Craters

Like the Moon and Mars, Mercury has ancient areas on its surface covered by craters.This is the most extensive type of terrain photographed by Mariner 10. The photomosaic shown below includes Mercury's north pole near the top of the image. Some heavily cratered regions are shown. Based on knowledge of the Moon and the density of the craters seen on Mercury, these surfaces are thought to be 4.1—4.2 billion years old (see the sidebar "The Late Heavy Bombardment," on page 103). Recently radar observations of Mercury from the Arecibo Observatory have spotted unusual echoes from the south and north poles, which have been interpreted by some scientists as signals from ice deposits in the floors of some permanently shadowed craters. If it is water, it is thought to have come from cometary impacts.

Caloris Basin is both the youngest and the largest impact crater on Mercury. At 800 miles (1,300 km) in diameter, it is almost as large as Hellas Basin on Mars, which is also the youngest and, at 870 miles (1,400 km) in diameter, the largest giant impact on Mars. The right half of Caloris Basin is shown along the left side of the photo shown in

Mercury's Borealis quadrangle includes the planets north pole near the top of the image, which is a mosaic of images taken by Mariner 10. (NASA/JPL/ Mariner 10)

the figure below, a mosaic of images taken by Mariner 10. This photo shows the concentric rings of a complex crater along with ancient streamers of crater ejecta. While Mars's Hellas has opposite it the immense Tharsis rise, a volcanic complex including the largest volcano in the solar system, opposite Mercury's Caloris is simply chaotic, jumbled terrain hills and faults. Though it is most intuitive to assume that the shock energy of the impact that produced Caloris

The right half of Mercury's largest impact crater, the Caloris Basin, lies on the left side of this photomosaic. (NASA/JPL/Mariner 10)

Chaos Region

Impact that creates Caloris Basin

Impact that creates Caloris Basin

Convergence of surface waves and eruption of compressional body waves create hilly and faulted chaos terrain antipodal to the impact traveled away from the impact and was focused on the other side of the planets, where all the wave fronts converged (shown in the figure above), there is no well-developed theory of the effect giant impacts have on the antipodal side of their planets.

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