In the Kuiper Belt, a puzzling lack of small craters



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Scientists studying photos taken by NASA's New Horizons spacecraft during its flyby of Pluto in 2015 and its giant moon Charon have discovered an anomaly in the crater size of both worlds that could have a significant impact on our understanding of the initial solar system.

Small craters were expected to be exponentially more frequent than larger craters. After all, it's like that on the Moon, Mars and other worlds studied for a long time.

But that was not the case. Instead, less than 13 kilometers in diameter – a size corresponding to an impactor about two kilometers wide – there was a crater shortage on both worlds. The discovery was made by a team led by Kelsi Singer, a scientist in planetary science at the Southwest Research Institute in Boulder, Colorado, USA, and was reported in the paper. Science.

According to Singer, this is an important discovery, as Pluto and Charon are inside a vast halo of comet-like bodies called the Kuiper belt. Some of these objects, known as Kuiper Belt Objects (KBOs), are vast, but the vast majority are probably far too small to be visible with the aid of terrestrial telescopes.

The objects however reveal themselves when they leave craters on other bodies such as Pluto and Charon, which were until this year the most distant bodies from which astronomers have ever obtained close-up images .

(The furthest now is Ultima Thule, which New Horizons flew over on January 1 of this year, but for which scientists are still analyzing their data.)

"Before we got there, we did not know how many [craters] there would be, "says Singer, noting that geological processes could have erased the evidence.

"But there were craters and, in our analysis, we found an unexpected lack of small craters."

The KBOs, she adds, would be remnants of the early solar system, and understanding their size distribution can help us understand the process of forming these objects and other objects.

One theory is that it happened through a cascade of collisions in which small objects merged to make larger ones, which then clashed to become larger, in a hierarchy of more and more collisions. bigger.

Another theory is the gravitational collapse model, in which dust density and ice-dust eddies collapse under their own gravity, instantly producing fairly large objects.

"More and more small objects are produced in the traditional idea of ​​moving from the smallest to the largest," says Singer.[so] if we see less, it could be related to the gravitational collapse model. "

Mike Brown, a global astronomer at the California Institute of Technology in Pasadena, California, USA, agrees. "I think this result is more or less consistent with a gravitational instability training process that starts with KBOs on the big [and] never form the little ones, "he says.

There is only one problem. At the end of January, while Singer's text was already in press, Japanese astronomers discovered a 2.6-kilometer Kuiper belt object, using a method that implied that There could be many more.

Not that astronomers actually saw the object. Instead, they saw her shadow as she moved through the sky, briefly blocking the light of a distant star.

The work, done by amateur astronomers organized by Ko Arimatsu of Japan's National Observatory of Astronomy, was in part a demonstration that KBOs of this size could actually be spotted by this technique.

His team not only proved that the method worked, but it also detected one in only 60 hours, using two 27.9 cm telescopes sweeping a field of only 2000 stars. The result was published in the journal Nature Astronomy.

The implication is that if such an object could be found as easily, there could be many more waiting to be discovered.

"Both results are clearly contradictory," says Brown, of Arimatsu's and Singer's findings.

The mystery is deeper and deeper: just as asteroids are known to hit each other and break into fragments, so do the KBOs that should collide and break.

"If this is the case, it is impossible to have a deficit in small body," says Alessandro Morbidelli, a specialist in planetary sciences at the Observatory of the Côte d'Azur, in Nice, France.

One possible solution is that the geological processes on Pluto and Charon have blurred the map by eroding or burying many small craters. But this seems unlikely, says Singer, because such processes should also have degraded larger craters, which did not happen.

"There are no half-eaten craters," she says. "You do not see craters partially filled. There may still be a cryptic process, but it is hard to imagine that there is a process that no one has ever seen before. [that] could be preferentially erase small craters. "

Brown agrees: "In the end, it's hard to disagree with the lack of craters," he says. "There are a lot of small KBOs, small craters are ubiquitous (with all the caveats regarding resurfacing, which, in my opinion, are handled properly)."

Arimatsu adds that the two studies may not be as inconsistent as they appear. "As you know, we have only found one object and there is still a great deal of uncertainty about our size distribution results," said Arimatsu.

What is needed, he adds, is an additional count of craters in the Kuiper belt, as well as more in-depth studies of stellar occultation such as that conducted by his team.

The singer is in agreement. She is surprised that her article is so controversial, but notes that the controversy can be beneficial for science.

"This is an additional motivation for studies and I am very much in favor of a more in-depth study of the Kuiper belt," she says.

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