of CO from C/2009 P1 (Garradd) as observed by the infrared spectrometer instrument on Deep Impact. The H 2 O outgassing, increased and peaked pre-perihelion and
then steadily decreased. CO, however, monotonically increased throughout the
entire apparition. This would suggest the choice of a “standard” molecule is far
from trivial.
The ratios of nine other species to CO are shown in Fig. 3.56 and are split into
three categories for visualization purposes. The abscissa is the time from perihelion
and the equinoxes are marked by bold vertical lines. The ratios are displayed as
running means with a window of 7 days (roughly 14 rotations of the nucleus) to
reduce effects of longitudinal variation. There are several points to note.
In the top panel, the two alcohols appear to be continuously increasing through
the perihelion passage. At the two equinoxes, with the Sun above the equator, a
comparison shows that the mixing ratios with respect to CO are factors of ~5 greater
post-perihelion. Methane shows somewhat similar behaviour apart from a rather
broad increase in production just before the pre-perihelion equinox.
In the bottom panel, the water and CO 2 mixing ratios appear almost anticorrelated in the time frame of Æ200 days with respect to perihelion. In the middle
panel, we can see that ammonia shows a behaviour similar to water while the other
species (HCN and OCS) increase with time. Given that several species are increasing
with time through perihelion, an alternative explanation may be that CO is becoming
gradually depleted in the active layers. What is apparent, however, is that the
changing production rates with time are not simple functions of heliocentric distance. It is also apparent that some ratios are not returning to the values found at the
start of the observations. This is particularly evident in the top panel. This suggests
that either the nucleus is evolving or that processes during the aphelion passage are
modifying the nucleus and returning it to its previous state. The latter is not
unthinkable. Sintering processes and longer term outgassing of more volatile species
are likely to occur during aphelion passage which might act to modify future
outgassing rates. However, this has not been proven.
3.7 Radiation Pressure on Gas Molecules and Radicals: The
Neutral Tail(s)
As we shall see in the next chapters, the dust and plasma tails of comets have been
well observed and studied. However, neutrals and radicals can also form tails as a
consequence of solar radiation pressure. The best studied example of this is the
sodium tail first identified by Cremonese et al. (1997) in C/1995 O1 (Hale-Bopp).
The acceleration of sodium in the anti-sunward direction is the result of the resonant
fluorescence mechanism operating at 589.0 and 589.6 nm (Fig. 3.50). The sodium
atoms absorb photons coming from one direction but re-emit isotropically. (There
has been some discussion in the literature in the 1980s as to whether the emission of
the photon is truly isotropic at the 10% level but this has not been proven and is
274
3 Gas Emissions Near the Nucleus
then steadily decreased. CO, however, monotonically increased throughout the
entire apparition. This would suggest the choice of a “standard” molecule is far
from trivial.
The ratios of nine other species to CO are shown in Fig. 3.56 and are split into
three categories for visualization purposes. The abscissa is the time from perihelion
and the equinoxes are marked by bold vertical lines. The ratios are displayed as
running means with a window of 7 days (roughly 14 rotations of the nucleus) to
reduce effects of longitudinal variation. There are several points to note.
In the top panel, the two alcohols appear to be continuously increasing through
the perihelion passage. At the two equinoxes, with the Sun above the equator, a
comparison shows that the mixing ratios with respect to CO are factors of ~5 greater
post-perihelion. Methane shows somewhat similar behaviour apart from a rather
broad increase in production just before the pre-perihelion equinox.
In the bottom panel, the water and CO 2 mixing ratios appear almost anticorrelated in the time frame of Æ200 days with respect to perihelion. In the middle
panel, we can see that ammonia shows a behaviour similar to water while the other
species (HCN and OCS) increase with time. Given that several species are increasing
with time through perihelion, an alternative explanation may be that CO is becoming
gradually depleted in the active layers. What is apparent, however, is that the
changing production rates with time are not simple functions of heliocentric distance. It is also apparent that some ratios are not returning to the values found at the
start of the observations. This is particularly evident in the top panel. This suggests
that either the nucleus is evolving or that processes during the aphelion passage are
modifying the nucleus and returning it to its previous state. The latter is not
unthinkable. Sintering processes and longer term outgassing of more volatile species
are likely to occur during aphelion passage which might act to modify future
outgassing rates. However, this has not been proven.
3.7 Radiation Pressure on Gas Molecules and Radicals: The
Neutral Tail(s)
As we shall see in the next chapters, the dust and plasma tails of comets have been
well observed and studied. However, neutrals and radicals can also form tails as a
consequence of solar radiation pressure. The best studied example of this is the
sodium tail first identified by Cremonese et al. (1997) in C/1995 O1 (Hale-Bopp).
The acceleration of sodium in the anti-sunward direction is the result of the resonant
fluorescence mechanism operating at 589.0 and 589.6 nm (Fig. 3.50). The sodium
atoms absorb photons coming from one direction but re-emit isotropically. (There
has been some discussion in the literature in the 1980s as to whether the emission of
the photon is truly isotropic at the 10% level but this has not been proven and is
274
3 Gas Emissions Near the Nucleus