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 30× the modern organic sulfur gas flux, haze forms at a CH4/CO2 ratio 20% lower than at 1× the modern organic sulfur flux. For a planet orbiting the M4V star, the impact of organic sulfur gases is more pronounced: at 1× the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2 ∼ 0.2, but at 30× 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. Key Words: Organic haze—Organic sulfur gases—Biosignatures—Archean Earth. Astrobiology 18, 311–329.

IDENTIFYING PLANETARY BIOSIGNATURE IMPOSTORS: SPECTRAL FEATURES OF CO AND O4 RESULTING FROM ABIOTIC O2/O3 PRODUCTION

“…The CO 2 content is similar to Earth's, f CO 2 = 3.6e-4, and the rest of the atmosphere consists of N 2 and trace photochemically produced species such as CO and O 3 . To self-consistently calculate temperature and ozone profiles, we used a version of the coupled photochemical-climate model Atmos (Arney et al 2016), based on photochemical and climate models originating with the Kasting group (Kasting & Donahue 1980;Segura et al 2005;Haqq-Misra et al 2008;Domagal-Goldman et al 2014) and upgraded to handle high-pressure O 2 -dominated atmospheres. We assume a completely desiccated planet (no H 2 O) orbiting at the inner edge of the HZ (a = 0.12 au) of a star with identical properties to GJ 876.…”

Earth: Atmospheric Evolution of a Habitable Planet

“…Meanwhile, high levels of CH4 and the spectral fingerprints of a haze may also provide a remotely detectable biosignature for inhabited worlds lacking O2 (Arney et al 2016(Arney et al , 2017.…”

Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres

“…We anticipate that abundance of isoprene, a hydrocarbon, in an atmosphere could lead to the presence of a haze layer in the atmosphere similar to the haze layer induced by organic molecules described in (Arney et al 2018). The presence of a haze layer may hinder detection of isoprene spectral features and should be quantified.…”

“…Gases such as CH3Cl (Segura et al 2005) and DMS (Arney et al 2018;Domagal-Goldman et al 2011;Pilcher 2003;Seager et al 2012) were suggested before as potential biosignature gases due to their large production rate by marine life on Earth and low destruction rate, which lead to relative stability in some atmospheres. Global annual production rates of isoprene are much higher than those of CH3Cl and DMS (production rates of major volatile secondary metabolites by life on Earth are compared in Figure 4).…”