In 2014, the Japanese Space Agency JAXA launched the Hayabusa 2 spacecraft to visit asteroid Ryugu. It arrived at the asteroid in June 2018 and studied it from orbit for over a year. Hayabusa 2 even dispatched four rovers to the asteroid’s surface. After departing, it flew past Earth in December 2020, dropping off a sample of Ryugu.

Of all the scientific results from that impressive mission, the most interesting one might be this: Asteroid Ryugu might not be an asteroid. It might be the remnant of a comet.

The Hayabusa 2 mission showed that asteroid Ryugu is a rubble pile asteroid. Instead of being one large, monolithic chunk of rock, it’s a conglomeration of smaller rocks. Like some other asteroids, it’s shaped like a spinning top. The asteroid’s rapid rotation forged it into this shape.

The authors say that “A widely accepted formation scenario for Ryugu is a catastrophic collision between larger asteroids and the subsequent slow gravitational accumulation of collisional debris.”

Hayabusa 2's Optical Navigation Camera-T captured this image of Ryugu from an altitude of about 64m. The image was taken on September 21, 2018, at around 13:04 JST. This is the highest resolution photograph obtained of the surface of Ryugu. Credit: JAXA
Hayabusa 2’s Optical Navigation Camera-T captured this image of Ryugu from an altitude of about 64m. On September 21, 2018, it took the photo at around 13:04 JST. This is the highest resolution photograph obtained of the surface of Ryugu. Credit: JAXA

A lot of Hyabusa 2’s evidence supported the idea that Ryugu is an asteroid, which astronomers assumed was the case since its discovery in 1999. But one thing stood out among the evidence that didn’t fit with the asteroid definition: Ryugu has a high concentration of organic matter.

If Ryugu is a rubble pile asteroid created from the collision of two smaller asteroids, then why does it have so much concentrated organic matter?

That question is at the heart of a new study published in The Astronomical Journal Letters titled “The Asteroid 162173 Ryugu: a Cometary Origin.” The lead author is Associate Professor Hitoshi Miura of Nagoya City University.

In their paper, the authors say that not only might Ryugu be the remnant of a comet, but similar rubble-pile asteroids might also be former comets. Astronomers call these objects Comet Asteroid Transition (CAT) objects.

Comets form in the distant cold reaches of the Solar System. Unlike asteroids, which are all rock, comets are icy and contain rock and frozen volatiles. The volatiles are mostly water ice, but comets also contain frozen carbon dioxide, ammonia, methane, and carbon monoxide. Astronomers sometimes call them “dirty snowballs.” Comets also have an unbound atmosphere. When they approach the Sun, the warmth melts some volatiles creating the atmosphere, and they sublimate into space. The atmosphere contains dust as well as volatile gases.

But after passing close to the Sun many times, some comets have lost all their volatiles to space. What’s left is just rock. Those are sometimes called extinct comets.

This figure from the study shows how a comet can transition to an asteroid. (a) shows the comet nucleus containing water ice and rock. (b) shows the ice sublimating from the outer layer as the comet enters the inner Solar System. (c) shows rocky debris accumulating on the surface and forming a porous mantle. (d) shows how the comet transforms into an asteroid after the remaining volatiles sublimate and escape through the porous mantle. Image Credit: Miura et al. 2022
This figure from the study shows how a comet can transition to an asteroid.

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