News Feature: “Celestial snowman” starts to reveal its secrets
Nola Taylor Redd
The distant rock has offered clues about planet formation and the state of the early solar system.
Within the cloud of icy rocks at the edge of the solar system lie objects that have remained virtually untouched since their formation more than four billion years ago. Last January, NASA’s New Horizons spacecraft made the first flyby of one such primitive sample, an object known as 2014 MU69 and nicknamed “Ultima Thule” (although that label has proved controversial*). After New Horizons’ successful flyby of Pluto in 2015, researchers were keen to study a primordial body that was within the craft’s reach. With MU69, that dream became a reality. The tiny object, one of only three possible destinations discovered after the mission launched, turned out to be an incredible target. “I really think we hit the jackpot,” says New Horizons principal investigator Alan Stern, of the Southwest Research Institute (SwRI) in Boulder, CO.
At first glance, MU69 looked much as researchers had imagined a pristine Kuiper Belt object (KBO) would appear, with a dark surface, rich in water ice and organic material, and relatively unscarred by craters. But when they looked closer, it offered plenty of surprises. From its shape to its spin to its composition, the distant rock is providing planetary researchers with a wealth of information about the conditions in the vicinity of the sun 4.5 billion years ago, and it’s even helping solve a decades-old puzzle about how the planets formed.
A Squashed Snowman
Researchers revealed their first results at the annual Lunar and Planetary Sciences conference in the Woodlands, TX, in March.† MU69 stretches roughly 35 kilometers (20 miles) from tip to tip and spins on its axis every 15.9 hours. Unlike most solar system inhabitants, which spin with their equator aimed at the sun, MU69 lies on its side, pole pointing sunward. The two lobes are roughly the same color, a reddish hue similar to other KBOs observed from Earth, which likely comes from materials known as tholins, formed when radiation from the sun modifies organic molecules.
Although the first image of MU69 suggested a pair of spheres, later observations revealed a big surprise. Thule is very thin, and Ultima even flatter: only 7 km thick and 22 km across. “If it’s not the flattest body in the solar system, it’s up there,” says team member William McKinnon, of Washington University in St. Louis, MO.
Most solar system objects this size are roughly spheroidal, with only a handful of exceptions. About a year before the flyby, Jeff Moore, New Horizons co-investigator at NASA Ames Research Center in Moffett Field, CA, had discussed the possibility that MU69 could be like some of the exceptions, showing images of the flattened moons of Saturn in a talk to fellow researchers. Forming in the disk of Saturn’s rings, rather than a more spherical cloud of particles, some of these moons have the appearance of a squashed walnut. “People scoffed,” Moore says. “They said, ‘whatever we’re seeing isn’t going to look like that.’”
But then New Horizons revealed the flattened hamburger lobes of Ultima and Thule. The similarity to Saturn’s moons suggests that they, too, were created in a swarm of particles spinning fast enough to form a disk. What’s more, the original axes of rotation for Ultima and Thule are almost parallel, only a few degrees apart.‡ The similar orientation suggests that the pair formed from a single cloud of material before joining together.
The final clue came from how the two lobes have retained their rounded shapes. If Ultima and Thule had slammed together at collision speeds typical of the Kuiper Belt, a few hundred meters per second, that would have been enough to deform or shatter them. Instead, they seem to have drifted gently into one another. Stern compares them to docking ships that came together without the violence common in the rest of the solar system.
This gentle docking maneuver makes sense if the pair started as part of the same cloud of material. Then they would have had similar speeds and wound up orbiting one another. As the pair orbited, other nearby material could have been ejected, reducing the system’s angular momentum and pulling the pair together; or something else entirely could have caused the two to become one. “That’s the bit I don’t think people have worked out yet,” says planetary formation researcher Harold Levison (SwRI), who is not on the New Horizons team but was present at the flyby.
A slow collision would mean that MU69’s interior probably remains loosely packed, a fluffy aggregate of ice and rock similar to the interior of comets (1). But had the lobes instead collided at high speed, their interiors would have been compacted.
Meanwhile, small lumps on the surface may be remnants from the birth cloud, the last pieces to accrete onto the KBO. Co-investigator Will Grundy, of Lowell Observatory in Flagstaff, AZ, says that the lumps appear to be homogeneous across the pair, again suggesting that they originated in the same cloud of debris.
Missing Craters
The early life of MU69 was a flurry of excitement. The two lobes formed and merged, and residual heat from the formation may have powered a bit of surface activity. But after half a billion years, most of the commotion had subsided. MU69 is too far out for the sun’s heat to change its surface. Apart from the occasional collision with a small piece of debris, the celestial snowman has remained essentially the same for the last four billion years. Although that may sound like a boring existence, it’s exactly why researchers wanted to visit the Kuiper Belt. Visiting MU69 is almost like traveling back in time to the birth of the solar system — with a few caveats.
When New Horizons flew by Pluto, researchers were stunned to see how few intermediate-sized craters§ covered the surface of the dwarf planet and its large moon, Charon. MU69 shows the same paucity, with only a handful of midsized excavations. “We see some very nice-looking craters on MU69, just not very many of them,” says co-investigator Kelsi Singer (SwRI).
