Vesta possesses features of both asteroids—of which it is one of the largest examples—and of planets. Recent observations by the Dawn mission have provided a detailed map of the asteroid's surface and a great deal of insight into its interior. This data shows Vesta to be part of the rubble of the early Solar System's history, with a structured interior similar to the terrestrial planets—Mercury, Venus, Earth, and Mars. Dawn recently departed Vesta orbit and is now headed to Ceres, the largest asteroid (and one of the five dwarf planets designated by the International Astronomical Union).

Even as Dawn is in transit, scientists are still going through data from its time at Vesta. A pair of studies published in Science reveals a new surprise: the possible presence of volatiles on the asteroid's surface. Additionally, Vesta's surface contains a much higher amount of hydrogen than expected. The asteroid's overall composition indicates that it may be the source of some meteorites found on Earth, and, like Earth, it received volatiles through a late bombardment.

Based on Dawn's observations, Vesta is chemically similar to the HED meteorites, and in fact is likely their source, as material could have been blasted free by collisions. ("HED" comes from three classes of meteorite structure: howardite, eucrite, and diogenite.) However, these meteorites are not high in volatiles, especially compared with the carbonaceous chondrite meteorites thought by many to be the origin of much of Earth's water.

The presence of volatiles on the asteroid—and especially hydrogen—suggests they were added to the asteroid later. This implies a common source for Earth's and Vesta's water, further cementing Vesta's status as a terrestrial world in its own right.

Hydrogen on the rocks

Radioactive decays and reactions from cosmic rays produce gamma rays and neutrons, which can be detected by Dawn provided they originate within a half-meter of the surface, using an instrument called GRaND (Gamma Ray and Neutron Detector). Based on GRaND measurements, researchers mapped the hydrogen content of Vesta's surface. The amount of hydrogen was surprising, since it is the lightest element and therefore extremely volatile. Other airless bodies, such as the Moon, are hydrogen-poor, as are the HED meteorites that originated on Vesta.

However, hints about the hydrogen's origin exist in its distribution. The researchers found hydrogen to be most abundant in regions with low albedo—where little light is reflected. Carbonaceous chondrite meteorites have similarly low albedo, but relatively high water content. This suggests the hydrogen on Vesta's surface came from the dissociation of water deposited by meteorite impacts early in the asteroid's history.

The pits of despair

Further evidence for volatiles came from strange pitted terrain on the otherwise smooth floors of craters. These pits lacked raised rims and were often irregular in shape, meaning they were not impact sites. (Craters are typically rounded with lifted edges.) The pits in the Marcia crater ranged in size from about 30 meters to 250 meters in diameter (though instrument limitations mean there might be smaller ones yet unobserved), with the largest pits occurring near the crater's center.

Pits of this type have never been seen on other airless objects, including Mercury and the Moon. However, they strongly resemble features found in relatively recent craters on Mars. On Mars, the formation mechanism involves volatiles, most likely the vaporization of water ice that collapsed the pits after an impact. This is akin to holes forming in bread dough, when carbon dioxide bubbles burst through the surface during the rising process.

As with the surprising amount of hydrogen, the volatiles could have been deposited by meteorite bombardment after Vesta's formation. The Marcia crater lies in a region low in hydrogen content, which lends further support to this model: as hydrogen is volatile, its leeching from the rocks would be part of the process that created the pits.

This data strongly supports the widely accepted model that Vesta is a planetesimal, a leftover fragment of the bodies that combined to make Earth and other terrestrial worlds. Additionally, the probable presence of water and other volatiles means Vesta's history echoes that of Earth, including the deposition of materials by carbonaceous chondrite meteorites. Further missions to asteroids should settle whether this is a common feature of the larger ones. But for now, the data indicates that Vesta in some ways resembles Earth more closely than it does the Moon.

Science, 2012. DOI: 10.1126/science.12225374 and 10.1126/science.12225354 (About DOIs).