The Japanese Hayabusa2 spacecraft took photos of the asteroid Ryugu while flying past it two years ago. The spacecraft later returned rock samples from the asteroid to Earth. Credit: JAXA
Rocks on Ryugu, a “rubble” near the Earth asteroid recently visited by the Japanese spacecraft Hayabusa2, appear to be losing much of their water before they came together to form the asteroid, new research shows.
Last month, the Hayabusa2 mission in Japan brought home a cache of rocks collected from a near-Earth asteroid called Ryugu. While analyzing the returned samples is in progress, researchers are using data from the spacecraft’s other instruments to reveal new details about the asteroid’s past.
In a study published in Natural Astronomy, researchers explain why Ryugu is not quite as rich in water-bearing minerals as some other asteroids. The study suggests that the ancient parent body from which Ryugu was formed probably dried out in a kind of warming event before Ryugu came into being, leaving Ryugu himself drier than expected.
“One of the things we are trying to understand is the distribution of water in the early solar system, and how the water was possibly delivered to Earth,” said Ralph Milliken, a planetary scientist at Brown University and co-author of the study said. “It is believed that waterborne asteroids played a role in this, so by studying Ryugu more closely and returning samples from it, we can better understand the abundance and history of waterborne minerals on these types of asteroids.”
According to Milliken, one of the reasons Ryugu was chosen as the destination was that it belonged to a class of asteroids that were dark in color and presumably contained water-bearing minerals and organic compounds. It is believed that these types of asteroids are possible parent bodies for dark, water and carbonaceous meteorites that occur on Earth, known as carbonaceous chondrites. These meteorites have been studied for decades in laboratories around the world, but it is not possible to determine with certainty from which asteroid a given carbonaceous chondrite meteorite may come.
The Hayabusa2 mission is the first time a sample of one of these intriguing asteroids has been collected directly and returned to Earth. But observations of Ryugu made by Hayabusa2 as it flew past the asteroid suggest that it may not be as water-rich as scientists originally expected. There are several competing ideas on how and when Ryugu may have lost some of his water.
Ryugu is a pile of rubbish – a loose composition of rock held together by gravity. Scientists think that these asteroids are likely to be formed as a result of debris left over as larger and firmer asteroids break apart due to a major impact event. It is therefore possible that the water signature seen on Ryugu today remains from a previously more water-rich parent asteroid that dried up due to a warming event of some kind. But it could also be that Ryugu has dried up after a catastrophic disruption and reform as a mess. It is also possible that Ryugu has had a few turns past the sun in the past, which could have heated it up and dried out the surface.
The Hayabusa2 spacecraft had equipment on board that could help scientists determine which scenario was more likely. During its encounter with Ryugu in 2019, Hayabusa2 fired a small projectile into the surface of the asteroid. The impact buried a small crater and exposed rock in the subsoil. Using a near-infrared spectrometer, which is able to detect water-bearing minerals, the researchers can then compare the water content of surface rock with that of the subsoil.
The data showed that the groundwater signature is very similar to that of the outer surface. This finding is consistent with the idea that Ryugu’s parent body has dried out, rather than the scenario in which Ryugu’s surface has dried out by the sun.
“You would expect high temperatures from the sun to occur mostly on the surface and not penetrate too far into the subsurface,” Milliken said. “But what we see is that the surface and subsoil are pretty much the same and that both are relatively poor, which again makes us think it’s Ryugu’s parent body that has changed.”
However, more work needs to be done to confirm the finding, the researchers say. The size of the particles excavated from the subsurface, for example, can influence the interpretation of the spectrometer measurements.
“The excavated material may have had a smaller grain size than that on the surface,” said Takahiro Hiroi, a senior research fellow at Brown and co-author of the study. ‘That grain size effect can make it look darker and redder than its coarser counterpart on the surface. It is difficult to rule out the grain size effect with remote sensing. ”
Fortunately, the mission is not limited to studying monsters remotely. Since Hayabusa2 successfully returned samples to Earth in December, scientists will take a closer look at Ryugu. Some of these monsters may soon be after the NASA Reflectance Experiment Laboratory (RELAB) in Brown, operated by Hiroi and Milliken.
Milliken and Hiroi say they are looking forward to seeing if the lab analysis confirms the team’s results on remote sensing.
“This is the double-edged sword of the sample yields,” Milliken said. ‘All the hypotheses we make using remote sensing data will be tested in the laboratory. It’s super-exciting, but maybe also a little nerve-wracking. One thing is for sure, we will surely learn a lot more about the links between meteorites and their parent asteroids. ”
Reference: “Thermally altered suburface material of asteroid (162173) Ryugu” by K. Kitazato, RE Milliken, T. Iwata, M. Abe, M. Ohtake, S. Matsuura, Y. Takagi, T. Nakamura, T. Hiroi, M. Matsuoka, L. Riu, Y. Nakauchi, K. Tsumura, T. Arai, H. Senshu, N. Hirata, MA Barucci, R. Brunetto, C. Pilorget, F. Poulet, J.-P. Bibring, DL Domingue, F. Vilas, D. Takir, E. Palomba, A. Galiano, D. Perna, T. Osawa, M. Komatsu, A. Nakato, T. Arai, N. Takato, T. Matsunaga, M Arakawa, T. Saiki, K. Wada, T. Kadono, H. Imamura, H. Yano, K. Shirai, M. Hayakawa, C. Okamoto, H. Sawada, K. Ogawa, Y. Iijima, S. Sugita, R. Honda, T. Morota, S. Kameda, E. Tatsumi, Y. Cho, K. Yoshioka, Y. Yokota, N. Sakatani, M. Yamada, T. Kouyama, H. Suzuki, C. Honda, N Namiki , T. Mizuno, K. Matsumoto, H. Noda, Y. Ishihara, R. Yamada, K. Yamamoto, F. Yoshida, S. Abe, A. Higuchi, Y. Yamamoto, T. Okada, Y. Shimaki, R Noguchi, A. Miura, N. Hirata, S. Tachibana, H. Yabuta, M. Ishiguro, H. Ikeda, H. Takeuchi, T. Shimada, O. Mori, S. Hosoda, R. Tsukizaki, S. Soldini, M. Ozaki, F. Terui, N. Ogawa, Y. Mimasu, G. Ono, K. Yoshikawa, C. Hirose, A. Fujii, T. Takahashi, S. Kikuchi, Y. Takei, T. Yamaguchi, S. Nakazawa, S. Tanaka, M. Yoshikawa, S. Watanabe and Y. Tsuda, January 4, 2021, Natural Astronomy.
DOI: 10.1038 / s41550-020-01271-2