November 16, 2012
NASA Mercury Probe Finds Unusual Groups of Ridges And Troughs On The Planet
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MESSENGER, in orbit around Mercury since March of last year, has discovered assemblages of tectonic landforms unlike any previously found on Mercury or elsewhere in the Solar System. Smithsonian scientist Thomas Watters published the findings in the December issue of Geology.
Mercury's surface is covered with deformational landforms, formed by faulting in response to horizontal contraction or shortening that occurred as the planet's interior cooled and the surface shrank. This caused blocks of crustal material to be pushed together. This type of land formation is so dominant on Mercury that extensional landforms caused by fault formation in response to horizontal stretching and pulling apart of crustal material has only been documented in the interiors of a few large impact basins.
The MESSENGER spacecraft has revealed families of extensional troughs, or graben, which are encircled by contractional wrinkle ridges arranged in circular rings. Such troughs can form complex patterns, which vary from the outlines of polygons inside the ridge rings to arcs that parallel the bounding ridges.
"The pattern of winkle ridges and graben resembles the raised edge and cracks in a pie crust," said Watters of the Center for Earth and Planetary Studies at the National Air and Space Museum.
Watters' "pie crust" analogy also fits the "ghost craters," which are associated with these collections of tectonic landforms. Impact craters that have been flooded and buried by lava flows are known as ghost craters, which are fully buried impact craters with thin volcanic deposits overlying the rim. These serve to concentrate contractional forces, leading to the formation of a ridge ring that reveals the outline of the buried crater.
"The special arrangement of the wrinkle ridges and graben in many of the ghost craters on Mercury is due to a combination of extensional forces from cooling and contraction of unusually thick lava flow units and contractional forces from cooling and contraction of the planet's interior," says Sean Solomon of the Columbia University's Lamont-Doherty Earth Observatory.
The reason why these systems of tectonic landforms in ghost craters on Mercury may not have been observed elsewhere in the Solar System is because of the eruption and rapid accumulation of very fluid lava flows into thick cooling units on a planet undergoing a high rate of global contraction.