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Home , 101955 Bennu, 162173 Ryugu, Dante Lauretta, Hayabusa, Rubble pile

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Diamonds in the rubble NEWS FEATURE Two ancient with distinctive diamond-like shapes are revealing clues not only about their own formation, but that of the as well.

Nola Taylor Redd, Science Writer

For the past three years, two have been studying a pair of the primitive solar system objects ever explored. The asteroids, Ryugu and Bennu, born of violent collisions in the belt, are some of the smallest objects ever visited by spacecraft. Ryugu is just over a kilometer across, and Bennu is about half as wide. But these small bodies are promising a huge bounty of data to astronomers over the next few years. Although the findings could revolutionize our under- standing of the history, formation, and composition of asteroids, they may also tell us a little about the solar system during its infancy. That’s because these carbon-rich (1) asteroids, with their coal-dark surfaces, are also among the darkest bodies known in the solar system. Although that can make it difficult to observe them from , studying The asteroid Bennu ejects particles from its surface in January 2019—a them could tell us about the building blocks of life that phenomenon captured in an image composed from multiple photos. Image credit: NASA/Goddard//Lockheed Martin. such asteroids may have delivered to Earth early in the history of the solar system. separated launch times, the spacecraft arrived at their Long Strange Trips respective targets only six months apart: in Before researchers could hope to study these - June 2018 and OSIRIS-REx in December. Almost im- oids, they first had to reach them. This, by itself, was an mediately, the spacecraft began to reveal secrets about enormous challenge. Only a handful of spacecraft their respective asteroids. have visited asteroids, and fewer still have managed to Both missions got up close and personal with their orbit one of these tiny, irregularly shaped objects. hosts. Not long after its arrival, OSIRIS-REx became the The spacecraft, launched by the Japa- world record holder for closest orbit, a record it went nese Aerospace Exploration Agency (JAXA) in 2003, on to break as it traveled even closer to Bennu. became the first to collect a sample of an asteroid. But Hayabusa2 dropped several small rovers which bounced the mission was beset by engine problems. Hayabusa’s across the surface of Ryugu, capturing photos and the prospects for returning with a sample remained un- public’s imagination. The Japanese craft went on to certain for much of its journey. “shoot” Ryugu with a special bullet that blew fine dusty On December 3, 2014, JAXA launched the material into the air that the spacecraft could capture, Hayabusa2 mission to Ryugu. A newly designed en- allowing it to probe beneath the asteroid’s surface. gine that relies on ionized xenon gas for propulsion “Before OSIRIS-REx and Bennu, we [had] never effortlessly sent the spacecraft on a 3.5-year, 280- visited such primordial asteroids,” says Yuchira Cho, million-kilometer journey—a striking contrast to the Haybusa2 team member. problems that plagued its predecessor. Hayabusa2 is already on its way back to Earth with About two years after the launch of Hayabusa2, a sample of Ryugu, whereas OSIRIS-REx has selected NASA sent its Origins, Spectral Interpretation, Resource the site from which to carve out a bit of the asteroid to Identification, Security- Explorer (OSIRIS-REx) carry back home. The clues they deliver could be hurtling from Earth to Bennu. Despite their widely revealing.

Published under the PNAS license.

