POTENTIAL TRANSIENT LIQUID WATER FLOW FEATURES IN FRESH CRATERS ON VESTA

Observations made by the Dawn spacecraft reveal unexpected potential transient liquid water flow features (PTFs) in fresh craters on Vesta. The PTFs have features similar to the headward alcoves, channels and distal aprons of the Martian gullies and related features on Earth (e.g. Malin & Edgett 2000). The craters have relatively fresh rims, suggesting they, and the PTFs, formed relatively recently. There is some correlation between craters containing PTFs and those containing pitted terrain, which Denevi et al. (2012) propose formed as volatile-bearing material degassed after impact heating. Cornelia crater contains good examples of PTFs. Cross-cutting relationships derived from a map of Cornelia, based on ~20 m/pixel images, make it possible to derive a geological history: firstly, an early damp stage, during which the PTFs formed; secondly, a transitional damp to dry stage, during which the pitted terrain formed and thirdly, a dry stage. There is considerable morphological evidence that flow of liquid water formed the PTFs in Cornelia and not dry granular flow, flow of impact melt or flow of liquid CO₂. The channels are not as sinuous as those on the Earth and Mars, but their formation on the steeply sloping crater walls under conditions of Vesta’s low gravity means that the channels may not need to divert around obstacles. The PTFs in Cornelia tend to form in the dark material dominated areas, which Reddy et al. (2012) conclude has a composition similar to carbonaceous chondrite (CC) and think originates from the impactor that formed the Veneneia impact basin. Results from Dawn’s GRaND instrument imply up to 10⁶ kg of water within the 150 g/cm² top portion of Vesta’s regolith (Prettyman et al. 2012). Vesta’s current surface temperatures and pressures make it inhospitable for liquid water. But, energy from a high velocity impactor impacting an area of CC could release the CC’s mineralogically bound water and provide temporarily increased temperatures and pressures in the newly formed crater, which would allow liquid water to briefly flow and form the PTFs before it spontaneously boils and evaporates. The potential for PTFs on the classically “dry” Vesta illuminates the possibility that liquid water could be present in previously unconsidered locations and have unconsidered influences on the history of our solar system.