2.9.3.3 Multi-volatile Models and Ice Fractionation
Multi-species models of the surface layers can be constructed numerically but
require numerous assumptions. The work of Marboeuf et al. (2012) is a recent
example. Here, dust and water ice are coupled so that sublimation of water ice
releases dust grains at the surface. If the dust grains are large, the gas drag is
insufficient to remove the dust and a dust mantle starts to form as in the Fanale
and Salvail approach. If drag is sufficient, the dust escapes. The super-volatiles such
as CO and CO 2 are not driving dust emission but pass through the porous layers of
the nucleus when heated by conduction. Hence, these species have sub-surface
sublimation fronts. An example of the evolution of the sublimation fronts over one
orbital period using parameters for 67P is shown in Fig. 2.34. The x axis gives the
fraction of the orbital period with perihelion defined at 0.5. It can be seen that the CO
sublimation front is typically 2–4 m below the surface whereas the CO 2 front is
significantly below the surface for much of the orbit but is closer to the surface near
perihelion when water sublimation is at its most intense.
The Marboeuf et al. model treats porosity, changes in pore size with condensation
and gas transport, variable conductivity arising from the pore property changes, and
the amorphous to crystalline water ice transition (which is discussed in the following
sub-section). For the 2012 version of the model, clathrates were also investigated.
The model is detailed and numerically sophisticated. Some alternative descriptions
of the surface layer were studied showing that the evolution and outgassing rates are
sensitive to the structure chosen. Other initial structures might also be envisaged
(e.g. coupling of volatile species) that could influence the results. The model is also
1D and hence both latitudinal and local spatial dependencies can arise in this model.
Nonetheless, the model illustrates that the sublimation fronts of super-volatiles may
be at considerable depth (Fig. 2.34).
Seiferlin (1991) pointed out the importance of sintering in water ice for thermal
conductivity. The rate of heat transfer in a porous medium depends partially on the
sizes of contacting area in the structure. Even here, there is the potential for
dynamical effects. Ice particles that come into contact produce ice bridges that
Fig. 2.36 The gas
production rate per unit area
for a simple energy balance
without conduction but
including heat loss arising
from the heat capacity of
non-volatile material. All
calculations are for the
sub-solar point. Solid line:
1.3 AU, dashed line:
1.6 AU, dot-dashed line:
2.0 AU
2.9 Surface Processes
101
Multi-species models of the surface layers can be constructed numerically but
require numerous assumptions. The work of Marboeuf et al. (2012) is a recent
example. Here, dust and water ice are coupled so that sublimation of water ice
releases dust grains at the surface. If the dust grains are large, the gas drag is
insufficient to remove the dust and a dust mantle starts to form as in the Fanale
and Salvail approach. If drag is sufficient, the dust escapes. The super-volatiles such
as CO and CO 2 are not driving dust emission but pass through the porous layers of
the nucleus when heated by conduction. Hence, these species have sub-surface
sublimation fronts. An example of the evolution of the sublimation fronts over one
orbital period using parameters for 67P is shown in Fig. 2.34. The x axis gives the
fraction of the orbital period with perihelion defined at 0.5. It can be seen that the CO
sublimation front is typically 2–4 m below the surface whereas the CO 2 front is
significantly below the surface for much of the orbit but is closer to the surface near
perihelion when water sublimation is at its most intense.
The Marboeuf et al. model treats porosity, changes in pore size with condensation
and gas transport, variable conductivity arising from the pore property changes, and
the amorphous to crystalline water ice transition (which is discussed in the following
sub-section). For the 2012 version of the model, clathrates were also investigated.
The model is detailed and numerically sophisticated. Some alternative descriptions
of the surface layer were studied showing that the evolution and outgassing rates are
sensitive to the structure chosen. Other initial structures might also be envisaged
(e.g. coupling of volatile species) that could influence the results. The model is also
1D and hence both latitudinal and local spatial dependencies can arise in this model.
Nonetheless, the model illustrates that the sublimation fronts of super-volatiles may
be at considerable depth (Fig. 2.34).
Seiferlin (1991) pointed out the importance of sintering in water ice for thermal
conductivity. The rate of heat transfer in a porous medium depends partially on the
sizes of contacting area in the structure. Even here, there is the potential for
dynamical effects. Ice particles that come into contact produce ice bridges that
Fig. 2.36 The gas
production rate per unit area
for a simple energy balance
without conduction but
including heat loss arising
from the heat capacity of
non-volatile material. All
calculations are for the
sub-solar point. Solid line:
1.3 AU, dashed line:
1.6 AU, dot-dashed line:
2.0 AU
2.9 Surface Processes
101