Avalanche on an asteroid!? What is the evolution of rubble-pile asteroids? - Avalanches, and the formation of top-shaped asteroids and rubble-pile satellites -

Nov. 25, 2022 | GATEWAY to Academic Articles

HYODO Ryuki / Dept. of Solar System Sciences, ISAS

On Earth, an avalanche in the Alps would not collapse Waikiki Beach. However, this common knowledge only applies on Earth (with a diameter of about 12,000 km), and the story becomes completely different for asteroids less than about 1 km in diameter. Many of the asteroids with diameters of less than about 1 km that have been observed have an enigmatic top-shape figure. They are also characterized by an equatorial bulge. HYODO Ryuki (JAXA) and SUGIURA Keisuke (ELSI) performed numerical simulations of the spin-up of rubble-pile spherical bodies (spherical object consisting of numerous pieces of rock). They found that when the rotation period of the asteroid becomes smaller than a critical value (e.g., spin period of about 3 hours) with a fast-enough spin acceleration, and when the constituent rocks have some degree of friction, a global avalanche occurs in a rotation axis-symmetric manner. This changes the shape of the asteroid from a sphere to a top-shape. The particles ejected by the avalanche are distributed in a disk-like structure on the equatorial plane of the top-shaped rubble-pile asteroid. The particle disk then spreads out and the rubble-pile satellite(s) form. It was also revealed that some of the disk particles re-accrete in the equatorial region of the asteroid, forming an equatorial bulge. These findings qualitatively well explain the observed features of rubble-pile asteroids.

Research Summary

There are numerous asteroids close to the Earth. The smaller they are, the more numerous they are. Asteroids with diameters of about 1 km or less include rubble-pile asteroids, which are not monolithic, but are made up of smaller rocks that are gravitationally assembled together. Many of the rubble-pile asteroids are top-shaped asteroids with an equatorial bulge (Fig. 1). For example, Ryugu (visited by JAXA's Hayabusa2), Bennu (visited by NASA's OSIRIS-REx^*1), and Didymos (NASA's DART and ESA-JAXA's HERA^*2) are all rubble-pile asteroids with such features. In addition, Didymos is orbited by a satellite named Dimorphos.

Such asteroids are thought to have brought water and building materials for life to the ancient Earth. Today, their collisions with the Earth could cause a critical situation for life on Earth. The orbital evolution of asteroids (i.e., the likelihood of collision with the Earth) greatly depends on their shape, constituent materials, and rotation state. Therefore, understanding the shape evolution and formation process of asteroids is important not only to investigate the origin of the Earth and life, but also for planetary defense (protecting the Earth from asteroid impact).

However, until now, it has been technically difficult to directly account for effects such as friction and cohesion between particles in simulations, and it has not been possible to simultaneously account for all of the observed features. In this study, we used advanced numerical simulations and the supercomputer^*3 to study the shape evolution of rubble-pile asteroids during their rotational acceleration. The dependence on the physical properties of the constituent particles (especially frictional forces) was also investigated.

Our results revealed the following evolution (see Figure 2 and movie).

1. When the rotation period of an asteroid becomes smaller than a critical value (e.g., the rotation period of about 3 hours)^*4 with a fast-enough spin acceleration and when the friction of constituent particles is large enough, a surface landslide (global avalanche) occurs symmetrically with the rotation axis. This changes the shape of the asteroid from spherical to top-shape. (The Earth's rotation period is almost 24 hours, so 3 hours is a very fast rotation.)
2. The surface material of the rubble-pile asteroid ejected by the avalanche is distributed in a disk-like structure on the asteroid’s equatorial plane.
3. The particle disk spreads both inward and outward by collision and self-gravity. Particles spreading outward accumulate by their own gravity, forming a rubble-pile satellite. Particles spreading inward selectively re-accrete in the equatorial region of the top-shaped rubble-pile asteroid. This forms an equatorial bulge.
4. Depending on the shape and surface conditions (particle size distribution and material properties) of the top-shaped rubble-pile asteroid and the rubble-pile satellite, the satellite's orbit could be greatly expanded, and the satellite could eventually be lost. In other cases, the satellite's orbit could stabilize without significant expanding. (Although this is not the main topic of this study and requires further research)

Avalanche on asteroid!? What is the evolution of rubble-pile asteroids? (Simulation movie) - Avalanches, and the formation of top-shaped asteroids and rubble-pile satellites - Based on Hyodo & Sugiura (2022) ApJL 937 L36

The above processes may explain the top-shaped figure, the equatorial bulge, and the presence or absence of satellites observed around the rubble-pile asteroids. The "degree" of each step strongly depends on the initial shape of the asteroid, spin-up process, various physical properties of its constituent particles, and their heterogeneity within the asteroid. Combining theoretical models (numerical simulations) such as those in this study with information on asteroid systems discovered through asteroid exploration can therefore combine to provide a clearer picture of each asteroid’s past and future. Future detailed studies of each asteroid are awaited.

Currently, Ryugu and Bennu do not have satellites. However, if these asteroids have evolved as described above, satellites existed around Ryugu and Bennu in ancient times. And the satellites may have been removed by now and are wandering somewhere in the Solar System. On the other hand, the satellite Dimorphos exists around Didymos. In this case, it would mean that the rubble-pile satellite has maintained a relatively stable orbit since its formation to the present.

Terminologies

• ^*1 OSIRIS-REx is a NASA-led asteroid sample return mission. Its target is the asteroid Bennu. It has already successfully collected samples and is scheduled to return to Earth in 2023.
• ^*2 The DART mission is a NASA-led asteroid mission that aims to experimentally change the orbit of an asteroid (Dimorphos) by intentionally impacting the spacecraft with the asteroid (Dimorphos forms a binary system with the asteroid Didymos) in preparation for the future risk of an asteroid impacting the Earth. The spacecraft successfully impacted the asteroid in September 2022. The ESA-led Hera mission, scheduled for launch in 2024, will visit the Didymos-Dimorphos system to study the impact crater formed by the DART mission and a change of the binary’s orbit in more detail (JAXA is also cooperating with the Hera mission).
• ^*3 Numerical simulations in this work were carried out on Cray XC50 (ATERUI II) at Center for Computational Astrophysics, National Astronomical Observatory of Japan.
• ^*4 The rotation period of asteroids changes due to torques resulting from the absorption and emission of solar energy. The rotation period of an asteroid can also change during a small meteorite impact or a close encounter with a planet. The critical value of the rotation period at which the asteroid begins to change its shape is about 3 hours (the asteroid rotates once in about 3 hours).

Information

Links

• ITYF
• Author‘s website (HYODO Ryuki)

Author

HYODO Ryuki
International Top Young Fellow (ITYF) at ISAS/JAXA. He specializes in planet formation theory and planetary exploration. He is actively involved in building the next generation of planetary exploration missions.