Single-Scattering Mechanism for Negative Polarization and Opposition Brightening of Atmosphereless Solar-System Objects
Near opposition, there are two ubiquitous phenomena observed for asteroids and other atmosphereless solar-system objects as well as interplanetary and cometary dust: the photometric opposition effect and the negative polarization confined to within 10 and 30 degrees from opposition, respectively. The coherent backscattering and shadowing mechanisms have been introduced to explain the phenomena sometimes showing up at extremely small phase angles. Coherent backscattering has been shown to contribute to both intensity and polarization; whereas, shadowing has been shown to contribute to the opposition effect only.We present a novel single-scattering mechanism to explain the substantial angular widths of the negative polarization branches (K. Muinonen et al., JQSRT 106, 360, 2007; J. Tyynela et al., JQSRT 106, 520, 2007). First, negative polarization is caused across the full phase-angle regime by the longitudinal component of the internal electric fields (parallel to the direction of incidence) induced in the single wavelength-scale scatterers. Second, after nulling the longitudinal components of the internal fields, both negative polarization and brightness enhancement can be seen to be caused by the coherence of transverse internal-field components. The mechanism has been verified for both spherical and nonspherical single scatterers with wavelength-scale sizes. With the help of the single-scattering mechanism, it becomes possible to analyze the dependence of the polarization branches on single-scatterer sizes, shapes, and complex refractive indices. The new mechanism bears considerable similarity to the coherent-backscattering mechanism in multiple scattering. We present a theoretical assessment of the information content in linear polarization observations of solar-system objects as well as of the interrelation of the coherent-backscattering and single-scattering mechanisms.
AAS/Division for Planetary Sciences Meeting Abstracts #39
- Pub Date:
- October 2007