WATER IN ASTEROIDS: AN OBSERVATIONAL PERSPECTIVE

Water is one of the most interesting and important materials in the Universe. The unique physical and chemical properties of this simple molecule ensure that, when present, it generally has a dramatic effect on its environment. In the solar nebula, because H and O were two of the most abundant elements in the solar nebula, H₂O in turn was a dominant molecule. At heliocentric distances (temperatures and pressures) where H₂O condensed to ice, it rivaled all other solids in mass, thereby affecting accretion. Given the well-known resonant delivery mechanisms of asteroids into near-Earth space, water-rich asteroids must also be considered as potential sources of terrestrial volatiles. In bodies large enough for significant internal heating, the presence of water influenced energy balance as well as the ensuing geochemistry. Asteroids retain a record of the initial distribution of H₂O and subsequent heating events in their surface compositions. The task of asteroid science is to decipher that record. Water has been detected on asteroids in two forms: structural OH/H₂O in hydrated minerals and ice. Both forms have been detected spectroscopically, and ice is also inferred from comet-like activity of some asteroids. Additionally, the dwarf planet Ceres may harbor a subsurface liquid water ocean. The distributions of these forms of water in terms of orbital and physical parameters largely fit within a framework of widespread incorporation of ice in early asteroids followed by ²⁶Al-driven internal processing. Some details, however, remain puzzling. For instance, hydration features on M-type asteroids (originally thought to be the metallic cores of differentiated asteroids) do not easily fit within the standard ²⁶Al heating paradigm, and ice detections at the surface asteroids with surface temperatures ~200K are difficult to explain given the fast sublimation rates of ice at these temperatures. In this talk, we will discuss in more detail these observational constraints concerning water on asteroids, with particular focus on recent spectral detections of ice in the outer Main Belt.