From Deep Impact to the Comet Interceptor: Integrating ground-based near-infrared spectroscopic measurements of volatiles with spacecraft cometary missions

Comet research has been a high priority in solar system exploration. The last two decades have witnessed three spacecraft missions to short-period comets (Stardust / NExT, Deep Impact / EPOXI, and Rosetta) as well as tremendously increased capabilities of astronomical remote sensing facilities. The primary motivation for  these extensive studies has been the need to understand comets as remnants from the protosolar nebula. These objects retain the volatiles (ices) from the cold regions in the protosolar disk, where they formed. Deciphering this cosmogonic heritage requires a strong synergy between mission findings and remote-sensing observations. Missions explore specific comets at unprecedented levels of detail that cannot be gained by other means. Whereas missions are necessarily restricted to only a few targets, remote sensing observations study comets as a population. This includes objects with a wide range of dynamical histories, nucleus sizes and shapes, gas production rates, and inner coma parent volatile compositions. 

This presentation is focused on the close and continually evolving synergy between missions and remote sensing near-infrared (near-IR) spectroscopic measurements of cometary volatiles. The near-IR samples a suite of molecules, such as CH4, C2H2, C2H4, C2H6, NH3, HCN, CO, OCS, H2CO, CH3OH, and CO2 (from space only). It also provides the easiest way to simultaneously measure H2O (the most abundant coma gas in most active comets) from the ground through its non-resonant fluorescence lines. In whole or in part these species are considered parent volatiles – those originally stored as ices in cometary nuclei. Abundances of isotopologues (e.g., HDO, CH3D) can be constrained in exceptionally bright comets.

Previous missions to short-period comets, especially Deep Impact / EPOXI and Rosetta, demonstrated the importance of ground-based near-IR observations of parent volatiles for pre-encounter characterization, parallel spacecraft and Earth-based measurements, or follow-up astronomical studies [1-11]. Extending this synergy, the Comet Interceptor mission (targeting a long-period comet [12]) can be supported by the modern near-IR spectrographs now operating at the NASA Infrared Telescope Facility, W. M. Keck Observatory, European Southern Observatory, and Gemini. These ground-based facilities offer high spectral resolving power, allowing emission lines from different molecules to be distinguished and establishing volatile abundances with unprecedented sensitivity. Examples of particular avenues for integration with mission results include:

1.    Detailed compositional measurements with emphasis on relative coma abundances among species (for example, C2H2/C2H6, CO/H2CO/CH3OH, NH3/H2O, etc.).

2.    Spatially resolved measurements of volatile column densities and gas rotational temperatures. These are diagnostic of the physical conditions in the inner collisional coma (to which near-IR observations are most sensitive), and to the role of various dynamic and thermodynamic processes that shape the coma environment. 

3.    Spatial studies also provide insights into heterogeneous outgassing and the sources of volatile release – directly from the nucleus versus release from icy grains ejected into the coma. Physical coma models [13] have been developed to evaluate the contributions of both nucleus and extended sources. Previously validated against EPOXI, Rosetta, and ground-based data, these models can be applied to the vastly different spatial scales and observing geometries of spacecraft measurements versus simultaneously or contemporaneously obtained ground-based data, thereby enabling unified interpretation of both.

4.    As done for the EPOXI target 103P/Hartley 2 [6], a near-IR spectral survey would provide a complete inventory of all emission lines detected in the L-band. Through comparison with molecular fluorescence models and available laboratory spectra, emission lines can be identified by measured frequency, intensity, and quantum assignment, and included in a database available to both mission and ground-based researchers.

In addition to providing direct mission support, telescopic near-IR measurements serve as a “bridge” between detailed findings on specific mission targets and taxonomic studies of a larger number of comets using established and uniformly implemented remote sensing techniques.

References

[1] Mumma et al. 2005, Science, 310, 270
[2] DiSanti et al. 2007, Icarus, 187, 240
[3] Dello Russo et al. 2014, Icarus, 238, 125
[4] Mumma et al. 2011, ApJ Letters, 734, L7
[5] Dello Russo et al. 2011, ApJ Letters, L8
[6] Dello Russo et al. 2013, Icarus, 222, 707
[7] Kawakita et al. 2013, Icarus, 222, 723
[8] Bonev et al. 2013, Icarus, 222, 740
[9] Feaga et al. 2023, LPI Contributions 2851, 2476
[10] Bonev et al. 2023, Astron. Journal, 166, 233
[11] Shou et al. 2024, AAS/DPS Meeting, 56, abstract 401.04
[12] Jones et al. 2024, Space Science Reviews, 220, 9
[13] Tenishev et al. 2024, Frontiers in Astron. and Space Sci., 11, 1484360