Laboratory experiments and models of diffusive emplacement of ground ice on Mars Troy L. Hudson, 1 Oded Aharonson, 1 and Norbert Schorghofer 2 Received 20 March 2008; revised 24 July 2008; accepted 4 November 2008; published 23 January 2009. [1] Experiments demonstrate for the first time the deposition of subsurface ice directly from atmospheric water vapor under Mars surface conditions. Deposition occurs at pressures below the triple point of water and therefore in the absence of a bulk liquid phase. Significant quantities of ice are observed to deposit in porous medium interstices; the maximum filling fraction observed in our experiments is 90%, but the evidence is consistent with ice density in pore spaces asymptotically approaching 100% filling. The micromorphology of the deposited ice reveals several noteworthy characteristics including preferential early deposition at grain contact points, complete pore filling between some grains, and captured atmospheric gas bubbles. The boundary between ice- bearing and ice-free soil, the ‘‘ice table,’’ is a sharp interface, consistent with theoretical investigations of subsurface ice dynamics. Changes of surface radiative properties are shown to affect ice table morphology through their modulation of the local temperature profile. Accumulation of ice is shown to reduce the diffusive flux and thus reduce the rate of further ice deposition. Numerical models of the experiments based on diffusion physics are able to reproduce experimental ice contents if the parameterization of growth rate reduction has expected contributions in addition to reduced porosity. Several phenomena related to the evolution of subsurface ice are discussed in light of these results, and interpretations are given for a range of potential observations being made by the Phoenix Scout Lander. Citation: Hudson, T. L., O. Aharonson, and N. Schorghofer (2009), Laboratory experiments and models of diffusive emplacement of ground ice on Mars, J. Geophys. Res., 114, E01002, doi:10.1029/2008JE003149. 1. Introduction [2] Communication between subsurface ice and the at- mosphere is an important component of Mars’s water cycle history [Mellon and Jakosky , 1995; Mellon et al., 2004]. This buried ice comprises a vast reservoir which may hold a significant fraction of the total water inventory that is able to exchange with the atmosphere and polar caps [Carr, 1996]. Mellon and Jakosky [1993] have predicted that ice can accumulate in soil pores from atmospherically derived water vapor. We reproduce this phenomenon at Mars-like con- ditions of low temperature and low pressure for the first time in laboratory experiments. [3] In the present climate, ice is stable in the shallow subsurface in latitudes poleward of about 60 degrees as predicted by models [e.g., Leighton and Murray , 1966; Mellon and Jakosky , 1993; Schorghofer and Aharonson, 2005], and confirmed by observations [e.g., Boynton et al., 2002]. It is hypothesized that the stability regions occupied higher latitudes when Mars’s obliquity was low, and were global when the obliquity was high [Mellon and Jakosky , 1995; Head et al., 2003]. The close agreement between present-day climate and model predictions is taken as evidence that the majority of the current subsurface ice is at or near its diffusive equilibrium depth with respect to the mean annual atmospheric vapor content. The observed hydrogen abundances in the middle to high latitudes, con- verted to water equivalent hydrogen, indicate that a signif- icant percentage (at least 60%) of the mass of the upper meter of the regolith is ice [Prettyman et al., 2004]. Litvak et al. [2006] and Feldman et al. [2007], however, suggest that the ice content of the ice-rich layer depends on latitude, and the ice content is consistent with the existence of interstitial ice in some latitude range. While some of this hydrogen may indicate pure ice layers deposited as precip- itation and subsequently buried, much of the buried ice may have been derived directly from atmospheric water vapor [Schorghofer, 2007]. [4] As the orbital and axial parameters of the planet change, the ice responds to changing temperatures and humidities, and the time scale of this response is modulated by the diffusivity of the regolith [Mellon and Jakosky , 1995; Hudson et al., 2007]. Diffusion coefficients for a variety of materials, including salt crusts, sand, dust, and mixtures with a wide particle size range, have been measured under simulated Mars surface conditions and have been found to lie within a range of 0.2–5 cm 2 s 1 [Hudson et al., 2007; JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E01002, doi:10.1029/2008JE003149, 2009 Click Here for Full Articl e 1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA. 2 Institute for Astronomy, University of Hawai’‘i at Ma noa, Honolulu, Hawaii, USA. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JE003149$09.00 E01002 1 of 21