The mystery of the origin of the Ryugu asteroid has been solved
Of all the scientific results of that impressive mission, the most interesting may be this: Asteroid Ryugu may not be a asteroid. It could be the remnant of a comet. The Hayabusa 2 mission proved that Ryugu is an asteroid with heaps of rubble. Instead of being a large monolithic chunk of rock, it is a conglomerate of smaller rocks. Like some other asteroids, it is shaped like a spinning top. The rapid rotation of the asteroid has forged it into this shape. The authors state that “a widely accepted formation scenario for Ryugu is a catastrophic collision between larger asteroids and the subsequent slow gravitational build-up of collisional debris.”
Much of the evidence for Hyabusa 2 has supported the idea that Ryugu is an asteroid, which astronomers have speculated since its discovery in 1999. But one thing stood out among the evidence that did not fit the definition of an asteroid: Ryugu has a high concentration of organic matter.
Cometary Origin ”. The lead author is Associate Professor Hitoshi Miura of Nagoya City University.
In their article, the authors state that not only Ryugu could be the remnant of a comet, but other similar asteroids formed from mounds of rubble could be former comets. Astronomers call these objects Comet Asteroid Transition (CAT).
Comets also have an unbound atmosphere. When they approach the Sun, the heat melts some birds, creating the atmosphere and sublimating it in space. The atmosphere contains dust and volatile gases. But after passing close to the Sun many times, some comets lost all their volatiles to space. What remains is only rock. These are sometimes called extinct comets. If Ryugu is indeed a former comet, can this explain Ryugu’s characteristics?
Ryugu rotates rapidly, which could stem from his previous life as a comet. "The sublimation of the ice causes the comet's core to lose mass and shrink, which increases its speed of rotation," lead author Miura said in a press release. "As a result of this spin-up, the cometary nucleus can acquire the rotation speed necessary to model a top shape".
The research team tested their hypothesis with numerical simulations. They calculated how long it would take for Ryugu to lose all his birds and become a rocky remnant. They also calculated the increase in rotation speed needed to shape the asteroid into what it is today. "Our calculation suggests that Ryugu was once an active comet and spent the rest of his dynamic life as a cumulative asteroid of rubble," they wrote in the study. "This scenario is consistent with the dynamic evolution of modern comets in the solar system".
This study focuses on asteroids with three characteristics: top shape, composition and morphology of rubble heaps and high concentration of substances organic. The results show that Ryugu and similar asteroids are comet-asteroid transition (CAT) objects. “CATs are small objects that were once active comets but have become extinct and seemingly indistinguishable from asteroids,” explains Miura. “Because of their similarities to both comets and asteroids, CATs could provide new insights into our solar system.”
Hayabusa 2 has returned its Ryugu samples to Earth, and another mission will soon do the same. NASA's OSIRIS-REx spacecraft visited asteroid Bennu, an asteroid very similar to Ryugu, and will return its samples to Earth in 2023. Analysis of these samples should confirm whether Ryugu and Bennu are asteroids or CAT.
