Figure 6-37 Major Impacts on Mercury (a) Caloris Basin. Messenger sent back this view of a huge impact basin on Mercury’s equator. The basin is the entire orange-colored portion of the figure. This image has been color enhanced to show the different surface compositions of Mercury. The orange regions around the edge of the basin are believed to be volcanic features. (b) Only about half the Caloris Basin appears here because it happened to lie on the terminator when the Mariner 10 spacecraft sped past the planet. Although the center of the impact basin is hidden in the shadows (just beyond the left side of the picture), several semicircular rings of mountains reveal its extent. (c) Unusual, Hilly Terrain. What looks like tiny, fine-grained wrinkles on this picture are actually closely spaced hills, part of a jumbled terrain that covers nearly 500,000 km2 (193,000 mi2) on the opposite side of Mercury from the Caloris Basin. The large, smooth-floored crater, Petrarch, has a diameter of 170 km (106 mi). This impact crater was produced more recently than Caloris Basin.