The diffusion coefficient of water vapor in unconsolidated porous media is measured
for various soil simulants at Mars-like pressures and subzero temperatures.
An experimental chamber which simultaneously reproduces a low-pressure,
low-temperature, and low-humidity environment is used to monitor water flux from an ice
source through a porous diffusion barrier. Experiments are performed on four types of
simulants: 40–70 µm glass beads, sintered glass filter disks, 1–3 µm dust (both loose and
packed), and JSC Mars–1. A theoretical framework is presented that applies to
environments that are not necessarily isothermal or isobaric. For most of our samples, we
find diffusion coefficients in the range of 2.8 to 5.4 cm^2 s^-1 at 600 Pascal and 260 K. This
range becomes 1.9–4.7 cm^2 s^-1 when extrapolated to a Mars-like temperature of 200 K.
Our preferred value for JSC Mars–1 at 600 Pa and 200 K is 3.7 ± 0.5 cm^2 s^-1. The
tortuosities of the glass beads is about 1.8. Packed dust displays a lower mean diffusion
coefficient of 0.38 ± 0.26 cm^2 s^-1, which can be attributed to transition to the Knudsen
regime where molecular collisions with the pore walls dominate. Values for the diffusion
coefficient and the variation of the diffusion coefficient with pressure are well matched by
existing models. The survival of shallow subsurface ice on Mars and the providence of
diffusion barriers are considered in light of these measurements
