In the Kuiper Belt, a baffling lack of small craters

Scientists studying photos snapped by NASA’s New Horizons spacecraft on its 2015 flyby of Pluto and its giant moon Charon have found an anomaly in the sizes of craters on these two worlds that may have important ramifications for our understanding of the early solar system.

It had been expected that smaller craters would be exponentially more frequent than larger ones. After all, that’s how it is on the moon, Mars, and other long-studied worlds. 

But that didn’t prove to be the case. Instead, below about 13 kilometres in diameter — a size corresponding to an impactor about two kilometres wide — there was paucity of craters on both worlds. The finding was made by a team led by Kelsi Singer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, US, and reported in the journal Science. 

It’s an important find, Singer says, because Pluto and Charon lie on the inner edge of a vast halo of comet-like bodies known as the Kuiper Belt. Some of these, known as Kuiper Belt objects (KBOs) are large, but the vast majority are probably far too small to be visible with earthly telescopes. 

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

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

“Before we got there, we weren’t sure how many [craters] there would be,” Singer says, noting that geological processes might have erased the evidence. 

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

KBOs, she adds, are believed to be relics of the early solar system, and understanding their size distribution can help us understand the process by which they and other objects formed.

One theory is that that this occurred via a cascade of collisions in which small objects merged into larger ones, which then collided to become still larger ones, in a hierarchy of ever-bigger collisions. 

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

“More small objects get produced in the traditional view of going from small to large,” Singer says, “[so] if we’re seeing fewer, that might be tied to the gravitational collapse model.” 

Mike Brown, a planetary astronomer at California Institute of Technology, Pasadena, California, US, agrees. “I think this result is more or less consistent with a gravitational instability formation process that starts out with KBOs on the large size [and] never forms the small ones,” he says. 

There’s just one problem. In late January, when Singer’s paper was already in press, Japanese astronomers found a 2.6-km Kuiper Belt object, using a method that implies that there might be a lot more of them out there. 

Not that the astronomers actually saw the object. Rather, they saw its shadow, as it moved across the sky, briefly blocking the light of a distant star.

The work, done by amateur astronomers organised by Ko Arimatsu of the National Astronomical Observatory of Japan, was partly a demonstration that KBOs of that size could indeed be spotted by this technique.{%recommended 1778%}

His team not only proved that the method worked, but also found one in a mere 60 hours of viewing time, using two 27.9-centimetre telescopes scanning a background field of only 2000 stars. The result was published in the journal Nature Astronomy.

The implication is that if one such object could be found that easily, there may be a lot more just waiting to be discovered. 

“The two results definitely are in conflict,” says Brown, of Arimatsu’s and Singer’s findings.

Further deepening the mystery is the fact that just as asteroids are known to hit each other and break into fragments, so too should KBOs collide and break up. 

“If this is the case, it’s impossible to have a deficit of bodies at small sizes,” says Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory, Nice, France.

One possible solution is that geological processes on Pluto and Charon have muddied the record by somehow eroding or burying many small craters. But that seems unlikely, Singer says, because such processes should also have degraded larger craters, something that hasn’t occurred.

“There are no ‘half-eaten’ craters,” she says. “You don’t see partially filled craters. There could always be some cryptic process, but it’s hard to imagine that there is some process nobody has ever seen before [that] could be preferentially erasing small craters.”

Brown agrees: “In the end, it’s hard to disagree with the lack of craters,” he says. “If there are many small KBOs, small craters would be all over the place (with all of the caveats about resurfacing, that I think are addressed nicely).” 

Arimatsu adds that the two studies may not be quite as incompatible as they look. “As you know, we discovered only one object, and there is still a large uncertainty in our size distribution results,” Arimatsu said. 

What’s needed, he adds, is additional crater-counting in the Kuiper Belt, as well as more stellar occultation studies such as his team conducted. 

Singer agrees. She was surprised that her paper turned out to be so controversial, but notes that controversy can be good for science. 

“It motivates more study, and I’m all for more study on the Kuiper Belt,” she says.