The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
Mary Ann Liebert for Astrobiology Society
Reason for embargo
Recognizing whether a planet can support life is a primary goal of future exoplanet spectral characterization missions, but past research on habitability assessment has largely ignored the vastly different conditions that have existed in our planet's long habitable history. This study presents simulations of a habitable yet dramatically different phase of Earth's history, when the atmosphere contained a Titan-like organic-rich haze. Prior work has claimed a haze-rich Archean Earth (3.8-2.5 billion years ago) would be frozen due to the haze's cooling effects. However, no previous studies have self-consistently taken into account climate, photochemistry, and fractal hazes. Here, we demonstrate using coupled climate-photochemical-microphysical simulations that hazes can cool the planet's surface by about 20 K, but habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (tau ~ 5 at 200 nm) even with the fainter young sun. We find that optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet. Hazes may even enhance planetary habitability through UV shielding, reducing surface UV flux by about 97% compared to a haze-free planet, and potentially allowing survival of land-based organisms 22.214.171.124 billion years ago. The broad UV absorption signature produced by this haze may be visible across interstellar distances, allowing characterization of similar hazy exoplanets. The haze in Archean Earth's atmosphere was strongly dependent on biologically-produced methane, and we propose hydrocarbon haze may be a novel type of spectral biosignature on planets with substantial levels of CO2. Hazy Archean Earth is the most alien world for which we have geochemical constraints on environmental conditions, providing a useful analog for similar habitable, anoxic exoplanets.
This work was performed as part of the NASA Astrobiology Institute's Virtual Planetary Laboratory, supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under solicitation NNH12ZDA002C and Cooperative Agreement Number NNA13AA93A. G. Arney was supported in part by the NASA Astrobiology Institute Early Career Collaboration Award. E.T. Wolf acknowledges NASA Planetary Atmospheres Program award NNH13ZDA001N-PATM and NASA Exobiology Program award NNX10AR17G for financial support. B. Charnay acknowledges support from an appointment to the NASA Postdoctoral Program, administered by Universities Space Research Association. E. Hébrard was supported by an appointment to the NASA Postdoctoral Program at NASA Goddard Space Flight Center, administered by Universities Space Research Association through a contract with NASA. Simulations were facilitated through the use of the Hyak supercomputer system at the University of Washington eScience Institute. We are grateful to C. McKay and three other anonymous reviewers whose comments substantially improved the quality of our manuscript. We thank R. Buick, D. Crisp, N. Kiang, and M. Parenteau for conversations and advice. Spectra shown in this work will be archived at the Virtual Planetary Laboratory online spectral database.
This is the author accepted manuscript. The final version is available from Mary Ann Liebert via the DOI in this record.
Vol. 16 (11), pp. 873-899