How do you get rocks to be rounded and smooth? Tumble them for a long time in water! At Hottah, Curiosity found the type of smooth and rounded gravel rocks that are produced by a few miles of tumbling in water. That means there was probably a flowing stream here, and that raises expectations for a past wet and habitable environment!
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These rocks at the Hottah outcrop resemble broken sidewalk. Look closely and you'll see rounded gravel mixed in the Martian cement. Geologists call this a sedimentary conglomerate. It is exposed bedrock made of smaller fragments cemented together by natural processes such as those that occur in streambeds.
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On Earth, cement is broken and lifted by things like tree roots and bulldozers. This sedimentary conglomerate at the Hottah site may have been tilted out of its ancient streambed by the shattering force of nearby meteor impacts!
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This golfball-size rock stands out against the smaller streambed gravel present at Hottah. Curiosity's science team estimates that the streambed that could have created Hottah's rounded rocks would have been at least knee-deep. In rover terms, water would have come up to the middle of Curiosity's wheels!
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Concrete Evidence of Water
The shaded, fan-shaped feature here is typical of areas where water has pushed debris down a slope. The black oval shows the area that was targeted for Curiosity's landing. On the ground, the rover found evidence that, yes, water did actually flow in this region!