This image of Mars is a 4-frame color mosaic taken by Viking Orbiter 1. This mosaic covers some of the oldest terrain on the Martian surface, eastern Noachis Terra. This image is taken looking south across Noachis Terra. Subtle parallel valleys can be seen at top center. These represent faults that opened up following the Hellas Basin impact. This enormous impact ejected so much material that the crust over 2000 km away slumped inwards to fill the hole. Water ice clouds can also be seen in this picture. A few, especially in the top left corner, form a distinctive wavelike pattern. These are called gravity waves, which form after air is forced to rise over the rims of craters. These wakes form on the lee side of craters, giving us a clue as to the direction of air flow in this region.