The subsurface thermal IR emission of asteroids becomes linearly polarized
when scattered across the surface, The disk integrated mid-IR Stokes parame
ters are computed by extending the new thermophysical model by Lagerros (19
96, Astron. Astrophys. 315, 625-632; 1998, Astron. Astrophys. 332, 1123-113
2). The small scale surface roughness is approximated by hemispherical segm
ent craters covering a smooth surface, Analytical solutions are used far th
e multiple scattered solar and thermally emitted radiation inside the crate
rs. The surface roughness enhances the emission toward the solar direction,
but randomizes the scattering planes which lowers the polarization, In gen
eral the predicted degree of linear polarization increases with higher refr
active index, higher absorption coefficient, and a more elongated shape of
the asteroid.
The asteroids (6) Hebe and (9) Metis were observed at lambda = 25 mu m with
ISOPHOT onboard the Infrared Space Observatory (ISO), The model absolute f
luxes are in good agreement with the photometric results. Although no linea
r polarization was detected, the upper limits together with the extended mo
del enable us to put useful constraints on the regolith properties of the t
arget asteroids. The derived detection limits are compared to model polariz
ation, by spanning a range in surface roughness, refractive index, and ther
mal inertia parameter space. The Metis observations favor a low refractive
index and high surface roughness, but the Hebe observations are inconclusiv
e since they coincided with a minimum in the polarization curve. (C) 1999 A
cademic Press.