The Japanese Space Agency JAXA launched the Hayabusa 2 mission in 2014 to visit the asteroid Ryugu. It arrived at the asteroid in June 2018 and spent nearly a year studying it from orbit. Hayabusa 2 even sent four rovers to the surface of the asteroid. Following its departure, it flew by Earth in December 2020, dropping off a sample of Ryugu.

The most intriguing scientific discovery from that outstanding journey may be this: Asteroid Ryugu may not be an asteroid. It could be the remains of a comet.

According to the Hayabusa 2 mission, asteroid Ryugu is a rubble pile asteroid. It is a cluster of tiny boulders rather than a single big, monolithic block of rock. It’s formed like a spinning top, like certain other asteroids. The rapid spinning of the asteroid shaped it into this shape.

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

A large portion of the information from Hyabusa 2 supported the assumption that Ryugu is an asteroid, which astronomers had believed since its detection in 1999. Among the findings that did not fit the asteroid description, one item stuck out: Ryugu had a significant concentration of biological materials.

Why does Ryugu have so much concentrated organic materials if it is a debris pile asteroid formed by the collision of two smaller asteroids?

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

The authors speculate in their research that not only could Ryugu be a comet remnant, but that comparable rubble-pile asteroids could also be former comets. These are known as Comet Asteroid Transition (CAT) objects by astronomers.

Comets develop in the Solar System’s furthest regions. Comets, unlike asteroids, are icy and include both rock and frozen volatiles. Water ice dominates the volatiles, although comets also contain frozen carbon dioxide, ammonia, methane, and carbon monoxide. Astronomers refer to them as “dirty snowballs” at times. Comets have an unrestricted atmosphere as well. When they get close to the Sun, the heat melts some of the volatiles that make up the atmosphere, and they sublimate into space. The atmosphere contains both dust and volatile gases.

However, after several close encounters with the Sun, some comets have lost all of their volatiles to space. All that’s left is rock. These are sometimes referred to as extinct comets.

Can Ryugu’s traits be explained if he is a former comet?

Ryugu rotates quickly, which could be attributed to its prior life as a comet. “Ice sublimation causes the comet’s nucleus to lose mass and shrink, increasing its rotational speed,” lead author Miura explained in a press release. “As a result of this spin-up, the cometary nucleus may gain the rotational speed needed to create a spinning-top shape.”

According to Dr. Miura, the high organic matter content can also be explained by the extinct comet idea. CO, CO2, methanol, carbonyl sulphide, formaldehyde, formic acid, methane, and cyanate are among the organic compounds found. “In addition, organic matter created in the interstellar medium is predicted to be present in the frozen components of comets.” As the ice sublimates, these organic compounds will be deposited on the stony debris left behind.”

Ryugu and other comets contain the same organics as carbonaceous chondrite asteroids, but they are concentrated on the surface. “The exceptionally high organic content estimated from albedo may be accounted for by the local concentration,” the research reads.

The research team used numerical simulations to evaluate their idea. They calculated how long it would take Ryugu to lose all of its volatiles and turn into a stony residue. They also computed the rotational speed increase required to mould the asteroid into what it is now.

“Our calculations show that Ryugu was once a comet and active for the first several tens of thousands of years before spending the balance of its dynamic existence as a rubble-pile asteroid,” the study concludes. “This concept is consistent with the dynamical evolution of present solar system comets.”

This research focuses on asteroids that have three characteristics: a spinning top, a rubble-pile composition/morphology, and a high concentration of organics. The findings indicate that Ryugu and other comparable asteroids are comet-asteroid transition objects (CATs). “CATs are small objects that were formerly active comets but have since become extinct and appear to be indistinguishable from asteroids,” Dr. Miura adds. “CATs could reveal new insights into our solar system because to their commonalities with both comets and asteroids.”

Hayabusa 2 returned Ryugu samples to Earth, and another mission will follow suit shortly. The OSIRIS-REx probe of NASA visited asteroid Bennu, which is remarkably similar to Ryugu, and will return samples to Earth in 2023. These samples should be analyzed to determine whether Ryugu and Bennu are asteroids or CATs.