Craters, bulgy mounds and a collar

When NASA’s New Horizons spacecraft returned its first images from its New Year’s Eve flyby of Kuiper Belt object Ultima Thule, 6.6 billion kilometres from Earth, many observers were startled to see that it looked amazingly like BB-8, the rolling droid of Star Wars fame.

But in a new, much more detailed image, it looks more like a chubby snowman seen from its side – complete with a dish-shaped face, dark eyes, a frown, and a string of buttons down its belly. 

Not that any of these features have anything to do with snow. The eyes, buttons, and probably the frown appear to be craters, their rims cast into sharp relief by slanting sunlight.

The new image, released on 25 January, is the first to be presented by NASA since 5 January, after which press operations were shuttered due to the US government’s 35-day partial shutdown.

It differs from prior images in part because it was taken from a different angle, with better lighting conditions. But it has also been sharpened by a process known as deconvolution, which enhances fine detail. 

It was taken when New Horizons was only 6700 kilometres away, just seven minutes before its closest approach.

Many of the newly detected features appear to be craters. In the case of the eyes and buttons, these craters are about 700 metres in diameter; the frown appears to be associated with a larger depression, about seven kilometres across, that might also be a crater.

But the arrangement of these craters raises questions about what formed them. 

“The fact that some are in lines is an argument against them being impacts,” says John Spencer, a planetary scientist from the Southwest Research Institute (SwRI) in Colorado, US, and a member of the New Horizons team. Instead, they could be collapse features that might reveal structures beneath Ultima Thule’s surface.{%recommended 8460%}

That said, it’s not clear that the clustering of these features near the terminator (the zone marking the border between day and night) truly indicates that similar craters might not exist nearby. 

“We do see hints of similar features away from the terminator,” Spencer says, “but whether they would cast shadows depends on the angle of the sunlight relative to the surface. [That] depends on the shape of the surface, which we’re still working on—it’s not necessarily a simple spherical shape.” 

Also intriguing is the fact that the larger lobe of Ultima Thule is covered with circular features that appear to be bulgy mounds, not craters, each of them remarkably smooth.

“They’re pretty intriguing,” Spencer says. “They might be smaller components that Ultima Thule was built from, or they might be something that happened to it after it was assembled.”

Even more dramatic is the bright “collar” surrounding the neck between Ultima Thule’s two lobes. This had been observed on the earlier image, but it stands out even more sharply on the new one. 

“We’re still debating whether it’s due to unusual slopes in the neck region – the gravity there is complicated – some marker of the collision that brought the two bodies together, or unusual temperatures due to shadowing and trapping of heat radiation,” Spencer says. “Everything is still on the table. We’ll get better 3D shape models, better colour and compositional data, and somewhat better images, which will hopefully help figure it out.”

Meanwhile, NASA is reopening – at least until 15 February, when the present injection of funding runs out and America may run into another partial government shutdown. But Spencer and other scientists from grant-funded organisations like SwRI aren’t quite as dependent on the vagaries of US politics as NASA itself. 

“We’ll be presenting more considered analysis at the Lunar and Planetary Science Conference in Houston [18-22 March], along with additional data,” Spencer says, “so expect a lot more helpful answers there.”

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