This image, from NASA's Mars Global Surveyor, shows a young lava flow (darker formation in lower middle part of picture) lying atop an older surface on Mars (lighter regions, upper right and lower left). The lava flow has visibly fewer impact craters than the background terrain, illustrating the general principle that crater numbers can reveal ages of surfaces. The 500-meter scale bar is approximately 500 yards long, or the length of 5 football fields.