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The Surface of Mercury: A Moon-Like World

The cratered face of Mercury looks very much like that of the Moon. There are, however, some differences. On Mercury, there are fewer craters in a given area and they are more widely separated than on the Moon. Material thrown from impact craters is not scattered as far because Mercury's gravity is greater than that of the Moon.

Caloris Basin

Caloris Basin

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(Figure 21: Mariner 10 Photomosaic of Caloris Basin. Atlas of Mercury, NASA 1976)

A Planet-Shaking Impact: Caloris Basin - Impact Site

In this photomosaic, one-half of the 1300-kilometer (810 mile) diameter circular basin lies hidden in darkness. This is the largest feature yet observed on Mercury and was produced by the impact of a large meteoroid.

What Might Have Happened: When the large meteoroid collided with Mercury and formed the Caloris basin, shock waves passed through the planet's crust and core. The waves converged at the surface opposite Caloris and shook the region violently.

"Weird Terrain" - Opposite Side Of Mercury

Directly opposite the Caloris basin is this jumbled landscape called "weird terrain" that may have been formed by shock waves from the impact that produced the basin. Similar terrain is found opposite huge impact basins on the Moon.

Weird Terrain

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(Mariner 10 photomosaic - NASA)

Discovery Scarp
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(Mariner 10 photograph- NASA)

A Shrinking Planet?

This long sinuous scarp, or cliff -- one of many on Mercury -- is about 550 kilometers (350 miles) long and as high as 3 kilometers ( 2 miles) in places. It appears to be a fault produced by compression of the crust. The shrinking of Mercury during formation of its core explains this surface compression.

Evidence of Past Volcanism?

Areas of smooth plains occur between craters and in low areas on Mercury, similar to the dark- and light-colored plains on the near side of the Moon. Before manned exploration of the Moon, these smooth light plains were thought to be volcanic deposits. As a result of returned samples from Apollo 16, it was found that much of the lunar light plains is really ejecta from craters.

Thus, it is possible that the smooth plains of Mercury may not be volcanic, but of impact origin. Evidence now favors volcanism, but the real answer will have to await a return trip to Mercury.


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