Asteroids hold many clues about the formation and evolution of planets and their satellites. Understanding their history can, therefore, reveal much about our solar system. While observations made from a distance using electromagnetic waves and telescopes are useful, analyzing samples retrieved from asteroids can yield much more detail about their characteristics and how they may have formed. An endeavor in this direction was the Hayabusa mission, which, in 2010, returned to Earth after 7 years with samples from the asteroid Itokawa.
The successor to this mission, called Hayabusa2, was completed near the end of 2020, bringing back material from Asteroid 162173 “Ryugu,” along with a collection of images and data gathered remotely from close proximity. While the material samples are still being analyzed, the information obtained remotely has revealed three important features about Ryugu. Firstly, Ryugu is a rubble-pile asteroid composed of small pieces of rock and solid material clumped together by gravity rather than a single, monolithic boulder. Secondly, Ryugu is shaped like a spinning top, likely caused by deformation induced by quick rotation. Third, Ryugu has a remarkably high organic matter content.
Of these, the third feature raises a question regarding the origin of this asteroid. The current scientific consensus is that Ryugu originated from the debris left by the collision of two larger asteroids. However, this cannot be true if the asteroid is high in organic content (which will confirmed once the analyses of the returned samples are complete). What could, then, be the true origin of Ryugu?
In a recent effort to answer this question, a research team led by Associate Professor Hitoshi Miura of Nagoya City University, Japan, proposed an alternative explanation backed up by a relatively simple physical model. As explained in their paper published in The Astrophysical Journal Letters, the researchers suggest that Ryugu, as well as similar rubble-pile asteroids, could, in fact, be remnants of extinct comets. This study was carried out in collaboration with Professor Eizo Nakamura and Associate Professor Tak Kunihiro from Okayama University, Japan.
Comets are small bodies that form on the outer, colder regions of the solar system. They are mainly composed of water ice, with some rocky components (debris) mixed in. If a comet enters the inner solar system— the space delimited by the asteroid belt “before” Jupiter—heat from the solar radiation causes the ice to sublimate and escape, leaving behind rocky debris that compacts due to gravity and forms a rubble-pile asteroid.
This process fits all the observed features of Ryugu, as 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.”
To test their hypothesis, the research team conducted numerical simulations using a simple physical model to calculate the time it would take for the ice to sublimate and the increase in rotational speed of the resulting asteroid due to it. The results of their analysis suggested that Ryugu has likely spent a few tens of thousands of years as an active comet before moving into the inner asteroid belt, where the high temperatures vaporized its ice and turned it into a rubble-pile asteroid.
Overall, this study indicates that spinning top-shaped, rubble-pile objects with high organic content, such as Ryugu and Bennu (the target of the OSIRIS-Rex mission) are comet–asteroid transition objects (CATs). “CATs are small objects that were once active comets but have become extinct and apparently indistinguishable from asteroids,” explains Dr. Miura. “Due to their similarities with both comets and asteroids, CATs could provide new insights into our solar system.”
Hopefully, detailed compositional analyses of the samples from both Ryugu and Bennu will shed more light on these issues. Make sure to stay tuned!
About Nagoya City University, Japan
Nagoya City University (NCU), a public university established in 1950, began with the Medical School and the Faculty of Pharmaceutical Sciences. Its origins, however, stretch back to the Nagoya School of Pharmacy, founded in 1884, and the Nagoya Municipal Women's Higher Medical School, founded in 1943. NCU has grown into an urban-style public university in the center of Nagoya, Japan, with around 4,000 students and 1,600 faculty members. In the last 60 years, NCU has graduated over 26,000 students. NCU continues to expand as an advanced education and research center to assist in the improvement of local health and welfare, as well as the development of the local economy and culture.
About Associate Professor Hitoshi Miura from Nagoya City University, Japan
Hitoshi Miura got his Ph.D. from the University of Tsukuba, Japan, in 2006. He joined Nagoya City University in 2013 as an Associate Professor. He has received awards from both the Planetary Science Society of Japan and Crystal Growth Society of Japan, as well as the 2019 Nagoya City University President’s Award. He specializes in theory of crystal growth and planetary science, and has over 45 publications to his name in these topics.
This study was partly supported by JSPS Kakenhi (numbers 19H00820 and 20K05347) and Daiko Foundation, as well as the Japanese Government Cabinet Office’s “National University Innovation Creation Project 2020” to Okayama University.
The Astrophysical Journal Letters
Method of Research
Subject of Research
The Asteroid 162173 Ryugu: a Cometary Origin
Article Publication Date