Character of the surface

The portions of Mercury imaged by Mariner 10 and Messenger look superficially like the Moon. Mercury is heavily pockmarked with impact craters of all sizes. The smallest craters visible in the highest-resolution Mariner photos are a

Meteorite crater surrounded by rays of ejected material on Mercury, in a photograph taken by the Messenger probe, Jan. 14,2008. A chain of craters crosses the centre of the rayed crater. NASA/ Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington few hundred metres in diameter. Interspersed among the larger craters are relatively flat, less-cratered regions termed intercrater plains. These are similar to but much more pervasive than the light-coloured plains that occupy inter-crater areas on the heavily cratered highlands of the Moon. There are also some sparsely cratered regions called smooth plains, many of which surround the most prominent impact structure on Mercury, the immense impact basin known as Caloris, only half of which was in sunlight during the Mariner 10 encounters but which was fully revealed by Messenger during its first flyby of Mercury in January 2008.

The most common topographic features on Mercury are the craters that cover much of its surface. Although lunarlike in general appearance, Mercurian craters show interesting differences when studied in detail. Mercury's surface gravity is more than twice that of the Moon, partly because of the great density of the planet's huge iron core. The higher gravity tends to keep material ejected from a crater from traveling as far—only 65 percent of the distance that would be reached on the Moon. This may be one factor that contributes to the prominence on Mercury of secondary craters, which are those made by impact of the ejected material, as distinct from primary craters formed directly by asteroid or comet impacts. The higher gravity also means that the complex forms and structures characteristic of larger craters—central peaks, slumped crater walls, and flattened floors—occur in

The double-ringed crater Vivaldi (right) on Mercury as seen by the Messenger probe on Jan. 14, 2008. Vivaldi is about 200 km (120 miles) across. The older depression covered by Vivaldi was not visible in images taken by Mariner 10. NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington

The double-ringed crater Vivaldi (right) on Mercury as seen by the Messenger probe on Jan. 14, 2008. Vivaldi is about 200 km (120 miles) across. The older depression covered by Vivaldi was not visible in images taken by Mariner 10. NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington smaller craters on Mercury (minimum diameters of about 10 km [6 miles]) than on the Moon (about 19 km [12 miles]). Craters smaller than these minimums have simple bowl shapes.

Mercury's craters also show differences from those on Mars, although the two planets have comparable surface gravities. Fresh craters tend to be deeper on Mercury than craters of the same size on Mars. This may be because of a lower content of volatile materials in the

Mercurian crust or higher impact velocities on Mercury, since the velocity of an object in solar orbit increases with its nearness to the Sun.

Craters on Mercury larger than about 100 km (60 miles) in diameter begin to show features indicative of a transition to the "bull's-eye" form that is the hallmark of the largest impact basins. These latter structures, called multiring basins and measuring 300 km (200 miles) or more across, are products of the most energetic impacts. Several dozen multiring basins were tentatively recognized on the imaged portion of Mercury; new Messenger images and laser altimetry will greatly contribute to the understanding of these remnant scars from early asteroidal bombardment of Mercury.

On the other side of the planet exactly 180° opposite Caloris, is a region of weirdly contorted terrain. It is interpreted to have been formed at the same time as the Caloris impact by the focusing of seismic waves from that event to the antipodal area on Mercury's surface. Termed hilly and lineated terrain, it is an extensive area of elevations and depressions. The crudely polygonal hills are

5-10 km (3-6 miles) wide and up to 1.5 km (1 mile) high. Preexisting crater rims have been disrupted into hills and fractures by the seismic process that created the terrain. Some of these craters have smooth floors that have not been disrupted, which suggests a later infilling of material. Once Messenger has mapped the entire globe of Mercury, a thorough search can be made for similarly disrupted terrain antipodal to other large basins on Mercury.

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