The dearth reveals a lot about MU69’s virtually unchanged environment over the last 4.5 billion years. Objects that would generate such craters would be tens to hundreds of meters across. These could form directly from gas and dust or be chipped off a larger body in a collision, but the shortage of craters suggests that neither of these processes prevailed in the Kuiper Belt. “It looks like it just didn’t initially make a lot of small objects and just kept not making them,” Singer says. New Horizons’ resolution made it difficult to definitively identify craters, but there aren’t many potential midsized scars that could have come from medium impacts. The largest potential crater scars Thule, stretching about halfway across the lobe’s diameter. Nicknamed Maryland, the crater has raised questions on the timing of its formation.
Although the colliding object would have been about 10 times smaller than the crater it excavated, the collision would still have been significant. One way to avoid breaking Ultima and Thule apart would be to have Maryland smash into the smaller lobe before the pair came together, but it’s hard to determine which came first. Based on her previous work with impact craters, Singer doesn’t think that the Maryland smackdown would have been that destructive. Moore agrees, pointing out that the survival of MU69 would depend on its interior. The porous interiors of comets act like Styrofoam, absorbing the impact energy of a collision, he says. If MU69 is similarly porous on the inside, it could have absorbed the blow from the impact without flying apart. “The impact that formed Maryland might have had essentially no effects on the two lobes of the binary,” Moore says.
A comet-like interior could also explain some geological differences between the two lobes. Although Ultima is relatively smooth, Thule has more features on its surface. Moore thinks many of those features may have formed when the two lobes merged. He says that the orientation of grooves, regional uplifts, and surface depressions on Thule make them more likely to be tied to its gentle bump with Ultima than the hit from Maryland. Because it’s smaller, Thule would have been harder hit by the impact than the larger Ultima. “Thule basically might have been split at the seams,” Moore says, allowing methane, nitrogen, and other volatile material to escape its interior. Movement of that material could have driven the processes that shaped Thule’s landscape.
Planet Building
Fully understanding the images of MU69 will take some time, but the first view is already helping shed some light on how planets formed. As rocky pieces move through the gas and dust around a newborn star, they should collide with one another to form larger and larger bodies. It is thought that they would have built up to objects like MU69, which then came together to form larger objects known as planetesimals, which in turn built the planets, from Mercury to Pluto (2). But there is a missing chapter to this story.
Simulations can turn bits of dust into so-called pebbles up to a meter in size and transform larger, kilometer-sized objects into planetary cores, but they had a problem in the middle, going from small pebbles to rocks kilometers across (3). The computer models suggested collisions would blow apart growing planetary embryos in this size range. Known as the meter-sized barrier, the problem has plagued planetary formation for decades.
In recent years, new theories have emerged to help with the problem. One idea, known as the streaming instability (4), suggests that gas in the disk creates drag, forcing these small objects to concentrate into clumps. Gravity then pulls the clumps together to create kilometer-sized objects in a matter of tens of thousands to hundreds of thousands of years, depending on the solar distance no midsized material is necessary. A popular extension of this theory, known as pebble accretion, suggests that smaller objects continue to fall onto the kilometer-sized objects to rapidly build the cores of giant planets.
Both the pancake shape of MU69 and its dearth of midsized craters seem to validate these theories. If large objects form via the streaming instability, there will be few projectiles in the range of tens of meters to create those craters. And according to David Nesvorny, who models early solar system formation at SwRI, the flat lobes of MU69 are also telling. The shapes are too flat to have come from the initial spin of a cloud of material. Such a large cloud would have had too much angular momentum to hold together and, instead, split into two objects. A loose collection of material in the outer solar system could have gathered together, as proposed by the streaming instability. Gravity would have pulled most of it together, creating objects flatter than a sphere but not quite pancake-like, then pebble accretion would have piled most of the remaining pieces on top, creating the flattened lobes visible today. Eventually, Ultima and Thule would have drawn closer together, gently docking with one another.
The researchers haven’t yet simulated the process for MU69 specifically, although they hope to. If correct it could solve the problem of how to build a planet. “That meter-sized barrier that we all stressed out about, we just jumped right over it,” says astronomer Kevin Walsh of SwRI. “We all suspected something like that but these observations from the Kuiper Belt really seem to clinch it.”
Previous models have always worked with spherical bodies rather than pancakes, in part because of limited computer power, according to McKinnon. “[Simulations] might have tracked angular momentum, but they certainly didn’t try to build shapes,” he says. The new results from MU69 have likely changed that. “Now I’m sure people will [attempt to simulate final shapes] because we have the capabilities to do this sort of stuff.”
MU69 may live at the edge of the solar system, but the processes that formed it probably worked throughout, Nesvorny says. Earth and other planets likely formed from similarly flattened objects. And the process is not limited to our planetary neighborhood. “Elsewhere in exoplanetary systems, it might be quite common to form pancake and contact binaries,” he says.
New Horizons will be sending back data on MU69 until the end of 2020, and plenty of further study will be needed, but it already has provided a wealth of insight about both the Kuiper Belt and the entire solar system. “We had no right to expect that we would get so much from this one small body,” Stern says, “but we have.”
