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 thousands of years, and then it moved into the inner asteroid belt. There, the higher temperatures quickly sublimated out any remaining volatiles and left only… well, a loosely held, spinning top of rocks.

Dr. Miura explains: Ice sublimation causes the nucleus of the comet to lose mass and shrink, which increases its speed of rotation. As a result of this spin-up, the cometary nucleus may acquire the rotational speed required for the formation of a spinning-top shape. Additionally, the icy components of comets are thought to contain organic matter generated in the interstellar medium. These organic materials would be deposited on the rocky debris left behind as the ice sublimates.

All this is a long-winded way to say that rubble-pile asteroids are likely a type of solar system body known as a CAT or comet-asteroid transition object. The categorization of small bodies in our solar system continues to become more of a spectrum and less of one or the other. And Dr. Miura notes: Due to their similarities with both comets and asteroids, CATs could provide new insights into our solar system.

Now we just wait for more results from the analysis of the physical samples to see if they confirm this hypothesis or not. We’ll update you as soon as they’re published.

More Information

Nagoya City University press release

“The Asteroid 162173 Ryugu: a Cometary Origin,” Hitoshi Miura, Eizo Nakamura, and Tak Kunihiro, 2022 January 31, The Astrophysical Journal Letters