the nucleus of a parent. It was suggested that the repeatability of the observed shells
reflected the rotation period of the nucleus (Schlosser et al. 1986; Sect. 2.4). More
recently, Schleicher et al. (2019) have observed jet-like activity in CN in 41P/Tuttle–
Giacobini–Kresák with the curvature of the jet probably being related to the rotation
of the nucleus. The spatial distribution of the observed CN was significantly different
from that of the OH daughter product of H 2 O which showed little evidence of jets.
C 2 and C 3 emissions broadly followed the emissions of CN suggesting source
inhomogeneity with respect to the main driving volatile.
While CN, C 2 , and C 3 are commonly observed and frequently show jet-like
structures in narrow-band imagery, interpretation in terms of surface volatile inhomogeneity is not straightforward because the parents of these species are not well
known and contributions from an extended dust source are also conceivable.
3.6 Compositional Variation with Heliocentric Distance
A potential diagnostic of the physico-chemical structure of the surface layer is the
variation of the outgassing rates of species with time as the comet orbits the Sun. The
changes in insolation ought to be reflected in the outgassing rates with the most
volatile species dominant at greater heliocentric distances and less volatile species
becoming increasingly evident as the comet approaches perihelion. Additional
complexity can arise from non-linear heat input as a consequence of the obliquity
of the nucleus (Fig. 2.13). Furthermore, if less volatile species are mixed with more
volatile ices then higher mixing ratios of the less volatile species might arise if the
more volatile species drags the less volatile species away from the surface in ice form
(as seen at 103P/Hartley 2 in Fig. 3.2, for example). The presence of clathrates,
where a “guest molecule” is initially held within a “cage” formed by a host molecule
or a lattice of host molecules, may also produce less straightforward variations in the
production rates of some species (e.g. Gautier and Hersant 2005; Luspay-Kuti et al.
2016).
The most comprehensive collection of observations of a single comet using
Earth-based techniques was completed by Biver et al. (2002b) on C/1995 O1
(Hale-Bopp) and included measurements of the production rates of OH, CO,
CH 3 OH, H 2 S, H 2 CO, HCN (and its minor tautomer, HNC), CS, and CH 3 CN from
7 AU inbound to 10 AU outbound for relatively large fields of view.
The chemical network model shows that the relative densities of neutrals emitted
from the nucleus are approximately constant in the coma out to ~10
3 km. The
Rosetta spacecraft was closer to the nucleus of 67P than this for much of its mission
and hence, this forms a potentially better data set for studies of parent species
variability. Ratioing of the density of one species to a standard species removes
dependencies on heliocentric distance and changes in the global production rates but
the key question is which species should provide the “standard”? While water is the
most abundant species, it is also exhibits strong diurnal and heliocentric variations
because of its relatively high free sublimation temperature. In Fig. 3.56, CO has been
272
3 Gas Emissions Near the Nucleus
reflected the rotation period of the nucleus (Schlosser et al. 1986; Sect. 2.4). More
recently, Schleicher et al. (2019) have observed jet-like activity in CN in 41P/Tuttle–
Giacobini–Kresák with the curvature of the jet probably being related to the rotation
of the nucleus. The spatial distribution of the observed CN was significantly different
from that of the OH daughter product of H 2 O which showed little evidence of jets.
C 2 and C 3 emissions broadly followed the emissions of CN suggesting source
inhomogeneity with respect to the main driving volatile.
While CN, C 2 , and C 3 are commonly observed and frequently show jet-like
structures in narrow-band imagery, interpretation in terms of surface volatile inhomogeneity is not straightforward because the parents of these species are not well
known and contributions from an extended dust source are also conceivable.
3.6 Compositional Variation with Heliocentric Distance
A potential diagnostic of the physico-chemical structure of the surface layer is the
variation of the outgassing rates of species with time as the comet orbits the Sun. The
changes in insolation ought to be reflected in the outgassing rates with the most
volatile species dominant at greater heliocentric distances and less volatile species
becoming increasingly evident as the comet approaches perihelion. Additional
complexity can arise from non-linear heat input as a consequence of the obliquity
of the nucleus (Fig. 2.13). Furthermore, if less volatile species are mixed with more
volatile ices then higher mixing ratios of the less volatile species might arise if the
more volatile species drags the less volatile species away from the surface in ice form
(as seen at 103P/Hartley 2 in Fig. 3.2, for example). The presence of clathrates,
where a “guest molecule” is initially held within a “cage” formed by a host molecule
or a lattice of host molecules, may also produce less straightforward variations in the
production rates of some species (e.g. Gautier and Hersant 2005; Luspay-Kuti et al.
2016).
The most comprehensive collection of observations of a single comet using
Earth-based techniques was completed by Biver et al. (2002b) on C/1995 O1
(Hale-Bopp) and included measurements of the production rates of OH, CO,
CH 3 OH, H 2 S, H 2 CO, HCN (and its minor tautomer, HNC), CS, and CH 3 CN from
7 AU inbound to 10 AU outbound for relatively large fields of view.
The chemical network model shows that the relative densities of neutrals emitted
from the nucleus are approximately constant in the coma out to ~10
3 km. The
Rosetta spacecraft was closer to the nucleus of 67P than this for much of its mission
and hence, this forms a potentially better data set for studies of parent species
variability. Ratioing of the density of one species to a standard species removes
dependencies on heliocentric distance and changes in the global production rates but
the key question is which species should provide the “standard”? While water is the
most abundant species, it is also exhibits strong diurnal and heliocentric variations
because of its relatively high free sublimation temperature. In Fig. 3.56, CO has been
272
3 Gas Emissions Near the Nucleus