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.