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The pale orange dot : the spectrum and habitability of hazy Archean Earth

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Date
01/11/2016
Author
Arney, Giada
Domagal-Goldman, Shawn D.
Meadows, Victoria S.
Wolf, Eric T.
Schwieterman, Edward
Charnay, Benjamin
Claire, Mark
Hébrard, Eric
Trainer, Melissa G.
Funder
NERC
European Research Council
Grant ID
NE/J022802/2
678812
Keywords
Haze
Archean Earth
Exoplanets
Spectra
Biosignatures
Planetary habitability
GE Environmental Sciences
QB Astronomy
QH301 Biology
DAS
BDC
R2C
SDG 13 - Climate Action
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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.
Citation
Arney , G , Domagal-Goldman , S D , Meadows , V S , Wolf , E T , Schwieterman , E , Charnay , B , Claire , M , Hébrard , E & Trainer , M G 2016 , ' The pale orange dot : the spectrum and habitability of hazy Archean Earth ' , Astrobiology , vol. 16 , no. 11 , pp. 873-899 . https://doi.org/10.1089/ast.2015.1422
Publication
Astrobiology
Status
Peer reviewed
DOI
https://doi.org/10.1089/ast.2015.1422
ISSN
1531-1074
Type
Journal article
Rights
© Giada Arney et al., 2016. Published by Mary Ann Liebert, Inc. This Open Access article is distributed under the terms of the Creative Commons Attribution Noncommercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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  • University of St Andrews Research
URI
http://hdl.handle.net/10023/9980

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