The thermal emission from boulders on (25143) Itokawa and general implications for the YORP effect

Abstract

Infrared radiation emitted from an asteroid surface causes a torque that can significantly affect rotational state of the asteroid. The influence of small topographic features on this phenomenon, called the YORP effect, seems to be of utmost importance. In this work, we show that a lateral heat diffusion in boulders of suitable sizes leads to an emergence of a local YORP effect which magnitude is comparable to the YORP effect due to the global shape. We solve a three-dimensional heat diffusion equation in a boulder and its surroundings by the finite element method, using the FreeFem++ code. The contribution to the total torque is inferred from the computed temperature distribution. Our general approach allows us to compute the torque induced by a realistic irregular boulder. For an idealized boulder, our result is consistent with an existing one-dimensional model. We also estimated (and extrapolated) a size distribution of boulders on (25143) Itokawa from close-up images of its surface. We realized that topographic features on Itokawa can potentially induce a torque corresponding to a rotational acceleration of the order 10 −7 rad day −2 and can therefore explain the observed phase shift in light curves.

The Yarkovsky effect for 99942 Apophis

“…Additionally, Lowry et al (2014) analysed implications of the detected YORP effect for Itokawa by using a detailed thermophysical model and concluded that the asteroid consists of two very different parts: (i) the main body with the density '1.75 g/cm 3 and (ii) the head with the density '2.85 g/cm 3 . While this striking difference may be in part due to their model incompleteness (e.g., Golubov et al, 2014;Ševeček et al, 2015), the two values again roughly bracket the expected interval of Apophis bulk density. We thus use the same bulk-density distribution function as shown in Fig.…”

The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt

“…The shape and surface properties of Arrokoth and any obliquity variations could change the strength and sign of the YORP torques (79)(80)(81)(82). Also, both YORP torques perpendicular to the spin axis and BYORP torques perpendicular to the orbit normal vector are minimized at high obliquities and binary inclinations, respectively (76, 81).…”

The Yarkovsky and YORP Effects

“…Clearly, if the collective crosssection of such surface features is large, details of conduction across them may have significant consequences. Ideally, the situation calls for a complete 3D heat transfer model (e.g., Golubov et al, 2014;Ševeček et al, 2015). Importantly, these studies indicate an overall tendency for YORP to spin objects up.…”

“…A surface boulder can give an object a locally small radius of curvature and 3D effects may become important. Full 3D conduction is computationally expensive (e.g., Golubov et al, 2014;Ševeček et al, 2015), but the potential consequences are significant. In this case, a general finite-element method is used to solve the heat diffusion problem.…”