[Paper Review] Radiative processes as diagnostics of cometary atmospheres

Dennis Bodewits, B. P. Bonev+2 more|arXiv (Cornell University)|Sep 6, 2022

Astro and Planetary Science4 citations

TL;DR

This paper reviews radiative processes across radio to X-ray wavelengths in cometary comae, demonstrating how spectral emissions serve as diagnostics for physical conditions and coma properties. It emphasizes multi-wavelength modeling to resolve discrepancies in volatile production rates—such as HCN—by integrating infrared, radio, and optical observations with advanced excitation models.

ABSTRACT

In this chapter, we provide a review of radiative processes in cometary atmospheres spanning a broad range of wavelengths, from radio to X-rays. We focus on spectral modeling, observational opportunities, and anticipated challenges in the interpretation of new observations, based on our current understanding of the atomic and molecular processes occurring in the atmospheres of small, icy bodies. Close to the surface, comets possess a thermalized atmosphere that traces the irregular shape of the nucleus. Gravity is too low to retain the gas, which flows out to form a large, collisionless exosphere (coma) that interacts with the heliospheric radiation environment. As such, cometary comae represent conditions that are familiar in the context of planetary atmosphere studies. However, the outer comae are tenuous, with densities lower than those found in vacuum chambers on Earth. Comets, therefore, provide us with unique natural laboratories that can be understood using state-of-the-art theoretical treatments of the relevant microphysical processes. Radiative processes offer direct diagnostics of the local physical conditions, as well as the macroscopic coma properties.These can be used to improve our understanding of comets and other astrophysical environments such as icy moons and the interstellar medium.

Motivation & Objective

• To synthesize current understanding of radiative processes across the electromagnetic spectrum in cometary comae.
• To address persistent discrepancies in volatile production rates (e.g., HCN) derived from infrared versus radio observations.
• To improve interpretation of spectroscopic data by integrating multi-wavelength observations with advanced modeling of excitation and radiative transfer.
• To bridge observational gaps between weakly active, distant comets and more active bodies, including active asteroids and dormant bodies.
• To support future missions and remote sensing by refining diagnostic tools for coma structure, volatile release, and non-LTE processes.

Proposed method

• Utilizes spectral modeling across radio, sub-mm, infrared, optical, and X-ray wavelengths to link observed emissions to underlying physical processes.
• Applies the Planetary Spectrum Generator (PSG) to simulate synthetic spectra at resolving power R = 5000, incorporating dust continuum and molecular emission lines.
• Employs high-resolution infrared and optical spectroscopy (e.g., from IRTF and McDonald Observatory) to compare fluorescence excitation models of CN and HCN.
• Integrates rovibrational and electronic transitions in the A²Π–X²Σ⁺ and B²Σ⁺–X²Σ⁺ systems to model CN fluorescence across the coma.
• Evaluates cascading effects in infrared fluorescence and their role in excitation imbalance between IR and radio-derived production rates.
• Combines ground-based optical observations of CO₂ with space-based IR detections to empirically constrain dissociation release rates and metastable state production.

Experimental results

Research questions

• RQ1 Why do HCN production rates derived from infrared rovibrational transitions exceed those from radio pure rotational transitions by a factor of ~2?
• RQ2 To what extent do cascading processes in infrared fluorescence contribute to the observed discrepancy in HCN excitation and derived production rates?
• RQ3 How can multi-wavelength observations (IR, optical, radio) be used to resolve inconsistencies in column density and production rate retrievals for cometary species?
• RQ4 What role do photodissociative excitation and precursor molecules (e.g., HCN) play in the excitation of CN radicals in the coma?
• RQ5 How can radiative diagnostics be extended to low-activity comets and dormant bodies to understand the evolutionary sequence of icy small bodies?

Key findings

• Discrepancies in HCN production rates between infrared and radio observations persist despite modeling of cascading effects, indicating unresolved assumptions in excitation modeling.
• High-resolution infrared and optical observations of CN in comet C/2014 Q2 support a new fluorescence model that includes both electronic and rovibrational transitions, improving excitation modeling across the coma.
• The IR spectrum can simultaneously probe CN and its likely precursor HCN, enabling direct comparison of excitation and release processes.
• Contemporaneous IR and optical observations allow intercomparison of model assumptions and parameters, enhancing confidence in fluorescence modeling for CN.
• Empirical constraints on CO₂ dissociation release rates are achievable by comparing ground-based optical and space-based IR observations, reducing uncertainties in metastable state production.
• Multi-wavelength spectroscopy is essential for resolving excitation imbalances and improving retrieval accuracy in low-density, tenuous cometary comae.