But a later mission, Mariner 9, showed in 1971 that the surface had extensive sand dunes, massive craters and huge lava flows.

ECONOMIST: Martian chronicles

The channels remained hidden because the once volcanically active region had been filled in by lava flows hundreds of millions years ago.

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The crater floor is made of relatively new volcanic rock, formed by lava flows that would have covered any evidence of ancient bodies of water.

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Lava flows extend for hundreds of kilometres across its surface.

ECONOMIST: Planetary science

First, he found dinosaur nests that had been built between lava flows 10-15 metres thick evidence that the animals were coping well with the volcanic activity rather than being weakened by it.

ECONOMIST: Mass extinctions

But Crawford and his colleagues believe the answer is to search for clues in relatively recent lava flows, because once cooled and covered over, they could preserve the mark of cosmic rays from the time they were exposed.

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