough et al. (2011) EPSL, 312 (3-4), 371-377. [7] R. V. Gough et al. (2014) EPSL, 393, 73-82.  [8] D. L. Nuding et al. (2014) Icarus.  [9] F. J. Martín-Torres et al. (2015) Nature Geoscience, 8 (5), 357-361.  [10] C. M. Dundas and A. S. McEwen (2015) Icarus, 254, 213-218.  [11] Ojha et al. (2015) Nature Geoscience, 8, 829-832.  [12]  E. G. RiveraValentín and V. F. Chevrier (2015) Icarus, 253, 156-158.  [13] R. Feistel and W. Wagner (2007) Geochimica et Cosmochimica Acta, 71 (1), 36-45. A B 
