Ultima Thule's Mystery Mounds Puzzle Scientists
The most distant object ever explored is weird.
Ultima Thule is lumpy, and scientists aren't sure why.
Just after midnight on Jan. 1, NASA's New Horizons spacecraft zoomed past Ultima Thule, a small, frigid object that lies about 1 billion miles (1.6 billion kilometers) beyond the orbit of Pluto.
The photos New Horizons captured during that epic encounter revealed a world unlike any other ever seen up close. The 21-mile-wide (33 km) Ultima Thule is bilobed and therefore resembles a reddish snowman — if that snowman had been flattened like a pancake.
"These are not spherical lobes at all," New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said Tuesday (April 23) during a presentation at the Outer Planets Assessment Group meeting at NASA headquarters in Washington, D.C.
"That caught us by surprise," Stern added. "I think it caught everybody by surprise."
New Horizons imagery also revealed a number of abutting mound-like features on the larger of the two lobes, which mission team members call Ultima. (The smaller lobe, naturally, is Thule.)
"They seem to be raised, but exactly what causes them we're not sure," Stern said. "It's still early days."
An early hypothesis held that the mounds resulted from convection of low-temperature ice, which was driven by the heat generated by the radioactive decay of aluminum-26. But further work suggests that this is an unlikely scenario, Stern said. The team now thinks the mounds may be the retained outlines of the small planetesimals that came together to form the Ultima lobe long ago.
"But there could be other processes as well," Stern said. "So, this is an active topic of debate."
Ultima Thule coalesced from a cloud of rocky, icy material far from the sun. These smaller chunks first formed two larger objects, which then apparently orbited a common center of mass as a binary pair, Stern said. These two bodies then slowly merged to form Ultima Thule (which is officially known as 2014 MU69).
The mission team has been able to put some "speed limits" on that merger, thanks to computer simulations. For example, if the two lobes had come together at about 22 mph (35 km/h), they likely wouldn't have merged at all; the collision would have been a glancing one, and Ultima and Thule would have gone their separate ways, Stern said.
A collision at 11 mph (18 km/h) would lead to a merger, but not one generating an object with two relatively intact lobes like Ultima and Thule; there would be considerable distortion, Stern said. So, the New Horizons team thinks the collision occurred at even slower speeds — perhaps around 5.5 mph (8.9 km/h).
Indeed, the result of simulations with a 5.5-mph merging speed "is strikingly like what we actually observe," Stern said.
Mission scientists have seen no evidence of any type of atmosphere on Ultima Thule, nor have they spotted any signs of satellites or ring systems, he added during Tuesday's talk. But the team isn't done looking on these fronts; it will take another 16 months or so for New Horizons to finish beaming all of its encounter data home to Earth.
The Ultima Thule flyby was the second such encounter for New Horizons, which famously cruised past Pluto in July 2015, providing the first good looks at that complex and surprisingly active world. And we may get further close-ups from New Horizons in the future. The spacecraft is in good health and has enough fuel to zoom past a third object, if NASA approves another mission extension, Stern has said.
Bland, no moons, no craters, no atmosphere. Welcome to Ultima Thule
The Kuiper Belt Object looks dull, but that’s what make it exciting.
In the first published results from the New Horizons spacecraft’s New Year’s Day 2019 flyby of an object officially called 2014 MU69, but more commonly known as Ultima Thule, scientists have reported that despite its unusual shape, it has a remarkably bland surface, without major variations in colour or composition.
Nor does it have any detectable moons, dust clouds, or atmosphere, the scientists report in the journal Science.
But that doesn’t mean it’s boring, says John Spencer, a planetary scientist with the Southwest Research Institute, Boulder, Colorado, US, who is one of the paper’s co-authors.
The most striking finding comes from the object’s odd shape, which features two flattened lobes joined edgewise via a narrow neck.
This structure, Spencer says, means that the two lobes began as separate objects that gradually spiraled inward toward each other in a merger that was more a kiss than a crash.
That’s important, because it gives us a snapshot of the process in which small objects came together to produce the larger objects that eventually made the planets, he says.
But the relative blandness of the surface is also important not for what’s there, but for what isn’t: craters.
A few craters exist, but compared to the destinations that other spacecraft such as NASA’s OSIRIS-REx and Japan’s Hayabusa2 have been visiting, Ultima Thule is nearly as smooth as the proverbial baby’s bottom.
