An ancient impact on Earth led to a cascade of cratering

A bevy of craters formed by material blasted from the carving of another, larger crater — a process dubbed secondary cratering — have finally been spotted on Earth. Several groupings of craters in southeastern Wyoming, including dozens of pockmarks in all, have the hallmarks of secondary cratering, researchers report February 11 in GSA Bulletin.

When an asteroid or another type of space rock smacks into a planet or moon, it blasts material from the surface and creates a crater (SN: 12/18/18). Large blocks of that material can be thrown far from the initial crater and blast out their own holes when they land, explains Thomas Kenkmann, a planetary scientist at the Albert Ludwig University of Freiburg in Germany. Astronomers have long observed secondary cratering on our moon, Mars and other orbs in the solar system, but never on Earth.

When Kenkmann and his colleagues first investigated a series of craters near Douglas, Wyo., in 2018, they thought the pockmarks were formed by fragments of a large meteorite that had broken up in the atmosphere. But Kenkmann and his team later discovered similar groups of craters of the same age, somewhere around 280 million years old, throughout the region.

Altogether, the team found more than 30 impact craters that range between 10 and 70 meters in diameter at six different locales. Based on subtle but distinct differences in the alignment of elliptical craters in the groups, the researchers suggest that the impactors that blasted each set of craters struck the ground from slightly different directions.

The impactors that created these secondary craters probably ranged between 4 and 8 meters in diameter and struck the ground at speeds between 2,520 and 3,600 kilometers per hour, Kenkmann says. Extrapolating the paths of these impactors back to their presumed sources suggests the original crater from which they flew straddles the Wyoming–Nebraska border northeast of Cheyenne.

The team’s evidence “comes together very well to make a compelling story,” says Gareth Collins, a planetary scientist at Imperial College London who was not involved in the new study.

The original crater was probably between 50 and 65 kilometers across and was created by an impactor 4 to 5.4 kilometers wide, Kenkmann and the team estimate. The crater is also probably buried under more than 2 kilometers of sediment that accumulated in the 280 million years since it formed. An equivalent amount of sediment eroded away to expose the secondary craters when the Rocky Mountains rose in the meantime.

“What a fortuitous discovery that these folks have made,” says Beau Bierhaus, a planetary scientist at Lockheed Martin Space Systems in Littleton, Colo. He likens the short geological interval during which these craters could be discovered to the brief period between the time a fossil is first exposed to the elements and when it is eroded to dust.

Scouring measurements of gravitational and magnetic fields in the region for anomalies could help reveal the buried crater, the researchers note. The team may also look for heavily fractured rock and other evidence of the ancient crater in sediment cores that have been drilled during oil and gas exploration in the region, Kenkmann says.