Wednesday , June 16 2021

Rate of asteroids jumped past the solar system



Before they can connect all these points, the team must continue to convince their colleagues that the asteroid impacts have really increased. "There is definitely a change in the impact rate, or at least I would put a lot of money into it," said Joshua Bandfield of the Institute of Space Sciences. "The problem," he said, "is how accurately can you tell?"

A Capsule of Lunar Time

Scientists have already suggested that the surface of our planet has been hit harder than usual for the last hundred million years. The history of the Earth extends for 4.5 billion years, but most of the large impact craters seem to have formed relatively recently. Still, geologists have classified this as a bias in the data: over time, they thought that weathering or tectonic activity only eliminated the traces of most of the old craters.

But by 2014, Mazrouei's graduate advisor, Rebecca Ghent, has created a way around this trend. While craters on Earth suffer from tectonic and weather plates, the lunar scars remain there, inert, like an astronaut's boot. "The moon is a time capsule," said Mazrouei.

When an impactor hits the moon, treading a crater on the surface, it throws stones in the air. These rocks then cover the area. Over a billion years, a drizzle of tiny micrometeorites hitting the moon will dissolve the stones into finer soils called regoliths. During this process, the proportion of rocks to regolith in a given crater tracks the time from the initial impact.

During the day, boulders and regolith absorb heat. At night, the boulders cling to this heat, glowing faintly with thermal radiation for hours. The regolito, on the other hand, cools quickly, like the sand of the beach after sunset. The amount of nighttime brightness indicates the relative turbulence of a crater, and therefore the method assumes its age.

Mazrouei and his 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. Between about 290 million years ago and now, its statistical analysis showed, large impacts shook the surface of the moon 2.6 times more than in the previous 710 million years. This jump occurred around the same time as the greatest mass extinction of the Earth, known as the "Great Death", when about 90% of all species were extinct. The causes of this extinction are under debate.

The team also made a census of 38 large craters on Earth to have a second independent set of records. But they knew that it would be difficult to prove that the fragmentary record of preserved ancient impacts on the earth's crust could be trusted. "The assumption has always been" Oh, this is erosion, "Bottke said. "How do you argue against that?"

So the team tried to measure hundreds of millions of years of erosion. Bottke brought Thomas Gernon, a geologist at the University of Southampton in the United Kingdom, and a specialist in kimberlite tubes: volcanic diamond-rich carrot structures that stand out on many of the same continental surfaces that also preserve old craters. Conveniently, kimberlites record the erosion they experience, which wears them from top to bottom.

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