Year: 2018
Type: review
Abstract: In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708. Table of Contents1. Introduction 1.1. Requirements for life 1.2. Exoplanet biosignature definitions 1.3. Biosignature categories2. Evaluating Planetary Habitability3. Overview of Terrestrial Exoplanet Modeling Studies 3.1. Observations of earth 3.2. Spectral models 3.3. Photochemical studies of terrestrial atmospheres 3.4. Earth through time4. Gaseous Biosignatures 4.1. Gaseous biosignature overview 4.2. Earth-like atmospheres 4.2.1. Oxygen (O2) 4.2.2. Ozone (O3) 4.2.3. Methane (CH4) 4.2.4. Nitrous oxide (N2O) 4.2.5. Sulfur gases (DMS, DMDS, CH3SH) and relation to detectable C2H6 4.2.6. Methyl chloride (CH3Cl) 4.2.7. Haze as a biosignature 4.2.8. Other gases 4.3. “False positives” for biotic O2/O3 and possible spectral discriminators 4.4. Biosignatures in other types of atmospheres 4.5. Effects of the host star spectrum on photochemistry 4.6. Impacts of flares and particle events on biosignature gases5. Surface Biosignatures 5.1. Photosynthesis 5.1.1. Principles of photosynthesis 5.1.1.1. Relationship between band gap wavelength and reductant in the generation of biogenic gases and pigment color 5.1.1.2. Uniqueness of OP 5.1.2. Photosynthetic pigments and the color of phototrophs 5.1.2.1. Structure 5.1.2.2. Light absorption 5.1.3. The vegetation “red edge” 5.1.4. Speculation about photosynthesis and pigment signatures on exoplanets 5.2. Retinal pigments 5.3. Alternative surface biosignatures: nonphotosynthetic pigments and reflectance features 5.4. False positive surface biosignatures 5.5. Chiral and polarization biosignatures 5.6. Fluorescence and bioluminescence6. Temporal Biosignatures 6.1. Oscillations of atmospheric gases 6.2. Oscillations in surface signatures7. Assessing Biosignature Plausibility 7.1. Chemical disequilibrium 7.2. Biomass estimation 7.3. Applications of network theory to biosignatures8. Cryptic Biospheres: “False Negatives” for Life?9. Prospects for Detecting Exoplanet Biosignatures10. Summary
Source: Astrobiology
Authors: Edward W. Schwieterman, Nancy Y. Kiang, Mary N. Parenteau, Chester E. Harman, Shiladitya DasSarma, Theresa M. Fisher, Giada N. Arney, Hilairy E. Hartnett, Christopher T. Reinhard, Stephanie L. Olson, Victoria S. Meadows, Charles S. Cockell, Sara I. Walker, John Lee Grenfell, Siddharth Hegde, Sarah Rugheimer, Renyu Hu, Timothy W. Lyons
Institutions: University of California, Riverside, Universities Space Research Association, NASA Astrobiology Institute, National Aeronautics and Space Administration, Blue Marble Space Institute of Science, Alternative Earths Astrobiology Center, Goddard Institute for Space Studies, Ames Research Center, Columbia University, University of Maryland, Baltimore, University System of Maryland, Institute of Marine and Environmental Technology, Arizona State University, Goddard Space Flight Center, Georgia Institute of Technology, University of Washington, University of Edinburgh, Santa Fe Institute, Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR), Cornell University, University of St Andrews, California Institute of Technology, Jet Propulsion Laboratory
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
Awards: FPDS BASE DESCRIPTION, MULTIPLE YEAR AWARD&INCREMENTAL FUNDING FOR YEARS 1 OF 5. THIS PROPOSAL IS ENTITLED, "THE VIRTUAL PLANETARY LABORATORY." THI, G465804250, THE CURRENT PROJECT WILL FOCUS ON KEY ASPECTS OF THE BIOLOGY OF HALORUBRUM LACUSPROFUNDI, AN EXTREMELY HALOPHILIC ARCHAEON ISOLATED FROM DEEP LAKE, A, G5540806446, G589152158, MULTIPLE YEAR AWARD&INCREMENTAL FUNDING FOR YEARS ONE OF FIVE. THIS PROPOSAL IS ENTITLED, "ALTERNATIVE EARTHS: EXPLAINING PERSISTENT INHABITATION ON
OpenAlex ID: W2616637335