How has Ryugu, the asteroid, lost its water? An interpretation is given by remote sensing data.

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New research indicates that Rocks on Ryugu, a “debris pile” near-Earth asteroid recently visited by Japan’s Hayabusa2 spacecraft, seems to have lost most of its water before coming together to form the asteroid.

A supply of rock samples from the near-Earth asteroid Ryugu was brought home by Japan’s Hayabusa2 spacecraft last month.

Though study of these returned samples has just started, researchers are using data from the other instruments of the spacecraft to learn new information about the history of the asteroid.

The researchers give an explanation for why Ryugu is not as abundant in hydrous minerals as some other asteroids in a report published in Nature Astronomy.

The study indicates that before Ryugu emerged, the ancient parent body from which Ryugu formed was possibly dried out by some kind of heating, rendering Ryugu itself drier than expected.

Ralph Milliken, a planetary scientist at Brown University and co-author of the report, said, “One of the things we’re trying to understand is the distribution of water in the early solar system and how that water might have gotten to Earth,” “Water-bearing asteroids are thought to have played a role in this. By studying Ryugu up close and bringing back samples from it, we can better understand the abundance and history of hydrous minerals on this type of asteroid.”

According to Milliken, one of the reasons Ryugu was picked as a priority is that it belongs to a class of asteroids that are dark in color and are assumed to contain hydrous minerals and organic compounds.

For dark, water- and carbon-bearing meteorites known on Earth as carbonaceous chondrites, certain forms of asteroids are considered potential parent bodies.

In laboratories around the world, these meteorites have been studied in detail for decades, but it is not possible to say with certainty which asteroid a specific carbonaceous chondrite meteorite may have come from.

The Hayabusa2 mission is the first time that a sample has been directly obtained and returned to Earth from one of these intriguing asteroids.

But Ryugu’s observations made by Hayabusa2 during its asteroid flyby indicate that the asteroid might not be as water-rich as originally predicted by scientists.

There are a variety of conflicting thoughts about how and when Ryugu could have lost some of its water.
Ryugu – a loose array of rocks kept together by gravity – is a debris pile. Scientists assume that when larger and more stable asteroids break away from a massive impact, these asteroids potentially form from debris left over. So it’s likely that all that’s left of a previously more water-rich parent asteroid that dried up due to some kind of heating is the water signature seen on Ryugu nowadays.

It could also, however, be that after a disastrous shattering and reforming, Ryugu dried up as a pile of rubble.

It is also likely that a few times in the past, Ryugu moved close to the sun, which may have heated it up and dried up its surface.

There was equipment on board the Hayabusa2 spacecraft that could help scientists decide which scenario is more likely.

Hayabusa2 launched a small projectile at the asteroid’s surface during its 2019 rendezvous with Ryugu.

A small crater and exposed rocks buried underground were formed by the impact. The researchers were then able to compare the water content of the surface rock with that of the subsurface by using a near-infrared spectrometer capable of detecting water-containing minerals.

The data shows that the subsurface’s water signature is very close to that of the atmosphere.

This result is consistent with the hypothesis that the parent body of Ryugu was desiccated, rather than the scenario that the sun desiccated Ryugu’s surface.

Milliken said, “You would expect the high-temperature heating from the sun to be mainly at the surface and not penetrate too far underground,” “But what we’re seeing is that the surface and subsurface are pretty similar and both are relatively low in water, which brings us back to the idea that it was Ryugu’s parent body that was altered.”

To validate this observation, however, further work needs to be done, the researchers note.

The size of the particles emanating from the subsurface, for example, may be

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