Polarization of comets at small phase angles |
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| Institution: | 1. Space Department, Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, USA;2. Koyama Astronomical Observatory, Kyoto Sangyo University Motoyama, Kita-ku, Kyoto 603-8555, Japan;3. Department of Physics, American University, Washington, DC, USA;4. Department of Physics and Astronomy, University of Missouri-St. Louis, St. Louis, MO, USA;5. Solar System Exploration Division, Planetary Systems Laboratory Code 693, NASA-Goddard Space Flight Center, Greenbelt, MD, USA;6. Goddard Center for Astrobiology, NASA-Goddard Space Flight Center, Greenbelt, MD, USA;7. University Space Research Association/NASA Postdoctoral Program, Columbia, MD, USA;1. South African Astronomical Observatory, 1 Observatory Road, Cape Town 7925, South Africa;2. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave. Cambridge, MA 02139, USA;3. Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA;4. Department of Astronomy, University of Cape Town, Rondebosch, Cape Town 7701, South Africa;5. Deutsches SOFIA Institut, Universität Stuttgart, Pfaffenwaldring 29, 70569 Stuttgart, Germany;6. SOFIA Science Center, NASA Ames Research Center, Mail Stop N211-1, Moffett Field, CA 94035, USA;7. Las Cumbres Observatory, 6740 Cortona Drive Ste. 102, Goleta, CA 93117, USA |
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| Abstract: | Comets Kopff (1982k) and Tempel 2 (1982d) were observed with a photopolarimeter. The observations spanned phase angles of 10-30° for Comet Kopff and of 45° for Comet Tempel 2. The Comet Kopff observations were near the phase angle where the position angle of the polarization switches from parallel to perpendicular to the scattering plane. The polarization showed no color dependence in the V, R, and I filters. The polarization of Comet Kopff as sa function of phase angle resembles polarization curves of asteroids. The minimum polarization was -1.5±0.1%. The data from Comet Kopff and four other comets were combined in a polarization versus phase-angle diagram. The coincidence of the data was remarkable. The collection of comet data determined a crossover phase angle of 22 ± 0° and a polarimetric slope of 0.34 ± 0.01% per degree. Mie models were unable to represent the group polarization curve, even though five parameters were varied over wide ranges. Assuming that multiple scattering provides the basis for the correlations between asteroid polarization and albedo, and that the multiple scattering process dominates the scattering from comet dust, the comet polarization implies an albedo. The two parameters of the comet polarization curve imply the same low albedo. |
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