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Curiosity, 2012: NASA says that water shaped the rocks on the left, in a photograph taken by the Mars rover Curiosity. For comparison, the agency released an image of rocks from the Earth (right).

NASA

Curiosity 2012: A wider view of the outcrop of a former streambed shows bedrock that scientists believe was likely exposed by meteorites striking the surface of Mars.

NASA

Spirit, 2010: When NASA's Spirit rover got stuck in Martian sand, it proved to be a lucky break: The spinning wheel churned up soil that provided evidence of rocks formed in the presence of water.

NASA

Opportunity, 2004: Opportunity discovered tiny mineral spheres nicknamed blueberries poking out of rocks that were likely formed by water. Researchers using Opportunity's science instruments identified them as concretions rich in the mineral hematite, deposited by water saturating the bedrock.

NASA

Pathfinder, 1997: The first mission to land a rover on Mars, Pathfinder touched down in Ares Vallis, an ancient flood plain in the planet's northern hemisphere. Among the 2.3 billion bits of data sent back by the lander and its rover, Sojourner, were 15 chemical analyses of rocks and soil, which suggested Mars had once had liquid water and a thicker atmosphere.

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Streams Of Water Once Flowed On Mars; NASA Says Photos Prove It