 NEAR SURFACE WATER VAPOR PRESSURE AND RELATIVE HUMIDITY ON MARS: NEW VALUES OBTAINED FROM THE PHOENIX MASS SPECTROMETER.  G. M. Martinez1, N. O. Renno1, J. H. Hoffman2, H. M. Elliott1, and E. Fischer1. 1Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143, USA, 2University of Texas at Dallas, Richardson, Texas, USA.    Introduction: Liquid water is a basic ingredient of  life as we know it. Therefore, in order to understand the habitability of Mars we must first understand the behavior of water on the red planet.  Liquid water can theoretically be found on Mars [1] in five different forms: (i) as pure liquid water on the surface of Mars in transient events, (ii) as monolayers of undercooled liquid interfacial water [2] on the surface, shallow subsurface (≤1 m), and deep subsurface (≥100 m), (iii) as liquid brines both on the surface  [3] and the shallow and deep subsurface, (iv) as melt water formed by a solid-state greenhouse effect in the shallow subsurface, and (v) as groundwater or aquifers (possibly of liquid brines) in the deep subsurface.  The formation of the above mentioned types of liquid water strongly depends on near surface values of water vapor pressure (e) and relative humidity (rh).       Here, we show new values for the near surface rh and e at the Phoenix landing site from the analysis of the Phoenix Mass Spectrometer [4] measurements.       Summary of near surface relative humidity measurements: Direct measurements of the near surface rh have previously been performed at polar latitudes by the TECP instrument aboard the Phoenix lander  [5], and currently at equatorial latitudes by the REMS instrument aboard Curiosity [6]. Unfortunately, the TECP rh measurements were pulled from the NASA Planetary Data System (PDS) due to uncertainties in the original calibration. To date, recalibrated TECP measurements, shown in [7], have not yet been made available in NASA PDS, possibly due to further concerns with the new calibration.  We have obtained independent values of rh and e from the Phoenix Mass Spectrometer. Such independent estimations are key to the understanding of the near surface water content and thus the water cycle at polar latitudes.       The Thermal Evolved Gas Analyzer: The thermal evolved gas analyzer (TEGA) instrument aboard Phoenix consists of two components, a set of eight ovens that heat samples of the ice-soil mixtures from the trench to release imbedded gases, and a Mass Spectrometer that serves as the analysis tool for the evolved gases.   Atmospheric measurements performed by the Phoenix Mass Spectrometer: In addition to soil experiments, the mass spectrometer conducted four sets of atmospheric measurements, which we used to determine the volume mixing ratio (VMR) of water vapor. On Sols 9, 11 and 16 measurements were made during the day; on Sol 12 measurements were made during the night.  We note that on Sol 16, measurements were made with the gas manifold heated to 35o C; the manifold was kept at ambient temperature during all other measurements. On each Sol, the duration of the measurements was approximately 1.7 hours.       The water vapor pressure can be obtained from the volume mixing ratio via e=VMR*P, where P is the atmospheric pressure. In addition, the relative humidity can be obtained via rh=e/es(T), where es is the saturation vapor pressure (known analytically) and T is the temperature.  Results:  Preliminary values of the water vapor pressure at the Phoenix landing site on Sols 9, 11, 12 and 16 show values around 10 Pa during the daytime, and 1 Pa at night. Such values are unexpectedly high, as previous measurements done by the TECP are between 0.01-1.6 Pa based on its first calibration [5], and between 0.01-0.16 Pa based on its second calibration  [7].  The implications of near surface values of e in the range 1-10 Pa are extremely relevant. On one hand, the rh could reach higher values than previously expected, which supports the formation of liquid brines on the surface and in the shallow subsurface. On the other hand, in order to match orbital measurements of the water column abundance with our values of the nearsurface e, it can no longer be assumed that the vertical profile of the water vapor mixing ratio is constant in the first few kilometers. Instead, there would be an excess of water vapor near the surface which points at the regolith as a key agent in controlling the diurnal cycle of water.  References:  [1] Martinez G. M. et al. (2013) Space Sci. Rev. (in press). [2] Mohlmann D. (2008) Icarus, 195(1), 131139. [3] Martinez G. M. et al (2012) Icarus, 221, 816830. [4] Hoffman J. H. (2008) J. Am. Soc. Mass Spectrom., 19, 1377-1383. [5] Zent A. P. et al. (2010) J. Geophys. Res., 115, 0014.  [6] Gomez-Elvira J. et al. (2012)  Space Sci. Rev., 1-58.  [7] Zent A. P. et al. (2012) 43rd LPSC, Abstract#2846.  
