Ryugu latest: asteroid is a partially dehydrated ‘pile of rubble’

A one-kilometre-wide asteroid known as 162173 Ryugu, currently being mapped by Japan’s Hayabusa2 spacecraft, looks like a giant “pile of rubble” shaped like a spinning top, planetary scientists report in a trio of papers in the journal Science.

The asteroid also has extremely low density, indicating that it is riddled with porous gaps, much like a chunk of Styrofoam or Swiss cheese. 

To determine the asteroid’s shape, the scientists used photographs from multiple angles, and laser range-finding mapping of its topography to create a 3D model. 

Its porosity was calculated by watching as the spacecraft fell to within 850 metres of the surface, then rose to 5.4 kilometres. Based on this, says Sei-ichiro Watanabe, of Nagoya University, Japan, it was possible to determine the asteroid’s mass, volume, and average density.

The third of these are so low that gaps of various sizes must make up half of the asteroid’s volume, he says – assuming it is made of the same type of minerals as a class of meteorites known as carbonaceous chondrites. 

The spinning top shape, he adds, is probably a leftover from its youth, when rapid rotation deformed it into a flattened shape with a prominent equatorial ridge that still persists, even though its rotation has now slowed to a sedate 7.6 hours. 

Why this ridge has not slowly flattened out, Watanabe says, is an open question, but he thinks it’s because the surface materials on Ryugu must not easily avalanche. 

“It seems the friction angle of materials on Ryugu is large enough, so the relaxation process would be rather slow,” he says.

In another study, a team led by Seiji Sugita, of the University of Tokyo, Japan, used data from the spacecraft’s near-infrared spectrometer to find that water-containing minerals are “ubiquitous” across Ryugu’s surface.

That said, these materials appear to have been “thermally metamorphosed” and partially dehydrated. 

Most likely, Sugita says, Ryugu stems from a parent body that was formed about 4.56 billion years ago, at the birth of the Solar System.{%recommended 8408%}

That body experienced a high level of rock-water reactions in its interior, which formed the hydrated minerals now found on the asteroid.

After that, however, it went through a partial dehydration process, possibly due to internal heating that caused some of these hydrated minerals to break down, thereby losing water to space.

Then, about a billion years ago, a large collision broke up the parent body. 

One possible parent body would have been the predecessor to the Polana collisional family, named for the 38-kilometre asteroid known as 1112 Polona, the parent of which broke up 1.4 billion years ago. The other would have been the parent body for 495 Eulalia, a similar-sized asteroid and the product of a break-up 800 million years ago.

Debris from the collision then re-accumulated into rubble piles, one of which became Ryugu, or perhaps its more immediate parent body, from which Ryugu was generated via yet another impact.

That said, it’s too soon to attempt to generalise these findings to other asteroids, Watanabe says, because Hayabusu2 is not the only mission currently investigating a near-earth asteroid. 

NASA’s OSIRIS-REx mission is currently orbiting the slightly smaller asteroid called 101955 Bennu, and is also expected to announce its initial findings very soon. 

“Comparative study of Ryugu and Bennu will tell us their commonalities and differences,” Watanabe says.

Meanwhile, he adds, Hayabusa2 is a sample-return mission, designed to collect material from the asteroid’s surface and bring it back to Earth for analysis by the end of next year. One goal of the current studies is to pick the right place from which to collect the sample – ideally one that helps us understand how the asteroid got its spinning-top shape.

Another goal is to understand how asteroids such as Ryugu helped bring water to Earth, says Kohei Kitazato, from the University of Aizu, Fukushima, and lead author of the third paper published in Science. 

“In order to know how much asteroids like Ryugu contributed to terrestrial water, it is important to understand how much water had been preserved in their interior,” he says.

“Our observations on Ryugu strongly suggest that the abundance of water bearing materials (and organics) on asteroids is controlled by dehydration due to radiogenic heat during its early history.

“This may have influenced how much water and organics Earth received from the asteroid belt, when life was born.”

Studying these asteroids may also be important to understanding the threats they may pose to our future, if someday we find ourselves faced with such a body on collision course with Earth.

In April, Watanabe says, an experiment called the Small Carry-on Impactor Experiment will shoot a 2.5-kilogram copper bullet at the asteroid in order to find out how its surface layer is glued together. The results could be useful if, someday in the future, we find ourselves faced with the question of how to divert such a body before it hits us.