Nasa scientists have discovered what might be the most compelling evidence yet of rocks formed in water on Mars.
The Opportunity rover has sent back pictures of rock slabs that appear to contain thin layers, say researchers.
On Earth, this feature is suggestive of sedimentary rocks that are the product of material deposited by water or wind.
The rover's alpha particle X-ray spectrometer should be able to examine the elemental chemistry of these rocks.
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