The latest picture of Ultima Thule reveals a remarkably smooth face

Ultima Thule has a new mug shot.

The closest-yet image of the ancient Kuiper Belt object, captured as the New Horizons spacecraft flew by January 1, shows a relatively smooth face unmarred by impact craters.  

“The thing is just not covered in craters,” says planetary scientist Kelsi Singer of the Southwest Research Institute in Boulder, Colo., of the image, released January 24.

That lack of impact scars suggests that the Kuiper Belt, a reservoir of ancient space rocks beyond the orbit of Neptune, has fewer small objects than scientists expected. If true, that could mean that the precursors to planets grew up fast, without leaving many protoplanetary crumbs behind.

Snapshots taken when New Horizons flew past Pluto and its moons in 2015 showed that those bodies are surprisingly smooth, too. Many of Pluto’s craters could have been covered by geologic activity on the dwarf planet, such as the movement of glaciers (SN Online: 10/15/15). But Pluto’s largest moon Charon is thought to be less active and so shouldn’t erase its craters (SN Online: 7/13/18). Singer and her colleagues have argued that lack of craters meant there were just not that many small objects available to smack into Pluto and Charon.

“If you can’t get geology to erase the craters on Charon, in particular, it’s kind of an inescapable conclusion that you have to just not make the craters in the first place,” Singer says.

The real test of this idea came with New Horizons’ flyby of Ultima Thule, whose official name is MU69. If MU69 lacked small craters that would mean relatively few small objects exist in the outer solar system for it to collide with, Singer and colleagues argue in a paper posted at arXiv.org in December.

The latest picture of MU69 shows just a few small craters along its top edge — where shadows make crater rims stand out in relief — probably left by objects about 100 meters wide. A large depression on the smaller of the object’s two lobes could be an impact crater left by an object about 700 meters wide.

A shortage of small objects that could dent bodies like MU69 could rule out some theories about how the planets and their predecessors formed. One idea about this time in the early solar system is that dust grains slowly glommed together to gradually build larger bodies. Another theory suggests that larger objects collided and smashed each other to smithereens. But both scenarios would probably have left many small objects floating around the Kuiper Belt today, Singer says.  

If protoplanets congealed directly out of the nebula of gas and dust that preceded the formation of the solar system, however, they could have grown to between tens and hundreds of kilometers wide relatively quickly, Singer says. That means there would be few small cosmic kibbles and bits left over.

Planetary scientist Alessandro Morbidelli of the Côte d’Azur Observatory in Nice, France, says it’s premature to draw any conclusions. Crater counts on Pluto and Charon are not reliable, he says. And while he agrees that MU69 is the “ultimate test,” he says higher resolution pictures are needed to see if its barely marred visage holds up under closer scrutiny. The current best image was taken seven minutes before New Horizons’ closest approach to MU69, when the spacecraft was still 6,700 kilometers away. Better images are on the spacecraft’s computer, waiting to be transmitted to Earth.

New Horizons will continue sending back data on MU69 until September 2020.