Volatile Vesta

On 16 July 2011 the Dawn spacecraft entered into orbit around the asteroid Vesta and has been providing us with a better understanding of the history and composition of the second most massive member of the main asteroid belt.  We now know that Vesta is a dry, differentiated protoplanet.

Two papers published this week in the journal Science report the discovery of volatiles around the equator of Vesta. The first paper, led by Thomas Prettyman, analyzed the composition of the asteroid’s regolith  using GRaND, the gamma ray and neutron detector on board Dawn. This instrument measures gamma rays and neutrons released by nuclear reactions and radioactive decays to depths of a few tens of centimetres in Vesta’s surface. This allows GRaND to derive the concentration and abundances for several elements (there is an excellent article here which explains how nuclear spectroscopy works).  The Prettyman et al. results indicate the presence of hydrogen in the form of hydroxyl and water bound with minerals in Vesta’s surface.

NASA’s Dawn mission detected hydrogen in a wide band around the equator of the giant asteroid Vesta. The hydrogen is likely in the form of hydroxyl or water bound to minerals in Vesta’s surface. (Credit: NASA/JPL-Caltech/UCLA/PSI/MPS/DLR/IDA)

Since Vesta accreted from volatile-poor material and also went through differentiation, the origin of the volatiles must be exogenous. There are two main processes that might have accreted volatiles onto the surface of Vesta:  (1) enrichment by solar wind, or (2) accretion from impacting objects.  Models show that the quantity of implanted volatiles by the solar wind do not match the amount found on Vesta’s surface from the Prettyman et al. analysis. Therefore the continuous infall of carbonaceous chondritic materials – carbon-rich rocks – containing OH-bearing phyllosilicates appears to be a viable alternative. If the collision speeds a low enough, the volatile content of the impactors should be preserved. In order to prove this theory, Prettyman et al. suggest that these impacts would have resulted in the formation of pitted terrain. Pitted terrains are characterized by irregular rimless depressions found around and within impact craters, and have a distinct morphology that can not be found on airless bodies.

Which is the subject of the second Science particle, led by Brett Denevi.  Analysing images taken during the low-altitude mapping orbit (LAMO) of Dawn, Denevi et al. clearly spotted pitted terrain on Vesta. Similar structures have been found on the surface of Mars, for which formation models suggest that the origin of the Martian pitted terrain may be due to sublimation of ice long after the impact event or erosion activated by rapid degassing of volatiles in a molten-breccia mixture after an impact event. Both of these scenarios require substantial abundances of volatiles.

Pitted terrain at the Marcia crater on Vesta. (Credit: Denevi et al. 2012)

The similarities between pitted terrains on Mars and Vesta suggest a similar origin. Given the dry environment in which Vesta likely formed, the bombardment of external material might have provided Vesta with the right amount of volatiles to allow the formation of its pitted terrain. This formation mechanism for pitted terrain, together with the low impact speeds, could explain both the occurrence of these features on Mars and Vesta. Vesta’s surface appears to be unique among airless bodies observed to date in the nature and degree of preservation of exogenic materials.

For more details, see

[Francesco Pignatale & Sarah Maddison]

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