According to the first published models of Vesta’s average global temperatures and illumination by the sun though generally thought to be quite dry, roughly half of the giant asteroid Vesta is expected to be so cold and to receive so little sunlight that water ice could have survived there for billions of years. According to Timothy Stubbs of NASA’s Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland, Baltimore County near the north and south poles, the conditions appear to be favorable for water ice to exist beneath the surface. Vesta probably does not have any significant permanently shadowed craters where water ice could stay frozen on the surface all the time, not even in the roughly 300-mile-diameter (480-kilometer-diameter) crater near the south pole.
Though temperatures on Vesta fluctuate during the year, the model predicts that the average annual temperature near Vesta’s north and south poles is less than roughly minus 200 degrees Fahrenheit (145 kelvins). Near Vesta’s equator, however, the average yearly temperature is roughly minus 190 degrees Fahrenheit (150 kelvins). Based on previous modeling, that is expected to be high enough to prevent water from remaining within a few meters of the surface. This band of relatively warm temperatures extends from the equator to about 27 degrees north and south in latitude. The modeling also indicates that relatively small surface features, such as craters measuring around 6 miles (10 kilometers) in diameter, could significantly affect the survival of water ice. So far, Earth-based observations suggest that the surface of Vesta is quite dry. However, the Dawn spacecraft is getting a much closer view. Dawn is investigating the role of water in the evolution of planets by studying Vesta and Ceres, two bodies in the asteroid belt that are considered remnant protoplanets, baby planets, whose growth was interrupted when Jupiter formed.