On the Prospects for Estimating the Properties of Particles in an Active Asteroid Exosphere by Features in the UV and Visible Reflectance Spectra

Abstract

The reflectance spectra of active asteroids (AAs) measured in the visible and near-UV ranges exhibit unusual features, which are apparently caused by the light scattering in an exosphere formed under active processes on an asteroid. To estimate the prospects for a quantitative interpretation of these features, we numerically simulated reflectance spectra of an AA enveloped by an exosphere composed of aggregate submicron particles of various composition and morphology, as well as homogeneous submicron particles. We assumed that the sizes of aggregates’ constituents correspond to those of grains in agglomerates of come tary and interplanetary dust. It has been shown that the scattering on aggregates of submicron grains produces interference features at wavelengths shorter than 0.6 μm, and the positions of these features are determined by both the sizes of these grains (rather than the aggregates themselves) and the real part of their refractive index. The structure of an aggregate and variations (up to ±20%) in the sizes of constituents weakly influence the position of these features. The shape of the spectrum at longer wavelengths also depends on the sizes of grains in the aggregates and serve as an additional criterion for estimating this parameter. Calculations per formed for aggregate particles absorbing in a short-wavelength range (which is typical of many materials that one may expect to find on AAs) show that the absorption significantly weakens the interference details appeared in this range. Hence, the attempts to detect strongly absorbing particles in the exosphere and to esti mate their properties by these spectral features cannot yield reliable results, as opposed to the simulations for weakly absorbing particles. The presence of homogeneous weakly absorbing submicron particles in the exo sphere of an AA results in a steady growth of the intensity at wavelengths shorter than 0.4−0.5 μm. Spectral measurements at the wavelengths shorter than 0.35 μm may help to estimate more reliably the properties of weakly absorbing particles, both aggregate and homogeneous, in the exospheres of AAs.