Dynamics of small bodies are secularly controlled by thermally induced forces and torques, known as the Yarkovsky and YORP effects [Rubincam, 2000; Vokrouhlický and Čapek, 2002]. Understanding these effects are crucial not only for scientific discovery but also for planetary defense initiatives.
We simulate the YORP torque acting on asteroid (162173) Ryugu during its one rotation on TDB 08/01/2018, by using a Finite Element Modeling approach 3-dimensional thermophysical model [Nakano and Hirabayashi, 2021]. To investigate how small topographic features on the surface of Ryugu affects the temperature distribution and resulting YORP torque, we consider two shape models having different number of facets (high- and low-resolution models). Figure 1 shows the evolution of the three YORP torque components, τω, τε, and τψ, over one rotation (see figure for descriptions). We note that the relatively large difference between the two lines at the initial rotation phase is due to an initial temperature setting; however, this converges quickly after 0.2 rotation.
Our result suggests that the difference in the shape model resolution has a negligible effect on the YORP torque during one rotation. This may potentially imply that small topographic features and surface roughness do not significantly contribute to the YORP effect, contrary to earlier studies [e.g., Statler, 2009], although they may be responsible for the flat diurnal temperature profile observed at some local spots on Ryugu [Okada et al., 2020]. If this is the case, using the 3-dimensional thermophysical model, we may be able to efficiently characterize the YORP effect, even with a low-resolution shape model. It is our next work to investigate whether this short timescale result also holds for the secular, long timescale spin state evolution.
References 1. Nakano and Hirabayashi, (2021) EPSC, 441 2. Okada et al., (2020) Nature, 579 3. Rubincam (2000) Icarus, 148 4. Statler, (2009) Icarus, 202 5. Vokrouhlický and Čapek (2002) Icarus, 159