Rosetta Alice Far Ultraviolet Observations of (2867) Steins

Details Rosetta Alice Far Ultraviolet Observations of (2867) Steins Rosetta Alice Far Ultraviolet Observations of (2867) Steins

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusm.p12a..04f&link_type=abstract

American Geophysical Union, Spring Meeting 2009, abstract #P12A-04

Physics

6000 Planetary Sciences: Comets And Small Bodies, 6205 Asteroids

Scientific paper

During Rosetta's flyby of the main-belt, E-type asteroid (2867) Steins on 5 Sept. 2008, the U.S. Alice UV imaging spectrometer was used to obtain the first far-ultraviolet (FUV) spectrum of an asteroid. A ten minute integration, averaging over a variety of geometries at closest approach, shows very good signal from 850 Å to 2000 Å representing the first spectrum of an E-type asteroid below the atmospheric cutoff. We find that the far ultraviolet albedo of Steins is very low, ˜5%, compared to its visible albedo, 41% (Keller et al. 2009; Weissman et al. 2008; Jorda et al. 2008), as is expected from the UV behavior of many refractory materials. We also find that the albedo does not show a dramatic color variation over the FUV spectral range; however, there is a pronounced dip near 1600 Å. In addition, Alice obtained the total FUV count rate integrated with 1 second resolution during the encounter to determine the average variation of reflected UV flux with phase angle. In comparison to the OSIRIS WAC data, Alice data show clear wavelength dependent phase reddening and that the opposition effect is greater in the FUV than in the visible. In addition to observing Steins at closest approach, a ˜22 hour exosphere search was conducted prior to closest approach with Steins in the Alice slit. As expected from the only existing model (Schläppi et al. 2008), a deep search for any exosphere (e.g., hydrogen, oxygen) yielded no obvious detections in our initial analysis. We have placed upper limits on an atomic hydrogen and oxygen exosphere at Steins. Jorda, L., et al. 2008. A. and Ap. 487, 1171. Keller, H. U., et al. 2009. Rosetta Steins Fly-by Scientific Workshop, Tegernsee, Germany. Schläppi, B., K. Altwegg, and P. Wurz 2008. Icarus 195, 674. Weissman, P.R., et al., 2008. Meteoritics and Planetary Science, 43, 1-10.

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