Simultaneous sublimation activity of primitive asteroids including (24) Themis and (449) Hamburga: Spectral signs of an exosphere and the solar activity impact

Abstract

We describe and analyze simultaneous sublimation activity of the main-belt primitive asteroids during their perihelion passages, which was detected by spectral methods. This phenomenon was observed for the first time in September 2012 for asteroids (145) Adeona, (704) Interamnia, (779) Nina, and Mars-crosser (1474) Beira (Busarev et al., 2015, 2016, 2018, 2019a). By our recent observations of (704) Interamnia, (24) Themis, and (449) Hamburga performed in March–April 2019, we confirmed the existence of this effect. (24) Themis and (449) Hamburga was detected as active for the first time. Never before, two or more asteroids have been observed active simultaneously. Activity on asteroids reveals itself by unusual spectral features (the maxima at approximately 0.40–0.50 and/or 0.60–0.70 μm), which appear in the reflectance spectra of the bodies when they approach perihelion. We explain them as manifestations of sublimation of ice materials and development of a subtle exosphere around the asteroids. Moreover, from numerical simulations of the light scattering in a medium containing ice and silicate/organic dust particles of complex structure, we found that some of these models fit well the reflectance spectra of active asteroids, which confirms the presence of a rarefied exosphere around these bodies. Our results show that the main conditions providing formation of a temporal sublimation-driven exosphere around primitive main-belt asteroids are: (1) the considerably higher content of H2O in their material than that previously believed from accounting for only the content of bound water in composition of carbonaceous chondrites, as possible components of asteroids; (2) frequent collisions of main-belt asteroids with smaller bodies and/or exposure to meteorite fluxes, which leads to excavation of ices and their subsequent sublimation; (3) the recurrent rise of the subsolar temperature on the surface of asteroids under proper orbital or dynamical parameters and the sublimation of subsurface ices (if present); and (4) the strengthening of solar activity (and the accompanying factors), which results in desiccation, destruction and/or dispersion of hydrated silicates and/or radiation damage of organic films isolating stocks of volatiles on or under the asteroid surface. According to our analysis, none of these factors produces a noticeable effect alone and can be marked as stronger or more frequent, but two or three of them working simultaneously may be effective.