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Colorful Surface of Asteroid Vesta through the Dawn Framing Camera

Wednesday, September 5, 2012
Le Corre

Vesta is the largest differentiated asteroid that is still mostly intact today and is considered to be a model for the initial stages of planetary differentiation. NASA’s Dawn mission entered orbit around Vesta on July 16, 2011 for a yearlong global characterization. The Framing Camera (FC) onboard the Dawn spacecraft has been mapping the asteroid in clear and seven narrow band filters (from 0.4 to 1 microns). Vesta exhibits global hemispherical scale dichotomy that is unlike any other asteroid we have observed in the asteroid belt. Large-scale variations in albedo have been observed before from ground-based telescopes and Hubble Space Telescope. Dawn FC observations during approach phase have confirmed this dichotomy. Apart from global color variations, several color terrains have been identified that are associated with compositional units using the latest color data. These units are classified as bright, dark, gray and orange/red materials. Enigmatic orange ejecta around two large impact craters, 34-km diameter Oppia, and 30-km diameter Octavia, are seen in FC color ratio images. Interestingly the orange ejecta around Oppia correspond to the olivine-rich unit observed by Gaffey (1997). Clementine color ratio images reveal that orange material around Oppia and Octavia forms an asymmetric ejecta extending southeast for Oppia and northwest for Octavia. The non-circular shape of Oppia crater coupled with preferential ejecta distribution suggests an impact on a slope. The steep visible slope and weaker 0.90-micron pyroxene absorption implies that another component or process has modified the original basaltic surface. Comparison with laboratory spectra of HEDs suggests that impact melt can mimic the color characteristics of orange material around Oppia crater. Preliminary understanding of the nature and origin of the orange/red terrains will be presented.

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