Directional characteristics of thermal-infrared beaming from atmosphereless planetary surfaces - a new thermophysical model

43 44 We present a new rough-surface thermophysical model (Advanced Thermophysical Model or 45 ATPM) that describes the observed directional thermal emission from any atmosphereless 46 planetary surface. It explicitly incorporates partial shadowing, scattering of sunlight, 47 selfheating and thermal-infrared beaming (re-radiation of absorbed sunlight back towards the 48 Sun as a result of surface roughness). The model is verified by accurately reproducing 49 ground-based directional thermal emission measurements of the lunar surface using surface 50 properties that are consistent with the findings of the Apollo missions and roughness 51 characterised by an RMS slope of ~32o. By considering the wide range of potential asteroid 52 surface properties, the model implies a beaming effect that cannot be described by a simple 53 parameter or function. It is highly dependent on the illumination and viewing angles as well 54 as surface thermal properties and is predominantly caused by macroscopic rather than 55 microscopic roughness. Roughness alters the effective Bond albedo and thermal inertia of the 56 surface as well as moving the mean emission away from the surface normal. For accurate 57 determination of surface properties from thermal-infrared observations of unresolved bodies 58 or resolved surface elements, roughness must be explicitly modelled, preferably aided with 59 thermal measurements at different emission angles and wavelengths. 60 61