Mars Pictures Reveal Frozen Sea

Mars' equator has long been a location where water is believed to be existent in the subsurface. River features exist, but no sea or pack ice has ever been evidenced. Here, a High Resolution Stereo Camera on the Mars Express spacecraft has taken pictures of massive, flat features that closely resemble ice floes found in the polar regions on Earth. So, it has been postulated that a vast amount of water flowed in the area, where pack ice formed atop it, then broke up, and then the entire structure froze. Dust, most likely volcanic ash, fell on water and ice alike in the area, which is why the pack-ice is differently colored than the material surrounding it. The water source is believed to have come from the north, as is shown through valleys carved by water down to the ice-flow like features.

http://news.bbc.co.uk/2/hi/science/nature/4285119.stm