www.pnas.org/cgi/doi/10.1073/pnas.2005076117 PNAS Latest Articles | 1of4 Downloaded by guest on September 26, 2021 Despite this potential window into the past, re- searchers know little about asteroids. The first one wasn’t discovered until 1801. It wasn’t until 1959 that researchers realized that many came from asteroids. More than 22,000 meteorites have been identified on Earth, including some originating from and the . But researchers only began classifying asteroids by type in the 1970s and 80s, and definitively identifying a specific has been a challenge. Then, in 1991, a spacecraft made the first flyby of an asteroid; since then, missions have buzzed more than a dozen of them. And so far, only the first Hayabusa mission has ever grabbed a piece of an asteroid ( in 2005) and brought it back to Earth. Like Ryugu and Bennu, Itokawa is a so-called rub- ble pile asteroid (2), the remains of a larger object that shattered in a collision. But whereas Ryugu and Bennu arerichincarbon,Itokawaisastonyasteroid,the source of the most common types of meteorites. The For scale, the rock casting a shadow on the bumpy boulder (Top Center) is 4 feet potato-shaped Itokawa has few craters or boulders (1.1 meter) long. The image is from the mission’s Orbital B phase in August on its surface. This finding led many to suspect that of 2019. Image credit: NASA/Goddard/University of Arizona. Hayabusa2 and OSIRIS-REx would encounter similar surfaces and have little trouble selecting samples from Solar System Historians their targets. That turned out not to be the case. Both About 4.5 billion years ago, a giant disk of rocks, gas, Ryugu (3) and Bennu (4) were covered with boulders and coalesced to form Earth and other . that make collecting samples a challenge. And their boulder-strewn surfaces weren’t the only surprises. Asteroids are some of the leftover pieces from this process and can provide insights into how our solar Diamond Delights system came to be. Most asteroids are found in the Radar and Hubble Space Telescope observations of between Mars and , a vast field Bennu had revealed its distinctive diamond shape well that looks nothing like Hollywood’s portrayal of it. The before OSIRIS-REx arrived. But Ryugu is farther away asteroids are usually widely separated, and collisions and its shape had remained indistinct. Studies from are few and far between. Asteroids have changed far Earth suggested a roughly spherical object, or one less than most other solar system bodies like Earth and with a dumpling-like appearance. Only when Hayabusa2 the Moon, making these small rocky bodies a valuable drew close to the asteroid did it become clear that Ryugu too is diamond shaped (5). “Until we got there, window into the early solar system. to the proximity of the asteroid, I don’t think we could really finally answer,” says Seiji Sugita, a professor at the University of Tokyo, Japan, and member of the Haybausa2 team. Both Ryugu and Bennu are small parts of larger asteroids, born when a massive object smashed into their parent bodies and blew off a cloud of debris. Rubble piles like Ryugu and Bennu (6) are made up of a variety of loosely bound rocks from both the surface and interior of the larger source asteroid. “We’re getting a glimpse at the previous body that was ca- tastrophically disrupted and then reaccumulated into this grab bag of rocks from that parent body,” says Erica Jawin, a researcher at the Smithsonian Museum of Natural History, in Washington, DC, and part of the OSIRIS-REx team. Gravity pulled debris together, and like an ice skater pulling his arms in, the asteroid be- gan to spin faster and faster, and the rocks preferen- tially accumulated near the of the asteroid, The intended touchdown (Arrow) of Hayabusa2 was near creating a bulge that gives the asteroids their di- ’ Ryugu s northern hemisphere, photographed here with amond top shape. an optical navigation camera in January of 2019. Image credit: JAXA, University of Tokyo, Kochi University, The ridges running around the of both Rikkyo University, Nagoya University, Chiba Institute of asteroids revealed a further surprise. Ryugu’s ridge is Technology, Meiji University, University of Aizu, AIST. prominent, whereas Bennu’s is muted. Several large

