It has long been thought that as the solar system grows older and stodgier, the number of asteroids and comets colliding with Earth and other planets has steadily gone down. But a new study reveals what appears to be a dramatic 2.5 times increase in the number of impacts striking Earth in the past 300 million years.
Earth’s surface is dotted with impact craters from the past billion years, but old craters are rarer than younger ones, a bias attributed to the crust-eating churn of plate tectonics, volcanism, and erosion. By looking at the moon, which doesn’t deal with the same forces but faces the same bombardment, scientists can probe the past of both bodies.
Scientists used a thermal camera on NASA’s Lunar Reconnaissance Orbiter to examine the number of large, heat-retaining rocks in the moon’s craters; those rocks are eventually ground to dust by minute meteorite impacts. By looking at previously dated craters, these rocks have been established as a reliable dating technique—the more intact the rocks, the younger the crater.
In the new study, the team found a surprising abundance of young craters, seemingly matching the number on Earth. That means, they write today in Science, that in its modern geological history, Earth is much better at retaining the features of impact craters than once thought, and that the recent proliferation coincides with an actual increase in the number of bombarding asteroids or comets.
But scientists still don’t know what caused the uptick. Perhaps several large asteroids collided or otherwise broke up some 300 million years ago, their chunks slowly migrating out from the asteroid belt to bombard Earth, the researchers say. And that could have included the giant impact, 66 million years ago, that wiped out most of the dinosaurs.