1. Introduction
Comets are primitive leftovers from the formation of the Solar System. As such, their composition provides clues to physics and chemistry operating during the protoplanetary disk phase (
Figure 1), as well as the preceding phases of star formation (e.g., [
1,
2]). Cometary nuclei consist of a combination of volatile ices and refractory carbonaceous material and silicates [
3]. The exact partition (whether comets are “dirty iceballs” or “icy dirtballs”) is still under debate, though results from the recent European Space Agency (ESA) mission Rosetta and other arguments favor an ice-to-dust mass ratio greater than unity (i.e., “icy dirtballs”) [
4].
The volatile composition of comets is usually dominated by water (H
O), with carbon dioxide (CO
) and carbon monoxide (CO) also being abundant at the 1–20% level compared to H
O [
5,
6]. One of the surprising results from the Rosetta mission was the discovery that molecular oxygen (O
) was very abundant in 67P/Churyumov-Gerasimenko (hereafter 67P), with O
/H
O = 110% [
7]. However, comets are also rich in simple organic species, such as methanol (CH
OH), ethane (C
H
), methane (CH
), and hydrogen cyanide (HCN). These species have been detected in comets since the 1990s using observations at infrared (IR) and sub-mm wavelengths [
5,
8]. Simpler radical species such as diatomic carbon (C
) and cyanide (CN) have been detected in comets at optical wavelengths since the 19th century. These species are likely released from the photodissociation of molecules such as acetylene (C
H
) and HCN, though the origin of many of these species is still not completely understood [
6,
9]. More recently, much more complex organic molecules such as ethanol (C
H
OH), formamide (NH
CHO), glycolaldehyde (CH
OHCHO), and acetaldehyde (CH
CHO) have been detected at sub-mm wavelengths (e.g., [
10]). Even glycine, the simplest amino acid, and phosphorous (P, predominantly traced back to PO [
11]) were detected by the Rosetta mission [
12].
The carbonaceous refractory phase of cometary material also potentially contains organic material, though this phase of cometary material is less understood than the volatile phase. There is observational evidence for polycyclic aromatic hydrocarbons (PAHs) present in cometary material (e.g., [
13,
14,
15,
16,
17,
18,
19]). In the past, the presence of PAHs in comets has been highly debated, though the detection of toluene at 67P by Rosetta [
20] has provided a firm basis for PAHs being present in comets. The Giotto mission flyby of comet 1P/Halley detected the presence of so-called CHON particles (so named because they were rich in carbon, hydrogen, oxygen, and nitrogen) [
21]. These CHON particles have been proposed as a possible source for carbon-bearing radicals like C
observed at optical wavelengths discussed above (e.g., [
22,
23]), as well as more complex molecules such as formaldehyde (H
CO) [
24].
As the chemical inventory of species detected in comets is rich in organic molecules, comets are of immense interest as possible sources of Earth’s organic material. For the purposes of this review, we adopt the strict chemical definition of organic matter, which encompasses molecules that contain C–H and/or C–C bonds, and focus on species that fit this definition. However, it should be noted that comets are also rich in non-organic species, such as carbon monoxide (CO), carbon dioxide (CO) and ammonia (NH), that, despite not being strictly organic, are still important in the formation of prebiotic molecules. In this review, we discuss the history of the study of organic matter in comets, recent advances in our understanding of cometary organic matter, and future prospects for the continued study of organic molecules in cometary nuclei.
2. Tools for Studying Cometary Organic Matter
Studies of cometary composition are often limited to remote sensing of the gas-phase coma (transient gravitationally unbound atmosphere) that surrounds the nucleus of the comet, owing to sublimation of the primary ices HO, CO, and CO. These methods employ spectroscopy at a variety of wavelengths to observe electronic, vibrational, rotational, and rovibrational transitions of molecules of interest in the gas phase. More recently, spacecraft missions have enabled studies of the solid phase of cometary material, most notably through sample return of refractory material (dust) by the Stardust mission and the lander Philae (associated with the Rosetta mission), which was able to perform in situ analysis on the comet’s surface. In situ missions also sample the coma gas through mass spectroscopy. We discuss these methods below.
2.1. Remote Sensing
2.1.1. Optical
Optical observations of comets (which we define as covering wavelengths from 300–1000 nm) have the longest history, with the first spectra of comets being obtained in the 19th century [
25,
26,
27]. Comets show a rich emission line spectrum at optical wavelengths, dominated by emissions from CN and C
(see
Figure 2). Many of these molecules are carbon-bearing and are likely fragments of more complex organic matter present in cometary ices and dust. Narrowband filter sets have also been developed so that imaging techniques can be used to isolate molecules of interest [
28].
