Solar insolation is the driving energy source for activity and heats the surface.
Some of this energy is re-radiated back into space as thermal emission but part of it
initiates sublimation of volatile material. Water is the dominant volatile by mass. But
more volatile species such as CO 2 are also present and the interaction between these
species to produce the observed gas emission is not understood. It has been inferred
that, in a well-mixed system, some form of fractionation should occur as modelled
by, for example, Herny et al. (2020), but it is not clear that such a type of process
actually does take place. Some energy is conducted through the uppermost layer into
the interior. The thermal conductivity is low but almost certainly non-negligible. The
relative absence of water ice on the surfaces of nuclei (at least in pure form) suggests
that sublimation is from below the surface with the surface itself being hot (above the
free sublimation temperature of water ice of ~200 K) and desiccated. Even where
water ice has been found on the surface, measured temperatures were well above the
free sublimation temperature (Groussin et al. 2013). Conduction, probably through
layers of differing conductivity, may therefore be a necessary part of the picture.
Diffusion of subliming gas from below, through the surface layer, is now assumed to
be important and this gas flow through a porous layer produces numerous physical
phenomena that complicate interpretation of the surface-coma interaction. Describing the surface of a nucleus as flat and homogeneous is no longer adequate. The
surface topography influences the heat balance not only through shadowing but also
through thermal re-radiation (sometimes referred to as self-heating), an effect that is
particularly important where steep slopes are prevalent.
In addition, dust is emitted from the surface. This is also an energy loss but the
importance of dust transport across the nucleus (sometimes referred to as “airfall” or
“dust hail”), resulting from particles not being sufficiently accelerated to escape the
nucleus into the coma, has now been established as a significant process affecting
cometary evolution (Sect. 2.10.8). The contradiction between sub-surface sublimation through a porous dessicated layer and the loss of that layer to produce dust
emission has not been satisfactorily resolved. The mathematically and physically
accurate description of these processes in a generalised model is by no means trivial.
2.9.2 Sublimation of Ices
The activity of comets close to the Sun is governed by the sublimation of ices from
the surface of the nucleus. The sublimation is a direct consequence of heating by
sunlight although there may be small additional internal heat sources.
In its simplest form, the equilibrium vapour pressure (p s ) of a gas in equilibrium
with its solid and/or liquid phase can be derived by equating the Gibbs free energy of
the two phases and substituting the change in entropy (ΔS) between the two states
with the equation
76
2 The Nucleus
Some of this energy is re-radiated back into space as thermal emission but part of it
initiates sublimation of volatile material. Water is the dominant volatile by mass. But
more volatile species such as CO 2 are also present and the interaction between these
species to produce the observed gas emission is not understood. It has been inferred
that, in a well-mixed system, some form of fractionation should occur as modelled
by, for example, Herny et al. (2020), but it is not clear that such a type of process
actually does take place. Some energy is conducted through the uppermost layer into
the interior. The thermal conductivity is low but almost certainly non-negligible. The
relative absence of water ice on the surfaces of nuclei (at least in pure form) suggests
that sublimation is from below the surface with the surface itself being hot (above the
free sublimation temperature of water ice of ~200 K) and desiccated. Even where
water ice has been found on the surface, measured temperatures were well above the
free sublimation temperature (Groussin et al. 2013). Conduction, probably through
layers of differing conductivity, may therefore be a necessary part of the picture.
Diffusion of subliming gas from below, through the surface layer, is now assumed to
be important and this gas flow through a porous layer produces numerous physical
phenomena that complicate interpretation of the surface-coma interaction. Describing the surface of a nucleus as flat and homogeneous is no longer adequate. The
surface topography influences the heat balance not only through shadowing but also
through thermal re-radiation (sometimes referred to as self-heating), an effect that is
particularly important where steep slopes are prevalent.
In addition, dust is emitted from the surface. This is also an energy loss but the
importance of dust transport across the nucleus (sometimes referred to as “airfall” or
“dust hail”), resulting from particles not being sufficiently accelerated to escape the
nucleus into the coma, has now been established as a significant process affecting
cometary evolution (Sect. 2.10.8). The contradiction between sub-surface sublimation through a porous dessicated layer and the loss of that layer to produce dust
emission has not been satisfactorily resolved. The mathematically and physically
accurate description of these processes in a generalised model is by no means trivial.
2.9.2 Sublimation of Ices
The activity of comets close to the Sun is governed by the sublimation of ices from
the surface of the nucleus. The sublimation is a direct consequence of heating by
sunlight although there may be small additional internal heat sources.
In its simplest form, the equilibrium vapour pressure (p s ) of a gas in equilibrium
with its solid and/or liquid phase can be derived by equating the Gibbs free energy of
the two phases and substituting the change in entropy (ΔS) between the two states
with the equation
76
2 The Nucleus