title: The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth.
abstract: 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 (τ ∼ 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 2.7-2.6 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 that 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 analogue for similar habitable, anoxic exoplanets. Key Words: Haze-Archean Earth-Exoplanets-Spectra-Biosignatures-Planetary habitability. Astrobiology 16, 873-899.
authors: 1 Astronomy Department, University of Washington , Seattle, Washington, USA .|2 NASA Astrobiology Institute Virtual Planetary Laboratory, University of Washington , Seattle, Washington, USA .|3 Astrobiology Program, University of Washington , Seattle, Washington, USA .|4 Now at: NASA Goddard Space Flight Center , Greenbelt, Maryland, USA .|5 Now at: NASA Postdoctoral Program, Universities Space Research Association , Columbia, Maryland, USA .
authors: Giada
authors: G
authors: Arney
authors: 2 NASA Astrobiology Institute Virtual Planetary Laboratory, University of Washington , Seattle, Washington, USA .|6 NASA Goddard Space Flight Center , Greenbelt, Maryland, USA .
authors: Shawn D
authors: SD
authors: Domagal-Goldman
authors: 1 Astronomy Department, University of Washington , Seattle, Washington, USA .|2 NASA Astrobiology Institute Virtual Planetary Laboratory, University of Washington , Seattle, Washington, USA .|3 Astrobiology Program, University of Washington , Seattle, Washington, USA .
authors: Victoria S
authors: VS
authors: Meadows
authors: 7 Department of Atmospheric and Oceanic Sciences, Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder , Boulder, Colorado, USA .
authors: Eric T
authors: ET
authors: Wolf
authors: 1 Astronomy Department, University of Washington , Seattle, Washington, USA .|2 NASA Astrobiology Institute Virtual Planetary Laboratory, University of Washington , Seattle, Washington, USA .|3 Astrobiology Program, University of Washington , Seattle, Washington, USA .|5 Now at: NASA Postdoctoral Program, Universities Space Research Association , Columbia, Maryland, USA .|8 Now at: University of California at Riverside , Riverside, California, USA .|9 Blue Marble Institute of Science , Seattle, Washington, USA .
authors: Edward
authors: E
authors: Schwieterman
authors: 1 Astronomy Department, University of Washington , Seattle, Washington, USA .|2 NASA Astrobiology Institute Virtual Planetary Laboratory, University of Washington , Seattle, Washington, USA .|3 Astrobiology Program, University of Washington , Seattle, Washington, USA .|10 Now at: Paris-Meudon Observatory , Paris, France .
authors: Benjamin
authors: B
authors: Charnay
authors: 2 NASA Astrobiology Institute Virtual Planetary Laboratory, University of Washington , Seattle, Washington, USA .|9 Blue Marble Institute of Science , Seattle, Washington, USA .|11 Department of Earth and Environmental Sciences, University of St Andrews , St Andrews, UK .
authors: Mark
authors: M
authors: Claire
authors: 6 NASA Goddard Space Flight Center , Greenbelt, Maryland, USA .|12 University of Exeter , Exeter, Devon, UK .
authors: Eric
authors: E
authors: Hébrard
authors: 6 NASA Goddard Space Flight Center , Greenbelt, Maryland, USA .
authors: Melissa G
authors: MG
authors: Trainer
mesh_terms: D001272:Atmosphere; D002980:Climate; D018534:Earth, Planet; D018559:Exobiology; D005118:Extraterrestrial Environment; D005740:Gases / Q000032:analysis; D008962:Models, Theoretical; D010316:Particle Size; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D014466:Ultraviolet Rays; D014867:Water
chemical_list: D005740:Gases; D014867:Water