Over 200 comets have been characterized at optical wavelengths (e.g., [
29,
30,
31,
32]). These studies have revealed several taxonomic groups, including comets that are depleted in carbon-chain species, and provided the first glimpse into variations in composition among comets (see
Section 3.9.2). However, due to the fragment nature of the observed species and our lack of understanding of the coma processes responsible for their release [
9,
33], interpretation of these taxonomic groupings in terms of the more complex organic matter that they likely trace is difficult.
Optical spectroscopy at high spectral resolution has also proven to be a powerful method for determining isotopic ratios for C and N in cometary material. The first isotopic measurements in C and N came from the CN molecule [
34,
35], with some carbon measurements also coming from observations of C
[
36]. More recently, NH
has been utilized to measure isotopic ratios in N [
37,
38,
39]. These measurements revealed Earth-like ratios in C, but ratios in N deviate from the terrestrial value by a factor of two and solar value by a factor of three. The reason for this discrepancy is not fully understood (see
Section 3.9.1).
2.1.2. Near Infrared (NIR)
NIR observations of comets were pioneered in the 1980s, resulting in the first direct detection of H
O from a comet [
40]. The development of ground-based high spectral resolution instruments in the 1990s resulted in the first observations of a wider volatile composition of comets at IR wavelengths (e.g., [
41,
42]), with individual organic molecules such as CH
, C
H
, and acetylene (C
H
) identified for the first time (see
Figure 3). NIR observations at high spectral resolution are the only way to directly study symmetric hydrocarbons such as C
H
, C
H
, and CH
in comets with remote facilities, as these species lack a permanent dipole moment and pure rotational transitions, precluding their measure at millimeter/sub-millimeter wavelengths (see
Section 2.1.3). Low spectral resolution IR observations are also diagnostic of organic matter, but to date have mostly been applied to observations of cometary surfaces visited by spacecrafts (see
Section 2.2).
2.1.3. Mm/Sub-mm
Observations at millimeter (mm) and submillimeter (sub-mm) wavelengths have a similar history to the NIR, with observations dating as far back as the 1970s (e.g., [
43,
44]). Advances in instrumentation in the 1990s first enabled the regular detection of a suite of molecules in comets at mm/sub-mm wavelengths. These observations have the advantage of being sensitive to more complex species than are observed at optical/NIR wavelengths, such as formamide and ethanol, through their rotational transitions. Sub-mm observations also have higher spectral resolution than optical/IR observations, enabling velocity-resolved studies of coma species via the analysis of their spectral line profiles. These observations measure molecular outflow velocities, and can reveal potential coma outgassing asymmetries. However, this often comes at the expense of spatial resolution on the sky. Early cometary observations were dominated by single-dish telescopes (e.g., [
45,
46,
47]) with an angular resolution on the scale of several to tens of arcseconds (corresponding to thousands of kilometers projected distance at the comet). The development of interferometers, including, but not limited to, the Atacama Large Millimeter/Submillimeter array (ALMA), with sub-arcsecond spatial resolution has revolutionized the study of organic molecules in comets, providing spatially resolved maps of the distributions of species such as HCN, H
CO, and CH
OH (see
Figure 4) (e.g., [
48,
49,
50]).
2.2. Spacecraft Missions
Spacecraft missions to comets have proven invaluable to the study of comets in general and specifically organic matter in comets, providing observations of the nucleus and compositional information only attainable in situ. Below we highlight a few missions with a particularly large impact on our knowledge of organic matter in comets.
2.2.1. The Halley Armada
The first comet to be studied via spacecraft flybys of the nucleus was 1P/Halley in 1986. A suite of spacecrafts flew by the comet, including ESA’s Giotto mission and the Soviet probe Vega 2. Giotto carried a mass spectrometer that measured the composition of the coma in situ for the first time [
51]. It also obtained IR spectra showing the presence of H
O, CO, CO
, and organic matter [
13]. Vega 2 provided optical and IR spectra of the inner coma, providing the first evidence for the presence of PAHs in comets (e.g., [
18]).
2.2.2. Stardust
NASA’s Stardust mission is to date the only mission to return cometary material for analysis in Earth-based laboratories [
53]. The spacecraft performed a flyby of comet 81P/Wild 2 on 2 January 2004, collecting dust particles from the coma in aerogel. These samples were then successfully returned to Earth two years later on 15 January 2006. With the ability to analyze the collected dust particles in state-of-the-art Earth-based laboratories rather than the necessarily limited instruments on a spacecraft, a wealth of studies have been performed, ranging from isotopic composition to the presence of organic material (e.g., [
16,
53,
54]), including the amino acid glycine [
55]. Related to these particles is the study of interplanetary dust particles (IDPs), which are collected from the upper layers of the Earth’s atmosphere (e.g., [
56,
57]). These particles may very well have a cometary origin, but determining a definitive link to a specific comet is difficult, though it has been attempted (e.g., [
58]).
