Green Valleys

Green Valley is a region on Mars that was chosen as the landing site of NASA's Pheonix lander. It is located at 68.35 degrees north, 233 degrees east. The valley is about 50 kilometers wide but only about 250 meters deep; either it was filled in or was never any deeper than that. The edges are not visible from the middle of the valley.
The ground within Green Valley is covered with polygonal features several meters across and roughly ten centimeters high, thought to be caused either by repeated freeze-thaw cycles or by the effects of wind-blown dust. Water ice is thought to be just below the surface.During the local winter as much as three feet of carbon dioxide ice is expected to appear on the surface.