About 2.5 billion years ago, microbes learned to harness plentiful solar energy to reduce CO$_2$ with H$_2$O, extracting energy and producing O$_2$ as waste. O$_2$ production from this metabolic process was so vigorous that it saturated its photochemical sinks, permitting it to reach "runaway" conditions and rapidly accumulate in the atmosphere despite its reactivity. Here we argue that O$_2$ may not be unique: diverse gases produced by life may experience a "runaway" effect similar to O$_2$. This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become potentially detectable. Planets orbiting smaller, cooler stars, such as the M dwarfs that are the prime targets for the James Webb Space Telescope (JWST), are especially favorable for runaway due to their lower UV emission compared to higher-mass stars. As an illustrative case study, we show that on a habitable exoplanet with an H$_2$-N$_2$ atmosphere and net surface production of NH$_3$ orbiting an M dwarf (the "Cold Haber World" scenario), the reactive biogenic gas NH$_3$ can enter runaway, whereupon an increase in the surface production flux of one order of magnitude can increase NH$_3$ concentrations by three orders of magnitude and render it detectable by JWST in just two transits. Our work on this and other gases suggests that diverse signs of life on exoplanets may be readily detectable at biochemically plausible production rates.
Spectral characterization of Super-Earth atmospheres for planets orbiting in the Habitable Zone of M-dwarf stars is a key focus in exoplanet science. A central challenge is to understand and predict the expected spectral signals of atmospheric biosignatures (species associated with life). Our work applies a global-mean radiative-convective-photochemical column model assuming a planet with an Earth-like biomass and planetary development. We investigated planets with gravities of 1g and 3g and a surface pressure of one bar around central stars with spectral classes from M0 to M7. The spectral si…
Nitrous oxide (N2O) -- a product of microbial nitrogen metabolism -- is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous investigations of N2O as a biosignature have examined scenarios using Earthlike N2O mixing ratios or surface fluxes, or those inferred from Earth's geologic record. However, biological fluxes of N2O could be substantially higher, due to a lack of metal catalysts or if the last step of the denitrification metabolism that yields N2 from N2O had never evolved. Here, we use…
(Abridged) Super Earth exoplanets are being discovered with increasing frequency and some will be able to retain stable H2-dominated atmospheres. We study biosignature gases on exoplanets with thin H2 atmospheres and habitable surface temperatures, by using a model atmosphere with photochemistry, and biomass estimate framework for evaluating the plausibilty of a range of biosignature gas candidates. We find that photochemically produced H atoms are the most abundant reactive species in H2 atmospheres. In atmospheres with high CO2 levels, atomic O is the major destructive species for some molec…
Oxygen and methane are considered to be the canonical biosignatures of modern Earth, and the simultaneous detection of these gases in a planetary atmosphere is an especially strong biosignature. However, these gases may be challenging to detect together in the planetary atmospheres because photochemical oxygen radicals destroy methane. Previous work has shown that the photochemical lifetime of methane in oxygenated atmospheres is longer around M dwarfs, but M dwarf planet habitability may be hindered by extreme stellar activity and evolution. Here, we use a 1-D photochemical-climate model to s…
The first potential exoplanet biosignature detections are likely to be ambiguous due to the potential for false positives: abiotic planetary processes that produce observables similar to those anticipated from a global biosphere. Here we propose a class of methylated gases as corroborative `capstone' biosignatures. Capstone biosignatures are metabolic products that may be less immediately detectable, but have substantially lower false positive potential, and can thus serve as confirmation for a primary biosignature such as O$_2$. CH$_3$Cl has previously been established as a biosignature candi…
Photochemistry induced by stellar UV flux should produce haze particles in exoplanet atmospheres. Recent observations indicate that haze and/or cloud layers exist in the atmospheres of exoplanets. However, photochemical processes in exoplanetary atmospheres remain largely unknown. We performed laboratory experiments with the PHAZER chamber to simulate haze formation in a range of exoplanet atmospheres (hydrogen-rich, water-rich, and carbon dioxide-rich at 300, 400, and 600 K), and observed the gas phase compositional change (the destruction of the initial gas and the formation of new gas speci…
Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane's short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that, in contrast to an Earth-like biosphere, known …
Chemical disequilibrium in exoplanetary atmospheres (detectable with remote spectroscopy) can indicate life. The modern Earth's atmosphere-ocean system has a much larger chemical disequilibrium than other solar system planets with atmospheres because of oxygenic photosynthesis. However, no analysis exists comparing disequilibrium on lifeless, prebiotic planets to disequilibrium on worlds with primitive chemotrophic biospheres that live off chemicals and not light. Here, we use a photochemical-microbial ecosystem model to calculate the atmosphere-ocean disequilibria of Earth with no life and wi…
Saturn's Moon Titan receives volatiles into the top of its atmosphere-including atomic oxygen-sourced from cryovolcanoes on Enceladus. Similar types of atmosphere exchange from one body to another, such as O2 and O3 sourced from TRAPPIST-1 d, could be introduced into the upper atmosphere of TRAPPIST-1 e and might be interpreted as biosignatures. We simulate this potential false-positive for life on TRAPPIST-1 e, by applying an external influx of water and oxygen into the top of the atmosphere using a coupled 1-D photochemical-climate model (Atmos), to predict atmospheric composition. In additi…
A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O$_2$, only a handful of gases have been considered in detail. Here we evaluate phosphine (PH$_3$). On Earth, PH$_3$ is associated with anaerobic ecosystems, and as such is a potential biosignature gas on anoxic exoplanets. We simulate CO$_2-$ and H$_2-$dominated habitable terrestrial planet atmospheres. We find that PH$_3$ can accumulate to detectable concentrations on planets with surface production fluxes of 10$^{10}$-10$^{14}$ cm$^{-2}$ s$^{-1}$ (corre…
It is widely anticipated that the James Webb Space Telescope (JWST) will be transformative for exoplanet studies. It has even been suggested that JWST could provide the first opportunity to search for biosignatures in an alien atmosphere using transmission spectroscopy. This claim is investigated, specifically for the proposed anoxic biosignature pair CH4-CO2. The most favourable known target is adopted (TRAPPIST-1e), with an assumed atmospheric composition similar to the Archean Earth. Compared to previous studies, a more systematic investigation of the effect that cloud/haze-layers have on t…
Ultraviolet-Driven Atmospheric Degeneracies Challenge Conventional Biosignature Frameworks for Terrestrial Planets with Ultracool M Dwarf Hosts: An Archean-Analog TRAPPIST-1 e Case Study
University of California, Riverside（加州大学河滨分校）；Blue Marble Space Institute of Science（蓝大理石太空科学研究所）；University of Maryland, Baltimore County（马里兰大学巴尔的摩县分校）；NASA Goddard Space Flight Center（NASA戈达德太空飞行中心）；SETI Institute（搜寻地外文明研究所）；NASA Ames Research Center（NASA艾姆斯研究中心）
We find that variations in the UV emissions of cool M-dwarf stars have a potentially large impact upon atmospheric biosignatures in simulations of Earth-like exoplanets i.e. planets with Earths development, and biomass and a molecular nitrogen-oxygen dominated atmosphere. Starting with an assumed black-body stellar emission for an M7 class dwarf star, the stellar UV irradiation was increased stepwise and the resulting climate-photochemical response of the planetary atmosphere was calculated. Results suggest a Goldilocks effect with respect to the spectral detection of ozone. At weak UV levels,…
Research for possible biosignature gases on habitable exoplanet atmosphere is accelerating. We add isoprene, C5H8, to the roster of biosignature gases. We found that formation of isoprene geochemical formation is highly thermodynamically disfavored and has no known abiotic false positives. The isoprene production rate on Earth rivals that of methane (~ 500 Tg yr-1). On Earth, isoprene is rapidly destroyed by oxygen-containing radicals, but its production is ubiquitous to a diverse array of evolutionarily distant organisms, from bacteria to plants and animals-few, if any at all, volatile second…
Detectability of Atmospheric Biosignatures in Earth Analogs with Varying Surface Boundary Conditions: Prospects for Characterization in the UV, Visible, Near-Infrared, and Mid-Infrared Regions
The Effects of M Star Age Dependent Ultraviolet Emission on Detecting and Interpreting Exoplanet Biosignatures
University of California Santa Cruz（加州大学圣克鲁兹分校）；SETI Institute（SETI研究所）；University of Washington（华盛顿大学）；NASA NExSS Virtual Planetary Laboratory（NASA NExSS虚拟行星实验室）；Arizona State University（亚利桑那州立大学）；University of Maryland, Baltimore County（马里兰大学巴尔的摩分校）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；United States Naval Observatory（美国海军天文台）
