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The far-ultraviolet albedo of Šteins measured with Rosetta-ALICE |
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| Authors: | Michael F A’Hearn Lori M Feaga |
| Institution: | a Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA b Service d’Aeronomie, Réduit de Verrières—BP3, Routes des Gatines, 91371 Verrières le Buisson Cédex, France c Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218-2695, USA d Southwest Research Institute, Department of Space Studies, Suite 300, 1050 Walnut St., Boulder, CO 80302-5150, USA e Southwest Research Institute, P.O. Drawer 28510, San Antonio, TX 78228-0510, USA f Applied Physics Laboratory, The Johns Hopkins University, 11100 Johns Hopkins Rd., Laurel, MD 20723-6099, USA g MPI für Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau 37191, Germany |
| Abstract: | During Rosetta’s flyby of the asteroid (2867) Šteins in 2008, we used the ALICE instrument to measure the first far-ultraviolet (FUV) reflectivity spectrum of an asteroid (850-2000 Å). It is very dark in the FUV, ∼4%, compared to its very high reflectivity (40%) at optical wavelengths. The FUV albedo does not exhibit a systematic color trend across the spectral range, but there is a broad absorption feature, not yet identified with a specific mineral, with maximum depth near 1650 Å. The shape of this feature implies a very low abundance of Fe2+ ions in the surface minerals. The FUV brightness exhibits a significant opposition surge at phases below 10°. The visible/FUV color gets much redder with increasing phase angle inside the opposition surge and gets gradually redder at larger phase angles. We also conducted a deep search for an exosphere of atoms sputtered from the surface and set upper limits on any column densities of oxygen and hydrogen atoms at the time of our observations. The upper limit for H is comparable to that predicted by the only theoretical prediction of which we are aware, while that for O is higher than predicted by about an order of magnitude. |
| Keywords: | Asteroid Ultraviolet Spectrum |
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