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Giant scarps (cliffs), called rupes (see PIA10609), are believed to have formed when Mercury’s interior cooled and the entire planet shrank slightly as a result. This figure, recently published in Science magazine, shows one of these scarps (white arrows) that is about 270 kilometers (170 miles) long. This portion of Mercury’s surface was seen during the Mariner 10 flybys, but this scarp, despite its large size, was not visible in the Mariner 10 photos because the Sun was nearly overhead at the time and, consequently, the scarp did not cast a discernible shadow. In contrast, MESSENGER acquired a mosaic of this area (see PIA10605) before the spacecraft’s closest approach to the planet, when this portion of the surface was near the terminator, the line between the sunlit dayside and the dark night side of the planet. Such lighting produced long shadows, enabling this rupes to be recognized for the first time.

Date Acquired: January 14, 2008
Image Mission Elapsed Time (MET): Mosaic of images from 108821370, 108821375, 108821397, and 108821402
Instrument: A: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)