TopicsRyugu’s fast-spinning past shaped
the asteroid’s western hemisphere!

The Astrophysical Journal Letters is an academic journal for research results in astronomy and astrophysics published in the United States. Our paper on the shape of asteroid Ryugu was recently included in the journal (publication date: March 26, 2019) entitled:

Hirabayashi, M., and 28 colleagues, “The western bulge of 162173 Ryugu formed as a result of a rotationally driven deformation process,” The Astrophysical Journal Letters, 2019, 874, 1, doi:10.3847/2041-8213/ab0e8b .

In this article, we will highlight our main findings from this research.

Ryugu has the shape of an abacus bead, which was introduced in detail in three recent papers led by members of the Hayabusa2 team –Sei’ichiro Watanabe, Seiji Sugita and Kohei Kitazato—who published in the academic journal, Science Magazine.

Prior to the spacecraft arriving at Ryugu, the asteroid’s shape was known to be simply “round-ish” but the angular abacus bead structure was not yet seen. How did such a shape arise? In truth, while there are various hypotheses, no certain explanation has been found. The most prominent idea is the high-speed rotation theory, which was described in detail in the paper by Sei’ichiro Watanabe.

In this new paper, we focused on the fact that while at first glance, the shape of Ryugu is roughly axisymmetric about the rotation axis, a closer inspection shows this is not exactly true. As introduced in the paper led by Seiji Sugita, the surface layer of Ryugu differs between the east and west regions, bordered by the Tokoyo and Horai Fossae. The region to the west, commonly known as the western bulge, has fewer craters and a smoother surface than the region to the east. In addition, the Ryujin Dorsum (equatorial ridge) has a very steep inclination angle where it passes through the western bulge (figure 1).

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    Figure 1: When viewed from a certain direction (90 degrees west), the angle of the equatorial ridge on Ryugu becomes very sharp. This feature is also discussed in the paper led by Sei’ichiro Watanabe. (Hirabayashi, M.+, 2019)

In this paper, we made a structural analysis of Ryugu to examine the formation of the western bulge. Using this, we could explore the result of structural relaxation that would occur after any changes to the shape of Ryugu that may have happened in the asteroid’s past. The detailed structural analysis suggested that the mysterious difference in surface properties between Ryugu’s east and west regions may have been due to structural changes (such as a landslide or internal alternations) during a period in Ryugu’s history when the asteroid was spinning at high speed (figure 2). This result supports the high speed rotation theory for explaining Ryugu’s odd shape.

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    Figure 2: While Ryugu was rotating at high speed, part of the asteroid’s interior suffered structural collapse (yellow). There is no collapse in the green area. The region where structural collapse did not occur overlaps with the region of the western bulge, indicated by the dashed line (Hirabayashi, M.+, 2019).

The ideas in this paper were triggered by the frequent discussions that were took place within the ONC (Optical Navigation Camera) and Science teams. After arriving at Ryugu, new images were sent from the spacecraft nearly daily and prompted a limitless series of questions that all began “why is this here?” Finding the answers to these questions was kind of like solving a puzzle.

Questions were discussed daily within the ONC and Science teams at regular meetings. “No – that can’t be it, lol!” was a key phrase for suggesting great ideas. Ideas can be hidden all over the place and tracking these down is fun. These active mid-summer discussions with the ONC team are all part of this paper.

Lastly, this paper should be considered to the beginning of further research, rather than a final outcome. It provides still more motivation to think about the new research that will come next. How was this abacus bead made in the Universe? We look forward to discovering more.

Masatoshi Hirabayashi (ONC Team Co-I、Auburn University)
Hayabusa2 Project