The Moon's surface is being mapped by NASA's Lunar Reconnaissance Orbiter spacecraft, to aid planning for future missions. On page 215, Speyerer et al. report how images taken by the orbiter's camera have been used to quantify the current rate at which lunar craters form as a result of surface impacts by comets, asteroids and associated fragments (E. J. Speyerer et al. Nature 538, 215–218; 2016).
The authors compared pairs of images of the Moon's surface taken at different times, and discovered that 222 craters had formed in the periods between the images being taken. They therefore estimate that about 180 craters of at least 10 metres in diameter form annually across the entire Moon. This is 33% more than would have been expected from a commonly used model of impact frequency.
By calculating the ratios of surface reflectance between pairs of images, Speyerer et al. uncovered distinct zones of subtly modified reflectance around the newly formed craters. The zones extend many crater widths out from the centre and are not visible to the naked eye (pictured are the ratios for a 12-m crater; dark regions reveal a zone that splays out up to 1,800 m from the centre). The authors propose that these zones are caused by impact-induced jets of melted and vaporized material formed early in the crater-formation process.
The researchers also detected thousands of subtle surface disturbances — changes in local reflectance that lack a resolvable crater rim. They interpret many of these as the scars of secondary impacts that churned up the upper few centimetres of the surface without forming a resolvable crater. Speyerer and colleagues therefore propose that the upper 2 centimetres of loose surface material on the Moon will be reworked in about 81,000 years, 100 times faster than previously predicted. Andrew MitchinsonFootnote 1
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Mitchinson, A. Moon churn. Nature 538, 177 (2016). https://doi.org/10.1038/538177a