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Why are secondary craters especially common on Mercury?
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Mercury
Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.
Secondary craters on Mercury—craters formed by the impact of material ejected from a primary crater—are particularly prominent due to the planet's high surface gravity.
Because Mercury's gravity is more than twice that of the Moon, material ejected during an impact event does not travel as far as it would on the lunar surface; it reaches only about 65 percent of the distance. This gravitational effect tends to concentrate the ejected debris closer to the primary impact site, which is a significant factor contributing to the prevalence of secondary craters on the Mercurian surface.1