How did the asteroid Ryugu lose its water? Remote sensing data provides an explanation


Asteroid Ryugu Hayabusa2 spaceship

The Japanese spacecraft Hayabusa2 took photos of the asteroid Ryugu as it flew alongside it two years ago. The spacecraft then returned rock samples from the asteroid back to Earth. Credit: JAXA

The rocks of Ryugu, a near-Earth “rubble pile” asteroid recently visited by the Japanese spacecraft Hayabusa2, appear to have lost much of their water before coming together to form the asteroid, new research suggests.

Last month, Japan’s Hayabusa2 mission brought home a cache of rocks collected from a near-Earth asteroid called Ryugu. As analysis of these returned samples has only just begun, researchers are using data from the spacecraft’s other instruments to reveal new details about the asteroid’s past.

In a study published in Nature astronomy, the researchers explain why Ryugu is not as rich in aquifer minerals as some other asteroids. The study suggests that the old parent body that Ryugu was formed from likely dried out during some sort of heating event before Ryugu was born, which left Ryugu himself drier than expected. .

“One of the things we’re trying to figure out is the distribution of water in the early solar system, and how that water was able to get to Earth,” said Ralph Milliken, planetary at Brown University and co. -author of the study. “Water-containing asteroids are believed to have played a role in this, so by studying Ryugu closely and returning samples, we can better understand the abundance and history of water-containing minerals on these types of asteroids.

One of the reasons Ryugu was chosen as the destination, says Milliken, is that it belongs to a class of asteroids that are dark in color and believed to contain aqueous minerals and organic compounds. These types of asteroids are believed to be possible parent bodies for the dark, water and carbon-containing meteorites found on Earth known as carbon chondrites. These meteorites have been studied in detail in laboratories around the world for many decades, but it is not possible to determine with certainty which asteroid a given carbonaceous chondrite meteorite may have come from.

The Hayabusa2 mission represents the first time that a sample of one of these intriguing asteroids has been directly collected and returned to Earth. But observations of Ryugu made by Hayabusa2 as it flew alongside the asteroid suggest it may not be as water rich as scientists initially expected. There are several competing ideas for how and when Ryugu lost some of his water.

Ryugu is a rubble heap – a loose conglomeration of rocks held together by gravity. Scientists believe these asteroids likely formed from debris left behind when larger, stronger asteroids were shattered by a large impact event. So it’s possible that the water signature seen over Ryugu today is all that remains of a previously water-rich parent asteroid that has dried out due to a heating event. But it could also be that Ryugu has dried out after a catastrophic disturbance and a replenishment into a rubble heap. It’s also possible that Ryugu had a few close laps in front of the sun in his past, which could have warmed it up and dried out its surface.

The Hayabusa2 spacecraft had equipment on board that could help scientists determine which scenario was more likely. During his date with Ryugu in 2019, Hayabusa2 fired a small projectile at the asteroid’s surface. The impact created a small crater and exposed rocks buried in the basement. Using a near-infrared spectrometer, capable of detecting mineral aquifers, researchers could then compare the water content of the surface rock with that of the subsoil.

The data showed that the signature of the groundwater was quite similar to that of the outermost surface. This finding is consistent with the idea that Ryugu’s parents’ body had dried up, rather than the scenario in which Ryugu’s surface was sun-dried.

“You would expect the high temperature heating of the sun to occur primarily on the surface and not penetrate too far into the basement,” Milliken said. “But what we see is that the surface and the subsoil are quite similar and both are relatively poor in water, which brings us back to the idea that it was Ryugu’s parental body that had been altered.

However, more work needs to be done to confirm the discovery, the researchers say. For example, the size of the particles extracted from the subsoil could influence the interpretation of spectrometer measurements.

“The excavated material may have a smaller particle size than what is on the surface,” said Takahiro Hiroi, senior research associate at Brown and co-author of the study. “This grain size effect could make it appear darker and redder than its coarser counterpart on the surface. It is difficult to rule out this grain size effect with remote sensing. “

Fortunately, the mission is not limited to studying samples from a distance. Since Hayabusa2 managed to return samples to Earth in December, scientists are set to take a closer look at Ryugu. Some of these samples may soon arrive at NASA Reflectance Experiment Laboratory (RELAB) at Brown, which is operated by Hiroi and Milliken.

Milliken and Hiroi say they can’t wait to see if lab tests corroborate the team’s remote sensing results.

“It’s the double-edged sword of sample return,” said Milliken. “All of these hypotheses that we make using remote sensing data will be tested in the lab. It’s super exciting, but maybe a little scary as well. One thing is for sure, we will certainly learn a lot more about the connections between meteorites and their parent asteroids. “

Reference: “Thermally Altered Subsurface 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, Nature astronomy.
DOI: 10.1038 / s41550-020-01271-2

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