its length, this can lead to significant non-radial flow. If the porosity is lower on one
side of a uniform slab, the gas outflow is reduced and significant pressure gradients
across the slab arise. In the case of 67P, the surface morphology that was observed
varies substantially across the surface (e.g. Figs. 2.69 and 2.73). Sublimation from
these areas would probably need to be through a porous layer in order to reduce the
detectability of any icy source by remote-sensing. If the morphology differences also
represent different porosities then lateral (non-radial) expansion occurs. This might
be a means of generating near-surface localised lateral flow to explain observations
such as the movement of boulders (El-Maarry et al. 2017b) or some of the smaller
formations that appear similar to wind-driven features (Fig. 2.100) on Earth and
Mars (Giacomini et al. 2016).
3.4.9 Nightside Outgassing
Most gas dynamics calculations of cometary outgassing include some form of
uniform outgassing from the nightside at production rates equivalent to 2%–7% of
the total production rate. Bieler et al. (2015a), for example, used 7% to match
ROSINA/COPS data at 67P while Marschall et al. (2016) deliberately did not use
a nightside gas emission at all but, in the latter case, the fitting of the gas to the
ROSINA/COPS measurements above the nightside of the nucleus was clearly
inadequate and additional gas emission was required.
We shall see below that dust emission indicates that nightside emission of dust is
necessary to explain the observations of the dayside to nightside dust column density
ratio. This dust emission must be driven by gas emission but it is not obvious which
volatile is responsible and how the process works.
Outgassing giving 7% of the total gas production rate from the nightside would
seem improbable if the driving volatile is water but ROSINA/COPS cannot distinguish between the species and hence it is conceivable that the major species
producing nightside emission is either CO or CO 2 . This should be evident in plots
of the mixing ratio as a function of the phase angle that spacecraft was measuring
at. However, the spacecraft trajectory that was flown was not well suited to making
this type of assessment because most measurements were acquired at the terminator.
Figure 3.38 shows a plot of the DFMS measurements of the ratio of CO 2 to H 2 O
binned in 5
bins of phase angle and it indicates that CO 2 production increases
rapidly relative to H 2 O as one nears the terminator. This is effectively another
expression of the initial plots made by Hässig et al. (2015) which showed abundant
CO 2 production over the southern (unilluminated) hemisphere in the early phases of
the Rosetta mission. While this provides some evidence, it is not unambiguous
because the clear trend at low phase angles is not evident at the very highest phase
angles. Hence, this is by no means conclusive. For comparison, a curve is given for
the same ratio but plotted against the absolute difference between the sub-observer
latitude and the sub-solar latitude. This shows a similar (but possibly better)
3.4 Gas Expansion
247
side of a uniform slab, the gas outflow is reduced and significant pressure gradients
across the slab arise. In the case of 67P, the surface morphology that was observed
varies substantially across the surface (e.g. Figs. 2.69 and 2.73). Sublimation from
these areas would probably need to be through a porous layer in order to reduce the
detectability of any icy source by remote-sensing. If the morphology differences also
represent different porosities then lateral (non-radial) expansion occurs. This might
be a means of generating near-surface localised lateral flow to explain observations
such as the movement of boulders (El-Maarry et al. 2017b) or some of the smaller
formations that appear similar to wind-driven features (Fig. 2.100) on Earth and
Mars (Giacomini et al. 2016).
3.4.9 Nightside Outgassing
Most gas dynamics calculations of cometary outgassing include some form of
uniform outgassing from the nightside at production rates equivalent to 2%–7% of
the total production rate. Bieler et al. (2015a), for example, used 7% to match
ROSINA/COPS data at 67P while Marschall et al. (2016) deliberately did not use
a nightside gas emission at all but, in the latter case, the fitting of the gas to the
ROSINA/COPS measurements above the nightside of the nucleus was clearly
inadequate and additional gas emission was required.
We shall see below that dust emission indicates that nightside emission of dust is
necessary to explain the observations of the dayside to nightside dust column density
ratio. This dust emission must be driven by gas emission but it is not obvious which
volatile is responsible and how the process works.
Outgassing giving 7% of the total gas production rate from the nightside would
seem improbable if the driving volatile is water but ROSINA/COPS cannot distinguish between the species and hence it is conceivable that the major species
producing nightside emission is either CO or CO 2 . This should be evident in plots
of the mixing ratio as a function of the phase angle that spacecraft was measuring
at. However, the spacecraft trajectory that was flown was not well suited to making
this type of assessment because most measurements were acquired at the terminator.
Figure 3.38 shows a plot of the DFMS measurements of the ratio of CO 2 to H 2 O
binned in 5
bins of phase angle and it indicates that CO 2 production increases
rapidly relative to H 2 O as one nears the terminator. This is effectively another
expression of the initial plots made by Hässig et al. (2015) which showed abundant
CO 2 production over the southern (unilluminated) hemisphere in the early phases of
the Rosetta mission. While this provides some evidence, it is not unambiguous
because the clear trend at low phase angles is not evident at the very highest phase
angles. Hence, this is by no means conclusive. For comparison, a curve is given for
the same ratio but plotted against the absolute difference between the sub-observer
latitude and the sub-solar latitude. This shows a similar (but possibly better)
3.4 Gas Expansion
247