Ion Chemistry and Composition of Planetary Atmospheres: Towards a Characterization of the early Earth and CO2-bearing Exoplanets

 

• Introduction

The anoxic Archean atmosphere was likely rich in CH₄, but the detailed composition remains difficult to assess from the limited geological records. CH₄-driven photochemistry may have produced haze particles, influencing the early Earth's atmospheric energy budget and gas-to-solid conversion^[1]. Precursors to haze particles such as HCN, CO₂, CH₄ or CO have been proposed, their reactivity in high-altitude photochemical regions remains largely unconstrained^[2].

Experimental simulations of early Earth and (exo)planet atmospheres have mostly focused on organic aerosol analogues (tholins). Previously, tholins formed from electrical discharges of N₂:CH₄:CO₂ gas mixtures containing low CH₄ abundances showed a negative correlation between CO₂ concentration and tholin yield^[1]. Seminal UV photolysis gas-phase experiments demonstrated the possibility for amino acid formation^[3] but gas-phase studies have been limited since. Recently, HCN formation pathways to tholin formation in the absence of oxygen were investigated but neutral-neutral pathways only were considered^[4].

In the present work, we performed low-temperature (150 K) plasma discharge experiments using an N₂:CH₄:CO₂ (95:4:1) gas mixture to reproduce conditions relevant to the ionized upper atmosphere of the Archean Earth and CO₂-containing exoplanets, and measured the formed molecular products with a quadrupole mass spectrometer (QMS). Our goals were to (i) evaluate the chemical impact of a small (1%) concentration of CO₂ on the gas-phase products in a largely reduced environment, and (ii) investigate an atmosphere with a C/O of 2.5, where CH₄-driven polymerization can occur despite partial CH₄ oxidation. Such transient reducing states - potentially induced by asteroid impacts - may have episodically raised atmospheric CH₄ concentration on the early Earth.

Methods

Plasma configuration and operation

Experiments were conducted using the COsmic SImulation Chamber (COSmIC) facility at NASA Ames Research Center^[5]. The COSmIC chamber uses a pulsed discharge nozzle (PDN) which cools down gas mixtures by expanding gases through a slit and generates a pulsed direct-current (DC) plasma discharge (600 𝜇s) within the jet-cooled gas expansion. The PDN consists of a copper slit plate anode, an insulator slit plate, and two cathode electrodes placed along the slit downstream of the alumina plate. Gas residence inside the plasma volume is ~4 𝜇s. A -1000 V high voltage generates the plasma discharge, producing electrons with energies ~2-12 eV. The gas is jet-cooled to ~150 K in the plasma cavity, enabling experimental simulation of atmospheric chemistry, resulting in the formation of complex organic molecules, at low temperature. Experiments were run using a N₂:CH₄:CO₂ (95:4:1) ultra-high purity (99.999%) gas mixture, controlled by an MKS 1479 mass flow controller at a rate of 2000 sccm.

Time-resolved quadrupole mass spectrometry (QMS):

Time-resolved measurements were performed by synchronizing spectra acquisition with the plasma frequency (10 Hz) through a TTL signal gating input. The QMS was placed to face the molecular beam resulting from the supersonic jet. Once extracted through the QMS, the gas phase products were then guided through a series of electromagnetic lenses before reaching the QMS microchannel plate detector.

Production and characterization of tholins:

A N₂:CH₄:CO₂ (95:4:1) plasma experiment was run for 18 hours in the COSmIC chamber to produce an early Earth/exoplanet tholin sample. The sample was deposited on silicon substrate, then collected in an argon-filled glovebox to minimize air exposure. After collection, mid-infrared spectra (4000–1000 cm^-1) were acquired using a Fourier transform Infrared spectrometer with a spectral resolution of 4 cm^-1.

Reaction network modeling and quantum chemistry calculations:

We used the multi-fluid physics 1D COSmIC Plasma Reactivity Simulation Model (CO-PRISM^[6]) to compute molecular abundances based on known reaction rates and branching ratios. CO-PRISM consists of a chemical network comprising 435 reactions, including 72 newly-added oxygen-bearing pathways. Species include radicals, neutrals, cations, and anions. In addition, ab initio calculations were conducted to study the pathways and structures of key molecular products. We determined the equilibrium structures with the coupled cluster singles and doubles with perturbative triples method. The harmonic vibrational frequencies were also computed to ensure global minima on the potential energy surfaces.

 

• Results

For the first time in an N₂:CH₄:CO₂ (95:4:1) gas mixture, neutral, cation, and anion gas phase products were probed by mass spectrometry at low temperature (150 K). Utilizing simultaneous modeling and quantum computations, our measurements have unveiled key ion-neutral pathways and molecular products. These include CH₃⁺, HCNH⁺, CH₃CH₂CNH⁺, H^-, CH₃^-, C₂H^- and CN^-. Furthermore, cyclic molecules (e.g., C₆H₅⁺ and C₆H₅O) were also detected, along with the unexpected formation of the dication N₂²⁺ and dianion NO₂^2-. This study highlights the essential role ion-neutral reactions, often largely neglected in photochemical models, play in atmospheric chemistry. Main results and interpretation for the chemistry of volatiles on the early Earth and CO₂-containing exoplanetary atmospheres will be discussed.

 

Acknowledgements

Funding for this project is provided through NASA SMD “Cold Solar System Objects” and “Laboratory Astrophysics Directed Work Package” Internal Scientist Funding Models. LS acknowledges support provided by the Tennessee Space Grant Consortium.

 

References

[1] Trainer, M.G. et al.  2004, Astrobiology 4.

[2] Carrasco, N. and Gautier, T. 2021, Chapter 4. Prebiotic Photochemistry: From Urey–Miller-like Experiments to Recent Findings, RSC.

[3] Sagan, C., and Khare, B.N. 1971, Science 173, 417-420.

[4] Pearce, B.K. et al. 2022, ACS Earth and Space Chemistry 6, 2385-2399.

[5] Sciamma-O’Brien, E. et al. 2014, Icarus, 243, 325-336.

[6] Dubois et al. 2025, The Planetary Science Journal, 6, 241.