Abstract
The Comet Interceptor Mission (ESA/JAXA) aims to visit a long-period comet or interstellar object. Its primary science goals are to characterize the object's shape, structure, and the composition of its surface and gas coma. The mission consists of three spacecraft, the primary and two accompanying ones. The focus of this paper is the unique synergetic activities between two mass spectrometers to investigate the chemical composition of the coma, one on the primary spacecraft and another on an accompanying spacecraft. Both can be operated in a complementary fashion, at different locations and at the same time, to sort out spatial from temporal effects. Relevant investigations of the coma composition and chemistry within the technical specifications of the instruments could address several unsolved questions. The Comet Interceptor Mission is posed to be an important mission for advancing our knowledge of comets, especially clues for understanding coma chemistry and composition.
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1. Introduction
The Comet Interceptor Mission [CI](ESA/JAXA) plans to visit a yet-to-be-discovered, long-period comet or interstellar object by waiting for up to several years at Earth's L2 Lagrange Point for a target of opportunity (Snodgrass & Jones 2019). Its primary science goals are to characterize the object's shape, structure, and the composition of its surface and gas coma. To be launched in 2029, the mission consists of three spacecraft, the primary A (ESA) and two accompanying ones, B1 (JAXA) and B2 (ESA). Relevant investigations of the coma composition and chemistry within the technical specifications of the instruments could address several unsolved questions concerning cometary volatiles, such as, (1) the relationship between ammonium salts (including 
) and the nitrogen inventory of cometary volatiles as well as their isotopic composition; (2) likely parents of phosphorous and its inner coma chemistry; (3) the nature of neutral sodium in the coma and tails, and its relationship to gas-phase species and dust particles, and (4) the source and chemistry of hydrogen halides.
The CI Mission will carry a number of instruments to measure the properties of the gas surrounding the nucleus. Included are plasma instruments to measure the characteristics of the charged particles in the coma. On board the A spacecraft is a neutral mass spectrometer (MANIaC, ESA) and the Plasma Suite (PS, JAXA) on the B1 spacecraft containing an ion mass spectrometer. The focus of this paper is the unique synergetic activities of these two mass spectrometers to investigate the chemical composition of the coma. Both can be operated in a complementary fashion, MANIaC at a further cometocentric distance than PS, at different locations and at the same time to sort out spatial (e.g., inhomogeneities) from temporal (e.g., scale lengths) effects (Figure 1).
Figure 1. Trajectory of the B1 Spacecraft (JAXA) (green arrow) with relevant features of the cometary environment.
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The expected results from these instruments and their relation to each other should shed light on how the neutral gas emitted by the comet nucleus and the resulting positively charged ions interact and how the composition of the nucleus can be deduced. In the next section, four such issues are highlighted that will be investigated by CI.
2. Potential Investigations by the CI Mission to Address Unsolved Comet Composition Issues
2.1. Nitrogen Inventory and Ammonium Salts
Investigating the relationship between ammonium salts and the nitrogen inventory of cometary volatiles as well as their isotopic composition is an important issue in modern comet studies. Comets are poor in N-bearing species; for example, the solar N/C ratio is 0.29 ± 0.12, whereas that of comet 67P/Churyumov-Gerasimenko is 0.035 ± 0.011 as measured by the Rosetta spacecraft, typical of comets. Ammonium salts have been suggested as the "missing N" (Poch et al. 2020) but have never been observed via ground-based facilities. Ammonium salts are formed between NH3 (base) and acids transferring 
from the acid to ammonia, including 
(ammonium chloride, 53, 55 Da) and 
(ammonium cyanide, 44 Da) within the PS spectrometer mass range. Obtaining the composition and other physico-chemical properties of ammonium salts as well as reaction pathways is a goal of CI.
