The Ultimate Ultima Thule photo reveals an incredibly smooth face



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Ultima Thule has a new photo.

The closest image of the ancient Kuiper Belt object, captured as the New Horizons spacecraft flew on January 1, shows a relatively smooth face, not marked by impact craters.

"It's not just covered in craters," says planetary scientist Kelsi Singer of the Southwest Research Institute in Boulder, Colorado, about the image, released Jan. 24.

This lack of impact scars suggests that the Kuiper Belt, a reservoir of ancient space rocks beyond Neptune's orbit, has fewer small objects than scientists had hoped for. If true, this may mean that the precursors of the planets grew rapidly without leaving many protoplanetary crumbs behind.

Snapshots made when New Horizons went through Pluto and its moons in 2015 showed that these bodies are surprisingly smooth as well. Many of Pluto's craters could have been covered by geological activity on the dwarf planet, such as the movement of glaciers (SN Online: 10/15/15). But Pluto's largest moon, Charon, is considered less active and therefore should not erase its craters (SN Online: 7/13/18). Singer and his colleagues argue that the lack of craters meant that there were not many small objects available to reach Pluto and Charon.

"If you can not get geology to erase the craters in Charon in particular, it's an inescapable conclusion that you have to just not do the craters in the first place," says Singer.

The real test of this idea came with the New Horizons flight from Ultima Thule, whose official name is MU69. If MU69 did not have small craters that would mean relatively few small objects in the outer solar system to collide, Singer and his colleagues argue in an article published in arXiv.org in December.

The last photo of MU69 shows only a few small craters along its upper edge – where the shadows make the edges of the craters stand out in relief – probably left by objects about 100 meters wide. A large depression in the smaller of the two lobes of the object may be an impact crater left by an object about 700 meters wide.

A shortage of small objects that could damage bodies like MU69 could rule out some theories about how the planets and their predecessors formed. One idea about this time at the beginning of the solar system is that the dust grains slowly come together to form larger bodies. Another theory suggests that larger objects collided and crushed each other into small pieces. But both scenarios would probably have left many small objects floating in the Kuiper belt today, Singer says.

If protoplanets freeze directly from the gas and dust nebula that preceded the formation of the solar system, however, they could have grown rapidly and were tens and hundreds of kilometers long, says Singer. That means there would be few small cosmic kibbles and bits remaining.

Planetary scientist Alessandro Morbidelli of the Côte d'Azur Observatory in Nice, France says it is premature to draw any conclusions. The crater count on Pluto and Charon is not reliable, he says. And while he agrees that MU69 is the "final test," he says that higher resolution images are needed to see if his poorly marked appearance holds up under closer scrutiny. The current best picture was taken seven minutes before the New Horizons' closest approach to MU69 when the spacecraft was still 6,700 km away. Better images are on the spacecraft's computer, waiting to be transmitted to Earth.

New Horizons will continue to submit data on MU69 through September 2020.

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