Thermophysical model of (951) Gaspra
Thermophysical model of (951) Gaspra
The thermal infrared emission from asteroids can be used for determining their physical properties, such as diameters, albedos, thermal inertia, emissivity and composition, surface roughness, and regolith thickness. A new thermophysical model of asteroids has been developed throughout a series of papers by J. Lagerros. A number of physical processes are introduced in the new model, instead of the empirical correction parameters used in the Standard Thermal Model (STM).
The study of asteroids is one important key to the understanding of the early solar system. The evolution of the asteroids and their interactions in the present-day solar system are also important issues. There are many open questions related to the distribution of mass, angular momentum, and chemical composition in the main-belt. The few images available of asteroids show highly irregular shapes, indicating violent histories of impacts and collisions. The traditional distinction between comets and asteroids seems to become increasingly more vague with new discoveries of borderline and transitional objects.
The observational techniques, instruments and telescopes are improving rapidly. New wavelength regions and fainter objects are made available. These developments require, however, a refinement of the traditional techniques to investigate asteroids.
A new thermophysical model of asteroids has been developed. The purpose is to facilitate the current and future investigations of asteroids in the mid- and far-infrared wavelengths, and beyond. Due to the constraints by the Earth's atmosphere, the asteroids are relatively poorly studied at these wavelengths, as compared to the visual spectral range.
Nevertheless, it is clear that many important physical properties of the asteroids can be explored by measuring their thermal emission. Asteroid diameters can be determined by radiometric methods. The bulk properties of the surface material can be investigated by measuring the thermal inertia. From different viewing geometries, the cooler morning side can be distinguished from the warmer afternoon side, and hence the sense of rotation can be determined. The thermal emission is strongly affected by the surface roughness and porosity of the surface material. Various minerals have several molecular bending and stretching modes, and crystal lattice vibrational modes, which could be used for spectral identifications in the mid- and far-infrared.
A thermophysical model is, however, required in order to achieve these and other goals. Although proven to be very successful in many respects, the current Standard Thermal Model (STM) of asteroids (Lebofsky and Spencer 1989, and references therein) has many limitations. This is because STM is based on a number of simplifications and makes use of empirical correction parameters to obtain agreement with observations. The aim of the new thermophysical model is to improve the physical understanding and to open a path for deriving real physical quantities from thermal emission of asteroids.
The Uppsala planetary system group
Johan Lagerros