Origin of Volatiles on Asteroid 4 Vesta

Orbital Mosaic of Asteroid Vesta. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Volatile elements make up the essential ingredients of life, and the isotopic compositions of volatile elements act as a tracer for their origin and evolution in the solar system. The element hydrogen has three isotopes: hydrogen (H), deuterium (D), and tritium (T). Hydrogen isotopic composition, usually expressed as the D/H ratio (because of the relative rarity of tritium), is one such important tracer for understanding the sources of water, H2O, in the inner solar system.

Asteroid 4 Vesta is one of the largest asteroids in the asteroid belt. NASA’s Dawn mission orbited Vesta from July 16, 2011, until September 5, 2012, and collected visible, infrared, gamma-ray, and neutron remote sensing data. However, volatile element abundances and isotopic compositions can be difficult to constrain from such remote sensing measurements. Fortunately, a group of meteorites called HEDs (short for howardites, eucrites, and diogenites) is thought to come from Vesta and thus provide us with samples to analyze in our laboratories. Previous determinations of D/H ratios for Vesta were based on analyses of phosphate minerals such as apatite and whitlockite in HED meteorites. The measured D/H ratios were similar to those of carbonaceous chondrite meteorites, supporting the hypothesis that water was added to Vesta from carbonaceous chondritic bodies that migrated inward from the outer solar system. However, phosphates are some of the last minerals to crystallize in the formation of HED meteorites, and their hydrogen isotopic compositions may have been altered by magmatic processes such as degassing (loss of volatiles to space), which can fractionate hydrogen and deuterium from one another.

In contrast, a recent study by Alice Stephant of the Instituto di Astrofisica e Planetologia (Italy) and colleagues focused on analyzing silicate minerals such as clinopyroxenes that crystallize early in eucrites. Their research showed that silicate minerals record a hydrogen isotopic composition that is significantly lighter (lower D/H ratio) than compositions of most carbonaceous chondrites (and lighter than the aforementioned phosphates). Their work suggests that at least some of the water on Vesta was deuterium poor. This lighter hydrogen isotopic composition implies that Vesta acquired at least some of its water from a deuterium-poor reservoir in the inner solar system as it formed. Chondritic meteorites do show a large range in their D/H ratio, however; therefore, later delivery of some chondritic meteorites with deuterium-poor composition could have also contributed to the volatile inventory of Vesta. Identifying the sources and the delivery mechanisms for water in the inner solar system is important for understanding how the terrestrial planets formed and evolved and how life may have originated. This work provides isotopic evidence suggesting that the inner solar system probably contained some indigenous water that accreted to the planets as they formed. READ MORE