A New Thermophysical Model
A new thermophysical model
of asteroids has been
developed throughout a series of papers by J. Lagerros.
A number of
physical processes previously neglected in the
Standard Thermal Model
are now considered. The new model predicts the thermal emission of
asteroids, from mid-infrared to microwave wavelengths.
The main topics that have been considered are
- Arbitrary shapes
- Irregular shapes
can be studied. Multiple
scattering of radiation between different parts of the
non-convex surfaces is considered. As input parameters to the
model the shape and spin state are used, which have
been estimated for many asteroids from visual light
curves.
- Heat conduction
- As the asteroid rotates and heat is
conducted into the surface, the day side
temperature is decreased and thermal emission is also taking
place on the night side. There is also a time lag between the
peak solar influx and the warmest time of the ``day''.
- Albedo patterns
- Some asteroids like
4 Vesta
have an
inhomogeneous surface, which causes substantial colour and
reflectance variations across the surface. Without direct
images it is difficult to distinguish shape effects and
effects due to colour variations. The visual and thermal light
curves are, however, expected to be out of phase if there are
spots on the surface.
- Beaming
- The thermal emission from atmosphereless solar system
bodies like the Moon, Mercury and the asteroids is observed to be
``beamed'' in the solar direction. The main cause for this is
probably the surface roughness. Surfaces filled with craters,
and stochastic surfaces have been considered.
- Emissivity
- The emissivity is affected by radiative transfer
processes in the porous regolith. At longer wavelengths there
is a drop in the observed emissivities, due to the size
distribution of particles as compared to the wavelength.
- Microwave emission
- At longer wavelengths the surface material
becomes semi-transparent. The microwave emission
enables the investigation of the thickness of the regolith and
physical properties at some depth below the surface.
- Polarization
- Johnson et at. (1983) obtained
0.2-0.6% of linear polarization in the 10 micron
emission of 1 Ceres. This can be explained by the
scattering of sub-surface emission in the regolith into the empty
space, which results in polarization according to Fresnel's
equations. The electrical vector becomes parallel to the scattering
plane, that is the plane containing the surface normal and the
direction of propagation. This is seen in the disk resolved
microwave emission from the Moon
(Clegg & Carter 1970) and
Mercury (Mitchell
& De Pater 1994).
Johan Lagerros