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NASA's Moon data sheds light on Earth’s asteroid impact history

(17 January 2019 - NASA Goddard) By looking at the Moon, the most complete and accessible chronicle of the asteroid collisions that carved our young solar system, a group of scientists is challenging our understanding of a part of Earth’s history.

The number of asteroid impacts to the Moon and Earth increased by two to three times starting around 290 million years ago, researchers reported in a paper in the journal Science.

They could tell by creating the first comprehensive timeline of large craters on the Moon formed in the last billion years by using images and thermal data collected by NASA’s Lunar Reconnaissance Orbiter (LRO). When the scientists compared those to the timeline of Earth’s craters, they found the two bodies had recorded the same history of asteroid bombardment—one that contradicts theories about Earth’s impact rate.

For decades, scientists have tried to understand the rate that asteroids hit the Earth by carefully studying impact craters on continents and by using radiometric dating of the rocks around them to determine the ages of the largest, and thus most intact, ones. The problem is that many experts assumed that early Earth craters have been worn away by wind, storms, and other geologic processes. This idea explained why Earth has fewer older craters than expected compared to other bodies in the solar system, but it made it difficult to find an accurate impact rate and to determine whether it had changed over time.

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NASA’s Landsat 7 satellite captured this image of Pingualuit Crater on August 17, 2002. In it, water appears blue, and land appears in varying shades of beige. With a diameter of 2.14 miles (3.44 kilometers), Pingualuit Crater holds a lake about 876 feet (267 meters) deep. (courtesy: NASA)

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A 2014 Lunar Reconnaissance Orbiter Camera image showing two similarly sized craters in Mare Tranquillitatis. Both are about 500 meters in diameter. One is littered with boulders and the other is not. This boulder discrepancy is likely due to age differences between the two craters. Image width is about 2 kilometers. North points up. (courtesy: NASA/GSFC/Arizona State University)

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Geologist-Astronaut Harrison H. Schmitt is photographed standing next to a huge, split boulder at Station 6 on the sloping base of North Massif during the third Apollo 17 extravehicular activity (EVA-3) at the Taurus-Littrow landing site. The "Rover" Lunar Roving Vehicle (LRV) is in the left foreground. Schmitt is the Apollo 17 Lunar Module pilot. This picture was taken by Commander Eugene A. Cernan on n December 13, 1972. (courtesy: NASA)

The Moon as Earth's Mirror

LRO's thermal radiometer, called Diviner, has taught scientists how much heat is radiating off the Moon’s surface, a critical factor in determining crater ages. By looking at this radiated heat during the lunar night, scientists can calculate how much of the surface is covered by large, warm rocks, versus cooler, fine-grained regolith, also known as lunar soil.

Large craters formed by asteroid impacts in the last billion years are covered by boulders and rocks, while older craters have few rocks, Diviner data showed. This happens because impacts excavate lunar boulders that are ground into soil over tens to hundreds of millions of years by a constant rain of tiny meteorites.

Paper co-author Rebecca Ghent, a planetary scientist at University of Toronto and the Planetary Science Institute in Tucson, Arizona, calculated in 2014 the rate at which Moon rocks break down into soil. Her work thus revealed a relationship between an abundance of large rocks near a crater and the crater’s age. Using Ghent’s technique, the team assembled a list of ages of all lunar craters younger than about a billion years.

“It was a painstaking task, at first, to look through all of these data and map the craters out without knowing whether we would get anywhere or not,” said Sara Mazrouei, the lead author of the Science paper who collected and analyzed all the data for this project while a Ph.D. student at the University of Toronto.

The work paid off, returning several unexpected findings. First, the team discovered that the rate of large crater formation on the Moon has been two to three times higher over approximately the last 290 million years than it had been over the previous 700 million years. The reason for this jump in the impact rate is unknown. It might be related to large collisions taking place more than 300 million years ago in the main asteroid belt between the orbits of Mars and Jupiter, the researchers noted. Such events can create debris that can reach the inner solar system.

The second surprise came from comparing the ages of large craters on the Moon to those on Earth. Their similar number and ages challenges the theory that Earth had lost so many craters through erosion that an impact rate could not be calculated.

“The Earth has fewer older craters on its most stable regions not because of erosion, but because the impact rate was lower about 290 million years ago,” said William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado and a co-author of the paper. “This meant the answer to Earth’s impact rate was staring everyone right in the face.”

Proving that fewer craters meant fewer impacts—rather than loss through erosion—posed a formidable challenge. Yet the scientists found strong supporting evidence for their findings through a collaboration with Thomas Gernon, an Earth scientist based at the University of Southampton in England who works on a terrestrial feature called kimberlite pipes.

These underground pipes are long-extinct volcanoes that stretch, in a carrot shape, a couple of kilometers below the surface. Scientists know a lot about the ages and rate of erosion of kimberlite pipes because they are widely mined for diamonds. They also are located on some of the least eroded regions of Earth, the same places we find preserved impact craters.

Gernon showed that kimberlite pipes formed since about 650 million years ago had not experienced much erosion, indicating that the large impact craters younger than this on stable terrains must also be intact. “So that's how we know those craters represent a near-complete record,” Ghent said.

Ghent’s team, which also included Southwest Research Institute planetary astronomer Alex Parker, wasn’t the first to propose that the rate of asteroid strikes to Earth has fluctuated over the past billion years. But it was the first to show it statistically and to quantify the rate. Now the team’s technique can be used to study the surfaces of other planets to find out if they might also show more impacts.

The team’s findings related to Earth, meanwhile, may have implications for the history of life, which is punctuated by extinction events and rapid evolution of new species. Though the forces driving these events are complicated and may include other geologic causes, such as large volcanic eruptions, combined with biological factors, the team points out that asteroid impacts have surely played a role in this ongoing saga. The question is whether the predicted change in asteroid impacts can be directly linked to events that occurred long ago on Earth.