Figure 9.25Mars Exploration Rover landing sites. (a) Spirit explored Gusev Crater, about 160 km in diameter, which is thought to have been filled with water, forming an ancient lake. A channel that might have supplied water to the crater is visible at the lower right. (b) Opportunity was sent to an area of Meridiani Planum where hematite—a mineral that often forms in water on Earth—is abundant. The image shows concentrations of hematite; the ellipse outlines the permissible landing area.