Figure 9.7A comparison of the surfaces of Earth, Mars, and Venus, all at the same scale. The topography of Mars, which shows the greatest range, was measured in 1998 and 1999 by a laser altimeter aboard the orbiting Mars Global Surveyor spacecraft. That of Venus, which shows the smallest range, was measured from 1990 to 1993 by a radar altimeter aboard the orbiting Magellan spacecraft. Earth’s topography, which is intermediate in range and dominated by continents and oceans, has been synthesized from altimeter measurements of the land surface, ship-based measurements of ocean depth, and gravity-field measurements of the seafloor surface from Earth-orbiting spacecraft.