The impacts of asteroids have a bad reputation here on Earth – it's the public relations victory of the dinosaurs' signature – but it's the moon that really has the scars of living in our confused neighborhood.
That's because Earth has an arsenal of forces that slowly wear away craters left by impacts. And this is frustrating for scientists who want to better understand the wreckage around our solar system. So a new study uses the lunar surface to show the history of things that have crashed on the Moon and Earth, uncovering signs that our neighborhood was very confused about 290 million years ago.
"It's a cool study that talks about our dynamic solar system and it's good to be out there," said Nicolle Zellner, a physicist at Albion College in Michigan, who was not involved in the new research at Space.com. "It will make people think and test, so it's exciting." [How the Moon Formed: 5 Wild Lunar Theories]
Earth and Moon are close enough in the scale of the solar system that lost asteroids would collide with each on the same frequency. (The Earth may attract some extras with its strongest gravity, and the Earth is likely to suffer more impacts because of its larger surface area – but in terms of impact per square mile, they must be as often.)
Scientists have identified only about 180 impact craters here on Earth, as opposed to hundreds of thousands of lunar impact craters. Earth cleans them with winds and rains, oceans and tectonic plates. "The moon is perfect for studying craters," said Sara Spaceou, a planetary scientist who led the new research during her Ph.D. at the University of Toronto, to Space.com. "Everything is there."
But to chart the history of impacts, scientists needed to not only identify craters but also estimate their ages. And this is much more difficult on the Moon than on Earth, since geologists can not experience the lunar craters directly.
So the team behind the new research has established what can be an amazing measure: how well nearby rocks retain heat during the long, cold lunar night. This may seem like a very random measurement. But when a large impactor hits the moon, it digs a crater and knocks out the surrounding landscape with pebbles from that material. Over time, these rocks are struck by small impactors that break them into smaller and smaller rocks, which eventually become dusty regoliths, so the team argued that the older craters would be surrounded by finer rocks and newer craters by larger rocks.
So when this landscape transitions from a 14-day lunar day to a 14-day lunar night, it changes temperature at different rates. "The idea is that large rocks can keep the heat at night, while the regolith or sand loses heat," said Mazrouei. "As craters age, they become less rocky." In turn, they calm down faster.
Mazrouei and his colleagues analyzed thermal imaging data from an instrument called Diviner aboard NASA's Lunar Reconnaissance Orbiter that has been circulating the Moon since 2009. The team identified 111 individual craters that they knew were less than 1 billion years old, analyzed. their signatures of heat and, using a model of how quickly the lunar boulders disintegrate, have estimated their age.
The result showed an intriguing pattern: a peak in impact rates about 290 million years ago, when crater formation rates appear to have more than doubled. This would suggest something significant changed in our solar system at that time – perhaps, the team proposed, a large space rock in the asteroid belt separating and wandering closer to Earth and the moon. And comparing the craters we know here on Earth with their results, the team sees similar patterns, suggesting that scientists have found a fairly representative collection of craters, albeit small.
Not everyone is convinced. "The results are intriguing, but I think the real support for these conclusions is very weak," said Jay Space, a planetary scientist at Purdue University, who was not involved in the new research. In particular, it is not sold in the boulder disintegration model they used – it does not think it adequately explains how this process accelerates as the rocks decay. And he does not see craters of Earth enough to withstand solid statistical analysis; he worries that they are working with a very small sample size. [The Moon: 10 Surprising Lunar Facts]
"That does not mean it's wrong, but it does not mean it's right either – we just do not know," Melosh said. "This is a noble attempt to go a little further than the media."
Zellner understands how difficult it is to study lunar craters: she worked with the glass droplets created by impacts and carried them to Earth in samples collected by Apollo astronauts. But dating this glass is still a challenge, even with laboratory technology, and all samples come from a small part of the surface of the moon. Orbital data put scientists more distant, but they cover the entire lunar surface – no method is perfect.
"We're doing the best we can with what we have now," Zellner said. "That's science, right? We put ideas around and then find ways to test these ideas, and the idea can stand the test of time or not. "
And all three scientists have offered convincing reasons why it's worth doing the work to discover the story of the moon's impact. First, of course, there is a self-interested approach: Earth craters may have some unpleasant side effects.
"Everyone is interested in the rate of formation of craters on Earth because we do not want to end up like the dinosaurs," Melosh said. The catastrophic impact consequences killed three out of every four species alive at the time, though extinctions left plenty of room for our own mammalian ancestors to thrive. "We should thank our lucky meteorite, but it was too bad for everyone else on the planet." Learn enough about impacts, theory says, and we can save our own skins next time.
For Zellner, there is also a more exotic appeal: learning more about our own solar system can help scientists understand not only our own neighborhood, but also the processes that shaped the alien solar systems that scientists are still discovering.
Mazrouei sees the work as an example of how different bodies of the solar system can shed light on one another. One of her coauthors is already eager to know how the BepiColombo mission for Mercury, armed with an instrument much like the moon now, will be able to add another dimension to the crater studies.
Earth is great for living, but scientists can not piece together their past from home. We need to study the moon and its surface with craters to understand what our planet has gone through, Mazrouei said. "We have been able to distill a lot of Earth's history as well."
The new research is described in an article published today (January 17) in the journal Science.