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The US space agency's robotic rover Opportunity has found initial evidence that rocks at a new Martian crater it is exploring were deposited in water.
The rover has conducted tests on a 30-cm-long rock called Lion Stone, which was probably tossed out by the impact that excavated Endurance Crater.
Lion Stone is peppered with spherical "concretions", exhibits fine layering and is rich in sulphur, NASA said.
The concretions probably formed when minerals precipitated out of water.
These tiny spheres were found in the rock outcrops and soil in Eagle Crater, where Opportunity touched down.
Layering is characteristic of sedimentary rocks, which can be formed by water or wind. Testing showed the layered rocks at Eagle Crater were deposited in water.
Only further investigation will determine which was responsible for layering seen in rocks at the 130m-wide Endurance Crater.
But the presence of sulphur in Lion Stone is a promising sign to mission scientists that the rocks Opportunity is now examining were deposited in water.
On Earth, rocks with as much salt as these Mars rocks either have formed in water or, after formation, have been highly altered by long exposure to water.