Mars Express' mooch around the red planet has yielded another set of snaps it felt worthy of adding to its Facebook wall. It's spent some time looking at the Reull Valliss, a dry river that runs for the better part of 932 miles (1,500km) through the Promethei Terra highlands -- and in some places is over 4.3 miles (7km) wide and nearly 1,000 feet (300m deep). Scientists think that at some point, there was plenty of water in the area, as the landscape shows signs of glaciatio
