The surface roughness of (433) Eros as measured by thermal-infrared beaming

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 poleon" illumination and viewing geometry. It is found that the surface roughness of (433) Eros is characterised by an RMS slope of 38 ± 8° at the 0.5-cm spatial scale associated with its thermalinfrared beaming effect. This is slightly greater than the RMS slope of 25 ± 5° 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°, and if the asteroids are observed at phase angles of less than 40°. They will likely exhibit NEATM beaming parameters that are lower than expected for a typical asteroid at all phase angles up to 100°.

Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis

“…This roughness is comparable to that of other asteroids [43][44][45][46] but rougher than that of the Moon [47]. The model itself does not set a spatial scale for this roughness; it can be anywhere between the resolution of the shape model used for the thermal analysis (~12 m) and the diurnal thermal skin depth (~2 cm, methods), though we expect that roughness at the smaller length scales dominates the thermal signature [43]. From the shape model of Bennu [3], we find RMS slopes that increase with decreasing spatial scale.…”

“…These results suggest that the thermal infrared beaming is most sensitive to the smaller end of the spatial scale (i.e., close to the thermal skin depth), and that Bennu is much rougher at these scales than at larger spatial scales. See [43] for more discussion of thermal infrared roughness.…”

Thermophysical Modeling of Asteroid Surfaces Using Ellipsoid Shape Models

“…Figure 1(b) shows thermal phase curves of objects with smooth and rough surfaces of varying thermal inertia. Figure 1(c) shows the thermal flux emitted as a function of sub-solar latitude and recreates the findings of Rozitis (2017). In general, the surface roughness of airless bodies are more easily estimated near opposition or, better yet, with disk-resolved observations that offer a greater range of viewing geometries.…”

“…In general, the surface roughness of airless bodies are more easily estimated near opposition or, better yet, with disk-resolved observations that offer a greater range of viewing geometries. The Rozitis (2017) study shows that the uncertainty in thermal inertia is slightly larger for an object observed at a large phase angle, as a consequence of the difficulty in estimating surface roughness. c Figure 1: Comparison of thermal flux emitted from an object varying it's thermophysical properties and shape, as a computed from the TPM described in section 3.…”

Thermophysical modeling of main-belt asteroids from WISE thermal data

“…Regarding future improvements on the TPM, it should happen based on studying the very few targets with ground-truth knowledge of physical properties and rich (thermal) data sets (e.g., Rozitis 2017). Or maybe with thermal infrared data from spacecraft missions (OSIRIS-REx data, Lauretta et al 2015).…”