An Optically Thin and Thick Dust Exosphere of Active Asteroids: Spectral Signs and Possible Formation Mechanisms

The detection and study of spectral signs of sublimation-driven dust activity (SDA) in 20 primitive-type asteroids of the Main Belt observed in the visible range allowed us to ascertain its temporal (or periodic) character. The asteroids exhibit SDA of relatively low intensity at minimal heliocentric distances, when their subsolar temperatures are, accordingly, highest (i.e., the surface temperature of the asteroid at the center of its sunlit hemisphere is close to the absolute maximum). Most likely, the major cause of SDA in these asteroids is that their interior contains H₂O ice, which becomes outcropped due to recent impact events. As follows from the numerical modeling of the reflectance spectra of active asteroids under consideration, the optically thin dust exosphere (DE), which was induced by SDA and envelopes these bodies, consists of submicron homogeneous particles of different composition and/or fractal-like aggregates of such particles. Taking into account an extremely weak gravitational field of asteroids that cannot hold this optically thin DE even for a short time (especially, if their sizes are less than ~10 km), we conclude that the DE of active primitive-type asteroids is supported by two jointly operating mechanisms. Namely, the electrostatic field of photoemission nature creates a plasma–dust sheath on a sunlit side of the asteroid (as in all atmosphereless celestial bodies), and SDA provides the additional ejection of dust particles from the surface into this layer, which enhances their amount to a detectable level; at the same time, the self-rotation of the body contributes to the spreading of dust around the asteroid.