The low surface thermal inertia of the rapidly rotating near-Earth asteroid 2016 GE1

Abstract

Context. Asteroids smaller than about 100 m in diameter are observed to rotate very fast, with periods often much shorter than the critical spin limit of 2.2 h. Some of these super-fast rotators can also achieve a very large semimajor axis drift induced by the Yarkovsky effect, which, in turn, is determined by internal and surface physical properties. Aims. We consider here a small super-fast-rotating near-Earth asteroid, designated as 2016 GE1. This object rotates in just about 34 s, and a large Yarkovsky effect has been determined from astrometry. By using these results, we aim to constrain the thermal inertia of the surface of this extreme object. Methods. We used a recently developed statistical method to determine the thermal properties of near-Earth asteroids. The method is based on the comparison between the observed and the modeled Yarkovsky effect, and the thermal conductivity (inertia) is determined via a Monte Carlo approach. Parameters of the Yarkovsky effect model are fixed if their uncertainty is negligible, modeled with a Gaussian distribution of the errors if they are measured, or deduced from general properties of the population of near-Earth asteroids when they are unknown. Results. Using a well-established orbit determination procedure, we determined the Yarkovsky effect on 2016 GE1 and confirm a significant semimajor axis drift rate. Using a statistical method, we show that this semimajor axis drift rate can only be explained by low thermal inertia values below 100 J m-2 K-1 s-1/2. We benchmarked our statistical method using the well-characterized asteroid Bennu and find that only knowing the semimajor axis drift rate and the rotation period is generally insufficient for determining the thermal inertia. However, when the statistical method is applied to super-fast rotators, we find that the measured Yarkovsky effect can be achieved only for very low values of thermal inertia: namely, 90% of the probability density function of the model outcomes is contained at values smaller than 100 J m-2 K-1 s-1/2. Conclusions. We propose two possible interpretations for the extremely low thermal inertia of 2016 GE1: a high porosity or a cracked surface, or a thin layer of fine regolith on the surface. Though both possibilities seem somewhat unexpected, this opens up the possibility of a subclass of low-inertia, super-fast-rotating asteroids.