Tom Brosz wrote:
20 and 75 cms seems to be a relatively thin layer of ice-free dirt. What keeps the ice from simply sublimating through the porous ground? Or isn't it that porous?

Alex writes:
From the article:
"The ice-rich regions are found only where it is cold. This is consistent with calculations that predicted where ice should be stable beneath the Martian surface."

Note also that the residual north polar cap is thought to consist almost entirely of H2O ice. It sits on the surface, not beneath a porous matrix, yet is does not sublimate. Not only pressure, but temperature is important as well, hence the zones of stability for surface and near-surface ice. In other words, the ice must lie at greater depths closer to the equator.

Alex R. Blackwell
University of Hawaii