Heat and Gas Diffusion in Comet Nuclei (pdf file 5.5 MB) - ISSI

18 2. The Structure of Comet Nuclei
Table 2.6: Some generic physical parameters for comet nuclei based on the
interstellar dust model
Quantity
Value
Bulk density (Ψ = 0.7 − 0.8)
300 − 500 kg m −3
Tensile strength (Ψ = 0.8)
5 − 10 × 10 3 Pa
Compressibility (Ψ = 0.8)
∼ 5 × 10 3 Pa
Dust thermal conductivity
Silicates 10 W m −1 K −1
Silic. + organic mantle
0.2 W m −1 K −1
Silic. + organic + amorph. ice mantle 1.5 × 10 −7 T W m −1 K −1
Silic. + organic + cryst. ice mantle
(< 100 K) 1.9 × 10 2 /T W m −1 K −1
Aggregate reduction factor (Ψ = 0.8)
(Hertz factor )
5 × 10 −4
Bulk thermal conductivity (Ψ = 0.8)
Amorphous ice mantle
0.75 × 10 −10 T W m −1 K −1
Crystalline ice mantle
0.4 − 1.5 × 10 −3 /T W m −1 K −1
Pore radius 1 µm
Chemical composition (by mass)
Sil : org : carb : ices 0.26 : 0.23 : 0.086 : 0.426
Bond albedo 0.04
In Table 2.6, we summarize the physical parameters for materials in
comet nuclei as derived from the aggregated interstellar dust model. The
unit (fully accreted) interstellar grains in the comet nucleus when considered
as spheres, can be described as silicate and organic refractory cores, with icy
material (including the small carbonaceous and PAH particles) as mantles.
The radii are respectively ˆr s = 0.07 µm, ˆr o = 0.101 µm, ˆr i = 0.139 µm. The
volume proportions used to provide these radii have been obtained from the
chemical proportions (by mass) given by:
Sil : Carb : Org. Refr. : H 2 O :CO : CO 2 :CH 3 OH : H 2 CO : Other
= 0.26 : 0.086 : 0.23 : 0.31 : 0.024 : 0.030 : 0.017 : 0.005 : 0.04.
When different theories give different results, we choose nominal values
rather than presenting a range. A similar set can be readily derived for
a range of porosities.
While the bond albedo is a consensus of observations, it is not much
lower than what was predicted from the interstellar dust model based on
the very porous character of the surface (Greenberg, 1998). We note that
the value of the pore radius is taken as 1 µm because it can be shown that
the pore size must be of the same order as particle size for a mean density
of 500 kg m −3 .