Before they can connect all those dots, though, the team needs to keep convincing their peers that the pitter-patter of asteroid impacts really did increase. “There’s definitely a change in the impact rate. Or, at least, I would bet a lot of money on it,” said Joshua Bandfield of the Space Science Institute. The rub, he said, is “how precisely can you tell?”

A Lunar Time Capsule

Scientists already had hints that our planet’s surface has been pounded more than usual over the past few hundred million years. Earth’s history stretches for 4.5 billion years, but most large impact craters appear to have formed relatively recently. Still, geologists typically chalked this up to a bias in the data: Over time, they thought, weathering or tectonic activity just rubbed away the traces of most old craters.

But in 2014, Mazrouei’s graduate-school advisor, Rebecca Ghent, devised a way around that bias. Whereas craters on Earth suffer plate tectonics and weather, lunar scars just sit there, inert, like the boot print of an astronaut. “The moon is a time capsule,” Mazrouei said.

When an impactor slams into the moon, stomping a crater into the surface, it launches boulders into the air. These boulders then litter the area. Over perhaps a billion years, a drizzle of tiny micrometeorites hitting the moon will dissolve the boulders into finer soils called regolith. During that process, the ratio of rocks to regolith in a particular crater tracks the time since the initial impact.

During the day, both boulders and regolith soak up heat. At night, the boulders hold on to this heat, glowing faintly with thermal radiation for hours. The regolith, on the other hand, cools quickly, like beach sand after sunset. The amount of nighttime glow indicates the relative rockiness of a crater and thus, the method assumes, its age.

Mazrouei and her colleagues identified 111 lunar craters with diameters of at least 10 kilometers. They then measured the rockiness around the craters using infrared data from NASA’s Lunar Reconnaissance Orbiter, estimating an age for each. From between about 290 million years ago and now, their statistical analysis showed, big impacts rattled the moon’s surface 2.6 times more often than they had in the preceding 710 million years. That jump occurred around the same time as Earth’s largest mass extinction, known as the “Great Dying,” when about 90 percent of all species went extinct. The causes of that extinction are under debate.

The team also made a census of 38 large craters on Earth so as to have a second, independent set of records. But they knew that it was going to be hard to prove that the fragmentary record of old impacts preserved on Earth’s crust could be trusted. “The assumption has always been ‘oh, that’s erosion,’” said Bottke. “How do you argue against that?”

So the team tried to measure hundreds of millions of years of erosion. Bottke brought in Thomas Gernon, a geologist at the University of Southampton in the U.K. and an expert on kimberlite pipes: carrot-shaped volcanic structures, rich in diamonds, which protrude down into many of the same continental surfaces that also preserve old craters. Conveniently, kimberlites record the erosion they experience, which wears them away from the top down.