Remote sensing data sheds light on when and how asteroid Ryugu lost its
water

Date:
January 5, 2021
Source:
Brown University
Summary:
Rocks on Ryugu, a 'rubble pile' near-Earth asteroid recently
visited by Japan's Hayabusa2 spacecraft, appear to have lost much
of their water before they came together to form the asteroid,
new research suggests.



FULL STORY ==========================================================================
Last month, Japan's Hayabusa2 mission brought home a cache of rocks
collected from a near-Earth asteroid called Ryugu. While analysis of
those returned samples is just getting underway, 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, researchers offer an explanation
for why Ryugu isn't quite as rich in water-bearing minerals as some
other asteroids. The study suggests that the ancient parent body from
which Ryugu was formed had likely dried out in some kind of heating event before Ryugu came into being, which left Ryugu itself drier than expected.

"One of the things we're trying to understand is the distribution of water
in the early solar system, and how that water may have been delivered to Earth," said Ralph Milliken, a planetary scientist at Brown University
and study co- author. "Water-bearing asteroids are thought to have played
a role in that, so by studying Ryugu up close and returning samples from
it, we can better understand the abundance and history of water-bearing minerals on these kinds of asteroids." One of the reasons Ryugu was
chosen as a destination, Milliken says, is that it belongs to a class
of asteroids that are dark in color and suspected to have water-bearing minerals and organic compounds. These types of asteroids are believed to
be possible parent bodies for dark, water- and carbon-bearing meteorites
found on Earth known as carbonaceous chondrites. Those meteorites have
been studied in great 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 come from.

The Hayabusa2 mission represents the first time a sample from 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 to be as water-rich as scientists
originally expected. There are several competing ideas for how and when
Ryugu may have lost some of its water.

Ryugu is a rubble pile -- a loose conglomeration of rock held together
by gravity. Scientists think these asteroids likely form from debris
left over when larger and more solid asteroids are broken apart by a
large impact event.

So it's possible the water signature seen on Ryugu today is all that
remains of a previously more water-rich parent asteroid that dried out due
a heating event of some kind. But it could also be that Ryugu dried out
after a catastrophic disruption and re-formation as a rubble pile. It's
also possible that Ryugu had a few close spins past the sun in its past,
which could have heated it up and dried out its surface.



==========================================================================
The Hayabusa2 spacecraft had equipment aboard that could help scientists
to determine which scenario was more likely. During its rendezvous with
Ryugu in 2019, Hayabusa2 fired a small projectile into the asteroid's
surface. The impact created a small crater and exposed rock buried in
the subsurface. Using a near-infrared spectrometer, which is capable
of detecting water-bearing minerals, the researchers could then compare
the water content of surface rock with that of the subsurface.

The data showed the subsurface water signature to be quite similar to
that of the outermost surface. That finding is consistent with the idea
that Ryugu's parent body had dried out, rather than the scenario in
which Ryugu's surface was dried out by the sun.

"You'd expect high-temperature heating from the sun to happen mostly at
the surface and not penetrate too far into the subsurface," Milliken
said. "But what we see is that the surface and subsurface are pretty
similar and both are relatively poor in water, which brings us back to
the idea that it was Ryugu's parent body that had been altered." More
work needs to be done, however, to confirm the finding, the researchers
say. For example, the size of the particles excavated from the subsurface
could influence the interpretation of the spectrometer measurements.

"The excavated material may have had a smaller grain size than what's
on the surface," said Takahiro Hiroi, a senior research associate at
Brown and study co-author. "That grain size effect could make it appear
darker and redder than its coarser counterpart on the surface. It's
hard to rule out that grain-size effect with remote sensing." Luckily,
the mission isn't limited to studying samples remotely. Since Hayabusa2 successfully returned samples to Earth in December, scientists are about
to get a much closer look at Ryugu. Some of those samples may soon be
coming to the NASA Reflectance Experiment Laboratory (RELAB) at Brown,
which is operated by Hiroi and Milliken.

Milliken and Hiroi say they're looking forward to seeing if the laboratory analyses corroborate the team's remote sensing results.

"It's the double-edged sword of sample return," Milliken said. "All of
those hypotheses we make using remote sensing data will be tested in the
lab. It's super-exciting, but perhaps also a little nerve-wracking. One
thing is for certain, we're sure to learn a lot more about the links
between meteorites and their parent asteroids."

========================================================================== Story Source: Materials provided by Brown_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. K. Kitazato, R. E. 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, M. A. Barucci, R. Brunetto,
C.

Pilorget, F. Poulet, J.-P. Bibring, D. L. 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, Y. Tsuda. Thermally altered subsurface material
of asteroid (162173) Ryugu. Nature Astronomy, 2021; DOI:
10.1038/s41550-020-01271-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/01/210105130127.htm

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