NEWS FEATURE

“Celestial snowman” starts to reveal its secrets NEWS FEATURE The distant rock has offered clues about planet formation and the state of the early solar system.

Nola Taylor Redd, Science Writer

Within the cloud of icy rocks at the edge of the solar At first glance, MU69 looked much as researchers system lie objects that have remained virtually un- had imagined a pristine Kuiper Belt object (KBO) would touched since their formation more than four billion appear, with a dark surface, rich in water ice and organic years ago. Last January, NASA’s New Horizons space- material, and relatively unscarred by craters. But when craft made the first flyby of one such primitive sample, they looked closer, it offered plenty of surprises. From an object known as 2014 MU69 and nicknamed “Ul- its shape to its spin to its composition, the distant rock is tima Thule” (although that label has proved controver- providing planetary researchers with a wealth of in- formation about the conditions in the vicinity of the sun sial*). After New Horizons’ successful flyby of Pluto in 4.5 billion years ago, and it’s even helping solve a 2015, researchers were keen to study a primordial decades-old puzzle about how the planets formed. body that was within the craft’s reach. With MU69, that dream became a reality. The tiny object, one of only A Squashed Snowman three possible destinations discovered after the mis- Researchers revealed their first results at the annual “ sion launched, turned out to be an incredible target. I Lunar and Planetary Sciences conference in the † really think we hit the jackpot,” says New Horizons Woodlands, TX, in March. MU69 stretches roughly principal investigator Alan Stern, of the Southwest Re- 35 kilometers (20 miles) from tip to tip and spins on its axis search Institute (SwRI) in Boulder, CO. every 15.9 hours. Unlike most solar system inhabitants,

Researchers have started to map out the various geological features of 2014 MU69, including troughs (black lines), scarp crests (notched lines), a feature circling unit mh dubbed “The Road to Nowhere,” and a large crater (lc) dubbed the Maryland Crater. Here, the magenta areas labeled pm are patterned material; green areas labeled rm are rough material, and the blue areas labeled um are undifferentiated material. Image credit: Wikimedia Commons/NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/ESA.

Published under the PNAS license. *After a naming campaign led by the SETI Institute labeled 2014 MU69 “Ultima Thule,” some astronomers and others objected. The age-old term, which essentially means the farthest point or far to the north or a land beyond known lands, was co-opted by the Nazi party during Adolf Hitler’s rise to power—part of that party’s vision of a mythical land of Aryan purity. But others, including Alan Stern of SwRI, supported the label, suggesting that it evoked exploration and that researchers shouldn’t let the Nazis hijack the term. † S. Porter et al., “A contact binary in the Kuiper Belt: The shape and pole of (486958) 2014 MU69” in Proceedings of the 50th Lunar and Planetary Science Conference. LPI Contribution No. 2132, id.2737 (2019).

