WATER ON VESTA: FIRST RESULTS FROM DAWN'S SURVEY ORBIT

Hydrogen, hydroxyl and water have been reported relatively recently to exist on and beneath the surface of the Moon, in spite of early studies of returned lunar samples indicating a dry Moon. Vesta is a similar object to the Moon in the inner Solar System and the Dawn mission in orbit about Vesta is searching for these molecules. We report on the initial findings and their implications. Water and other volatiles on airless solid surfaces in the inner solar system seem to be short-lived due to high temperatures, low pressures, high-energy irradiation and bombardment, but their presence would carry profound scientific implications. Dawn at Vesta carries several instruments useful for this search: the Gamma Ray and Neutron Detector (GRaND), for detection and mapping of hydrogen, and the Visual-Infrared (VIR) imaging spectrometer, sensitive to OH, H₂O and other volatile molecules. The multispectral imaging system provides geologic and topographic context for these observations. These instruments are sensitive to different properties and build their results on different time scales, and so their findings have to be integrated into a broader discussion. Volatiles can come from at least three different sources: 1) indigenous, i.e., from the original material; 2) infall, from comets and some asteroidal material; and 3) surface chemistry, from solar wind implantation. Evidence suggests that all three of these could be sources for the Moon, and it can be argued to be true also for Vesta. Vesta should retain water more easily, including just below the surface, than the Moon, given its colder environment. Vesta apparently has differentiated, and it may have had more water originally than the Moon, considering their different origin hypothesis. Vesta should be impacted at least as much as the Moon by “wet” asteroidal and cometary material. The solar wind also strikes the exposed surface of Vesta with about the same energy per particle as for the Moon, although the flux density should be lower. Thus, the same chemical reactions could take place on Vesta as for the Moon, but perhaps somewhat slower. Observations of Vesta so far show little or no evidence of volatiles, although these observations are very difficult from the Earth. Dawn has a huge advantage by being in orbit and for a relatively long period.