Ryugu May Be Extinct Comet

Intrigued by the presence of high levels of organic matter on Ryugu, scientists propose that rubble pile asteroids may be extinct comets.

IMAGE: The full disc of the asteroid 162173 Ryugu, as it appeared to the Hayabusa2 spacecraft at 03:50 UTC on 26 June 2018. The photograph was taken by the spacecraft’s Optical Navigation Camera — Telescopic (ONC-T) at a distance of 20 kilometres (12 miles). CREDIT: JAXA, ONC team: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.

Rubble pile asteroids. Here at CosmoQuest, we have a love-hate relationship with these objects, mostly due to our work cataloging All The Rocks on Bennu. Our work helped the [OSIRIS-REx] site selection team decide just where to punch Bennu in the face and collect a sample. Okay, okay. It was supposed to be a gentle boop. But later analysis determined it was more of a punch. Anyway, that sample is still on its way back to Earth, but Bennu wasn’t the only asteroid sample collection mission happening back in 2020.

Over in Japan, scientists were excited to retrieve their sample of the asteroid Ryugu, taken by the Hayabusa2 mission. That sample has since been divided between Japan and the rest of the planetary science community for analysis, and a few preliminary results have already been published. However, there was a lot of data taken remotely prior to the sample collection that is also being analyzed, and one of the results shows that Ryugu has a “remarkably high organic matter content.”

Friendly reminder: ‘organic content’ only means stuff that contains a lot of carbon. Ryugu isn’t harboring alien life. At least, we doubt it is.

Back to that composition. Here’s where it gets interesting. Rubble pile asteroids like Ryugu and Bennu are basically spinning tops of loosely held together rocks. Sampling them was incredibly easy because they’re so loosely bound. And these characteristics led to the conclusion that they were created when larger asteroids collided, probably in our solar system’s more violent past. But if there is a whole lot of organic matter in those asteroids, which hasn’t been confirmed in the sample analysis, yet, then that formation hypothesis cannot be correct.

And that brings us to a new paper published in The Astrophysical Journal Letters that proposes that rubble pile asteroids may actually be extinct comets, and simulations seem to confirm that this is indeed a possibility. The work, led by Hitoshi Miura, calculated how long it would take for a comet’s ice to sublimate and subsequently increase the object’s rotational speed such as we see with Ryugu. The results suggest that Ryugu probably was an active comet for tens of…