2002 Mars Odyssey's planet wide survey had found hydrogen rich soil
near the Martain poles. The only plausible way of storing hydrogen
in the quantities that the data suggested is as underground water-ice.
of 2003, the orbiters were joined by an array of international landers
and rovers - 'Spirit', 'Opportunity' and 'Beagle 2'. In May 2004,
the Opportunity rover found the first direct evidence, in the layered
rock sediments, that liquid water once existed at Endurance Crater,
enough water to have sustained life as we know it. Later it's ‘twin’
rover ‘'Spirit' found minerals in the rocky basin of Gusev
Crater which can only form in the presence of liquid water.