Though typically believed to be quite dry, roughly half of the giant asteroid Vesta could be so cold that water ice may have survived there for billions of years.
"Near the north and south poles, the conditions appear to be favorable for water ice to exist beneath the surface," confirmed Timothy Stubbs of NASA's Goddard Space Flight Center in Greenbelt, Md.
Stubbs, along with Yongli Wang of the Goddard Planetary Heliophysics Institute at the University of Maryland, said his assessment was based on a new model of Vesta's average global temperatures and illumination by the sun.
Nevertheless, Vesta, which is ranked as the second-most massive object in the asteroid belt between Mars and Jupiter, 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.
Indeed, the asteroid isn't a good candidate for permanent shadowing because it tilts on its axis at about 27 degrees, which is even greater than Earth's tilt of roughly 23 degrees. In contrast, the moon, which does have permanently shadowed craters, is tilted at only about 1.5 degrees. As a result of its significant tilt, Vesta has seasons, and every part of the surface is expected to see the sun at some point during Vesta's year.
Though temperatures on Vesta fluctuate during the year, the new model estimates the average annual temperature near Vesta's north and south poles at less than roughly minus 200 degrees Fahrenheit (145 kelvins). That is the critical average temperature below which water ice is thought to be able to survive in the top 10 feet or so (few meters) of the soil, which is called regolith.
Near Vesta's equator, however, the average yearly temperature is roughly minus 190 degrees Fahrenheit (150 kelvins). Based on previous modeling, 150 kelvins is thought 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.
"On average, it's colder at Vesta's poles than near its equator, so in that sense, they are good places to sustain water ice," said Stubbs.
"But they also see sunlight for long periods of time during the summer seasons, which isn't so good for sustaining ice. So if water ice exists in those regions, it may be buried beneath a relatively deep layer of dry regolith."
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.
"[Yes], the bottoms of some craters could be cold enough on average - about 100 kelvins - for water to be able to survive on the surface for much of the Vestan year [about 3.6 years on Earth]... Although, at some point during the summer, enough sunlight would shine in to make the water leave the surface and either be lost or perhaps redeposit somewhere else."
So far, Earth-based observations suggest that the surface of Vesta is quite dry. Fortunately, the Dawn spacecraft is getting a much closer view by investigating the role of water in the evolution of planets as it studies Vesta and Ceres, two bodies in the asteroid belt that are considered remnant protoplanets – baby planets whose growth was interrupted when Jupiter formed.
Dawn is searching for water using the gamma ray and neutron detector (GRaND) spectrometer, designed to identify hydrogen-rich deposits that could be associated with water ice. The spacecraft recently entered a low orbit which is well suited to collecting gamma ray and neutron data.
"Our perceptions of Vesta have been transformed in a few months as the Dawn spacecraft entered orbit and spiraled closer to its surface," explained Lucy McFadden, a planetary scientist at NASA Goddard and a Dawn mission co-investigator.
"More importantly, our new views of Vesta tell us about the early processes of solar system formation. If we can detect evidence for water beneath the surface, the next question will be is it very old or very young, and that would be exciting to ponder."