2.2. Phosphorus Sources
Although phosphorus has a low abundance in the solar system, it is a key element in all living organisms and is an essential prebiotic species in life's origin (e.g., Boice & de Almeida 2012). Ground-based observations of comets have never detected P-bearing species; the only detections coming from in situ measurements by spacecraft missions. Molecules bearing phosphorus have been discovered in the dust component in comets 1P/Halley and 81P/Wild 2 (Sandford et al. 2008). Atomic phosphorus was first detected in the gas phase in comet 67P/Churyumov-Gerasimenko (Altwegg et al. 2016) with the molecule PO being the dominant reservoir of phosphorus since PO/PN > ∼10 and PO/PH3 > 3.3 (Rivilla et al. 2020).
The CI Mission will investigate the parents of phosphorus, including PH3 (34 Da) and PO (47 Da), and the inner coma chemistry of simple P-bearing molecules likely to be found in comets and important for prebiotic chemistry.
2.3. Sodium Sources
First seen in comet Mrkos in 1960, neutral sodium tails have been observed in several comets (e.g., Hyakutake, Hale-Bopp, ISON, NEOWISE) and are thought to be a common feature. Comet Hale-Bopp displayed two types of neutral sodium tails; a straight, narrow, anti-sunward tail and a broad, diffuse tail superimposed on the dust tail (Cremonese et al. 2002).
Dust with a sodium-bearing icy mantle is a likely distributed coma source of sodium in the diffuse tail. In the inner coma, sodium atoms are likely dissociation products. Possible gas-phase parents within the mass range of the PS spectrometer are NaOH (40 Da), NaH (24 Da), and Na2(46 Da). Photo rates of these molecules lead to typical scale-lengths (at 1 au) in the inner coma of NaOH (18 km), NaH (7 km), and Na2 (31 km); making them best suited for detection by the close approaching B1 spacecraft. Investigating the nature of neutral sodium in the coma and tails, and its relationship to gas-phase species and refractory dust particles will be a goal of the CI Mission.
2.4. Hydrogen Halides Sources
Rosetta detected hydrogen halides, including HF (20 Da) and HCl (36, 38 Da), in 67P/Churyumov-Gerasimenko and suggested distributed sources released from dust particles covered with a halogen-enriched icy mantle in the inner coma (De Keyser et al. 2017). Icy aggregates with a size on the order of 100 μm or larger and with speeds well below the gas speed are likely delivering most of the hydrogen halides to the coma but constitute a minor fraction of the total mass production.
The large electron affinity of halides affects inner coma chemistry, so investigating the source and chemistry of hydrogen halides within the PS spectrometer mass range, HCl and HF, will be an important mission aim.
3. Discussion
The existence of distributed sources of coma species make it difficult to assess the fraction of minor volatiles in the nucleus composition. In the inner coma, dust mantle material may not have completely sublimated so that abundance measurements will underestimate the material's abundance. Farther away from the nucleus, photodissociation, photoionization, charge exchange, and other loss processes might become important. Models of coma chemistry are needed to analyze CI's observations of minor species abundances for these effects.
4. Conclusions
Several important cometary composition issues can be investigated using results from the instruments onboard the CI spacecraft; namely, the nitrogen inventory and related ammonium salts, and the sources and inner coma chemistry of phosphorus, sodium, and hydrogen halides. Quantitative studies of these issues using an advanced coma chemistry model (SUISEI, Boice 2017) have begun, including properties of icy dust grains as well as gaseous species. Necessary for this study is basic data for photolytic and gas-phase processes for ammonium salts, hydrogen halides, and phosphorous species, many that are not well known.
The Comet Interceptor Mission is posed to be an important mission for advancing our knowledge of comets, especially clues for understanding coma chemistry and composition to resolve outstanding issues of cometary composition in a long-period comet. Seeking answers to the issues posed in this paper will be the direction of future work.
This work was partially supported by NSF Planetary Astronomy Program grant No. 0908529 and JSPS KAKENHI grant No. JP20K14541.