Mindful of the recent work on axial tilt I’ve reported in these pages, I was interested to learn that Vesta’s axial tilt is just a bit greater than the Earth’s, about 27 degrees. We’ve been pondering the consequences of such obliquity on planets in the habitable zone, but in Vesta’s case, the issue isn’t habitability but water ice. For spurred by the Dawn mission, scientists are looking at whether permanently shadowed craters on the asteroid’s surface would allow water to stay frozen all year long. Unlike the situation on the Moon, the answer on Vesta (on the surface at least) seems to be no.
Earth’s axial tilt is 23.5 degrees, but the Moon’s is a scant 1.5 degrees, making the shadow in some lunar craters permanent, a fact that has led to speculation that ice in these locations could be of use to future manned missions there. In contrast, Vesta’s obliquity means that it has seasons, so that every part of the surface becomes exposed to sunlight at some point in the year. Even so, says Timothy Stubbs (NASA GSFC), “Near the north and south poles, the conditions appear to be favorable for water ice to exist beneath the surface” [italics mine].
Image: New modeling shows that, under present conditions, Vesta’s polar regions are cold enough (less than about 145 K) to sustain water ice for billions of years, as this map of average surface temperature around the asteroid’s south pole indicates. (The white dashed line marks Vesta’s south polar circle.) Figure reprinted from the paper in Icarus by T.J. Stubbs, T.J. and Y. Wang. Image credit: NASA/GSFC/UMBC.
The polar regions on Vesta are under scrutiny as the Dawn mission continues its close look. Observations from the Earth have suggested a bone-dry Vesta, but in its current low orbit, Dawn is using its gamma ray and neutron detector (GRaND) spectrometer to look for hydrogen-rich deposits that might flag the presence of water ice below the surface. Because Dawn’s targets — Vesta and Ceres — are both considered remnant protoplanets, it will be important to learn whether there is sub-surface water on Vesta. The next issue to ponder will be whether any water that is discovered has arrived recently or goes back to the earliest days of the Solar System.
Modeling has shown that water ice should be able to survive in the top few meters of regolith when surface temperatures are less than 145 K or so, and while Vesta’s equator sees average yearly temperatures around 150 K — too warm to sustain water ice within meters of the surface — the regions near the asteroid’s north and south poles are cold enough to allow its survival for billions of years. The temperature dividing lines show up at about 27 degrees north latitude and 27 degrees south latitude.
As for the surface, this JPL news release quotes Stubbs on the matter:
“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.”
Ceres, the second of Dawn’s destinations, should prove a study in contrasts. As compared with Vesta’s dry surface, Ceres may have seasonal polar caps of water frost and even a thin atmosphere. Some models show a layer of 100 kilometers of icy material including water and ammonia, and allow the presence of liquid water beneath. Recall that this is the most massive body in the main asteroid belt, making up perhaps as much as a third of the mass of the entire main belt. Hubble observations have suggested that the topography here is low, with a lack of deep craters that indicates flow in the crust to even out the landscape. Dawn will be able to give us better estimates of Ceres’ mass and help us understand how that mass is distributed.
The paper on Vesta is Stubbs and Wang, “Illumination conditions at the Asteroid 4 Vesta: Implications for the presence of water ice,” Icarus Vol. 217, Issue 1, pp. 272-276 (January, 2012).