The Argyre Basin

The largest craters or impact basins on Mars may be buried beneath the northern smooth plains. In the southern hemisphere, the bombardment history typical of all the inner planets is recorded by a few large basins, such as Argyre. The ring of mountains surrounding the basin probably rises 5 kilometers (3 miles) above the basin floor. The Mars Global Surveyor mapping mission has provided accurate elevation data that have answered many questions about Mars.

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Viking mosaic from U.S. Geological Survey

Rather than the relatively blocky craters found on the Moon, the oldest terrain on Mars shows degradation of crater rims and evidence of erosion by running water. A comparison of the number of craters in the Martian highlands versus those on the Moon suggests that even the earliest crust of Mars may have long since been buried by more recent volcanic deposits.

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A Young Crater

Even the youngest craters on Mars display lobate ejecta patterns that may indicate subsurface water or ice. This 30-kilometer (18-mile) crater is the result of a meteorite impact on smooth plains of the northern hemisphere, blanketing the surrounding terrain for up to 70 kilometers (42 miles) around.

See Water? for more on craters.