2of4 | www.pnas.org/cgi/doi/10.1073/pnas.2005076117 Redd Downloaded by guest on September 26, 2021 impact craters scar the ridges on both asteroids, fea- tures which Kevin Walsh, a planetary modeler at the Southwest Research Institute in Boulder, CO, and an OSIRIS-REx team leader, calls “shocking”: The craters are evidence that the equators on these asteroids are older than expected. The finding casts doubt on our understanding of how and when these asteroids formed. Planetary re- searchers thought that the asteroids probably didn’t achieve their diamond shape until after they had mi- grated, about a billion years after their birth, to the quieter, less violent neighborhood nearer to Earth. If so, impact craters on the equatorial ridges should be small and less plentiful than if the craters had formed back in the asteroid belt. The thinking now is that the impacts that created the massive craters most likely came early in the life of Upper Left Ryugu and Bennu, when they were still in the asteroid The small asteroid , MASCOT ( ), separates from the Hayabusa2 spacecraft in October belt, but after the ridges were formed. “These aster- 2019 and heads to the surface of Ryugu to collect data. oids might have gotten their shape when they were Image credit: JAXA, University of Tokyo, Kochi born, or at least a very long time ago,” he says. That’s University, Rikkyo University, Nagoya University, Chiba something researchers didn’t expect. Institute of Technology, Meiji University, University of Aizu, AIST. A Field of Boulders Both Ryugu and Bennu had another surprise in store collisions or impacts. Bennu’stwolargestrocksmayap- for mission researchers. Instead of surfaces that were pear different because they are made up of different relatively smooth like Itokawa, both asteroids featured material, or their differences may arise from weathering fields of boulders. According to Hayabusa2 team processes, with the largest boulder having been exposed member Sho Sasaki, Ryugu boasts more than 4,400 to the surface longer. The massive boulders and their boulders larger than 5 meters in diameter. Bennu has smaller siblings could provide a glimpse into the condi- more than 3,100 boulders that are at least a meter tions present at the start of the solar system. across, Jawin says, with large boulders spread across the surface in similar proportions to Ryugu. Peering Inside Extremely large boulders are rare; Bennu has two The largest boulders may catch the eye, but the ’ giants, and Ryugu has only one. Bennu s biggest boul- midsized rocks can also provide valuable information. ders are 58 meters and 95 meters across. The smaller of One class of objects, described as having “a crumbly, the two is bright, composed of relatively fine-grained cauliflower-type appearance,” was visible in the night- material. Observations revealed a meter-size chunk of time hops made by Mobile Asteroid Surface Scout embedded rock, or clast, but overall the rock seems to (MASCOT), a lander dropped on Ryugu’s surface which have very little surface variation. The boulder protrudes sent images back to its mother ship for a few short about three meters above the surface, Jawin says. . A second type of rock, with smooth, angular Bennu’s larger rock is a dark, partially buried object features and linear fractures, was only visible during the with several clasts embedded in its surface. The tex- descent. Together, the two types dominate the surface ture appears hummocky, a geological term that de- of Ryugu. Fine rocky material, expected to be found in scribes lines in the rock similar to small knolls. When the shines directly overhead, the stone giant abundance on the asteroid, was surprisingly absent. becomes the hottest part of the asteroid, about 20 The low-flying spacecraft revealed that roughly 5% degrees warmer than the rest of the surface, Jawin of the 500 boulders larger than 12 meters on Ryugu “ says, and spectral studies of the rock suggest that it are cracked. Fractured boulders are also common ” may be made up of . across the surface of Bennu, Walsh says. Researchers Ryugu has its own distinctive 160-meter rock, which are still trying to determine what cracked the rocks. Hayabusa2 researchers named Otohime, for a princess Impacts and can cause rocks to split, but the from a Japanese folktale. Otohime is a dark rock lo- leading theory is that repeated heating and cooling cated near the asteroid’s south pole, with sharp edges caused by exposure to the sun caused the cracks. and smooth surfaces. Its vertical face is the brightest The new observations may be the first direct evi- surfaceonRyugu. dence of such a process on a solar system object other “We have two very distinct boulders on Bennu,” says than Earth. “We expect the stresses induced at the Jawin, and first casual impressions suggest that “one of very center of the materials are going to want to those looks very similar to the large boulder on Ryugu.” propagate, creating linear fracturing in boulders,” says The trio of large boulders on the two asteroids could researcher and OSIRIS-REx team member Jamie provide insights about the bodies that birthed Ryugu and Molaro of the Institute head- Bennu.Allthreearetoolargetohaveformedthrough quartered in Tucson, AZ.

