Thanks, Nick. The reason I ask about the composition of the layers is elsewhere (usenet) several methods of extraction have been suggested.

I suggested a dome and using solar energy (read cloche). I didn't expect it to be very efficient, but over time a robotic system could electrolysis enough hydrogen from water to do a Mars Direct type mission with.

Paul Dietz suggested something similar using microwaves to heat the soil. "a system using a vapor collecting hood + a microwave
heater"

Dr John Stockton proposed: " Drill a 1 cm well some few metres into the ice-bearing layer and drop a heater in. After a few days, lower an outlet tube into the middle of
the cavity, and pump local air down outside it to push the water up the tube. Cast it into ingots within a lightly-pressurised bag."

I think Stockton's method would work with a vapor collection hood...

--Chris Vancil