Modern Mars may be a frozen wasteland, but it has a hot and steamy past. NASA's Opportunity rover has started examining its most significant target yet, Mars's giant Endeavour crater. The initial results suggest the structure was once exposed to hot water.
Opportunity drove for three years before reaching the 22-kilometre-wide crater. Its rim exposes rocks from the earliest epoch of Martian history, when liquid water was abundant.
"We're looking at this phase of Opportunity's exploration as a whole new mission," says David Lavery of NASA Headquarters in Washington DC.
After arriving at the crater rim in early August, Opportunity investigated a rock about 1 metre across informally named Tisdale 2. It turns out to contain large quantities of zinc, more than in any other rock examined by Opportunity or its now defunct sister rover Spirit.
On Earth, hot water is known to sometimes contain a large quantity of dissolved zinc, which it can then deposit to form zinc-rich rocks. The zinc-rich rock on Endeavour's rim hints that hot water was once present there too, possibly heated by the massive impact that formed the crater, according to rover chief scientist Steve Squyres of Cornell University in Ithaca, New York.
Bright veins
The rover team is still unsure about exactly what kind of environment the rock formed in, including whether the hot water was in liquid or vapour form.
Opportunity images have also revealed bright veins cutting through other rocks on the rim. These could be places where water flowed through cracks in the rock, leaving mineral deposits behind, although the rover has not yet measured their composition.
Before examining them, the rover is first heading along the crater rim to the north-east of Tisdale 2. Measurements from orbit suggest there are clay minerals in that direction, which require water to form.
Opportunity's handlers are relishing the prospect of exploring terrain that neither Mars rover has seen before. "The excitement level within the engineering and science teams is way up," says rover scientist Ray Arvidson of Washington University in St Louis, Missouri.
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