Redd PNAS Latest Articles | 3of4 Downloaded by guest on September 26, 2021 Cracked boulders on Bennu reveal a little more “We Only Need One” about what’s going on inside. Molaro says that Bennu’s Although the bounty of boulders may provide signif- cauliflower-like boulders appear brittle. “They really icant scientific insights, they have also caused a just look like they are crumbling and falling apart,” few problems for the two sample return missions. she says. Some of the rough surfaces have bright spots Hayabusa2 was slated to begin the first of three sample that could be the result of material landing on the collections in October 2018. However, the boulders boulders. Others appear to have inclusions, small bits caused a delay as researchers rushed to find a safe way of material encased by the larger rocks which may to collect fine material that was unexpectedly rare on help to advance the fragmentation process. the surface. Hayabusa2 didn’t gather its first sample Ryugu’s rocks also hold hidden treasures. MASCOT until February 2019, nearly 18 months after its arrival. carried not only a camera but four colored LED lights, Like its predecessor, the spacecraft fired a small pro- allowing it to capture images in the dark. By shining jectile at the asteroid and scooped up material blown different colors of light on the rocks, the LEDs revealed out from the surface. Its collection complete, it left inclusions of a variety of colors too dim to see in - Ryugu for Earth in December 2019 with a planned ar- light, some of them shining as much as four times rival date in December 2020. brighter than the surrounding material. “We are not sure Bennu faced similar challenges. OSIRIS-Rex mis- what these inclusions are,” Hayabusa2 team member sion planners had expected to find the equivalent of half a football field worth of space to navigate among Stefanus Schroder said last year at the Lunar and Plan- the boulders. Instead, they found open spaces to be etary Sciences Conference in The Woodlands, TX. rare, with only a handful of regions between 5 and Back on Earth, researchers compared the Ryugu 20 meters across that were devoid of large, potentially rocks to various meteorites, and found that, under hazardous boulders. similar conditions, those rocks are similar to the Murchi- In December 2019, OSIRIS-REx finally announced son , which fell to Earth near Murchison, its landing site, a region named Nightingale near Australia, in 1969. The red and blue inclusions in the Bennu’s north pole. Nightingale sits in a crater within a meteorite both contain (7) that had been altered crater. “We made our final decision based on which by water at some time in the past, Schroder says. Olivine site has the greatest amount of fine-grained material is present in a class of meteorites known as carbona- and how easily the spacecraft can access that material ceous chondrites, and the similar inclusions could sug- while keeping the spacecraft safe,” OSIRIS-REx prin- gest a link between those meteorites and asteroids like ciple investigator , a planetary researcher ’ Ryugu. However, because MASCOT couldn tmake at the University of Arizona in Tucson, announced at the spectral observations that shed light on the aster- American Geophysical Union conference in San Diego, ’ oid s mineral composition, the identification of ol- CA, in December. The spacecraft began reconnaissance ivine remains tentative. flights over Nightingale and a backup site and has be- When Hayabusa2 fired a ball at the surface gun rehearsals for its touch-and-go sample collection of Ryugu, it uncovered even more information about attempt, which is scheduled for August 2020. Unlike the asteroid. The layers of material revealed in the Hayabusa2, OSIRIS-Rex’s sampling arm will reach down subsequent crater strengthened the case for a rubble to the asteroid’s surface, blow nitrogen gas at it, and pile asteroid of different material. Intriguingly, the collect the dislodged material. Then it will leave Bennu resulting crater was larger than anticipated, calling for Earth sometime around March 2021. into question the scaling laws used to date the aster- Despite the intricacies of the maneuver, OSIRIS- oid duo (8). REx researchers remain confident that they will be “Perhaps the ridge did form relatively recently, able to safely collect material from the asteroid for one erasing all previous craters and showing only the last of the sites they have identified. Jawin knows it will be 10 million years of cratering on the surface,” says a major feat, perhaps more challenging than originally Walsh, who called the new research “a big result, and thought, but remains hopeful that they’ll retrieve just surprising.” enough. “We only need one,” she says.

1 R. Jaumann et al., Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites. Science 365, 817–820 (2019). 2 K. J. Walsh, asteroids. Annu. Rev. Astron. Astrophys. 56, 593–624 (2018). 3 K. Kitazato et al., The surface composition of asteroid from Hayabusa2 near-infrared . Science 364,272–275 (2019). 4 D. S. Lauretta et al., OSIRIS-REx Team, The unexpected surface of asteroid (101955) Bennu. 568,55–60 (2019). 5 S. Watanabe et al., Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-A spinning top-shaped rubble pile. Science 364, 268–272 (2019). 6 D. N. DellaGiustina et al., Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis. Nat. Astron. 3, 341–351 (2019). 7 V. E. Hamilton et al., OSIRIS-REx Team, Evidence for widespread hydrated minerals on asteroid (101955) Bennu. Nat. Astron. 3, 332– 340 (2019). 8 M. Arakawa et al., An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime. Science, 368, 67–71 (2020).

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