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Dawn spacecraft offers first look at giant asteroid's chemistry


This image shows an area within the Rheasilvia Basin in the south polar area of the giant asteroid Vesta. In this image, a younger, darker blanket of material ejected by an impact is in contact with a brighter, hummocky deposit marked by craters. The brighter material, which appears to be older, also shows crisscrossing, linear features. The image covers an area about 12 miles by 12 miles (20 kilometers by 20 kilometers).



A cutaway diagram of the GRaND instrument. Credit: UCLA

The NASA Dawn spacecraft’s close-up study of the giant asteroid Vesta is offering researchers their first look at the elemental composition of this ancient protoplanet. Vesta is the second-most massive body in the main asteroid belt and has remained intact since its formation more than 4.5 billion years ago. Dawn’s Gamma Ray and Neutron Detector (GRaND) will determine the chemical composition of Vesta, providing new information about how Vesta formed and evolved. Tom Prettyman from the Planetary Science Institute in Tucson, Arizona, is the lead for GRaND as well as Dawn’s Geochemistry Team.

The NASA Dawn spacecraft has been acquiring science data in orbit around Vesta since July 2011. In December, Dawn reached its lowest altitude orbit with an average distance of about 130 miles (210 kilometers) from Vesta’s surface. Vesta’s diameter is about 330 miles (530km), just a little bit larger than the width of Arizona. Dawn plans to stay in this orbit for 110 days before ascending to higher altitudes and departing to its next destination, the dwarf planet Ceres.

At low altitudes, GRaND is detecting strong neutron and gamma-ray signals that scientists will analyze to map the elemental composition of the entire surface of Vesta, Prettyman said. Unlike Dawn’s framing camera and visible and infrared mapping spectrometer, GRaND can see in the dark, Prettyman added. Consequently, GRaND is sensitive to the composition of Vesta’s surface at high northern latitudes, presently in polar night.

GRaND measures the abundance of elements found in planetary surfaces, such as hydrogen (H), iron (Fe), magnesium (Mg), and silicon (Si). The data will help scientists determine how hydrogen was delivered to the surface of Vesta — for example, by the solar wind or by carbon-rich materials hitting the asteroid. Measurements of rock-forming elements, including Fe, Mg, and Si, will help them understand the volcanic processes that shaped Vesta.

After five weeks of mapping in the spacecraft’s low-altitude orbit, global-scale variations in Vesta’s elemental composition are apparent, Prettyman said. Vesta’s varied surface distinguishes it from smaller asteroids, which are typically uniform in composition. Vesta, which underwent complex geochemical processes, forming a core, mantle, and crust, seems more like a terrestrial planet than an asteroid.

GRaND’s initial observations are tantalizing; however, their interpretation will require additional accumulation of data and further analysis by Dawn’s Geochemistry Team. First results will be reported soon after mapping at low altitudes is complete.