Mars Global Surveyor:
Mars' poles are covered with layered
ice caps. The layers are the result
of seasonal deposition and sublimation
of ice
(and some wind-blown sand).
Mars Polar Lander, which failed at
Martian entry or landing, was supposed to land
near the polar cap and perform geology
and climate experiments (including digging
to look for water ice).
The
Mars Phoenix mission
is the rebirth of MPL.