YORP Effect on Asteroid 162173 Ryugu: Implications for the Dynamical History. Issue 12 (2nd December 2021) - Kanamaru, Masanori
Abstract: Asteroid 162173 Ryugu is a carbonaceous asteroid that was visited by Japan's Hayabusa2 spacecraft in 2018. The formation mechanism of the "spinning‐top" shape of Ryugu is a vital clue to the dynamical history of the near‐Earth asteroid. In this study, we address the long‐term evolution of its spin state induced by the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack (YORP) effect, that is, the thermal recoil torque that changes the rotation period and spin‐pole direction. Given the current orbit, spin state, and three‐dimensional shape of Ryugu observed by Hayabusa2, we computed the YORP torque exerted on Ryugu using a simplified thermal model assuming zero thermal conductivity. Despite variations in the meter‐scaled topography, all 20 shape models that we examined indicate that the spin velocity of Ryugu is currently decreasing at a rate of ( − 6.3 to − 0.42 ) × 10 − 6 $(-6.3\enspace \mathrm{to}\enspace -0.42)\times {10}^{-6}$ deg/day 2 . Our findings also suggest that the thermal torque is responsible for maintaining the spin pole upright with respect to the orbital plane. Therefore, the YORP effect may explain the significant spin‐down from an earlier period of 3.5 hr to the present period of 7.6 hr. The corresponding time scale of the spin‐down is estimated to be 0.58–8.7 million years, depending on the input shape models. This time scale is comparable to the formation period of the largest crater, Urashima (5–12 Ma), or the western bulge (2–9 Ma) as derived from previous studies on crater statistics in Ryugu. Thus, its rotation may have started to decelerate as a consequence of major resurfacing events. Plain Language Summary: The Japanese spacecraft Hayabusa2 visited the kilometer‐sized asteroid, Ryugu, and found it to have a spinning‐top shape with a diamond‐like cross‐section. For an aggregate of rock fragments to deform in this way, fast rotation is required. The current rotation period of Ryugu is 7.6 hr, and its rotational speed is not sufficiently fast to deform the entire body. It is proposed that Ryugu rapidly spun in the past and has since slowed down. Light has no mass, but it does have momentum and produces a faint pressure on the surface of the asteroid. As Ryugu is heated by the Sun, thermal radiation is emitted from its surface, resulting in a breaking effect and changing the rotational speed of the asteroid over millions of years. We found that the radiation pressure is a reasonable mechanism for the spin‐down of the asteroid and thus provides a clue to Ryugu's history. Key Points: Thermal recoil torque (i.e., the YORP effect) plays a dominant role in the spin‐down of asteroid 162173 Ryugu The estimated spin‐down time scale (0.58–8.7 million years) indicates a period of major change in the topography of Ryugu The YORP effect is responsible for keeping the spin‐pole of Ryugu perpendicular to the orbital plane