VESTA’S SACRED SPRING

The Vestals of ancient Rome performed purification rituals in which water was sprinkled on the temple of the perpetual fire. Thus, the selection of “Vesta” by Gauss as the name of the fourth main-belt asteroid, discovered in 1807, foreshadowed the nature of this small body: “Fire” and water appear to have shaped the surface of Vesta. Based on telescopic observations, data recently acquired by the NASA Dawn mission, and many other lines of evidence, Vesta is connected to a large group of magmatic meteorites - the Howardite, Eucrite and Diogenite (HED) clan - thought to have been ejected from the crust and upper mantle of a single, parent body. Their low K/Th ratios and igneous petrology support the hypothesis that Vesta formed from volatile-poor materials that melted and differentiated to form a core, mantle, and crust early in the history of the solar system. Yet, the Dawn mission discovered that the upper surface – the regolith – of this “dry” asteroid contains extensive deposits of dark, hydrated materials along with geomorphological evidence for alteration of the surface by volatiles. Hydrogen, in the form of hydroxyl, is found in low albedo regions and in geologic settings that suggest hydrous material was delivered by the infall of carbonaceous chondrites onto Vesta’s bright, basaltic surface. In fact, howardites, thought to be lithified samples of Vesta’s ancient regolith, often contain carbonaceous chondrite clasts. These clasts contain hydrous phyllosilicates, minimally altered by impacts. Intense heating produced by occasional, high-velocity impacts may have violently released “lattice water” from Vesta’s regolith. Such processes may have formed pitted regions within relatively young craters such as Marcia, similar to that seen in some martian craters. Given Vesta’s location within the main belt, perhaps the presence of a carbonaceous chondrite component is not surprising; however, exactly how and when this material was delivered and its influence on Vesta’s evolution is unknown. That a few eucrites appear to have crystalized from magmas containing dissolved water and the presence of quartz veinlets in one of them deepens the mystery. The discovery of hydrated minerals on the surface of a basaltic asteroid raises new questions about the delivery of water to the asteroid belt and inner solar system.