NEWS FEATURE Diamonds in the rubble NEWS FEATURE Two ancient asteroids with distinctive diamond-like shapes are revealing clues not only about their own formation, but that of the solar system as well. Nola Taylor Redd, Science Writer For the past three years, two spacecraft have been studying a pair of the most primitive solar system objects ever explored. The asteroids, Ryugu and Bennu, born of violent collisions in the asteroid 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 Earth, 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/University of Arizona/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: Hayabusa2 in Before researchers could hope to study these aster- 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 Hayabusa 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-Regolith 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 25, 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 meteorites came from asteroids. More than 22,000 meteorites have been identified on Earth, including some originating from Mars and the Moon. But researchers only began classifying asteroids by type in the 1970s and 80s, and definitively identifying a specific parent body 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 (25143 Itokawa 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 dust coalesced to form Earth and other planets. 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 asteroid belt between Mars and Jupiter, 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 equator 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 equators 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 25, 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 lander, 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.