GEOMORPHOLOGICAL EVIDENCE FOR IMPACT-INDUCED, LOCALIZED, TRANSIENT FLOW OF WATER ON VESTA

Vesta, the second most massive asteroid, has long been perceived as anhydrous. However, recent studies suggesting the presence of hydrated minerals and past sub-surface water have challenged this perception (Sarafian et al., 2013; De Sanctis et al., 2012; McCord et al., 2012; Prettyman et al., 2012; Reddy et al. 2012; Treiman et al, 2004). Yet, direct geologic indications of water activity on Vesta’s surface were unexpected. Herein we show evidence that transient water flowed on the surface, in a debris-flow-like process, and left distinctive geomorphologic features. Based on detailed analysis of high-resolution (~20 m/ pixel) images obtained by the Dawn mission, we identify a class of locally occurring, interconnected and curvilinear systems of gullies on the walls of young (< 100s Ma) impact craters, ending in lobate deposits near the crater floors. As curvilinear systems only occur within impact craters, we propose that they formed by a particulate-dominated transient flow of water that was released from buried ice-bearing deposits by impact-induced heating and melting. Curvilinear systems may be formed in a minimum of ~26 minutes. This interpretation is in accordance with the occurrence of pitted terrain on lobate deposits and crater floors. Vestan pitted terrain is analogous to Martian pitted terrain and is interpreted to result from the degassing of volatiles (Denevi et al., 2012). We also identify a linear class of gully systems, which are morphologically distinct from the curvilinear systems, and are interpreted to form by dry flow of granular material. Craters containing curvilinear systems are clustered in two regions of Vesta’s surface, whereas linear systems are evenly spread across the surface. This indicates that the proposed buried ice-bearing deposits are likely localized in extent. Together with the newly expanded understanding of the distribution and behavior of water on other rocky bodies, such as Earth’s Moon (e.g. Saal et al., 2008), and elsewhere in the asteroid belt (e.g. Küppers et al., 2014; Hsieh & Jewitt, 2006), our results support the new paradigm that there is a continuum of bodies in the solar system with many intermediate states of hydration. The varied hydrologic processes that occur within this new paradigm suggest the evolution of our solar system is more complex than previously thought.