NASA's Dawn spacecraft spent 14 months photographing Vesta from close range. Now researchers have completed a global map of the asteroid's surface geology.
Planetary geologists are always seeking to know the sequence of events that shaped a solar-system body, and now they have a much better idea of what happened when on asteroid 4 Vesta. A detailed geologic map, published in the December issue of Icarus, shows features large and small revealed by NASA's Dawn spacecraft during its 14-month-long survey of Vesta in 2011-12. It represents 2½ years of effort by David A. Williams (Arizona State University), R. Aileen Yingst (Planetary Science Institute), and 12 others.
Shown below, the global map reveals that this oblong, 573-by-446-km body is divided into provinces pegged to the formation of its three largest craters. Roughly a third of Vesta's surface (shown as brown and tan hues) comprises old, heavily cratered areas in the northern hemisphere. These predate the excavation of a huge basin, 400-km-wide Veneneia, near Vesta's south pole.
A wide northern band (shown in purple) represents terrain emplaced after Veneneia but before the formation nearby of Rheasilvia, which is even larger — half the diameter of Vesta itself. Ejecta from that blast are shown in blue hues. A final veneer (yellow-greens) followed the formation of Marcia crater, 68 km wide and one of the asteroid's youngest features.
Vesta also exhibits clusters of ridges and troughs, which are depicted in the map as black lines. A cluster in the northern hemisphere, named Saturnalia Fossae, likely resulted from internal stresses during the impact event that formed Veneneia. An even denser set, called Divalia Fossae, extends east-west right along the equator and seems to be associated with Rheasilvia.
Getting the Timetable Right
It's the "when" part of Vesta's history that's given Williams and his colleagues trouble. They deduce one set of ages when using a model based on an assumed cratering rate within the asteroid belt — and very different ages when attempting to extrapolate from cratering rates on the Moon and from the ages of lunar samples.
For example, the Veneneia blast occurred either about 2.1 or 2.7 billion years ago, according to the asteroid or lunar model, respectively. Likewise, Rheasilvia could have been excavated as recently as 1.0 billion years ago (asteroid model) or as long as 3.5 billion years (lunar model). Adding to the confusion, the asteroid model implies a significantly older age for Marcia (about 390 million years) than does the lunar model (120 million years).
Just as lunar samples returned by Apollo missions removed uncertainty regarding the Moon's age, in principle geochemists would be able to accurately date Vesta if they could only get their hands on samples of its surface. And, incredibly, they can! Three closely-related basaltic meteorite groups known as howardites, eucrites, and diogenites (HEDs) are such close spectral matches to Vesta that the big asteroid (most likely the Rheasilvia basin) must have been their launch site.
But laboratory tests suggest that these "Vestoids" have very old ages, dating to at least 4 billion years ago. These correspond to when the rocks solidified from a basaltic magma. Big cratering events on the Moon create such powerful shock waves that the target rocks' isotopic clocks get reset. But apparently that didn't happen on Vesta, because collision speeds among asteroids are generally much lower, no more than about 5 km per second.
Consequently, while the mappers can tell which impacts came first and which ones later, in a relative sense, they're unable to pin down when specific events occurred. Perhaps the surface of even larger 1 Ceres, which Dawn will reach in early March 2015, holds additional clues to the ages of asteroids' surfaces.
Read about Dawn's amazing adventures around Vesta in Sky & Telescope's November 2011 issue