Material composition assessment and discovering sublimation activity on asteroids 145 Adeona, 704 Interamnia, 779 Nina, and 1474 Beira

Abstract

Spectrophotometric observations of 145 Adeona, 704 Interamnia, 779 Nina, and 1474 Beira—asteroids of close primitive types—allowed us to detect similar mineralogical absorption bands in their reflectance spectra centered in the range 0.35 to 0.92 μm; the bands are at 0.38, 0.44, and 0.67–0.71 μm. On the same asteroids, the spectral signs of simultaneous sublimation activity were found for the first time. Namely, there are maxima at ∼0.35–0.60 μm in the reflectance spectra of Adeona, Interamnia, and Nina and at ∼0.55–075 μm in the spectra of Beira. We connect this activity with small heliocentric distances of the asteroids and, consequently, with a high insolation at their surfaces. Examination of the samples of probable analogues allowed us to identify Fe3+ and Fe2+ in the material of these asteroids through the mentioned absorption bands. For analogues, we took powdered samples of carbonaceous chondrites Orgueil (CI), Mighei (CM2), Murchison (CM2), and Boriskino (CM2), as well as hydrosilicates of the serpentine group. Laboratory spectral reflectance study of the samples of low-iron Mg serpentines (<2 wt % FeO) showed that the equivalent width of the absorption band centered at 0.44–0.46 μm strongly correlates with the content of Fe3+ in octahedral and tetrahedral coordinations. Our conclusion is that this absorption band can be used as a qualitative indicator of Fe3+ in the surface matter of asteroids and other solid celestial bodies. The comparison of the listed analog samples and the asteroids by parameters of the spectral features suggests that the silicate component of the asteroids' surface material is a mixture of hydrated and oxidized compounds, including oxides and hydroxides of bivalent and trivalent iron and carbonaceous-chondritic material. At the same time, the sublimation activity of Adeona, Interamnia, Nina, and Beira at high surface temperatures points to a substantial content of water ice in their material. This contradicts the previously existing notions on the C-type and similar asteroids as bodies containing water only in the bound state. Moreover, since the sublimation process simultaneously occurs in four primitive-type bodies at small heliocentric distances, we may suppose that this phenomenon is common for the main-belt asteroids.