– the depth and density of the uppermost inert layer (if it exists, although I would
argue that this is almost proven)
– the changing volatile mixing ratio with depth and detection of a sub-surface
recondensation layer
– the determination of the positions of species-specific sublimation fronts (should
they exist)
– the depth at which internal properties are no longer influenced by the irradiation
of the surface
It should be noted that models have been developed that purport to derive these
quantities but none of these have actually been measured to confirm and/or constrain
the models. It is also notable that coring down to at least 1 m and preferably 3 m was
the initial target of the Rosetta mission when it was still a sample return mission in
the 1980s. It is perhaps telling that we have not been able to refine this significantly
other than to confirm what we already knew.
It is also evident from the work on Rosetta data that our understanding of the
microphysics of the surface and sub-surface layers remains rudimentary. This has
left two major uncertainties in our understanding of comets.
First, we do not know how the volatile component is associated with the refractory component of cometary material. This is vital to understanding how comets
formed and may ultimately provide key information on the solar system formation
process. One can envisage ices surrounding or encapsulating the dust component,
ices within dust matrices, ices as a component isolated from the refractories, and so
on. There is no strong evidence supporting any of these models.
Second, the thermophysical behaviour of the surface and sub-surface layers
remain inadequately constrained. We have values for the thermal inertia but the
thermal conductivity and heat capacity are unknown, the importance of gas transport
and re-condensation are unknown, and the refractory to volatile ratio inside the
nucleus remains a subject of fierce debate. Without these, models of cometary
evolution are, at best, poorly constrained.
Hence, one of the primary objectives for future missions must be to improve our
knowledge of thermophysical parameters since data interpretation and our understanding of cometary activity heavily depends on the output of thermophysical
computer simulations of the upper cometary surface layers ( 1 m). These parameters strongly depend on microphysical details of the cometary surface, such as e.g.,
the material composition, the morphology of the material (structure, arrangement of
the particles, coordination number, void space) and the mixing of the different
components (silicates, organics and ices). Besides the thermal modelling aspect,
also the macroscopic behaviour of the cometary surface is influenced by the microphysical properties of the material, such as the tensile strength (Gundlach et al.
2018), compressive strength (Lorek et al. 2016; Schräpler et al. 2015), and the
thermal conductivity (Gundlach and Blum 2012).
This also illustrates that laboratory work has an important role to play. The
properties of ice-refractory mixtures can be studied under idealised conditions to
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8 Future Investigations of Comets
argue that this is almost proven)
– the changing volatile mixing ratio with depth and detection of a sub-surface
recondensation layer
– the determination of the positions of species-specific sublimation fronts (should
they exist)
– the depth at which internal properties are no longer influenced by the irradiation
of the surface
It should be noted that models have been developed that purport to derive these
quantities but none of these have actually been measured to confirm and/or constrain
the models. It is also notable that coring down to at least 1 m and preferably 3 m was
the initial target of the Rosetta mission when it was still a sample return mission in
the 1980s. It is perhaps telling that we have not been able to refine this significantly
other than to confirm what we already knew.
It is also evident from the work on Rosetta data that our understanding of the
microphysics of the surface and sub-surface layers remains rudimentary. This has
left two major uncertainties in our understanding of comets.
First, we do not know how the volatile component is associated with the refractory component of cometary material. This is vital to understanding how comets
formed and may ultimately provide key information on the solar system formation
process. One can envisage ices surrounding or encapsulating the dust component,
ices within dust matrices, ices as a component isolated from the refractories, and so
on. There is no strong evidence supporting any of these models.
Second, the thermophysical behaviour of the surface and sub-surface layers
remain inadequately constrained. We have values for the thermal inertia but the
thermal conductivity and heat capacity are unknown, the importance of gas transport
and re-condensation are unknown, and the refractory to volatile ratio inside the
nucleus remains a subject of fierce debate. Without these, models of cometary
evolution are, at best, poorly constrained.
Hence, one of the primary objectives for future missions must be to improve our
knowledge of thermophysical parameters since data interpretation and our understanding of cometary activity heavily depends on the output of thermophysical
computer simulations of the upper cometary surface layers ( 1 m). These parameters strongly depend on microphysical details of the cometary surface, such as e.g.,
the material composition, the morphology of the material (structure, arrangement of
the particles, coordination number, void space) and the mixing of the different
components (silicates, organics and ices). Besides the thermal modelling aspect,
also the macroscopic behaviour of the cometary surface is influenced by the microphysical properties of the material, such as the tensile strength (Gundlach et al.
2018), compressive strength (Lorek et al. 2016; Schräpler et al. 2015), and the
thermal conductivity (Gundlach and Blum 2012).
This also illustrates that laboratory work has an important role to play. The
properties of ice-refractory mixtures can be studied under idealised conditions to
440
8 Future Investigations of Comets