Curvilinear, interconnecting gullies and associated flow features as evidence for transient water flow on Vesta

Abstract

The traditional view that airless small solar system bodies are completely dry is changing (e.g. Hsieh & Jewitt 2006). Vesta too has evidence for water: meteorites contain evidence for aqueous alteration (Treiman et al. 2004 & Warren et al. 2013) and water in primitive melts (Sarafian et al. 2012). Additionally, pitted terrain, formed by degassing of volatile-bearing material, is found in some craters (Denevi et al. 2012). Here we show that surface morphology in some craters, which contain pitted terrain, indicates that liquid water transiently flowed down the crater walls and formed curvilinear, interconnecting gullies, along with associated flow features inside and outside of the craters. Curvilinear gullies often end in lobate deposits, which are sometimes covered by pitted terrain. This association is evidence that volatiles are involved in the formation of curvilinear gullies. Well-defined examples of these morphologies are found in Cornelia and Marcia craters. Curvilinear gullies are qualitatively and quantifiably distinct from linear gullies, which are formed by dry granular flow of material. There is no physical mechanism identified that can explain dry granular flow forming both types of gullies. Moreover, curvilinear gullies are inconsistent with an impact melt origin. We further show that water can flow transiently on the surface of Vesta, which has no atmosphere and a low average surface temperature of ~145 K (Stubbs & Wang 2012). Sub-surface ice-bearing deposits can be retained and stable for billions of years on Vesta (Stubbs & Wang 2012). However, any sub-surface ice-bearing deposits are likely present in localized areas since curvilinear gullies and associated flow features are found in a minority of craters on Vesta, which are clustered into two groups. Since curvilinear gullies and associated flow features are always found in association with impact craters, it is proposed that an impact excavates to the ice-bearing deposit level and also increases the temperatures and pressures so that part/ all of the ice-bearing deposit melts. Once water leaves the ice-bearing deposit and reaches the crater wall it can transiently flow under a temporary protective evaporating and/ or freezing barrier before it finally all evaporates and is lost. Water at low temperatures and pressures has an equivalent/ greater erosion rate and flows faster than water under terrestrial conditions (Conway et al. 2011). Thus, it is plausible that these features can be carved by a transient flow of liquid water under vestan surface conditions. After the transient flow, loss, through evaporation, of the water that pools at the bottom of the craters contributes to pitted terrain formation. Evidence that Vesta is partly hydrated on a localized scale suggests that we should expect the possible presence of water on other small solar system bodies, especially as we explore deeper into the asteroid belt.