abundances of several minor species measured for 9 comets including 103P/Hartley
2. All the observations were made by teams centred around M. Mumma and
G. Villanueva. Variations in the relative abundances of factors of 3 or more are
seen for some species. Although visually the coma observed during the EPOXI
fly-by of P/Hartley 2 was unusual because of the large numbers of water ice particles
seen, the chemical composition seen from the ground was not particularly unusual
(Fig. 3.18).
It was already inferred from Giotto data that there are substantial amounts of
hydrocarbons in the gaseous emissions from comets. The repetitive appearance of
specific peaks in the mass spectra (alternating peaks 16 and 24 Daltons apart) were
interpreted as being indicative of a specific polymer, polyoxymethylene (POM),
which is polymerized formaldehyde (Huebner 1987) although Mitchell et al. (1989)
later showed that this regular pattern is generally characteristic of any kind of
CHO-bearing molecules that include POM-like structures. Wright et al. (2015)
reported detection of CHO-bearing molecules at the surface of 67P using the
PTOLEMY instrument onboard the Philae lander but it was considered unlikely
that this was POM by Altwegg et al. (2017).
There are possibly issues with POM itself being a parent of the observed
fragmentation pattern. It is known to decompose into formaldehyde just above
room temperature and its lifetime in the gas coma of a comet is expected to
be short (Le Roy et al. 2012). However, other laboratory studies (Butscher et al.
2019) continue to suggest that POM is a plausible organic component of the
surface material and that specific forms can remain sufficiently stable to be detected
in the inner coma. Garrod (2019) has noted that these complex organic molecules are
likely to be a product of cosmic-ray processing of surface layers well before entry
into the inner Solar System.
PTOLEMY measurements also indicated an apparent absence of aromatic compounds such as benzene, a lack of sulfur-bearing species, and very low concentrations of nitrogenous material although many of these species are evident in the
higher sensitivity measurements made by ROSINA on the Rosetta orbiter
(e.g. Schuhmann et al. 2019). There was however a positive detection of toluene
from PTOLEMY data acquired at the surface and confirmed with ROSINA data
(Altwegg et al. 2017). The low abundance of NH 3 in coma mass spectra may be
attributable to the integration of ammonia into ammonium salts which now appear to
have been unambiguously detected in data from two instruments (ROSINA and
VIRTIS) on Rosetta (Altwegg et al. 2020; Poch et al. 2020).
Of the simple organics, methanol was detected from ground through its infrared
emissions (Hoban 1993; Davies et al. 1993) and has been extensively monitored at
sub-mm wavelengths (e.g. Biver et al. 2002b). Formic acid (HCOOH), another
organic identified in both comets and the interstellar medium (ISM), has now been
detected (Favre et al. 2018) along the line of sight towards the TW Hydrae protoplanetary disc with ALMA (cf Fig. 1.1) indicating that another link in the chain
between the ISM and comet formation can now be verified by measurement. We will
return to the subject of organics in Sect. 4.14 where we shall also discuss the
detection of glycine in the coma by Altwegg et al. (2016).
3.3 Minor Species
209
2. All the observations were made by teams centred around M. Mumma and
G. Villanueva. Variations in the relative abundances of factors of 3 or more are
seen for some species. Although visually the coma observed during the EPOXI
fly-by of P/Hartley 2 was unusual because of the large numbers of water ice particles
seen, the chemical composition seen from the ground was not particularly unusual
(Fig. 3.18).
It was already inferred from Giotto data that there are substantial amounts of
hydrocarbons in the gaseous emissions from comets. The repetitive appearance of
specific peaks in the mass spectra (alternating peaks 16 and 24 Daltons apart) were
interpreted as being indicative of a specific polymer, polyoxymethylene (POM),
which is polymerized formaldehyde (Huebner 1987) although Mitchell et al. (1989)
later showed that this regular pattern is generally characteristic of any kind of
CHO-bearing molecules that include POM-like structures. Wright et al. (2015)
reported detection of CHO-bearing molecules at the surface of 67P using the
PTOLEMY instrument onboard the Philae lander but it was considered unlikely
that this was POM by Altwegg et al. (2017).
There are possibly issues with POM itself being a parent of the observed
fragmentation pattern. It is known to decompose into formaldehyde just above
room temperature and its lifetime in the gas coma of a comet is expected to
be short (Le Roy et al. 2012). However, other laboratory studies (Butscher et al.
2019) continue to suggest that POM is a plausible organic component of the
surface material and that specific forms can remain sufficiently stable to be detected
in the inner coma. Garrod (2019) has noted that these complex organic molecules are
likely to be a product of cosmic-ray processing of surface layers well before entry
into the inner Solar System.
PTOLEMY measurements also indicated an apparent absence of aromatic compounds such as benzene, a lack of sulfur-bearing species, and very low concentrations of nitrogenous material although many of these species are evident in the
higher sensitivity measurements made by ROSINA on the Rosetta orbiter
(e.g. Schuhmann et al. 2019). There was however a positive detection of toluene
from PTOLEMY data acquired at the surface and confirmed with ROSINA data
(Altwegg et al. 2017). The low abundance of NH 3 in coma mass spectra may be
attributable to the integration of ammonia into ammonium salts which now appear to
have been unambiguously detected in data from two instruments (ROSINA and
VIRTIS) on Rosetta (Altwegg et al. 2020; Poch et al. 2020).
Of the simple organics, methanol was detected from ground through its infrared
emissions (Hoban 1993; Davies et al. 1993) and has been extensively monitored at
sub-mm wavelengths (e.g. Biver et al. 2002b). Formic acid (HCOOH), another
organic identified in both comets and the interstellar medium (ISM), has now been
detected (Favre et al. 2018) along the line of sight towards the TW Hydrae protoplanetary disc with ALMA (cf Fig. 1.1) indicating that another link in the chain
between the ISM and comet formation can now be verified by measurement. We will
return to the subject of organics in Sect. 4.14 where we shall also discuss the
detection of glycine in the coma by Altwegg et al. (2016).
3.3 Minor Species
209