Pluto and Charon lack small craters



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Scientists studying crater images on Pluto and moon, Charon, by New Horizons, discovered that the Kuiper belt should contain fewer small objects than expected – suggesting that the outer solar system has not changed much since he started.

When NASA's New Horizons mission was sped up by Pluto and its moon, Charon, in 2015, it captured nearly half of each world with incredible detail. Each pixel can capture entities as small as 76 meters on Pluto and 154 meters on Charon. For reference, this is equivalent to discerning the Godzilla hulk on the surface of these worlds (according to the movie: Godzilla measured 75 meters high in 1984 but reached 150 meters by 2014).

They were not monsters that the scientists were looking for, but the marks left by some of the smaller objects in the Kuiper Belt. The sparse ring of icy rocks that surrounds our solar system can represent the primordial remains of the planet's formation. But until now, scientists did not know how these objects were virgin. March 1st ScienceKelsi Singer (Southwestern Research Institute) and his colleagues have unearthed the craters of some of the oldest surfaces of Pluto and its moon, and their discoveries suggest a Kuiper belt that does not virtually untouched since its inception.

Count craters

Count craters on Charon

Scientists studying the craters and the geology of Pluto and Charon found fewer small craters than they had hoped. One of the areas examined by the team was the smooth and geologically stable "Vulcan Planitia" on Charon, shown here.
NASA / JHUAPL / LORRI / SwRI

In general, whether asteroids, stars, or galaxies, there are many more little things than big things in the universe. But what Singer's team discovered on Pluto and Charon is that craters with a diameter smaller than a certain diameter, especially less than 13 kilometers in diameter, were surprisingly few in number.

When a rock crushes on a larger body, the crater that it creates is usually much larger than the rock itself. For Pluto and Charon, a crater shortage of less than 13 kilometers in diameter results in a shortage of Kuiper Belt objects extending over less than 1 to 2 kilometers. Co-author Alex Parker (also from the Southwest Research Institute) has some uncertainty about the details of this translation. However, no matter what craters create on these worlds, they tend to be bigger.

Scientists claim that geological processes such as cryovolcanism and glacial activity could not have simply smoothed the surfaces of Pluto and Charon, as such resurfacing would have erased the largest craters with the smaller ones. The team concluded that no known process would preferentially erase smaller craters. In addition, the number of craters of Pluto and Charon is consistent with similar studies conducted on the moons of Jupiter and Saturn: small craters are also lacking on these moons.

Too little or too much? And why it's important

Kuiper belt object

Kuiper Belt object artist concept.
Ko Arimatsu

However, the number of craters does not exactly match the results of a recent star-monitoring survey called OASES, which looked for fast blinking stars suggesting that a distant object was passing in front of them. After 60 hours of observation of 2,000 stars, Ko Arimatsu (National Astronomical Observatory of Japan) and his colleagues discovered that a single occultation event corresponded to an object of about 1 kilometer in the Kuiper belt. As even one discovery was unexpected, scientists explained Nature Astronomy that the discovery indicates an excess – not a shortage – of these objects the size of one kilometer in the external solar system.

"I'm not convinced yet by any of these blackout detections," says Parker. "For each of these types of events with one or two players, there are countless alternative possibilities for what they may have been other than occultations of stars by objects of the Kuiper belt. "

For his part, Arimatsu says: "I suppose additional studies on resurfacing processes are needed because Pluto and Charon (and also Ultima Thule, probably) seem to be geologically unique and that unexpected resurfacing processes can occur."

At the same time, says Arimatsu, "we have not discovered AN object and the results of the size distribution are still very uncertain." It is even possible, he adds, that the concealment of his team is not totally incompatible The Horizons team found.

The issue of the number of objects in the Kuiper belt the size of a kilometer may seem a bit like asking how many angels are dancing on the head of a pin. But the answer has potentially considerable implications for our understanding of the formation of the planet in the early solar system.

If there are few objects the size of one kilometer, it could mean that the Kuiper belt is so sparse that objects have not spent too much time since 4 billion of years to cross each other. After all, collisions usually create a lot of little things from a few great things. And if the collisions were relatively rare, what we see now is what was there at the beginning. That is, the Kuiper Belt would represent the solar system much as it existed during its formation.

While the Kuiper Belt has remained intact since its inception, the relative size of the objects it contains could illuminate the formation of the planet. For example, the size distribution found by Singer and his colleagues confirms the idea that giant planets were formed when gravitational instabilities led to explosive growth.

What the future holds

Both teams agree that future blackout investigations, such as the Transneptunian Automated Blackout Survey (TAOS-II), are essential to fully understand the Kuiper Belt. TAOS-II, for example, will capture the sky at a speed of 20 frames per second. It's a little faster than the OASES configuration, which captured 15.4 frames per second, but that changes what TAOS-II will see.

"High cadences are absolutely necessary because shadows cast by objects of this size and distance are not the same as the ones we usually see every day around us," says Parker. As the shadow of an object from the Kuiper belt sweeps the Earth, observers see the light of the star blocking it fade and lighten, fade and illuminate, a double pattern called Fresnel Diffraction. "The characteristic flicker of these diffraction fringes in the shade is what provides a strong confirmation of the properties of the occulting object."

Plus, TAOS-II is designed to capture not just one occultation event but hundreds of them. By combining many observations, astronomers will finally gain a much better understanding of the population of the Kuiper Belt.

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