www.pnas.org/cgi/doi/10.1073/pnas.1913629116 PNAS | September 17, 2019 | vol. 116 | no. 38 | 18749–18752 Downloaded by guest on September 26, 2021 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 sys- tem’s angular momentum and pulling the pair to- gether; or something else entirely could have caused In 2006, NASA launched the New Horizons spacecraft, here being inspected at “ ’ ’ the Kennedy Space Center in Merritt Island, FL, a year prior to launch. After its the two to become one. That s the bit I don t think 2015 Pluto encounter, New Horizons headed for MU69. Image credit: NASA. people have worked out yet,” says planetary forma- tion researcher Harold Levison (SwRI), who is not on the New Horizons team but was present at the flyby. which spin with their equator aimed at the sun, A slow collision would mean that MU69’s interior MU69 lies on its side, pole pointing sunward. The probably remains loosely packed, a fluffy aggregate of two lobes are roughly the same color, a reddish hue ice and rock similar to the interior of comets (1). But similar to other KBOs observed from Earth, which had the lobes instead collided at high speed, their likely comes from materials known as tholins, formed interiors would have been compacted. when radiation from the sun modifies organic Meanwhile, small lumps on the surface may be molecules. remnants from the birth cloud, the last pieces to accrete Although the first image of MU69 suggested a pair onto the KBO. Co-investigator Will Grundy, of Lowell of spheres, later observations revealed a big surprise. Observatory in Flagstaff, AZ, says that the lumps appear Thule is very thin, and Ultima even flatter: only 7 km to be homogeneous across the pair, again suggesting thick and 22 km across. “If it’s not the flattest body in the that they originated in the same cloud of debris. solar system, it’s up there,” says team member William McKinnon, of Washington University in St. Louis, MO. Missing Craters Most solar system objects this size are roughly The early life of MU69 was a flurry of excitement. The spheroidal, with only a handful of exceptions. About a two lobes formed and merged, and residual heat from year before the flyby, Jeff Moore, New Horizons co- the formation may have powered a bit of surface ac- investigator at NASA Ames Research Center in Moffett tivity. But after half a billion years, most of the com- motion had subsided. MU69 is too far out for the sun’s Field, CA, had discussed the possibility that MU69 heat to change its surface. Apart from the occasional could be like some of the exceptions, showing images collision with a small piece of debris, the celestial of the flattened moons of Saturn in a talk to fellow re- snowman has remained essentially the same for the searchers. Forming in the disk of Saturn’s rings, rather last four billion years. Although that may sound like a than a more spherical cloud of particles, some of these boring existence, it’s exactly why researchers wanted moons have the appearance of a squashed walnut. to visit the Kuiper Belt. Visiting MU69 is almost like “ ” “ ‘ People scoffed, Moore says. They said, whatever traveling back in time to the birth of the solar system— ’ ’ ’” we re seeing isn t going to look like that. with a few caveats. But then New Horizons revealed the flattened When New Horizons flew by Pluto, researchers hamburger lobes of Ultima and Thule. The similarity to were stunned to see how few intermediate-sized cra- Saturn’s moons suggests that they, too, were created ters§ covered the surface of the dwarf planet and its in a swarm of particles spinning fast enough to form a large moon, Charon. MU69 shows the same paucity, disk. What’s more, the original axes of rotation for with only a handful of midsized excavations. “We see Ultima and Thule are almost parallel, only a few de- some very nice-looking craters on MU69, just not very ‡ grees apart. The similar orientation suggests that the many of them,” says co-investigator Kelsi Singer (SwRI).

‡ W. McKinnon et al., “A pristine “contact binary” in the Kuiper §K. Singer et al., “Impact craters on 2014 MU69: Implications for Belt: Implications from the New Horizons encounter with 2014 the geologic history of MU69 and Kuiper Belt population size- MU69 (“Ultima Thule”)” in Proceedings of the 50th Lunar frequency distributions” in Proceedings of the 50th Lunar and Planetary Science Conference. LPI Contribution No. 2132, and Planetary Science Conference. LPI Contribution No. 2132, id.2767 (2019). id.2239 (2019).

18750 | www.pnas.org/cgi/doi/10.1073/pnas.1913629116 Redd Downloaded by guest on September 26, 2021 Astronomers are starting to piece together how Ultima and Thule joined together, as explained in this graphic composed by postdoc James Tuttle Keane at California Technical Institute in Pasadena. Image credit: NASA/JHUAPL/SwRI/James Tuttle Keane.

