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Astron. Astrophys. 332, 1123-1132 (1998)

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Thermal physics of asteroids

IV. Thermal infrared beaming

Johan S.V. Lagerros *

Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden (WWW: http://www.astro.uu.se/)

Received 17 November 1997 / Accepted 13 January 1998

Abstract

The so called beaming is an important factor when studying the thermal emission from the atmosphereless bodies of the solar system. The emission is non-Lambertian and has a tendency to be "beamed" in the solar direction. The small scale surface roughness is probably the main source of the effect.

In this paper, the problem is studied theoretically by adding roughness to Lambertian surfaces, and investigate the change in the emission. The radiative transfer problem in the visual and IR is considered and solved simultaneously with the heat conduction problem. This gives the temperature variations over the surface, as caused by shadows and varying slopes on the surface. The beaming is derived by comparing the emission from the rough and smooth surface.

Two types of roughness approaches are considered. Analytical solutions to the temperature variations are given for surfaces covered with hemispherical segment craters. Numerical methods are used for stochastic surfaces. The latter approach to the roughness is probably closer to the real surfaces, but the former is far easier to apply in practise. The qualitative results of the two surface types are the same. The beaming is enhanced by the increasing the root mean square (r.m.s.) slope, or by increasing the albedo. The beaming produced by the two approaches for a given r.m.s. slope is rather close for a 60 % crater coverage in the spherical crater case.

The combined effects of heat conduction and the surface roughness is studied. In many cases it is, however, possible to treat them separately which is of major practical importance.

As an example, IRAS data (Tedesco et al., 1992) of the asteroid 3 Juno is compared to model thermal light curves produced, with good agreement.

Key words: minor planets, asteroids – radiation mechanisms: thermal – infrared: solar system – Moon – planets and satellites: individual: 3 Juno

* e-mail: Johan.Lagerros@astro.uu.se

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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