A file photo from the Lunar Reconnaissance Orbiter showing a young crater (right) surrounded by excavated boulders and rocks, and an older, eroded crater (left) with no recognisable boulders. The boulders around the young crater retain heat during the lunar night while the fine-grained soils of older craters lose their heat much more rapidly, allowing researchers to determine the ages of impact craters across the Moon’s surface. Image: NASA/GSFC/Arizona State University
Data from NASA’s Lunar Reconnaissance Orbiter indicate the number of crater-forming impact events on the Moon doubled or tripled during the last 290 million years or so. A similar increase is seen in the number of Earth impacts, both possibly the result of an asteroid collision more than 300 million years ago that showered the inner solar system with rocky debris.
Despite the crater-erasing effects of erosion on Earth, scientists have long wondered why relatively few craters remain in older terrains given Earth and Moon presumably experienced the same levels of bombardment over the history of the solar system. The new study provides a possible answer: the number of major impacts older than a billion years, both on Earth and the Moon, was much lower than previously thought.