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The moon has been hiding a big secret about Earth in plain sight.
Evidence of ancient asteroid strikes is difficult to find on our planet. In fact, fewer than 200 craters are known to science. The commonly understood reason for this has been that Earth quickly erodes, buries, and otherwise hides even major impact sites.
The moon, however, acts like a time capsule because it has no air, water, or active geology, so its craters don't vanish. And it happens to be right next door to our planet, which means that whatever happened to the moon reflects what also happened to Earth.
So a new study published this week in the journal Science took a close look at the biggest lunar craters. By counting and dating such impact sites, the researchers suggest, an approximate history of asteroid strikes on Earth can be reconstructed.
"The moon is like a time capsule, helping us understand the Earth," William Bottke , a space scientist at the Southwest Research Institute, said in a press release about the new research. "We found that the moon shared a similar bombardment history, which meant the answer to Earth's impact rate was staring everyone right in the face."
The team of researchers behind the work studied highly detailed images of the moon and thermal data recorded by NASA's Lunar Reconnaissance Orbiter (LRO) to deduce the ages of the largest craters. When their analysis was complete, they had 111 major impact sites dated across about 1 billion years.
The team's findings suggested that the moon has fewer craters left by big strikes during the oldest 700 million years of that time period, whereas big strikes were about 260% more frequent in the latter third.
This led the researchers to conclude that wind and water on Earth probably didn't hide as many major impact sites as scientists previously thought.
"Earth has fewer older craters on stable terrains not because of erosion, but because the impact rate was lower prior to 290 million years ago," Bottke said in the release.
Bottke and his colleagues did not suggest that their discoveries mean we should worry about any huge asteroid strikes coming soon. Rather, the uptick may have been a temporary surge driven by "the breakup of one or more large asteroids in the inner and/or central main asteroid belt," they wrote.
The work should prove helpful to researchers who are trying to understand the Earth's history and its complex relationship with life.
"Our findings also have implications for the history of life, which is punctuated by extinction events and rapid evolution of new species," Bottke said. "Though the forces driving these events are complicated, asteroid impacts have surely played a role in this ongoing saga."
The researchers at Southwest Research Institute gave 1.3 billion years' worth of lunar impact data to animators at System Sounds , who compressed that history into two roughly one-minute movies ( one of which is not shown here).
The animation above illustrates 111 of the moon's larger impact craters as sound and color, and in the order that they occurred. Smaller craters are represented by higher-pitched and quieter sounds, while the biggest are louder and lower-pitched.
In the last third of the video, you'll notice that there's an uptick in lower and louder sounds.
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