Mars as it might have appeared more than 2 billion years ago, tipped 50 degrees from its orientation today and with an ocean filling the lowland basin that today occupies the north polar region. The largest feature on the planet, the Tharsis bulge, is at center of left image. In right image, today's polar ice cap is shown in the middle of the ocean to indicate where the pole is today. The pictures were generated using Viking Orbiter images and topographic data from the Mars Orbiter Laser Altimeter on board the Mars Global Surveyor spacecraft. (Taylor Perron/UC Berkeley)