Scarps

The most important landforms on Mercury for gaining insight into the planet's otherwise largely unseen interior workings have been its hundreds of lobate scarps. These cliffs vary from tens to over a thousand kilometres in length and from about 100 metres (330 feet) to 3 km (2 miles) in altitude. Viewed from above, they have curved or scalloped edges, hence the term lobate. It is clear that they were formed from fracturing, or faulting, when one portion of the surface was thrust up and overrode the

A double-ringed crater on Mercury filled with plains material, in an image taken by the Messenger probe on Jan. 14, 2008. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington adjacent terrain. On Earth such thrust faults are limited in extent and result from local horizontal compressive (squeezing) forces in the crust. On Mercury, however, these features range across all of the surface that has been imaged so far, which implies that Mercury's crust must have contracted globally in the past. From the numbers and geometries of the lobate scarps, it appears that the planet shrank in diameter by at least 3 km (2 miles).

A scarp on Mercury, as seen by the Messenger probe on Jan. 14,2008. The scarp (upper left) curves downward, ending in the large impact crater at the bottom. The region shown is about 200 km (120 miles) across. NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington

A scarp on Mercury, as seen by the Messenger probe on Jan. 14,2008. The scarp (upper left) curves downward, ending in the large impact crater at the bottom. The region shown is about 200 km (120 miles) across. NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington

Moreover, the shrinkage must have continued until comparatively recently in Mercury's geologic history—that is, since the time Caloris formed—because some lobate scarps have altered the shapes of some fresh-appearing (hence comparatively young) impact craters. The slowing of the planet's initial high rotation rate by tidal forces would have produced compression in Mercury's equatorial latitudes. The globally distributed lobate scarps, however, suggest another explanation: later cooling of the planet's mantle, perhaps combined with freezing of part of its once totally molten core, caused the interior to shrink and the cold surface crust to buckle. In fact, the contraction of Mercury estimated from cooling of its mantle should have produced even more com-pressional features on its surface than have been seen, which suggests that the planet has not finished shrinking.

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