Influence of CO versus CH$_4$ on organic haze formation in atmospheres of diverse terrestrial exoplanets

Abstract: Context. Terrestrial exoplanets are expected to host secondary, high-metallicity atmospheres derived from outgassing of volatiles such as N2, CO2, H2O, CH4, and CO. Photochemical organic hazes are likely to form in such environments, significantly affecting atmospheric observations and planetary habitability. Aims. We investigate haze formation in representative terrestrial exoplanet atmospheres and assess how CH4 versus CO as the primary carbon source affects haze production rates, particle properties, and chemical complexity. Methods. We performed six laboratory simulations by exposing gas mixtures at a few mbar to glow discharge at 300 K. Each atmosphere contained 75% N2, CO2, or H2O, 10% of each of the other two gases, and 5% CH4 or CO. Gas-phase products were analyzed with a residual gas analyzer, and solid products were characterized by production rate, particle density, atomic force microscopy, Fourier-transform infrared spectroscopy, and very high-resolution mass spectrometry. Results. CH4 experiments produced more diverse gas-phase species and much higher haze yields than the corresponding CO experiments. CO-derived hazes showed a narrow particle size range of 10-80 nm, whereas CH4-derived hazes were denser and chemically more complex. The identified molecular formulas suggest growth pathways linked to gaseous precursors such as HCN, CH2O, and C2H4. Conclusions. The atmospheric redox state critically controls haze formation in simulated terrestrial exoplanet atmospheres. CH4 is significantly more effective than CO in initiating organic growth, leading to higher haze production rates and greater chemical complexity. These results provide useful constraints for exoplanet atmospheric modeling and spectral interpretation, and further support the possibility that reducing atmospheres may facilitate prebiotic organic chemistry relevant to the emergence of life.

• The paper demonstrates that CH4-driven atmospheres yield 5–90 times more haze mass and chemical complexity than CO-based experiments.
• The experimental design systematically varied dominant gases and carbon sources, revealing how redox state modulates photochemical haze formation.
• Analytical techniques like FTIR, AFM, and VHRMS confirmed that CH4 hazes exhibit diverse functional groups, larger particle sizes, and implications for exoplanet spectral modeling.

Influence of CO versus CH$_4$ on Organic Haze Formation in Diverse Terrestrial Exoplanet Atmospheres

Introduction

This study provides a comprehensive experimental analysis of photochemical haze formation pathways in atmospheres relevant to terrestrial exoplanets, emphasizing the distinct roles of CO and CH$_4$ as primary carbon sources. The work addresses compositional diversity expected in secondary, high-metallicity exoplanetary atmospheres and quantifies how redox state and available carbon species modulate haze production rate, particle properties, and chemical complexity. Laboratory simulations were conducted using a systematic suite of six representative atmospheric mixtures that isolate the effects of dominant background gases (CO$_2$, N$_2$, or H$_2$O) and carbon source identity (CO vs. CH$_4$).

Figure 1: Initial gas mixing ratios for the six experiments, each utilizing a dominant species (CO$_2$, N$_2$, or H$_2$O; 75%), two background gases (10% each), and 5% of either CH$_4$ or CO.

Experimental Design and Analytical Methodology

Six controlled plasma discharge experiments were executed with initial mixtures set to simulate plausible exoplanet atmospheres. Each scenario was structured to have 75% of a dominant molecule (CO$_4$0, N$_4$1, or H$_4$2O) and 10% each of the other two, supplemented by 5% of either CH$_4$3 or CO to mimic variable carbon speciation under contrasting redox regimes. This design enables direct assessment of the mechanistic influence of carbon source across oxidizing/reducing conditions.

Gas-phase products were resolved and semi-quantified with residual gas analyzer (RGA) mass spectrometry, employing iterative Monte Carlo deconvolution to identify newly-formed species. Solid-phase haze particles were analyzed for yield, bulk density (via pycnometry), particle size distribution (via AFM), functional group structure (via FTIR), and soluble molecular constituency (via very high-resolution mass spectrometry).

Gas-Phase Chemistry: Carbon Speciation Dictates Product Complexity

Mass spectral deconvolution of the gas phase reveals that CH$_4$4 as a carbon source drives a vastly more complex and diverse array of photochemical products compared to CO. This pattern holds regardless of dominant background gas. The CH$_4$5-containing atmospheres yield multiple classes of hydrocarbons, N-bearing organics, and O- and N/O-heteroatomic compounds. In particular, reduced species such as NH$_4$6, H$_4$7, HCN, and saturated/unsaturated hydrocarbons are consistently more abundant when CH$_4$8 is present.

Figure 2: Gas-phase mass spectrum deconvolution for N$_4$9-rich/CH$_2$0 illustrates complexity and species-level partitioning in the plasma, distinguishing instrument background and true photoproducts.

Relative gas-phase product abundances clearly stratify experiments by carbon source and background gas oxidation state, as shown in the comparison of all six mixtures.

Figure 3: Gas-phase formation yields for all experimental combinations, highlighting greater molecular diversity and abundance in CH$_2$1-containing mixtures compared to CO analogs.

HCN emerges as a key intermediate, but haze production is not simply proportional to HCN abundance—pointing to a multi-precursor haze nucleation/growth mechanism.

Haze Particle Yields, Density, and Size: Strong Dependence on Redox State

Quantified haze yields display a dramatic dichotomy. The three CH$_2$2-based experiments yield orders of magnitude more mass than any of the CO-based trials. Specifically, haze production rates in N$_2$3-rich/CH$_2$4 and H$_2$5O-rich/CH$_2$6 cases exceed corresponding CO-based experiments by factors of ~80–90, and even the relatively oxidizing CO$_2$7-rich/CH$_2$8 experiment outproduces its CO analog by a factor of five.

Figure 4: Haze yield as a direct function of both dominant background composition and carbon source.

Bulk analysis indicates that only the CH$_2$9 experiments produce sufficient mass for collected measurement; particle densities are in the range of 1.50–1.51 g cm$_2$0, at the upper end of previously reported values for exoplanet haze analogs, pointing to growth of larger, more polar molecules under these regimes.

The CO-based hazes, by contrast, yield substantially fewer and smaller (10–80 nm) near-spherical particles, as shown by AFM.

Figure 5: AFM characterization of particle size distributions for CO-based haze samples across three dominant backgrounds.

Chemical and Molecular Composition: Functional and Structural Diversity as a Function of Precursors

FTIR analysis demonstrates that CH$_2$1-driven hazes possess extensive functional group diversity, including saturated and unsaturated C–H, N–H, O–H, as well as various C=N, C=O, and other heteroatom features. Spectra of CO-based hazes are depleted in C–H and instead dominated by unsaturated, oxidized functionalities, highlighting the suppressive effect of oxidizing conditions on hydrocarbon growth.

Figure 6: Comparative infrared spectra exhibit clear trends in vibrational signatures consistent with underlying redox-driven chemical differences between CH$_2$2- and CO-derived hazes.

Very high-resolution mass spectrometry mapping of haze analogs produced under N$_2$3-rich/CH$_2$4 and H$_2$5O-rich/CH$_2$6 conditions reveals thousands of unique molecular formulas, most with CHON composition, with long homologous series reflective of polymerization/oligomerization chemistry initiated by simple gas-phase species such as HCN, CH$_2$7O, and C$_2$8H$_2$9.

Figure 7: VHRMS results for N$_2$0-rich/CH$_2$1 and H$_2$2O-rich/CH$_2$3 hazes, with molecular assignment spectra, compositional breakdown by CHN, CHO, and CHON classes, and van Krevelen diagrams revealing unsaturation and heteroatom incorporation trends.

Implications for Exoplanetary Spectroscopy, Atmospheric Modeling, and Prebiotic Chemistry

The pronounced efficiency gap in haze yield between CH$_2$4- and CO-driven atmospheres directly underscores the role of atmospheric redox state as a master parameter controlling haze formation pathways. This finding has immediate consequences for radiative transfer models and interpretation of exoplanet transmission and emission spectra with JWST, ELTs, and future direct-imaging missions, as haze optical depth, particle microphysics, and chemical composition all modulate spectral appearance.

Compositional analyses suggest that reducing, CH$_2$5-enriched atmospheres not only supply thick, optically active haze layers, but also foster synthesis of prebiotically relevant molecules—including amino acid and nucleobase analogs—via gas-to-solid growth mechanisms, with VHRMS revealing plausible precursors for biochemistry. This supports the theoretical hypothesis that reducing environments more readily provide the chemical heterogeneity and complexity needed for origin-of-life chemistry, and that such systems are less likely to suffer the surface UV transparency penalty observed for oxidizing, haze-poor atmospheres.

Atmospheric microphysical modeling must incorporate true, experimentally constrained haze densities and particle sizes reported here to correctly simulate particle settling rates, vertical distributions, and wavelength-dependent scattering. Similarly, compositional dependence of FTIR features and functional group connectivity will be critical for interpreting observed spectral features, and for distinguishing between haze-rich CH$_2$6 exoplanets and relatively clear CO/CO$_2$7-dominated analogs.

Conclusion

This experimental study robustly demonstrates that CH$_2$8 is a vastly more effective initiator of organic haze production in terrestrial exoplanet atmospheres than CO, independent of the dominant background gas. CH$_2$9 atmospheres exhibit both higher haze yields (by factors of 5–90) and substantially greater chemical complexity. The strong compositional and physical partitioning between CO- and CH$_4$0-initiated hazes constrains forward models of atmospheric evolution, spectral characterization, and assessments of planetary habitability, highlighting the interplay between atmospheric chemistry, photochemistry, and particle microphysics. The results reinforce the paradigm that the atmospheric redox state is the first-order control on haze properties, with key implications for exoplanet observability and potential prebiotic chemical inventories (2604.03575).