The ancient parent body of the near-Earth ‘rubble pile’ asteroid Ryugu had likely dried out in some kind of heating event before the asteroid came into being, according to an analysis of observations of Ryugu’s subsurface material by the Near-Infrared Spectrometer (NIRS3) onboard JAXA’s Hayabusa-2 spacecraft.
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.
“Ryugu is a rubble pile, a loose conglomeration of rock held together by gravity,” said co-author Dr. Ralph Milliken, a planetary scientist at Brown University, and colleagues.
“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.”
In April 2019, the Hayabusa-2 mission successfully completed an artificial impact experiment on Ryugu, which provided an opportunity to investigate exposed subsurface material and determine which scenario was more likely.
Using the NIRS3 instrument, which is capable of detecting water-bearing minerals, the mission scientists 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,” Dr. 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.”
The findings appear in the journal Nature Astronomy.
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K. Kitazato et al. Thermally altered subsurface material of asteroid (162173) Ryugu. Nat Astron, published online January 4, 2021; doi: 10.1038/s41550-020-01271-2
