Related update, September 2015: From The New York Times, “Researchers studying the surface of Mars have noticed thousands of dark streaks appearing on canyon walls and hillsides during Martian summer months. Now they have found definitive evidence of salty liquid water flowing within some of these seasonal streaks.”
A time-lapse animation of Palikir Crater, below, shows how the streaks extend and darken during warmer months on Mars, then gradually fade as temperatures cool.
There’s more Mars and some Europa in the archives. Plus: First Evidence for Water Ice Clouds Found Outside Solar System.