Thermal physics of asteroids .3. Irregular shapes and albedo variegations

A new thermophysical model for asteroids is described, which allows for (almost) arbitrary shapes, albedo variegations, beaming, and heat conduction. Multiple scattering is considered in both the visual and the infrared wavelength region. The computational efforts are reduced to acceptable levels by using view factor algebra for the multiple scattering, and an iterative method for solving the heat conduction equation. This enables a statistical investigation, here using a set of Gaussian random shapes, spanning a range of various geometries. The model is also applied to the asteroids 243 Ida, 951 Gaspra, 4179 Toutatis, 4769 Castalia, and the moons of Mars, Phobos and Deimos, where shapes are known from radar measurements or direct imaging. The self heating parameter is introduced in order to measure the degree of concavity of a given shape. The self heatings of the five objects with known shapes are rather low, which means that only a small portion of the radiation energy participates in the multiple scattering process on larger scales. In relation to the thermal beaming, the self heating on small scales is computed for a few surface roughness models, and is found to be substantially larger. Thermal and visual light curves for the synthetic shapes are compared to each other, as well as to the curves produced by fitted ellipsoids. The differences between the models can be substantial, and in general becomes more pronounced at shorter wavelengths. There are systematic trends with increasing self heating, but the interpretation is that this is due to the increased asymmetry rather than the concavities themselves playing an important role. A statistical test to detect albedo variegations is proposed, based on the comparison between thermal and visual light curves. The sensitivity of the test is investigated over a wide range of shapes and albedo patterns. The probability of detecting albedo spots is the highest at wavelengths close to the emission peak.