“If you look at the average asteroid, they are covered in craters,” Spencer says. “They are completely beaten up.”
That’s because asteroids lie much closer to the sun in a region where there is not only a dense concentration of objects, but where orbits are easily stirred up by Jupiter’s gravity.
“That means that they’re coming in at each other at crazy angles,” Spencer explains.
The result is that they have not only experienced a great many collisions during their lifetimes, but that when they collide, they often hit hard.
Most asteroids, in fact, are shards of larger bodies ripped apart by ancient impacts, then peppered by innumerable smaller ones.
“They are not intact, pristine objects,” Spencer says.
Ultima Thule, however, lies in a far-distant part of the solar system known as the Cold Classical Kuiper Belt, a region beyond Neptune where objects sedately circle in widely spaced, well-ordered orbits. Like cars on the freeway, they are all cruising along at the same speed in basically the same direction, and therefore don’t run into each other very often.
This means that Ultima Thule’s surface remains largely undisturbed since the dawn of the solar system.
“We’ve never seen that before,” Spencer says.
But that doesn’t mean its surface is as dull a cue ball. Rather, it is mottled, as though made up of sub-units, each about five kilometres across. Eight of these have been counted on the larger lobe, and that’s only on the parts of it that New Horizons was able to see as it zipped past.
“You get the idea that it’s built of different pieces,” Spencer suggests.
One of his colleagues, he adds, has described it as looking like monkey bread, a pastry-like snack in which balls of dough are baked together in a single pan, fusing into a lumpy loaf.
Ultima Thule obviously wasn’t baked in a cosmic bread pan, so it’s an open question how its component parts came to fit so smoothly together. Perhaps they were initially soft and pliable and sort of smushed into each other as they came together, Spencer speculates.
But at the moment, he concedes, “We’re scratching our heads.”
As for other findings, the lack of an atmosphere or any detectable amounts of gases venting from Ultima Thule’s surface was no surprise.
The object is far out from the sun and has been in the same orbit for four-and-half-billion years. If it was ever going to have vented gases, it presumably would have done so a long time ago.
But it would have been silly to waste a once-in-a-lifetime opportunity to find surprises.
“We looked for things even if we were not expecting to see them,” he says.
The lack of moons, on the other hand, was a disappointment, because their orbits would have allowed scientists to calculate Ultima Thule’s mass, and from that its density, a useful clue to its interior composition.
Not that the scientists are certain that no moons exist.
“We’re still searching,” says Spencer. The spacecraft, he adds, is continuing to download data, though most of the high-priority science stuff has already been received.
The next round of scientific analyses will probably be published in August or September.
After that, the scientists will turn their attention to seeking a fresh destination for New Horizons, for yet another flyby, reachable with the spacecraft’s remaining fuel.
Finding such a destination, will be tough, however, because there aren’t all that many objects that far out, and those that do exist are so dim that they are hard to find.
“I think it’s a long shot,” Spencer says, “but we’re certainly leaving no stone unturned. We’ll give it our best shot.”
Initial results from New Horizon's exploration of distant Kuiper Belt Object 2014 MU69
On 1 January 2019, the New Horizons Spacecraft conducted a flyby of (486958) 2014 MU69 - a distant object orbiting in the outer reaches of the Solar System. In a new report, Alan Stern and colleagues present the first results from the flyby, showing that MU69 is an ancient relic that has remained largely untouched - even by the heat of the Sun - since its formation roughly 4.5 billion years ago. After passing Pluto in 2015, the New Horizons spacecraft continued into the Kuiper Belt, a region of the outer solar system beyond the orbit of Neptune populated by small icy bodies. MU69 is a Cold Classical Kuiper Belt Object, a class of objects thought to be largely undisturbed since the Solar System's formation due to their stable orbit and feeble heating from the distant Sun. As such, objects like MU69 preserve clues about the early history of the Solar System. The authors describe results from the New Horizons data that was transmitted back to Earth in the first few weeks after the flyby. Their analysis indicates that MU69 has a flattened bi-lobate shape, probably formed by the gentle collision of two smaller objects. Discrete geological units were identified on the surface, but there is little variation in color and composition. No moons, rings or dust clouds were found orbiting MU69; nor is there any evidence of an atmosphere. According to Stern et al., the insights gained in this study are based on only about 10% of the total data collected during the flyby; the full data transmission from the spacecraft to Earth is expected to be complete in 2020.