2.2.3. Rosetta
The Rosetta mission provided huge leaps forward in our understanding of comets and their molecular composition (see
Figure 5). Impactful instruments of the many on board include the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) mass spectrometer and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) IR spectrometer, as well as the Cometary Sampling and Composition (COSAC) and Ptolemy mass spectrometers on the Philae lander. ROSINA was able to perform an in situ analysis of the coma gas around the target comet, 67P/Churyumov-Gerasimenko. These observations provided extensive measurements of species previously known in comets, but also discovered many new species never before detected in comets. These include complex organic molecules that are beyond the grasp of current Earth-based observation techniques, such as glycine [
12], due to their low abundance and complex spectra. COSAC and Ptolemy on the Philae lander were able to perform a similar in situ analysis of the surface material. Although the Philae lander only operated for a few days due to power constraints created by a non-optimal landing configuration, COSAC and Ptolemy still provided the first measurements directly from the surface of a cometary nucleus (see [
59] for a complete review of Philae results). The VIRTIS instrument provided NIR spectroscopy of the surface of 67P revealing the presence of water and CO
ice [
60,
61,
62], as well as an organic absorption feature around 3.2 microns ([
63,
64], see
Figure 6).
4. Implications
Remote sensing observations at a variety of wavelengths have revealed a number of organic molecules present in comets. The presence of these relatively simple organic molecules hinted at the potential presence of more complicated species, a supposition confirmed by the Rosetta mission, which greatly expanded the list of known organic molecules in comets. Apart from providing an inventory of organic molecules, these results also have implications for the origin and formation of organic matter, as well as their potential delivery to the terrestrial planets.
Many of the simple organic molecules, such as C
H
, H
CO, and CH
OH, are efficiently formed via hydrogen addition reactions on grain surfaces involving C
H
and CO. The relative abundances of these species in comets show evidence for this formation mechanism in the protosolar disk. While CO/H
CO/CH
OH ratios show a large scatter suggesting variable efficiency in hydrogenation of CO (or a large role of evolutionary/sublimation effects on observed abundances), C
H
/C
H
ratios are more constant and suggest a very efficient hydrogenation process for C
H
on grain surfaces, if this is indeed the formation process for C
H
in the protosolar disk [
8]. Correlations between the abundances of different organic molecules and mapping of spatial distributions in the coma can reveal links between the formation of different organic molecules and their colocation in the nucleus. HCN is highly correlated with hydrocarbons, specifically C
H
, and these species often have similar spatial distributions when observed in the IR, suggesting a common release mechanism from the nucleus (see
Figure 11) [
8]. Species like H
CO and CH
OH show weaker correlations and different spatial distributions, often indicative of an extended source of production ([
8,
48], see also
Figure 4 and
Figure 7). These correlations (or lack thereof) could therefore indicate different ice phases in the nucleus (for instance apolar vs. polar, see
Section 3.4), and how the organic molecules segregate into these different ice phases could have ramifications for their formation and incorporation into cometary nuclei.
Several species (in addition to the known photochemical products observed at optical wavelengths discussed in
Section 3.9) have distributed sources in cometary comae, including H
CO, HNC, and sometimes CH
OH. While for CH
OH this may point to incorporation of this molecule into water-rich ice grains [
96,
97,
99], the distributed sources for H
CO and HNC point to a more complex progenitor contained in the cometary dust ([
48,
49]
Section 3.1 and
Section 3.2). Understanding the nature of these distributed sources can yield clues to the nature of complex organic material in cometary dust grains.
The organic content of comets suggests they could be an important source of the Earth’s current biosphere. Using the measurements of organic matter in comet 67P by Rosetta, Rubin et al. 2019 [
137] and Altwegg et al. 2019 [
65] argued that comets could account for all of the organic material currently present in Earth’s biosphere.
The presence of complex organic matter in comets suggests that these molecules form readily in protoplanetary disks and/or the ISM. The organic inventory of comets exceeds the complexity of the ISM, which could point to interstellar inheritance as an origin for some Solar System ices. In particular, the presence of glycine in both Rosetta observations and the Stardust samples suggests amino acids can form in a protoplanetary disk/ISM environment.
