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For many at the Johns Hopkins Applied Physics Laboratory, January 1 this year didn't mean a New Year's celebration. Instead, it meant the first arrival of data from New Horizons' visit to a small Kuiper Belt object. But, like its earlier flyby of Pluto, the probe was instructed to grab all the data it could and deal with getting it back to Earth later. The full set of everything New Horizons captured won't be available for more than a year yet. But with 10 percent of the total cache in hand, researchers decided they had enough to do the first analysis of 2014 MU69.
2014 MU69 is thought to preserve material as it condensed in the earliest days of the Solar System's formation. And everything in the New Horizons' data suggests that this is exactly what it has done. With the exception of one big crater temporarily named “Maryland” and the gentle collision that created its two-lobed structure, the object appears to have been largely untouched by more than 4 billion years of the Solar System's existence.
The dawn of time
The Kuiper belt is a sparse donut of small bodies near the outer edges of the Solar System. The bodies there are formed primarily of icy materials, most of which would otherwise remain gases in the warm, inner regions of the Solar System. Some of them, like Pluto, are large enough and/or have a complex collision history, which can ensure that they undergo geological changes that alter the materials that were present at their formation.
But 2014 MU69 is much smaller; early estimates placed it at under 50km in diameter. This raised the possibility that it could preserve the materials present at its formation over 4 billion years ago. Bodies like 2014 MU69, collectively termed planetesimals, also contributed to the formation of the outer planets and larger Kuiper Belt Objects. So New Horizons' study of 2014 MU69 provided a potential opportunity to better understand the conditions present at the start of the Solar System, including those that went into building larger bodies. But that would only hold true if 2014 MU69 hadn't been changed during its time in the Solar System.
To study the object, New Horizons came equipped with seven scientific instruments, including cameras, spectrometers to map its surface composition, and even particle and dust collectors to capture any material that 2014 MU69 was releasing into space. The spacecraft's closest approach was about 3,500km from 2014 MU69, which allowed it to capture images that could resolve features 50km across (and, in some cases, even smaller).
Even as the data was still coming down, the New Horizons team used it to build and refine models of the body's surface. They tested whether imaging data was consistent with the dips and curves of the model and altered the model if they weren't. The results confirm those of the earliest images: 2014 MU69 is really two objects smushed together gently until they connected via a constricted “neck.” Researchers had nicknamed 2014 MU69 “Ultima Thule,” and in this paper they refer to the larger lobe as Ultima and the smaller one as Thule.
Ultima may be the larger of the two in most dimensions, but it is also squashed flat. This gives it the appearance of a thick pancake about 20km across but only 7km thick. Thule is a bit more well-rounded, with dimensions varying between 10 and 14 kilometers, but it is also home to the most obvious impact crater on the body. That crater has been nicknamed “Maryland,” and other surface features have been tagged with the names of states where the New Horizons team works. (All of these names are temporary and will almost certainly be replaced.)
A quiet place
There's no indication that the two lobes were brought together violently. Both lobes seem to have retained their original shapes, as there aren't any signs of compression or fracture, and their orientation suggests the two spent enough time close to each other to become tidally locked before merging. The New Horizons team suggests that the two were brought together at speeds similar to what you'd expect based on their mutual gravitational pull (which, given their size, is small). However, Ultima and Thule seem to have formed somewhat separately, as the two lobes can be distinguished based on the color of their surface materials.
2014 MU69, like other Kuiper Belt objects, is reddish in color; New Horizons confirmed that the reddish tint continues into the near infrared. The brighter areas on the surface tend to be at the edge of slopes, such as in the Maryland crater and the neck between the lobes. This, the researchers suggest, may be the result of loose material piling up at the base of a slope.
While the instruments picked up plenty of indications of water ice, there was no indication of the nitrogen and methane ices that are prevalent on Pluto. Temperature estimates suggest that the whole body should have equilibrated to about 42K, which is warm enough to cause these materials to boil off; the orbit of 2014 MU69 also ensures that some areas will spend decades in the sunlight, causing localized heating that should enhance this process.
There was no indication of any material coming off the surface at the moment, however, suggesting that the water ice was stable and other volatiles were already gone. While this may be due to the limitations of New Horizons' instruments, the researchers estimate that, at most, 2014 MU69 would have lost about 10cm of material from its surface during its entire history.
Finally, there's a remarkable absence of obvious impacts aside from the large Maryland crater. Beyond that, there are some pits that are about 1km across, but these aren't clearly the product of impacts. And there's absolutely nothing in the size range between the two.
Overall, 2014 MU69 looks exactly like what we'd hope for: a world that underwent some major changes immediately after its formation but has since become static, preserving its state largely as it was billions of years ago. Hopefully, more details on that state are sitting in storage on New Horizons. Because we're not likely to send something back to 2014 MU69 any time soon.