Spinning Top-Shaped Potentially Hazardous Asteroid Ryugu May Be Dead Comet

Research could have solved mysteries surrounding the origins of the spinning top-shaped asteroid Ryugu, by suggesting that it is the extinguished remains of a long-dead comet.

In 2020 the space mission Hayabusa2 returned samples and images from the space rock Asteroid 162173 or Ryugu, classified as a potentially hazardous asteroid (PHA) by NASA's Center for Near-Earth Objects Studies. This data revealed the asteroid's spinning top shape, the fact that it is a loose pile of gravitationally bound rubble, and that it is rich in organic material.

Since then, researchers have attempted to explain these characteristics with a single-origin theory. The current scientific consensus is that Ryugu originated from the debris left by the collision of two larger asteroids. But this theory fails to explain why the asteroid is high in organic content.

Now a team of scientists led by Hitoshi Miura, associate professor at Nagoya City University, Japan, believes they may have found a solution to this puzzle that addresses all of these characteristics, including the high organic content of Ryugu.

Findings published in The Astrophysical Journal Letters suggest Ryugu is the remains of a dead or extinguished comet that lost its ice content in a process that could have gifted it its unique characteristics.

Lead author Miura told Newsweek: "Depending on whether Ryugu was originally an asteroid or a comet, it experienced a very different environment.

"Asteroids formed in warm regions relatively close to the sun. On the other hand, comets formed in a cooler environment away from the sun. To assume off the top of one's head that Ryugu was originally an asteroid is to overlook the possibility that Ryugu may have been in a cold environment."

Miura said that the theory of cometary origin provides an unprecedented perspective on where Ryugu formed and how it evolved in the solar system. The researcher says it will make a big difference in interpreting the results of the analysis of the material brought back from Ryugu by Hayabusa2.

Explaining Ryugu's Unique Characteristics

Comets are small bodies that form on the outer edge of the solar system, its coldest region. They are composed predominantly of water ice, with some rocky debris mixed in. If a comet enters the inner solar system, passing by Jupiter, the heat from the solar radiation causes the ice to sublimate and escape as gas. This gives comets their characteristic tail and also leaves behind rocky debris that compacts due to gravity and forms a rubble-pile asteroid.

Miura explained that the simulations they performed showed that Ryugu has had plenty of time to form in this way. The scientist said: "According to our model, the time required for the sublimation and loss of ice from a comet nucleus with a radius of about 1.2 km [0.75 miles] is about 100,000 years. This is sufficiently short compared to the history of the solar system, about 4.6 billion years.

"Therefore, the transition from comets to asteroids is quite possible during the evolution of the solar system."

The next characteristic the team's model had to address was the asteroid's spinning top-type shape, something that could be caused by the rocky body spinning rapidly during its formation. The team found that sufficient angular momentum could have been provided by escaping gases.

Miura said: "When ice sublimates, the remaining rocky components gravitationally fall to the center of the comet nucleus, and the entire celestial body shrinks."

This accelerates the rotation of the object in the same way that a figure skater spins up by wrapping their arms around their body.

The researcher said: "Based on our computational setup, it is clear that the almost complete sublimation of ice can accelerate the comet's rotation by a factor of about four times its initial rate.

"Ryugu is thought to have taken its present shape as a result of the centrifugal deformation caused by its high-speed rotation. Our model can explain the high-speed rotation necessary for this deformation."

The final and arguably most challenging characteristic the team's model had to address was the fact that the asteroid is rich in organic material.

Miura told Newsweek: "The ice that makes up comets are thought to contain a small amount of organic matter. The refractory organic matter remains in place after the ice sublimates.

"Therefore, after the sublimation of ice, organic matter should be concentrated together with rock components."

This means that this model suggests Ryugu, and the similarly shaped, rubble-pile-asteroid Bennu, are actually comet–asteroid transition objects (CATs). This is exciting, Miura suggests, because due to their similarities with comets and asteroids, CATs could provide insights into the conditions of the solar system in its earliest moments when the planets were forming.

The researcher said: "It was surprising that a relatively simple physical model could naturally explain all the major features of Ryugu. This suggests that a similar process, the transition from comet to asteroid, may have occurred universally in the solar system.

"Currently, NASA's OSIRIS-REx asteroid probe is bringing back to Earth samples collected from another asteroid, Bennu, which is thought to have a similar shape to Ryugu. We are hopeful that our model can be applied also to Bennu."

Asteroid Ryugu Comet
(Left) an image of the asteroid Ryugu captured at a distance of 12 miles in 2018. (Right) A stock illustration of a comet. New research indicates that the spinning-top-shaped asteroid may be the remains of a long-dead comet. JAXA/ Getty/Trifonov_Evgeniy/ University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST.