CO₂-Dominated Gas Coma

• CO₂-dominated gas coma is defined by CO₂ outgassing that surpasses water vapor, forming in comets, protoplanetary disks, and planetary atmospheres.
• Comprehensive models reveal that photodissociation and electron impact dissociation of CO₂ drive unique emission features, such as enhanced green (5577 Å) lines.
• Observational diagnostics, including green-to-red emission ratios and production rate measurements, provide insights into volatile evolution and planetary system formation.

A CO$_2$-dominated gas coma refers to an astronomical environment—most prominently in comets, but also arising in protoplanetary disks, interstellar objects, and some debris disks and planetary atmospheres—where gas-phase carbon dioxide is the principal volatile constituent by number density and/or production rate, surpassing both water vapor and other volatiles such as CO. Such comae arise under diverse physical scenarios: sublimation-driven activity in comets at large heliocentric distances, volatile stratification during planetary formation, and the aftermath of energetic events (e.g., giant impacts) in circumstellar disks. The presence and dominance of CO$_2$ in the coma crucially impact observable emission features, physical and chemical evolution, and our interpretations of volatile delivery and atmospheric history in planetary systems.

1. Fundamental Processes and Emission Mechanisms

CO$_2$-dominated comae emerge through competitively efficient gas-phase release mechanisms and distinct photochemical pathways. In cometary environments, CO$_2$ sublimation occurs at lower temperatures than H$_2$O (but above CO), allowing CO$_2$ to dominate outgassing at moderate to large heliocentric distances (typically $r_H > 2$–3.5 au). The modeling of emission features relies on key molecular processes:

• Photodissociation: CO$_2$ absorbs solar photons in the 955–1165 Å range, resulting in nearly unit quantum yield for O($^1$S) (atomic oxygen in the metastable excited state), fueling the characteristic green (5577 Å) emission line. The relevant reaction,

$$\mathrm{CO}_2 + h\nu \rightarrow \mathrm{CO} + \mathrm{O}(^1S)$$

dominates the green emission in CO$_2$-abundant comae, while H$_2$O photodissociation primarily generates O($^1$D) for the red-doublet (6300, 6364 Å) (Raghuram et al., 2014).

• Electron Impact Dissociation: In dense inner coma regions (e.g., comet 67P), electron impact can be the principal process, with the cross-section for CO$_2$–e$^-$ dissociation being up to $\sim$40× higher than for H$_2$O. Even at moderate CO$_2$/H$_2$O ($\sim$3%), CO$_2$ is found to dominate OI emission, especially in regions with locally enhanced CO$_2$ (Bodewits et al., 2016).
• Thermal Sublimation Physics: The sublimation rate $Z$ of CO$_2$ ice is exponentially dependent on local temperature,

$$Z \propto \exp\left(-\frac{E_\mathrm{bind}}{kT}\right)$$

where $E_\mathrm{bind}$ is the binding energy. CO$_2$ outgassing can proceed at lower $T$ than H$_2$O, suppressing water vapor production in some CO$_2$-dominated comae (Cordiner et al., 25 Aug 2025).

• Radiative Transfer and Emission Profiles: Modeling of infrared emission proceeds via

$$I_{ul} = \frac{2 h c \nu_{th}}{\lambda^4} \frac{p_u}{\frac{w_u}{w_l} p_l - p_u} \left[1 - e^{-\tau_{ul}}\right]$$

with $I_{ul}$ the line intensity, $p_{u,l}$ and $w_{u,l}$ the level populations and statistical weights, and $\tau_{ul}$ the optical depth (Bockelée-Morvan et al., 2016). CO$_2$-dominated environments often produce strong, optically thick $^{12}$CO$_2$ emission (at $T\sim$450 K) overlain by colder, optically thinner $^{13}$CO$_2$ emission, as revealed by JWST observations in protoplanetary disks (Vlasblom et al., 17 Dec 2024).

2. Diagnostics: Observational Signatures and Ratios

The predominance of CO$_2$ radically alters both spectroscopic diagnostics and physical interpretation:

• Green-to-Red Doublet Ratio (G/R): In classical cometary analysis, the G/R ratio (5577 Å / [6300,6364] Å), with the canonical value $\sim$0.1, is diagnostic of a water-dominated coma. Elevated G/R ratios ($>$0.1) are a hallmark of increased CO$_2$ abundance, with ratios approaching or exceeding 0.5 indicating near parity or dominance (Raghuram et al., 2014). In cases of roughly equal CO$_2$ and H$_2$O abundance, up to 50% of the red-doublet emission can arise via CO$_2$ photodissociation, undermining the use of G/R as a linear CO$_2$ indicator.
• Production Rate Ratios:
  • $$Q(\mathrm{CO}_2)/Q(\mathrm{H}_2\mathrm{O}) \sim 0.1$$–0.5** typifies many CO$_2$-dominated comae.
  • $Q(\mathrm{CO})/Q(\mathrm{H}_2\mathrm{O})$ is generally lower ($\lesssim 0.03$ median), while in exceptional comets and interstellar objects, $Q(\mathrm{CO}_2)/Q(\mathrm{H}_2\mathrm{O})$ can reach 8 [$2508.18209$], or even exceed 30 in rare cases (e.g., C/2016 R2, $$Q(\mathrm{CO}_2)/Q(\mathrm{H}_2\mathrm{O})\approx32$$) (McKay et al., 2019).
• Coma Morphology and Kinematics: CO$_2$ comae can be extended (e.g., $\sim$3 arcmin/350,000 km in 3I/ATLAS, (Lisse et al., 21 Aug 2025)), with centrally peaked but nearly isotropic profiles. In some comets (e.g., 67P), plume-like enhancements in [OI] and CN—absent in OH—directly map regions of locally high CO$_2$/H$_2$O (Bodewits et al., 2016).

3. Origins and Evolutionary Environments

CO$_2$-dominated gas comae manifest across a spectrum of Solar and extrasolar environments, each providing distinct formation and evolutionary insights:

• Comets at Large Heliocentric Distance: Beyond 2–3.5 au, water ice is thermally inert, but CO$_2$ remains active. Observations with Rosetta/VIRTIS-H have shown perihelion increases in $Q(\mathrm{CO}_2)/Q(\mathrm{H}_2\mathrm{O})$ from $1$–$3\%$ (devolatilized north) to $14$–$32\%$ (volatile-rich south), interpreted as exposure of less processed layers following dust ablation (Bockelée-Morvan et al., 2016). The spatial fan-shaped anisotropy of CO$_2$ and water bands correlates with the rotation axis and local illumination.
• Exocomets and Debris Disks: In the Fomalhaut belt, ALMA observations show CO coincident with dust, with inference that exocometary ices are CO+CO$_2$ rich—matching Solar System comets’ fraction (4.6–76%) (Matrà et al., 2017). In the aftermath of giant impacts (e.g., HD 23514), hot ($\sim$900 K), CO$_2$-rich gas and sub-$\mu$m silica grains are co-located in the inner AU, implying efficient CO$_2$ self-shielding, ongoing replenishment, and relevance for models of volatile retention in planetary formation (Su et al., 26 Jun 2025).
• Protoplanetary Disks: JWST-MIRI spectra of the compact disk CX Tau reveal CO$_2$ lines dominating over H$_2$O, explained as a consequence of radial drift and the sequential inward delivery and sublimation of ice-rich pebbles; $^{12}$CO$_2$ (450 K) traces the upper inner disk, $^{13}$CO$_2$ (200 K) the outer or deeper layers (Vlasblom et al., 17 Dec 2024).
• Interstellar Objects: JWST and SPHEREx confirm that 3I/ATLAS has a resolved, extended, symmetric CO$_2$ gas coma and an exceptionally high $$Q(\mathrm{CO}_2)/Q(\mathrm{H}_2\mathrm{O})=8.0 \pm 1.0$$, over 6$\sigma$ above the extrapolated Solar System distribution (Cordiner et al., 25 Aug 2025, Lisse et al., 21 Aug 2025).

4. Physical and Chemical Evolution

CO$_2$-dominated environments drive and reflect key evolutionary processes:

• Seasonality and Surface Processing: On cometary nuclei, seasonal insolation influences CO$_2$ exposure: intense illumination can ablate dust caps to expose deeper, volatile-rich strata (e.g., southern hemisphere of 67P), transiently elevating CO$_2$ output (Bockelée-Morvan et al., 2016).
• Thermal Lag and Stratification: The time delay between perihelion and peak volatile production in comet 67P is modeled as a conduction timescale $\tau = (\rho c x^2)/k$, where $x$ is the ablation depth; this places volatile fronts at depths of a few centimeters (Bockelée-Morvan et al., 2016).
• Outgassing and Atmospheric Evolution in Planets: In early magma oceans (e.g., TRAPPIST-1 planets), CO$_2$ is less soluble than H$_2$O in silicate melt and outgasses first, leading to a transient CO$_2$-dominated state. The governing mass-balance equation,

$$M_i^{\mathrm{moa}} = k_i F_i M^{\mathrm{crystal}} + F_i M^{\mathrm{liq}} + \frac{4\pi r_p^2}{g} p_{i,\mathrm{mass}}$$

and its evolution via coupled ODEs, quantitatively tracks the partitioning and subsequent evolution toward H$_2$O- or CO$_2$-dominated atmospheres. A CO$_2$-rich envelope inhibits H$_2$O escape (via diffusion-limited flux), potentially extending magma ocean solidification times by up to $2\times10^8$ years (Carone et al., 13 Dec 2024).

5. Implications for Activity, Spectroscopy, and Planetary System Formation

• Diagnostic Power and Limitations: Elevated G/R ratios, broad green emission line widths, optically thick CO$_2$ bands, and strong IR features provide robust diagnostics for CO$_2$ dominance (Raghuram et al., 2014, Vlasblom et al., 17 Dec 2024). However, at high CO$_2$ levels, traditional proxies for water production (e.g., red-doublet emission) systematically overestimate H$_2$O unless the CO$_2$ contribution is accurately subtracted (Raghuram et al., 2014).
• Coma Dynamics in Extreme Environments: In interstellar objects (3I/ATLAS) and certain Solar System comets (C/2016 R2), a strikingly low H$_2$O abundance relative to CO$_2$ and CO points to primitive, hypervolatile-rich formation scenarios or extensive cosmic irradiation altering the ice budget (Cordiner et al., 25 Aug 2025, McKay et al., 2019). The broad, symmetric, and extended CO$_2$ comae in such bodies contrast with more jet-dominated morphologies in water-driven activity.
• Retention and Destruction Processes: In inner debris disks, the photochemical lifetime of CO$_2$ is extremely short under intense UV unless self-shielding or dust shielding operates, or continuous replenishment from impact-driven vaporization sustains detectable column densities (e.g., $N(\mathrm{CO}_2)\gtrsim10^{18}$ cm⁻² extends the effective lifetime via $\Gamma_\mathrm{eff}=S(N)\Gamma_\mathrm{pd}$) (Su et al., 26 Jun 2025). These mechanisms allow coexistence of hot CO$_2$ gas with fragile, sub-$\mu$m dust in luminous environments.
• Planet and Disk Evolution: The observation that Fomalhaut’s exocomets and Solar System comets share similar CO+CO$_2$ ice fractions suggests pervasive ISM inheritance of ice compositions across extrasolar and Solar System bodies (Matrà et al., 2017). In protoplanetary disks, radial drift–induced CO$_2$ enrichment in the inner disk could precondition forming planets with a higher C/O ratio and altered volatile inventory (Vlasblom et al., 17 Dec 2024).

6. Quantitative Summary Table

System/Context	$Q(\mathrm{CO}_2)/Q(\mathrm{H}_2\mathrm{O})$	Notable Features
67P S. hemisphere (perihelion)	14–32%	Seasonal ablation, pristine volatile layers (Bockelée-Morvan et al., 2016)
Solar System median	12%	From large comet sample (Pinto et al., 2022)
Interstellar 3I/ATLAS	$8.0 \pm 1.0$	Extreme ratio, resolved CO$_2$ coma (Cordiner et al., 25 Aug 2025)
C/2016 R2 (PanSTARRS)	3230%	CO, N$_2$, CO$_2$-dominated, water-depleted (McKay et al., 2019)
Fomalhaut exocomets	4.6–76% (CO+CO$_2$) ice fraction	Comparable to Solar System comets (Matrà et al., 2017)
HD 23514 (giant impact disk)	—	Hot (900 K) CO$_2$, persistent via self-shielding (Su et al., 26 Jun 2025)

7. Broader Astrophysical Significance

CO$_2$-dominated gas comae provide critical insight into the physical chemistry of planetary system formation, the structure and evolution of cometary and exocometary reservoirs, and the potential for volatile delivery to habitable-zone planets. The diversity of environments exhibiting CO$_2$ dominance underscores the necessity of multi-wavelength diagnostics, coupled physical-chemical emission modeling, and detailed consideration of local physical conditions (e.g., temperature profile, ice stratigraphy, photochemistry, and shielding mechanisms). These systems test and refine models of volatile inheritance, processing, and loss—not only within our Solar System but also in exoplanetary disks, interstellar objects, and debris environments.

The prevalence of CO$_2$-dominated comae at large heliocentric distances, the links between CO$_2$ and water outbursts and erosion, and the evidence from both local and extrasolar contexts collectively advance our understanding of planetary system chemical evolution, volatile budget constraints, and atmospheric histories across diverse astrophysical settings.