Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres

Abstract

Early Earth may have hosted a biologically-mediated global organic haze during the Archean eon (3.8-2.5 billion years ago). This haze would have significantly impacted multiple aspects of our planet, including its potential for habitability and its spectral appearance. Here, we model worlds with Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf (GJ 876) and show that organic haze formation requires methane fluxes consistent with estimated Earth-like biological production rates. On planets with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and CH3SCH3), photochemistry involving these gases can drive haze formation at lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting the sun, at 30x the modern organic sulfur gas flux, haze forms at a CH4/CO2 ratio 20% lower than at 1x the modern organic sulfur flux. For a planet orbiting the M4V star, the impact of organic sulfur gases is more pronounced: at 1x the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2 ~ 0.2, but at 30x the organic sulfur flux, the CH4/CO2 ratio needed to form haze decreases by a full order of magnitude. Detection of haze at an anomalously low CH4/CO2 ratio could suggest the influence of these biogenic sulfur gases, and therefore imply biological activity on an exoplanet. When these organic sulfur gases are not readily detectable in the spectrum of an Earth-like exoplanet, the thick organic haze they can help produce creates a very strong absorption feature at UV-blue wavelengths detectable in reflected light at a spectral resolution as low as 10. In direct imaging, constraining CH4 and CO2 concentrations will require higher spectral resolution, and R > 170 is needed to accurately resolve the structure of the CO2 feature at 1.57 μm, likely, the most accessible CO2 feature on an Archean-like exoplanet.

accepted for publication in Astrobiology

References in corpus (9)

• Extreme Water Loss and Abiotic O${}_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs
• The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
• Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth
• Pale Orange Dots: The Impact of Organic Haze on the Habitability and Detectability of Earthlike Exoplanets
• Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations
• Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production
• The Effects of Refraction on Transit Transmission Spectroscopy: Application to Earth-like Exoplanets
• Developing Atmospheric Retrieval Methods for Direct Imaging Spectroscopy of Gas Giants in Reflected Light I: Methane Abundances and Basic Cloud Properties
• Atmospheric Retrieval for Direct Imaging Spectroscopy of Gas Giants In Reflected Light II: Orbital Phase and Planetary Radius

Cited by in corpus (32)

• Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life
• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST
• Aerosols in Exoplanet Atmospheres
• The Case and Context for Atmospheric Methane as an Exoplanet Biosignature
• Sulfur-driven Haze Formation in Warm CO2-rich Exoplanet Atmospheres
• Distinguishing between wet and dry atmospheres of TRAPPIST-1 e and f
• Effects of UV stellar spectral uncertainty on the chemistry of terrestrial atmospheres
• Understanding planetary context to enable life detection on exoplanets and test the Copernican principle
• Differentiating Modern and Prebiotic Earth Scenarios for TRAPPIST-1e: High-resolution Transmission Spectra and Predictions for JWST
• Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres
• Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres
• Estimating the Ultraviolet Emission of M dwarfs with Exoplanets from Ca II and H$\alpha$
• Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures
• Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature
• Observational Constraints on the Great Filter
• Testing Earth-like atmospheric evolution on exo-Earths through oxygen absorption: required sample sizes and the advantage of age-based target selection
• Disruption of a Planetary Nitrogen Cycle as Evidence of Extraterrestrial Agriculture
• Relative abundances of CO2, CO, and CH4 in atmospheres of Earth-like lifeless planets
• H${}_2$-dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets
• Surface and Temporal Biosignatures
• The Impact of Planetary Rotation Rate on the Reflectance and Thermal Emission Spectrum of Terrestrial Exoplanets Around Sun-like Stars
• The Role of Atmospheric Exchange in False-Positive Biosignature Detection
• Retrievals Applied To A Decision Tree Framework Can Characterize Earth-like Exoplanet Analogs
• Earth: Atmospheric Evolution of a Habitable Planet
• Projections of Earth's technosphere: Scenario modeling, worldbuilding, and overview of remotely detectable technosignatures
• Controls of Atmospheric Methane on Early Earth and Inhabited Earth-like Terrestrial Exoplanets
• Projections of Earth's Technosphere: Strategies for Observing Technosignatures on Terrestrial Exoplanets
• On the importance of laboratory experiments for interpreting exoplanet observations
• Evolution and Observable Properties of Rocky Planet Atmospheres
• Searching for technosignatures in exoplanetary systems with current and future missions
• Detectability Simulations of a NIR Surface Biosignature on Proxima Centauri b with Future Space Observatories
• Habitable zone predictions and how to test them