Water and Ice on Asteroids

The object 1 Ceres that is now classified as dwarf planet was the first asteroid where spectral signatures of water were found. Near 3.0 microns an absorption feature indicates the presence of water. From that it was estimated that between 10 and 15% water of hydration may be found on Ceres surface (Lebofsky, 1978 [195]). This was the first evidence of water substance on the surface of an object in the main asteroid belt between Mars and Jupiter. Hydration means that a water molecule is added to the crystal structure of a mineral. This creates a new mineral, usually called a hydrate. 

Lebofsky et al., 1981 [196] pursued these studies of Ceres by high-resolution Fourier spectra (1.7-3.5 microns) and medium-resolution spectrophotometry (2.7-4.2 microns). The 3-micron feature is compared with the 3-micron bands due to water of hydration in clays and salts and they conclude that the spectrum of Ceres shows a strong absorption at 2.7-2.8 microns due to structural OH groups in clay minerals. The dominant minerals on the surface of Ceres are therefore hydrated clay minerals structurally similar to terrestrial montmorillonites. There is also a narrow absorption feature at 3.1 microns which is attributable to a very small amount of water ice on Ceres. 

Fanale and Salvail, 1989 [126] studied the presence of water ice on Ceres based on models assuming temperatures at the surface and subsurface, water fluxes, ice depths, water supply rates as a function of time, latitude and regolith properties. It was found that ice could persist in a transient state only above 80 degrees latitude. 

A study of the spectral signature of water described above showed that 66 percent of the C-class asteroids in the sample investigated have hydrated silicate surfaces. Differences in hydration among different types of asteroids may be understood by the fact that the early Sun had a very strong wind which heated the protoasteroids. 

This effect was less pronounced for the more distant particles (Jones et ah, 1990 [175]). Ices of volatile elements should be more abundant in the external region of the belt, where pristine materials have not undergone drastic modification processes after the accumulation in the planetesimal swarms between those of Mars and of Jupiter. There is no evidence of any water, water ice or aqueous alteration materials on the D-type asteroids, which are considered the least altered objects (Barucci, Fulchignoni and Lazzarin, 1996 [19]). D-type asteroids have a very low albedo and appear reddish and are found in the outer asteroid belt. It has been suggested that they have a composition of 

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