In June 2018, the Japanese Aerospace Exploration Agency (JAXA) spacecraft and sample return mission Hayabsua-2 arrived at the near-Earth asteroid 162173 Ryugu. The first scientific results from the mission, published this week in a trio of papers in the journal Science, portray Ryugu as a porous ‘pile of rubble.’
This image of the asteroid Ryugu was captured by the Optical Navigation Camera – Telescopic (ONC-T) on JAXA’s Hayabusa-2 spacecraft on June 26, 2018, from a distance of 13.7 miles (22 km). Image credit: JAXA / University of Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / Aizu University / AIST.
In one of the three studies, Dr. Sei-Ichiro Watanabe of JAXA and Nagoya University and co-authors presented the first up-close observations of Ryugu’s mass, shape and density.
“The asteroid’s low density suggests it has a highly porous interior and is a ‘rubble pile’ of loosely aggregated rocks, which formed into a spinning-top shape during a period of rapid spin,” they said.
“From the remote sensing of the asteroid, we identified potential landing sites for the spacecraft best suited for sample collection that could further inform how Ryugu got its spinning-top shape.”
Hayabusa-2 image of the asteroid Ryugu as seen from a distance of 3.7 miles. A particularly large crater is visible near the center of the image. Image credit: JAXA / University of Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / University of Aizu / AIST.
In a second study, University of Aizu’s Dr. Kohei Kitazato and colleagues used Hayabusa-2’s near-infrared spectrometer (NIRS3) to survey the surface composition of Ryugu.
They discovered that hydrated minerals are ubiquitous across the dark asteroid’s surface.
The NIRS3 spectral data are similar to known thermally-and/or shock-metamorphosized carbonaceous chondrite meteorites.
“Just a few months after we received the first data we have already made some tantalizing discoveries,” said Dr. Seiji Sugita, a researcher at the University of Tokyo and the Chiba Institute of Technology and co-author of the three studies.
“The primary one being the amount of water, or lack of it, Ryugu seems to possess. It’s far dryer than we expected, and given Ryugu is quite young — by asteroid standards — at around 100 million years old, this suggests its parent body was much largely devoid of water too.”
Global map and images of Ryugu: (A) geologic map of Ryugu; impact craters are indicated with circles; (B) an oblique view of Ryugu, showing the circum-equatorial ridge (yellow arrows), trough (blue arrows) extending from the equatorial region through the south polar region to the other side of Ryugu, and the large and bright Otohime Saxum (red arrow) near the south pole; the location of the poles and the spin direction are indicated with white arrows; (C) asymmetric regolith deposits on imbricated flat boulders on the northern slope of the circum-equatorial ridge of Ryugu; small yellow arrows at the edges of regolith deposits indicate the direction of mass wasting; the large yellow arrow indicates the current geopotential gradient from high to low; the direction of geopotential gradient is consistent with the mass wasting. Image credit: Sugita et al, doi: 10.1126/science.aaw0422.
Combining the results from the previous two studies, as well as through observation of the geological features of Ryugu, Dr. Sugita and colleagues — in a third study — attempted to constrain the origin of Ryugu.
“Small asteroids, such as Ryugu, are estimated to have been born from much older parent bodies through catastrophic disruption and reaccumulation of fragments during the Solar System evolution,” the scientists said.
“Ryugu likely formed as rubble, ejected by an impact from a larger parent asteroid.”
“The preponderance of materials on the asteroid with little water signature suggests that a dominant part of its original parent body was also ‘water poor’.”
They believe Ryugu comes from a parent asteroid several tens of kilometers wide, most likely in the asteroid families Polana or Eulalia.
“Thanks to the parallel missions of Hayabusa-2 and OSIRIS-REx, we can finally address the question of how these two asteroids came to be,” Dr. Sugita said.
“That Bennu and Ryugu may be siblings yet exhibit some strikingly different traits implies there must be many exciting and mysterious astronomical processes we have yet to explore.”
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S. Watanabe et al. Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu – A spinning top-shaped rubble pile. Science, published online March 19, 2019; doi: 10.1126/science.aav8032
K. Kitazato et al. The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy. Science, published online March 19, 2019; doi: 10.1126/science.aav7432
S. Sugita et al. The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes. Science, published online March 19, 2019; doi: 10.1126/science.aaw0422
