Asymmetry Parameters of the Phase Function for Large Particles with Multiple Internal Inclusions

Details Asymmetry Parameters of the Phase Function for Large Particles with Multiple Internal Inclusions Asymmetry Parameters of the Phase Function for Large Particles with Multiple Internal Inclusions

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1706m&link_type=abstract

American Astronomical Society, DPS meeting #28, #17.06; Bulletin of the American Astronomical Society, Vol. 28, p.1122

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Scientific paper

It is well known that the phenomenological Hapke model of bidirectional reflectance by particulate media often yields negative values for the asymmetry parameter of the single-scattering phase function. It has been hypothesized that the negative asymmetry parameters result from a complicated internal structure of planetary regolith particles. In this paper we test this hypothesis by theoretically computing asymmetry parameters for isolated and densely packed particles with multiple internal inclusions and size typical of lunar regolith particles. The asymmetry parameters are calculated using the Monte Carlo ray-tracing method coupled with Fraunhofer diffraction and the static structure theory [JQSRT 52, 95 (1994)]. The results of our computations show that it is essentially impossible to make asymmetry parameters of regolith particles even slightly negative by filling the particles with large numbers of internal inclusions in the form of voids and/or grains with a refractive index substantially different from that of the host medium. Asymmetry parameters are positive even for densely packed composite particles with no internal absorption and extreme values of the internal scattering coefficient. Furthermore, they are sharply increased by even modest absorption inside composite particles, by reducing the refractive index contrast between the inclusions and the host particles, and/or by decreasing the packing density. Also, the negative asymmetry parameters retrieved with the Hapke model contradict the results of laboratory phase function measurements for large heterogeneous particles of terrestrial and interplanetary origin [Astron. Astrophys. 126, 240 (1983)] and phase functions retrieved for interplanetary dust particles [Astron. Astrophys. 163, 269 (1986)]. Thus, the negative asymmetry parameters need another explanation rather than assuming that they are real and are the result of the complicated internal structure of regolith particles. Besides the opposition-effect term, the Hapke model is nothing more than an approximate solution of the radiative transfer equation which inherently violates the energy conservation law. Therefore, in many cases the negative asymmetry parameters are likely to be numerical artifacts resulting from approximations made in the model.

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