for the remainder of the rotation (the nightside). Here, ξ p is a penetration scale length.
In the Urquhart and Jakosky formulation, a dependence on the angle of incidence is
included and, in the simplest case, there would be a relationship to the cosine of i.
(They also substitute Ω N with 2π/P where P is the period.) Given the low thermal
inertia, it is conceivable that this type of volume absorption could play a role if ξ p is
significantly greater than x 1 /10. This type of process was first implemented in comet
models by Davidsson and Skorov (2002a, b).
Solid-state greenhouse effect (Matson and Brown 1989) occurs as a consequence
of ice in its purest form being translucent. This results in the impinging solar energy
being deposited below the physical surface. It is now well established that this
phenomenon can lead to unusual effects. The most spectacular is the formation of
geyser-like activity on Mars arising from basal sublimation of CO 2 in response to
sunlight penetrating a 1 m thick layer of CO 2 ice (Kieffer et al. 2006). In this
example, thermal re-radiation from below the ice layer is effectively trapped by
the CO 2 ice above and generates a sort of pressure cooker until the pressure is
released via a crack in the ice with subsequent violent outgassing. There is no
evidence that metre-thick translucent ice layers are present on comets but the
principle may operate on smaller length scales. One can imagine ices being semitranslucent over millimetre scales so that basal heating of an ice layer can occur. This
might be a mechanism for more explosive loss of larger amounts of material. This
would probably have had to be on sub-decimetre scales to avoid detection by
Rosetta’s imaging system.
The high porosity of cometary nuclei has effectively been confirmed by Rosetta
observations of the bulk density (Sect. 2.3). On the other hand, the high porosity
refers to the nucleus as a whole and it is not completely clear that this holds for the
uppermost surface layer. However, if it does, there are implications for the thermal
behaviour of the surface layer.
The thermal conductivity of a structure is reduced as voids within the structure
become more and more significant as the porosity, Ψ , increases. Porosity is defined
through
Ψ ¼
V v
V
ð2:115Þ
where V v /V is the ratio of the volume of the voids to the volume of the whole. A
useful example is the porosity of a skeleton of spherical particles of radius a, which
is simply
Ψ ¼ 1 À N p
4
3
πa
3
ð2:116Þ
where N p is the number of particles per unit volume. However, the effect on thermal
conductivity is mitigated by thermal re-radiation within the porous structure. Hence,
the pore size distribution within the structure also plays a role. This results in the
effective conductivity becoming a complex function of the radiation field. The
96
2 The Nucleus
In the Urquhart and Jakosky formulation, a dependence on the angle of incidence is
included and, in the simplest case, there would be a relationship to the cosine of i.
(They also substitute Ω N with 2π/P where P is the period.) Given the low thermal
inertia, it is conceivable that this type of volume absorption could play a role if ξ p is
significantly greater than x 1 /10. This type of process was first implemented in comet
models by Davidsson and Skorov (2002a, b).
Solid-state greenhouse effect (Matson and Brown 1989) occurs as a consequence
of ice in its purest form being translucent. This results in the impinging solar energy
being deposited below the physical surface. It is now well established that this
phenomenon can lead to unusual effects. The most spectacular is the formation of
geyser-like activity on Mars arising from basal sublimation of CO 2 in response to
sunlight penetrating a 1 m thick layer of CO 2 ice (Kieffer et al. 2006). In this
example, thermal re-radiation from below the ice layer is effectively trapped by
the CO 2 ice above and generates a sort of pressure cooker until the pressure is
released via a crack in the ice with subsequent violent outgassing. There is no
evidence that metre-thick translucent ice layers are present on comets but the
principle may operate on smaller length scales. One can imagine ices being semitranslucent over millimetre scales so that basal heating of an ice layer can occur. This
might be a mechanism for more explosive loss of larger amounts of material. This
would probably have had to be on sub-decimetre scales to avoid detection by
Rosetta’s imaging system.
The high porosity of cometary nuclei has effectively been confirmed by Rosetta
observations of the bulk density (Sect. 2.3). On the other hand, the high porosity
refers to the nucleus as a whole and it is not completely clear that this holds for the
uppermost surface layer. However, if it does, there are implications for the thermal
behaviour of the surface layer.
The thermal conductivity of a structure is reduced as voids within the structure
become more and more significant as the porosity, Ψ , increases. Porosity is defined
through
Ψ ¼
V v
V
ð2:115Þ
where V v /V is the ratio of the volume of the voids to the volume of the whole. A
useful example is the porosity of a skeleton of spherical particles of radius a, which
is simply
Ψ ¼ 1 À N p
4
3
πa
3
ð2:116Þ
where N p is the number of particles per unit volume. However, the effect on thermal
conductivity is mitigated by thermal re-radiation within the porous structure. Hence,
the pore size distribution within the structure also plays a role. This results in the
effective conductivity becoming a complex function of the radiation field. The
96
2 The Nucleus