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...