Abstract
Asteroid 162173 (Ryugu) is a carbonaceous asteroid that was visited by
Japan’s Hayabusa2 spacecraft in 2018. The formation mechanism of
spinning-top shape of Ryugu is an essential clue to the dynamical
history of the near-Earth asteroid. In this study, we address the
spin-state evolution of Ryugu induced by the
Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, i.e., the
thermal recoil torque that changes the rotation period and spin-pole
direction.
Given the current orbit, spin state, and
three-dimensional shape observed by Hayabusa2, we computed the YORP
torque exerted on Ryugu using a simplified thermal model approximating
zero thermal conductivity. Despite differences in meter-scaled
topography, all 20 shape models that we examined indicate that the spin
velocity of Ryugu is currently decreasing at a rate of
(-0.42—6.3)*10-6 deg/day2. Our
findings also suggest that the thermal torque on the asteroid is
responsible for maintaining the spin pole upright with respect to the
orbital plane.
Therefore, the YORP effect could explain
the significant spin-down from a period of 3.5 h initially to 7.6 h
currently. The corresponding time scale of the rotational deceleration
is estimated to be 0.58–8.7 million years, depending on the input shape
models. This time scale is comparable to e.g., 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. It is
considered that the rotation of the asteroid started to decelerate in
the wake of the major crater formation or the resurfacing event on the
western hemisphere.