Here's a 1024x768 wallpaper of a region of Mars called Mangala Valles taken by the ESA's Mars Express spacecraft. The region is located on the southwest Tharsis Bulge, and it looks like it was significantly shaped by liquid water some time in the distant past. Similar regions are here on Earth, where volcanic activity causes underground water and ice to suddenly burst forth in a catastrophic flood. In addition to the large outflow channels, there are smaller ones off to the side that could have been caused by rainfall.