Much of the Martian surface is covered with a thick ice-rich, mantle layer that has fallen from the sky a number of times in the past.
[10] [11] The Phoenix (spacecraft) discovered water ice with made direct observations since it landed in a field of polygons.
The mantle layer lasts for a very long time before all the ice is gone because a protective lag deposit forms on the top.
After a certain amount of ice disappears from sublimation the dust stays on the top, forming the lag deposit.
This compares to a 30-meter depth over the whole planet for the water locked up in the north and south polar caps.