Sublimation-driven dust activity of main-belt asteroids: Detection by spectral methods and analysis with numerical modeling of the reflectance spectra

Abstract

We review the results of the spectrophotometric and multi-band photometric observations of main-belt asteroids aimed at searching for signs of sublimation-driven dust activity (SDA) and the analysis of these signs with numerical simulations of light scattering. The reflectance spectra of 17 main-belt asteroids out of 112 objects, which were observed near perihelion after 2012, exhibited the signs of activity as a temporary dust exosphere. Among these 17 asteroids, there are 12 primitive asteroids of C, F, B, P, and D spectral types with low-temperature mineralogy and a low geometric albedo (<0.10) and 5 asteroids predominantly of X-type with mixed mineralogy and a relatively high geometric albedo (>0.10). In our view, the most important result of these observations is that SDA was almost simultaneously detected in several asteroids for each of the observation periods, which apparently suggests that this phenomenon is frequent and widespread in the Solar System. We also discuss why in ground-based observations the spectral method is more sensitive than the direct imaging method in detecting the signs of a thin dust exosphere on main-belt asteroids. The registered reflectance spectra of asteroids were analyzed in comparison with numerically modeled spectra of an active asteroid enveloped by a dust exosphere containing particles of various composition and morphology. This comparison made it possible to obtain an insight into the properties of a formed temporary dust exosphere. Particularly, it has been found that the dust exosphere of an active asteroid may contain aggregate particles of different composition and structure rather than just homogenous submicron ice and dust particles. A relatively stable though temporary SDA in a primitive-type asteroid is apparently indicative of water-ice deposits located near the surface of the body. As our investigations suggest, on main-belt primitive asteroids, the mechanism of SDA often operates in conjunction with meteoroid impacts and the electrostatic field formed near the sunlit surface of the bodies. At the same time, flares and eruptive events on the Sun may considerably influence the shape and extension of the dust exosphere.