 EXPERIMENTAL INVESTIGATION OF DIURNAL WATER VAPOR CYCLES AT THE MARTIAN SURFACE AND IMPLICATIONS FOR DELIQUESCENCE-DRIVEN BRINE FORMATION. R A. Slank, H. N. Farris, V. F. Chevrier, Arkansas Center for Space and Planetary Science, University of Arkansas, 346 Arkansas Ave., Fayetteville, AR 72701, rslank@uark.edu.   Introduction:  In 2001, Mars Odyssey mapped hydrogen distribution across the Martian surface, leading scientists to discover large quantities of water ice bound as ice caps in the polar regions and as well as buried just beneath the surface in the mid to high latitudes [1].  Variability in the parameters of Mars' orbit allows for mass transport of seasonal water vapor from the equatorial regions to the poles.  The stability of this water ice on Mars is largely due to the diffusive and thermal properties of the regolith.  While diffusion can explain this process over large timescales, it is the dynamics of water vapor for small timescales (day to year), which remain unknown.  Local effects, mostly related to phase changes of water, including adsorption [2], formation of liquid [3], interaction with hygroscopic salts, like perchlorates [4, 5, 6] through hydration, deliquescence, and dissolution [3, 6, 7], the presence of an ice layer [2], and the presence of certain minerals, like ferrihydrite (Fig. 1) can all be attributed to this lack of clarity.  Real-time experiments are to be conducted in a simulation chamber that can mimic Martian pressure, temperature, and atmospheric composition. These experiments are vital in understanding how the water vapor diurnal cycle can account for the creation of liquid water at the surface of Mars.  Figure 1: Comparison between top: the distribution of nanophase ferric oxides as seen by Mars Express OMEGA (high abundance: white, low: blue) [8] and bottom: the humidity in the atmosphere, ranging from 0 (blue) to ~30 (red), as observed by MGS-TES in the equatorial regions [9, 10].  The similarity of both maps suggests the ferric oxides abundant in the regolith could control the atmospheric humidity through adsorption and desorption [11].  Chamber Specifications:  The Ares chamber (Fig. 1) is approximately 3 ft tall and 2 ft wide and made of stainless steel.  The lower portion is surrounded by coils for cooling and then insulated with ceramic.  The lid is raised and lowered by a motor and bolted down to the base for a vacuum seal during experiments.  Chamber Instrumentation:  The chamber (Fig. 2) has an iR Photonics fiber optic cable (1 to 5μm) connected to a Nicolet 6700 FTIR Spectrometer, an array of six Vaisala hygrometers (identical to those onboard MSL Curiosity) wired to a Fuji Electric digital output, a Logitech webcam, a Dino-lite digital microscope (30 μm per pixel), five Omega thermocouples, and an LED light.  All the feedthrough ports for the equipment were replaced with the proper, stainless steel flanges for low-pressure experiments.  This eliminated the air leaks present at various access points around the chamber, allowing us to reach lower pressures than with the previous system.  The chamber also has an input and output connected to a Sterling chiller which pumps a glycol mixture through the cooling coils surrounding the chamber.  Lastly, there are two feed-through ports, which accommodate a vacuum pump, to achieve a desired pressure range, and gas exchange for replacing the atmosphere with N2 or CO2 (Fig. 3).   Figure 2: Exterior view of the Ares Chamber    Figure 3: Schematic of exterior of the Ares Chamber   Methodology: A series of preliminary experiments were performed in the chamber, before the modifications were made, at ambient temperature (~24°C) and pressure (1 bar).  A sample of Martian soil simulant (varying masses and types) and a hygroscopic salt (varying masses) were mixed together and placed in the chamber along with a humidity buffer (saturated salt solution) to control the relative humidity in the chamber.  The chamber was then closed and the sample was observed using an FTIR Spectrometer (1 to 5 μm).  For this abstract, we focus on water vapor transfer between the atmosphere and a column of regolith: JSC Mars 1 with varying weight % of Ca(ClO4)2.  Twelve experiments will be run (Table 1) to understand the range over which deliquescence may account for brine formation and to isolate the effects of variables like sample depth, salt concentration, presence and depth of an ice layer, and localized effects of minerals like nanophase ferric oxides.  This process is heavily dependent on diurnal fluctuations in relative humidity and temperature of the environment.  An array of thermocouples and hygrometers will enable us to observe a gradient of temperatures and relative humidities over the height of the column.  In addition, by sublimating ice below a column of regolith at depths corresponding to observations by Phoenix (~5-15 cm) [12], we seek to understand the role that heat transfer and phase changes of water play in Martian soils.       Experiment # Regolith Type Salt Weight % Depth of Sample (cm) 1 JSC Mars 1 1 2  2 JSC Mars 1 w/ nanophase ferric oxide layer 1 2  3 JSC Mars 1 w/ ice layer 1 2 4 JSC Mars 1 1 10  5 JSC Mars 1 w/ nanophase ferric oxide layer 1 10  6 JSC Mars 1 w/ ice layer 1 10 7 JSC Mars 1 5 2  8 JSC Mars 1 w/ nanophase ferric oxide layer 5 2  9 JSC Mars 1 w/ ice layer 5 2 10 JSC Mars 1 5 10  11 JSC Mars 1 w/ nanophase ferric oxide layer 5 10  12 JSC Mars 1 w/ ice layer 5 10 Table 1: Table of planned experiments.  Experiments will be run at 2 different depths and will have 2 different salt weight percents.      Conclusion: It is well known that perchlorates can deliquesce at the surface of Mars, accounting for small amounts of transient, metastable liquid brines.  Exposing complex subtleties in diffusion, adsorption/desorption cycles, and deliquescence processes on Mars, specifically the limits to which this liquid formation is possible has important implications for liquid stability and habitability at equatorial regions (MSL), future missions to Mars, and the continuing search for liquid water.   References:  [1] Boynton, W.V. et al. (2002) Science 297. [2] Chevrier V. F. et. al (2008) Icarus, 196, 459-476. [3] Sears D. W. G. et al. (2005) GRL, 32. [4] Hecht, S. P. et al. (2009) Science, 5936, 64-67. [5] Nuding, D. L. et al. (2014) Icarus, 243, 420-428. [6] Chevrier V. F. et. al (2009) GRL, 36. [7] Chevrier V. F. et. al (2008) GRL, 35. [8] Poulet, F. et al. (2007) JGR, 112. [9] Jakosky, B. M. et al. (2005) Icarus, 175, 58-67. [10] Smith, M. D. (2002) JGR, 107. [11] Pommerol, A. et al.  (2009) Icarus, 204, 114-136. [12] Mellon M. T. et. al (2004) Icarus, 169, 324-340. 
