ISOPHOT Polarization Measurements of the Asteroids (6) Hebe and (9) Metis at 25 μm

Details ISOPHOT Polarization Measurements of the Asteroids (6) Hebe and (9) Metis at 25 μm ISOPHOT Polarization Measurements of the Asteroids (6) Hebe and (9) Metis at 25 μm

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..142..454l&link_type=abstract

Icarus, Volume 142, Issue Icarus, pp. 454-463.

Astronomy and Astrophysics

Astrophysics

6

Scientific paper

The subsurface thermal IR emission of asteroids becomes linearly polarized when scattered across the surface. The disk integrated mid-IR Stokes parameters are computed by extending the new thermophysical model by Lagerros (1996, Astron. Astrophys. 315, 625-632; 1998, Astron. Astrophys. 332, 1123-1132). The small scale surface roughness is approximated by hemispherical segment craters covering a smooth surface. Analytical solutions are used for the multiple scattered solar and thermally emitted radiation inside the craters. The surface roughness enhances the emission toward the solar direction, but randomizes the scattering planes which lowers the polarization. In general the predicted degree of linear polarization increases with higher refractive index, higher absorption coefficient, and a more elongated shape of the asteroid. The asteroids (6) Hebe and (9) Metis were observed at λ=25μm with ISOPHOT onboard the Infrared Space Observatory (ISO). The model absolute fluxes are in good agreement with the photometric results. Although no linear polarization was detected, the upper limits together with the extended model enable us to put useful constraints on the regolith properties of the target asteroids. The derived detection limits are compared to model polarization, by spanning a range in surface roughness, refractive index, and thermal inertia parameter space. The Metis observations favor a low refractive index and high surface roughness, but the Hebe observations are inconclusive since they coincided with a minimum in the polarization curve.

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