UC Berkeley

Aerosols in planetary atmospheres play a critical role in radiative transfer and thus also in determining the penetration depth of UV radiation and atmospheric temperatures. For early Mars and an anoxic early Earth, aerosols may have significantly influenced the stability of liquid water at the surface, as well as climate and habitability, yet the study of aerosol formation remains poorly constrained by models and experiments, making conclusions difficult. Significant progress can be made in reducing these uncertainties by providing additional laboratory constraints and tests for the photochemical and microphysical models used to generate the greatly varying predictions about aerosol formation in terrestrial-like atmospheres. Photochemistry experiments measuring gas- and condensed-phase species were conducted to determine (1) the extent to which a photochemical haze could have formed in Mars' and Earth's early atmospheres and (2) whether such aerosols might be depleted in carbon-13, providing a potentially false biosignature in organic matter in the Martian and terrestrial rock and meteorite records.

To gain a greater understanding of the fundamentals of aerosol formation in the CO₂-rich atmospheres of early Earth and Mars, methane (CH₄) and mixtures of CH4 and carbon dioxide (CO₂) were irradiated with UV light in a static stainless steel reaction chamber, leading to the formation of aerosols. HeNe laser scattering was used to detect in situ the presence and relative amounts of aerosol produced, as well as the induction time required to form aerosols from the irradiation of the gases. Online mass spectrometry measurements were used to monitor the time evolution of gas phase species and, by difference, the relative amounts of carbon in the aerosol formed during an experiment. For comparison, particle samples were also collected during the irradiation experiments for "offline" analysis by elemental analyzer-isotope ratio mass spectrometry (EA-IRMS); the IRMS peak areas were used to quantify the relative amounts of carbon in the aerosols. From these three types of measurements, particle production was observed to be higher as the pressure of initial reactant CH₄ was increased from 5 to 200 Torr, but little sensitivity to the CH₄/CO₂ ratio between 0.34 and 5 for a given CH₄ pressure was observed. This behavior is in contrast to what has been predicted by photochemical models, which suggest that aerosol production rates decrease as CO₂ levels increase. Furthermore, aerosol formation was detectable at a CH₄/CO₂ ratio down to 0.25 by in situ HeNe light scattering and down to 0.34 by offline EA-IRMS analysis of collected particles. These experimental ratios are significantly lower than the threshold ratios of 0.6 to 1 predicted by photochemical models for particle production. This suggests that aerosol production in CO₂-rich atmospheres with feasible sources of CH₄ from mantle outgassing or methanogenic bacteria was likely to have been much more favorable than predicted by models. Further, more sophisticated photochemical models of early Earth's atmosphere will need to include reactions of organic species with oxygen in order to better predict the minimum CH₄/CO₂ threshold at which particles form as a function of CH₄/CO₂.

In addition to affecting atmospheric radiative transfer, organic aerosols that formed photochemically in the atmosphere could have deposited on the surface, and would thus have been incorporated in the rock record. If such aerosols were significantly lighter in carbon-13 than the CH₄ from which they formed, such a light isotopic signature might potentially be mistaken as a biomarker. To test this hypothesis, aerosols from the UV irradiation of CH₄ and mixtures of CH₄ and CO₂ were collected and analyzed for the carbon-13 isotopic composition by EA-IRMS. Compared to the isotopic composition of the reactant CH₄, the particle samples were depleted in ¹³C by 6±3 ‰. While this is likely too small to account for the large isotopic depletions in ¹³C observed between 2.5 to 3.0 billion years ago (which have been interpreted as the rise of methanotrophic bacteria), experimentation at lower temperatures is recommended to address whether the overall kinetic isotope effects in photochemical aerosol production might be larger at the temperatures in the upper atmosphere where CH₄ photolysis would have occurred.