Sublimation-Driven Dust Activity of Primitive-Type Asteroids as a Sign of the Presence of Н2О Ice

Abstract

In the period December 12, 2022, to February 28, 2023, we performed a series of UBVRI observations of 65 asteroids, including 50 primitive-type objects, which could be observed near perihelion at that time, and calculated their approximated reflectance spectra. The observations were aimed at searching for spectral signs of sublimation-driven dust activity in the asteroids and formation of a transient dust exosphere, which are caused by the presence of Н₂О ice in their interior and the highest subsolar temperature. Based on the analysis of the accuracy and shape of approximated reflectance spectra of the primitive-type asteroids, a set of which was conventionally restricted by the geometric albedo value not exceeding 0.10, we found sublimation-driven dust activity in asteroids 164 Eva, 360 Carlova, and 750 Oskar by distinct short-term variations in the reflectance of these asteroids in the U band. During one of the nights, similar spectral changes and sublimation-driven dust activity related to them were detected in asteroid 629 Bernardina; the geometric albedo of this asteroid is somewhat higher (0.14 or 0.19), which may be indicative of a heterogeneous composition of its material and, apparently, a local outcropping of water ice on its surface. Moreover, in three asteroids of a high-temperature type, 757 Portlandia, 1121 Natascha, and 1687 Glarona, noticeable variations were detected in the entire spectral range; they may be connected with ejection of submicron dust particles from the surface by some other mechanisms. To substantiate and interpret the results of observations, we numerically modeled the reflectance spectra of conditional asteroids of taxonomic types C and S (with low- and high-temperature mineralogy, respectively), which are enveloped by an optically thin dust exosphere containing aggregate or homogeneous submicron particles of different composition. On the basis of this modeling, we introduce a quantitative criterion to identify a weak sublimation-driven dust activity in primitive-type asteroids in the visible range.