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M. Kreslavsky (Brown Umiversity), Yu. Shkuratov, V. Kaydash (Kharkov Astronomical Institute, Ukraine), G. Videen (Space Science Institute), J. Bell (Cornell University), M. Wolff (Space Science Institute), M. Hubbard (Cornell University), K. Noll, A. Lubenow (Space Science Institute)
The Hubble Space Telescope carried out an extensive set of observations of Mars during the 2003 opposition (GO 9738). For the first time, such observations included imaging polarimetry. The observations were made at 5 phase angles (approximately 6, 8, 10, 14, 16 deg) with the ACS/HRC camera in blue and two UV wide spectral filters combined with polarized filters (effectively 435 nm, 330 nm, 250 nm). Polarimetric calibration of the ACS is problematic due to rather strong instrumental polarization and their spectral dependence. Using calibration observations of a standard star in addition to observations of Mars, we derive a polarimetric calibration, which allowed measurements of linear polarization degree with absolute accuracy better than 0.5% and detection of features in the images with polarimetric contrast as small as 0.2%. We map the distribution of linear polarization degree and polarization plane orientation over the Martian disk. These distributions and their dependence on phase angle and wavelength are the result of scattering by the Martian surface and atmosphere. In addition to this well understood pattern, we observe transient ``polarization clouds" best expressed in the ultraviolet light. These polarization features are associated with some cloud features that are semitransparent in the UV and almost invisible in the blue light. These clouds lie at the diffuse edges of denser clouds. Strong polarization (up to 3% at 8 deg phase angle) of these optically thin aerosols indicates strong single-particle scattering polarization. Probably, these clouds are made of micron-scale particles of regular shape and similar size. Further quantitative analysis of the results obtained involving quantitative models of light scattering promise further understanding of the nature and properties of Martian aerosols.
The author(s) of this abstract have provided an email address for comments about the abstract: misha@mare.geo.brown.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.