The Surface Roughness of (433) Eros as Measured by Thermal-Infrared Beaming

TL;DR

This study demonstrates how thermal-infrared observations during specific geometries can effectively constrain the small-scale surface roughness of asteroid (433) Eros, revealing a rougher surface than laser measurements suggest and providing insights into surface processes.

Contribution

The paper introduces a method to constrain asteroid surface roughness using thermal-infrared data during pole-on geometries, improving over previous degeneracy issues.

Findings

Surface roughness of Eros characterized by RMS slope of 38 ± 8° at 0.5 cm scale.

Surface roughness is slightly greater than laser ranging extrapolations suggest.

Thermal-infrared beaming effect can constrain surface roughness for high obliquity asteroids during pole-on observations.

Abstract

In planetary science, surface roughness is regarded to be a measure of surface irregularity at small spatial scales, and causes the thermal-infrared beaming effect (i.e. re-radiation of absorbed sunlight back towards to the Sun). Typically, surface roughness exhibits a degeneracy with thermal inertia when thermophysical models are fitted to disc-integrated thermal-infrared observations of asteroids because of this effect. In this work, it is demonstrated how surface roughness can be constrained for near-Earth asteroid (433) Eros (i.e. the target of NASA's NEAR Shoemaker mission) when using the Advanced Thermophysical Model with thermal-infrared observations taken during an "almost pole-on" illumination and viewing geometry. It is found that the surface roughness of (433) Eros is characterised by an RMS slope of 38 $\pm$ 8{\deg} at the 0.5-cm spatial scale associated with its thermal-infrared beaming effect. This is slightly greater than the RMS slope of 25 $\pm$ 5{\deg} implied by the NEAR Shoemaker laser ranging results when extrapolated to this spatial scale, and indicates that other surface shaping processes might operate, in addition to collisions and gravity, at spatial scales under one metre in order to make asteroid surfaces rougher. For other high obliquity asteroids observed during "pole-on" illumination conditions, the thermal-infrared beaming effect allows surface roughness to be constrained when the sub-solar latitude is greater than 60{\deg}, and if the asteroids are observed at phase angles of less than 40{\deg}. They will likely exhibit NEATM beaming parameters that are lower than expected for a typical asteroid at all phase angles up to 100{\deg}.