Another thing we thought was water on Mars actually isn't water

Back in 2011, NASA scientists announced they had spotted compelling evidence that water sometimes still flowed on Mars. Now, researchers are backpedaling on these watery conclusions. 

To NASA's credit, the 2011 evidence looked quite convincing. Images captured by the Mars Reconnaissance Orbiter — a NASA satellite orbiting Mars — showed telltale dark streaks running down various mountains, valleys, and craters on Mars. They look strikingly similar to features formed on Earth's surface by flowing water.

But, according to a new study in the journal Nature Geoscience, it turns out these dark streaks are made mostly of "granular flow" — sand and perhaps rocks falling downhill — rather than water flowing down valleys during the warmer Martian summers.  Read more...

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