0–70 eV electrons. The thermal evolution of interstellar ice analogues composed of
water, carbon dioxide, ammonia, and formaldehyde has been investigated by
Vinogradoff et al. (2015). Formation of hexamethylenetetramine (HMT –
C 6 N 4 H 12 ) was observed in the organic refractory residue left after ice sublimation.
This molecule is suspected to be present in various astrophysical contexts involving
nitrogen-rich photochemistry (Pirali et al. 2014).
4.15 Refractory to Volatile Ratios
The term “dust-to-gas ratio” has been used in the past to refer to the ratio of the mass
lost in refractory, non-volatile material to that lost as gaseous material. Prior to the
Rosetta mission, this quantity, Q d /Q g , was also considered as a proxy for the ratio of
refractory to volatile materials in the cometary nucleus itself. The refractory to
volatile ratio (RVR) is an important number in solar system evolution models and
hence considerable effort has been made to establish values.
Direct measurements of the interior were attempted by using the CONSERT
experiment. Brouet et al. (2016b), for example, performed laboratory measurements
showing that the real part of the permittivity measured by CONSERT (ε
0
¼ 1.27 at
90 MHz) could be achieved by porosities of ~80% with a low dust to ice ratio.
However, these measurements indicated a fairly weak dependence on the refractory
to volatile ratio.
The complexity of determining the RVR without actually sampling the interior of
the nucleus and measuring it directly should not be underestimated. Two attempts to
combine space-borne measurements of the dust and gas production have been made.
McDonnell et al. (1991) provided a number for Q d /Q g (the value was two) by
combining Giotto data of the gas production with the measured dust production
rate but it was clear to all at the time that the error bar on this number was very large.
Subsequent work by Thomas and Keller (1991) showed that the available data were
sufficiently uncertain that an error bar of at least one order of magnitude could be
associated with McDonnell’s value. Rotundi et al. (2015) reported a dust to gas ratio
of 4 Æ 2 at 67P but based on sampling a small range of the dust size distribution at
high heliocentric distances early in the mission and the authors suggested that this
value might change as the comet became more active.
On the other hand, RVR ~ 1 is a number that intuitively feels right although there
is actually only limited justification for this. Attempts by ground-based observers
have given values of ≲ 1 (e.g. Singh et al. 1992) but the assumptions necessary to
reach these results were no less significant than those used in the analysis of Giotto
data. We noted above that Pontoppidan et al. (2014) studied dense interstellar
medium and protostellar envelopes and gave a rock/ice ratio of ~0.67. The difficulty
in arriving at an accurate number is immediately evident by looking at the two parts
of the ratio.
The total water vapour production of a comet can be estimated from the hydrogen
emission at Lyman-α (e.g. Fig. 3.48) or from the OH emission using, for example,
392
4 Dust Emission from the Surface
water, carbon dioxide, ammonia, and formaldehyde has been investigated by
Vinogradoff et al. (2015). Formation of hexamethylenetetramine (HMT –
C 6 N 4 H 12 ) was observed in the organic refractory residue left after ice sublimation.
This molecule is suspected to be present in various astrophysical contexts involving
nitrogen-rich photochemistry (Pirali et al. 2014).
4.15 Refractory to Volatile Ratios
The term “dust-to-gas ratio” has been used in the past to refer to the ratio of the mass
lost in refractory, non-volatile material to that lost as gaseous material. Prior to the
Rosetta mission, this quantity, Q d /Q g , was also considered as a proxy for the ratio of
refractory to volatile materials in the cometary nucleus itself. The refractory to
volatile ratio (RVR) is an important number in solar system evolution models and
hence considerable effort has been made to establish values.
Direct measurements of the interior were attempted by using the CONSERT
experiment. Brouet et al. (2016b), for example, performed laboratory measurements
showing that the real part of the permittivity measured by CONSERT (ε
0
¼ 1.27 at
90 MHz) could be achieved by porosities of ~80% with a low dust to ice ratio.
However, these measurements indicated a fairly weak dependence on the refractory
to volatile ratio.
The complexity of determining the RVR without actually sampling the interior of
the nucleus and measuring it directly should not be underestimated. Two attempts to
combine space-borne measurements of the dust and gas production have been made.
McDonnell et al. (1991) provided a number for Q d /Q g (the value was two) by
combining Giotto data of the gas production with the measured dust production
rate but it was clear to all at the time that the error bar on this number was very large.
Subsequent work by Thomas and Keller (1991) showed that the available data were
sufficiently uncertain that an error bar of at least one order of magnitude could be
associated with McDonnell’s value. Rotundi et al. (2015) reported a dust to gas ratio
of 4 Æ 2 at 67P but based on sampling a small range of the dust size distribution at
high heliocentric distances early in the mission and the authors suggested that this
value might change as the comet became more active.
On the other hand, RVR ~ 1 is a number that intuitively feels right although there
is actually only limited justification for this. Attempts by ground-based observers
have given values of ≲ 1 (e.g. Singh et al. 1992) but the assumptions necessary to
reach these results were no less significant than those used in the analysis of Giotto
data. We noted above that Pontoppidan et al. (2014) studied dense interstellar
medium and protostellar envelopes and gave a rock/ice ratio of ~0.67. The difficulty
in arriving at an accurate number is immediately evident by looking at the two parts
of the ratio.
The total water vapour production of a comet can be estimated from the hydrogen
emission at Lyman-α (e.g. Fig. 3.48) or from the OH emission using, for example,
392
4 Dust Emission from the Surface