This mosaic of NAC images shows the impact crater Hokusai, located on Mercury at a latitude of 58° North. The crater has an impressive system of rays, which extend as much as a thousand kilometers (more than 600 miles) across the planet and are the longest that have yet been identified on Mercury.
Such rays are formed when an impact excavates material from beneath the surface and throws that material outward from the crater. These bright rays, consisting of both ejecta and secondary craters that form when the ejected material re-impacts the surface, slowly begin to fade as they are exposed to the harsh space environment. Mercury and other airless planetary bodies are being constantly bombarded with micrometeoroids and energetic ions, producing an effect known as space weathering. Craters with bright rays are thought to be relatively young because the rays are still visible, indicating that they have had less exposure to such weathering processes than craters that lack rays.
Although the extent of some of Hokusai's rays have been determined, images acquired during MESSENGER's three Mercury flybys have not yet shown all of Hokusai's rays. During MESSENGER's orbital observations, which will begin in March 2011, MDIS will acquire high-resolution color images of Mercury's entire surface. This global color map will allow the full extent of the extensive systems of rays emanating from Hokusai and other young craters to be mapped for the first time.
Date Acquired: October 6, 2008
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Scale: The diameter of Mercury is 4,880 kilometers (3030 miles). Hokusai has a diameter of 95 kilometers (59 miles)
Photo credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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