Earth’s geologic record contains fewer large craters the further back in time researchers look. A common hypothesis asserts that tectonic activity and weathering from wind and water erode large, old craters. Sara Mazrouei and Rebecca Ghent from the University of Toronto and their colleagues tested that idea by examining the population of craters found on the Moon’s surface. As Earth’s closest astronomical neighbor, the Moon should have sustained strikes by the same population of impactors that hit Earth. The researchers’ analysis suggests that erosional rates may not be as important for crater retention as previously thought.
Mazrouei and her colleagues took advantage of the Diviner instrument on NASA’s Lunar Reconnaissance Orbiter to assemble a data set of craters, highlighted in the figure, that have diameters larger than 10 km and that are less than 1 billion years old. When the researchers compared the lunar craters with the terrestrial ones compiled in the Earth Impact Database, they found a common size–frequency distribution. The similar trends suggest that Earth may not have a preservation bias: Its large craters may accurately represent its impact history. From their analysis, the authors found that the rate of terrestrial impacts increased about 2.6 times beginning roughly 290 million years ago, which they suggest may be due to the breakup of a large asteroid in the belt surrounding the solar system.
An earlier study by Terrence J. Blackburn of MIT and colleagues also found erosion to be negligible. It pegged the erosion rate of continental land terrains at a few meters of material per million years—not enough erosion over the past 650 million years to erase impact craters that have diameters greater than 20 km and depths of at least 2 km. Interestingly, the terrestrial data set has no craters older than 650 million years, whereas about 15% of lunar craters are in that age range. Mazrouei and her colleagues propose that glaciation from about 720 million to 650 million years ago associated with the Snowball Earth hypothesis (see the article by Raymond T. Pierrehumbert, Physics Today, January 2011, page 33) could have removed vertical kilometers of surface material and expunged all but the deepest terrestrial impact craters. (S. Mazrouei et al., Science, in press, doi:10.1126/science.aar4058.)