The dearth reveals a lot about MU69’s virtually them more likely to be tied to its gentle bump with unchanged environment over the last 4.5 billion years. Ultima than the hit from Maryland. Because it’s Objects that would generate such craters would be smaller, Thule would have been harder hit by the im- tens to hundreds of meters across. These could form pact than the larger Ultima. “Thule basically might directly from gas and dust or be chipped off a larger have been split at the seams,” Moore says, allowing body in a collision, but the shortage of craters suggests methane, nitrogen, and other volatile material to that neither of these processes prevailed in the Kuiper escape its interior. Movement of that material could Belt. “It looks like it just didn’tinitiallymakealotofsmall have driven the processes that shaped Thule’s objects and just kept not making them,” Singer says. landscape. New Horizons’ resolution made it difficult to definitively identify craters, but there aren’t many potential midsized Planet Building scars that could have come from medium impacts. The Fully understanding the images of MU69 will take largest potential crater scars Thule, stretching about some time, but the first view is already helping shed halfway across the lobe’s diameter. Nicknamed Mary- some light on how planets formed. As rocky pieces land, the crater has raised questions on the timing of its move through the gas and dust around a newborn formation. star, they should collide with one another to form Although the colliding object would have been largerandlargerbodies.Itisthoughtthatthey about 10 times smaller than the crater it excavated, would have built up to objects like MU69, which then the collision would still have been significant. One way came together to form larger objects known as to avoid breaking Ultima and Thule apart would be to planetesimals, which in turn built the planets, from have Maryland smash into the smaller lobe before the Mercury to Pluto (2). But there is a missing chapter to pair came together, but it’s hard to determine which this story. came first. Based on her previous work with impact cra- Simulations can turn bits of dust into so-called ters, Singer doesn’t think that the Maryland smackdown pebbles up to a meter in size and transform larger, would have been that destructive. Moore agrees, kilometer-sized objects into planetary cores, but they pointing out that the survival of MU69 would depend had a problem in the middle, going from small pebbles on its interior. The porous interiors of comets act like to rocks kilometers across (3). The computer models Styrofoam, absorbing the impact energy of a collision, suggested collisions would blow apart growing plane- he says. If MU69 is similarly porous on the inside, it tary embryos in this size range. Known as the meter- could have absorbed the blow from the impact without sized barrier, the problem has plagued planetary for- flying apart. “The impact that formed Maryland might mation for decades. have had essentially no effects on the two lobes of the In recent years, new theories have emerged to help binary,” Moore says. with the problem. One idea, known as the streaming A comet-like interior could also explain some instability (4), suggests that gas in the disk creates geological differences between the two lobes. Al- drag, forcing these small objects to concentrate into though Ultima is relatively smooth, Thule has more clumps. Gravity then pulls the clumps together to features on its surface. Moore thinks many of those create kilometer-sized objects in a matter of tens features may have formed when the two lobes of thousands to hundreds of thousands of years, merged. He says that the orientation of grooves, re- depending on the solar distance no midsized material gional uplifts, and surface depressions on Thule make is necessary. A popular extension of this theory, known

Redd PNAS | September 17, 2019 | vol. 116 | no. 38 | 18751 Downloaded by guest on September 26, 2021 as pebble accretion, suggests that smaller objects about, we just jumped right over it,” says astronomer continue to fall onto the kilometer-sized objects Kevin Walsh of SwRI. “We all suspected something to rapidly build the cores of giant planets. like that but these observations from the Kuiper Belt Both the pancake shape of MU69 and its dearth of really seem to clinch it.” midsized craters seem to validate these theories. If Previous models have always worked with spher- large objects form via the streaming instability, there ical bodies rather than pancakes, in part because of will be few projectiles in the range of tens of meters to limited computer power, according to McKinnon. create those craters. And according to David Nesvorny, “[Simulations] might have tracked angular momen- who models early solar system formation at SwRI, the tum, but they certainly didn’t try to build shapes,” he flat lobes of MU69 are also telling. The shapes are too says. The new results from MU69 have likely changed flat to have come from the initial spin of a cloud of that. “Now I’m sure people will [attempt to simulate material. Such a large cloud would have had too much final shapes] because we have the capabilities to do angular momentum to hold together and, instead, this sort of stuff.” split into two objects. A loose collection of material in MU69 may live at the edge of the solar system, the outer solar system could have gathered together, buttheprocessesthatformeditprobablyworked as proposed by the streaming instability. Gravity throughout, Nesvorny says. Earth and other planets would have pulled most of it together, creating ob- likely formed from similarly flattened objects. And the jects flatter than a sphere but not quite pancake-like, process is not limited to our planetary neighborhood. then pebble accretion would have piled most of the “Elsewhere in exoplanetary systems, it might be quite remaining pieces on top, creating the flattened lobes common to form pancake and contact binaries,” he says. visible today. Eventually, Ultima and Thule would New Horizons will be sending back data on have drawn closer together, gently docking with MU69 until the end of 2020, and plenty of further one another. study will be needed, but it already has provided a The researchers haven’t yet simulated the process wealth of insight about both the Kuiper Belt and the for MU69 specifically, although they hope to. If correct entire solar system. “We had no right to expect that it could solve the problem of how to build a planet. we would get so much from this one small body,” “That meter-sized barrier that we all stressed out Stern says, “but we have.”

1 S. A. Stern et al., Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object. Science 364, 6441 (2019). 2 N. T. Redd, Inner workings: Newborn stars don’t have enough dust to build planets. What are the missing ingredients? Proc. Natl. Acad. Sci. U.S.A. 116, 7605–7607 (2019). 3 S. Weidenschilling, Aerodynamics of solid bodies in the solar nebula. Mon. Not. R. Astron. Soc. 180,57–70 (1977). 4 A. Youdin, J. Goodman, Streaming instabilities in protoplanetary disks. Astrophys. J. 620, 459 (2005).

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