The Influence of the Exosphere of an Active Asteroid on the Polarization of Scattered Light and the Possibility of Estimating Its Properties from Ground-Based Measurements

Celestial bodies which have orbital and physical characteristics typical of asteroids, but episodically exhibit the signs of cometary activity are of particular interest, because the knowledge of the nature of these bodies is necessary to understand the processes of how the Solar System formed and how water was delivered to the terrestrial planets. In the estimation of the properties of an exosphere of an active asteroid (AA) from remote-sensing data, polarimetry can play a prominent role, since the polarization of scattered light is sensitive to the properties of particles in the medium. Numerical simulations of the light scattering by particles in the exosphere around an AA have shown that, depending on the wavelength of the scattered light, the refractive index of particles, and their morphology, the light scattering in the exosphere may both weaken the polarization of light reflected by the surface and enhance it. At the same time, the spectral gradient of polarization may change both towards larger positive values and towards negative ones. At phase angles less than 30 degrees, which are typical of observations of the Main-belt asteroids, the changes in the polarization induced by scattering in the exosphere are small and vary only slightly for particles of different properties. Nevertheless, if the polarizations of light reflected by an asteroid changes relative to the canonical values, this may indicate the presence of the exosphere. At larger phase angles, the influence of scattering in the exosphere on the polarization of an AA is more noticeable, which makes the use of polarimetry promising for studying activity of near-Earth asteroids. This effect should also be taken into account when estimating the albedo of an asteroid by the polarization maximum (according to the Umov law), if a manifestation of activity in this particular asteroid can be expected.