Vesta: Exploration, Predictions, and Surprises

Kurzfassung

Introduction: The Dawn mission (led by PI Chris Russell) has been selected for implementation and will launch in 2006. It will arrive at Vesta in July 2010 and explore that protoplanet in detail for about a year before leaving for Ceres. Although samples believed to have originated from Vesta (HEDs) have been well studied in Earthbased laboratories, this is the first time in history we will have the opportunity to actually test what we think we have learned from these stones from the sky. Certainly some predictions will prove to be wrong, and overlooked and unexpected details will surprise us. In order to sharpen our skills in preparation for the year at Vesta it is worthwhile to examine and re-examine what we “know” and what we might find. Examples are given here, but we hope the community will be motivated to seriously probe ongoing and new issues. Ancient Differentiated Body: We naturally expect to find some of the oldest basalt flows of the solar system (4.5 Ga). We may also find source areas to be volcanoes or rifts. Alternatively, the closest basaltic analogue may turn out to be the heavily cratered southern highlands of Mars. Large craters and basins (such as at Vesta’s south pole) will have exposed the lower crust and perhaps mantle, thus providing the stratigraphy of this differentiated protoplanet. Such a geologic context will resolve with certainty the relationship between Eucrites and Diogenites. More importantly, it will identify and characterize other (unsampled?) rock types that constrain Vesta’s geologic evolution. Predictions are welcomed. Remanent Magnetic Field: Vesta is expected to have a core that also formed early. Although a currently active dynamo is not likely, any remanent magnetic signature frozen in by the quickly cooled protoplanet will certainly be detected. Assuming the 4.5Ga age of ALH84001 represents the age of the ancient Martian crust, then the crust of Vesta and Mars are roughly contemporaneous, and Vesta presents an exceptional laboratory with which to study early magnetization processes. Fresh Surface: The strong ferrous absorption bands observed in Vesta’s optical spectrum have long been interpreted to indicate the surface is relatively fresh, or unweathered by the solar wind and space environment. This is hard to reconcile with the growing evidence that significant spaceweathering has occurred on other, smaller asteroids (Eros, Gaspra). If a recent large impact event has resurfaced the upper few millimeters of the surface, such an event will be readily apparent from orbit. Alternatively, if an unexpectedly strong magnetic field is observed, it might protect the surface from interaction with much of the solar wind. Moons of Vesta: Given the suite of small Vestoids associated with Vesta (Binzel and Xu., Science,260,1993, 186), it is possible, if not probable, that several moonlets will be seen as we approach the protoplanet. Larger moonlets conceivably might be detected prior to arrival at Vesta using adaptive optics with modern large telescopes.