5. Future Directions for the Study of Cometary Organic Matter
Both remote sensing observations and directed missions to comets have provided unique insights into cometary organic matter, and both types of studies continue to be vital to provide further understanding. Remote sensing observations are limited to simpler organic molecules, but provide the statistical sampling needed to understand the cometary population as a whole. On the other hand, space missions provide detailed analysis of a specific target, quantifying organic matter that can currently only be detected by being in close proximity (e.g., glycine).
Future missions to comets will build upon the success of Rosetta and Stardust. Comet Interceptor is an ESA mission that will perform a flyby of either a dynamically new (first passage through the inner Solar System) or interstellar object to be determined, providing the first close up study of any comet in either of these dynamical classes, though as a flyby it will be more limited in the scope of its investigations than Rosetta [
138]. The next major step in the study of cometary organic molecules is to build off the success of Stardust and Philae and return a sample of material directly from the cometary surface, preferably through a cryogenic sample return, in order to preserve the structure of the nucleus ices. The Comet Astrobiology Exploration Sample Return (CAESAR) was a proposed mission to the Rosetta target comet 67P in order to obtain a sample from the surface and bring it back to Earth. This mission was a finalist in the latest call for NASA New Frontiers Class proposals but was not selected. A similar mission concept called AMBITION has been proposed as part of ESA’s Voyage 2050 program [
139].There is no currently selected cometary sample return mission planned for launch, cryogenic or otherwise. In the meantime, analysis of the vast amount of data obtained by Rosetta will continue, providing new insights into the composition of cometary organic matter.
The next generation of remote sensing facilities will provide similar leaps forward in our understanding of cometary organic matter. ALMA has already provided detailed studies of species such as H
CO, CH
OH, and HCN, and will continue to do so. Sub-mm observations to search for the more complicated organic molecules detected by ROSINA will be able to identify different isomers (for instance dimethyl ether (CH
OCH
) and ethanol), which could not be differentiated with ROSINA. The iSHELL instrument on the NASA IRTF has provided similarly vast improvements in the ability to quantify the composition of comets at IR wavelengths. The next generation of 30-m class telescopes will extend remote sensing observations of cometary volatiles to much fainter targets than currently possible. The James Webb Space Telescope (JWST) will be able to observe the aromatic and aliphatic C–H stretch transitions from hydrocarbons in the 3.3–3.4 micron region [
140]. Recent observations of the interstellar comet 2I/Borisov gave us our first insight into the organic composition of comets from other star systems. While some deviations from Solar System comets have been noted (C
depleted, CO enriched), 2I/Borisov is overall fairly similar [
141,
142,
143,
144]. With survey telescopes such as the Vera Rubin Observatory going online in coming years, more opportunities to discover and characterize the organic composition of interstellar comets should follow with more new discoveries.
Specific avenues for further study include (but are not limited to):
(1) Release of organic matter into the coma.Past results at IR and sub-mm wavelengths have shown that while many organic molecules are released directly from the nucleus (C
H
, HCN), others have a distributed source. H
CO has a wide range of abundances in comets, and ALMA observations have shown distributed sources for multiple comets to date. Further observations are needed to understand H
CO abundances in comets and how they connect to more complex organic matter such as POM, CHON particles, or polymers. Similarly, the origin of HNC is not well understood, though it is clear that it is not released directly from the nucleus. The identity of the CH
OH extended source observed for some comets also warrants further study. A detailed study of the scattering properties of dust grains can also reveal the presence of organic matter [
145,
146].
(2) Further development of compositional taxonomies. While the optical taxonomy [
29,
30] has a large sample size (>200 comets), its meaning for the organic composition of comets is not well understood due to a lack of knowledge of the origin of the observed radicals. On the other hand, IR and sub-mm abundance patterns are more straightforward to interpret, but suffer from small sample sizes (∼50). While trends are emerging [
8], the recent discovery of C/2016 R2 (PanSTARRS), a comet dominated by CO with high N
and low H
O abundances [
147,
148,
149,
150,
151], has revealed that there is still much that is not understood about cometary composition. More observations at all wavelengths are needed to better interpret compositional taxonomies and what they imply about the organic composition of cometary nuclei.
(3) Cryogenic sample return.The next giant leap in the study of cometary organic matter will be the return of a (preferably cryogenic) sample to laboratories on Earth through a mission such as AMBITION, as discussed above. This will enable the detection and characterization of complex organic matter that has been measured in meteorites, but will only be studied in comets through a sample that is returned to Earth.
Comets are rich in organic matter and are vital tools to understanding the formation of organic matter in the ISM/protosolar disk and subsequent delivery to the terrestrial planets. Past studies have revealed much about the organic inventory of comets, and the future is bright for continued insights into the organic composition of cometary nuclei.