Description:
This is a double-ring crater (double-ring basin) photographed on Mercury.
What force created this “basin within a basin”? No one knows. This strange landscape, spanning 160 km, was captured by the MESSENGER spacecraft operating on the innermost planet of the solar system. Although double or multiple ring basins are relatively rare, they have also been photographed on Mars, Venus, Earth, and the Moon. Mercury itself has several such basins, including the massive Caloris Basin, the Rembrandt Basin, and the mysterious Raditladi Basin. Craters on planets and moons are generally caused by the impact of a single asteroid or comet fragment. Since it is unlikely for an impact to occur again exactly at the center of an existing crater, large double-ring basins are usually attributed to subsequent volcanic lava flows within the crater. Perhaps the second ring was formed by the effects of melting and material flow during the impact. One clue to the origin of the double ring is that the central area of the basin appears smoother than the area between the rings. MESSENGER has completed its final flyby of Mercury, but it will still return in March 2011 and attempt to enter orbit.
NASA Website:
http://antwrp.gsfc.nasa.gov/apod/astropix.html
http://antwrp.gsfc.nasa.gov/apod/ap091007.html
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