O2 and O3 have been long considered the most robust individual biosignature gases in a planetary atmosphere, yet multiple mechanisms that may produce them in the absence of life have been described. However, these abiotic planetary mechanisms modify the environment in potentially identifiable ways. Here we briefly discuss two of the most detectable spectral discriminants for abiotic O2/O3: CO and O4. We produce the first explicit self-consistent simulations of these spectral discriminants as they may be seen by JWST. If JWST-NIRISS and/or NIRSpec observe CO (2.35, 4.6 um) in conjunction with C…
Wavelength Requirements for Life Detection via Reflected Light Spectroscopy of Rocky Exoplanets
University of Washington（华盛顿大学）；Northern Arizona University（北亚利桑那大学）；NASA NExSS Virtual Planetary Laboratory, University of Washington（美国国家航空航天局 NExSS 虚拟行星实验室，华盛顿大学）；Astrobiology Program, University of Washington（天体生物学项目，华盛顿大学）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；JHU Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；Lunar and Planetary Laboratory, University of Arizona（亚利桑那大学月球与行星实验室）；Purdue University（普渡大学）；Southeastern Universities Research Association（东南大学研究协会）；NASA Ames Research Center（美国国家航空航天局艾姆斯研究中心）；Louisiana State University（路易斯安那州立大学）；Chapman University（查普曼大学）；University of California, Riverside（加州大学河滨分校）；Bard College（巴德学院）；California Institute of Technology（加州理工学院）
There's more to life in reflected light: Simulating the detectability of a range of molecules for high-contrast, high-resolution observations of non-transiting terrestrial exoplanets
The search for life on planets outside our solar system will use spectroscopic identification of atmospheric biosignatures. The most robust remotely-detectable potential biosignature is considered to be the detection of oxygen (O_2) or ozone (O_3) simultaneous to methane (CH_4) at levels indicating fluxes from the planetary surface in excess of those that could be produced abiotically. Here, we use an altitude-dependent photochemical model with the enhanced lower boundary conditions necessary to carefully explore abiotic O_2 and O_3 production on lifeless planets with a wide variety of volcani…
Following the discovery of the first exoplanet candidate transiting a white dwarf (WD), a "white dwarf opportunity" for characterizing the atmospheres of terrestrial exoplanets around WDs is emerging. Large planet-to-star size ratios and hence large transit depths make transiting WD exoplanets favorable targets for transmission spectroscopy - conclusive detection of spectral features on an Earth-like planet transiting a close-by WD can be achieved within a medium James Webb Space Telescope (JWST) program. Despite the apparently promising opportunity, however, the post-main sequence (MS) evolut…
We present a study of the photochemistry of abiotic habitable planets with anoxic CO$_2$-N$_2$ atmospheres. Such worlds are representative of early Earth, Mars and Venus, and analogous exoplanets. H$_2$O photodissociation controls the atmospheric photochemistry of these worlds through production of reactive OH, which dominates the removal of atmospheric trace gases. The near-UV (NUV; $>200$ nm) absorption cross-sections of H$_2$O play an outsized role in OH production; these cross-sections were heretofore unmeasured at habitable temperatures ($<373$ K). We present the first measurements of NUV…
Some atmospheric gases have been proposed as counter indicators to the presence of life on an exoplanet if remotely detectable at sufficient abundance (i.e., antibiosignatures), informing the search for biosignatures and potentially fingerprinting uninhabited habitats. However, the quantitative extent to which putative antibiosignatures could exist in the atmospheres of inhabited planets is not well understood. The most commonly referenced potential antibiosignature is CO, because it represents a source of free energy and reduced carbon that is readily exploited by life on Earth and is thus of…
Preparing for the Early eVolution Explorer: The Impact of Flare Temperature on Ozone Column Depth in Earth-Like Atmospheres
John Hopkins University（约翰霍普金斯大学）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；SETI Institute（搜寻地外文明研究所）；NASA Ames Research Center（美国国家航空航天局艾姆斯研究中心）；University of Colorado（科罗拉多大学）；Johns Hopkins Applied Physics Laboratory（约翰霍普金斯大学应用物理实验室）；Arizona State University（亚利桑那州立大学）
Molecular oxygen (O2) paired with a reducing gas is regarded as a promising biosignature pair for the atmospheric characterization of terrestrial exoplanets. In circumstances when O2 may not be detectable in a planetary atmosphere (e.g., at mid-IR wavelengths) it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy to infer the presence of O2. However, O3 production has a nonlinear dependence on O2 and is strongly influenced by the UV spectrum of the host star. To evaluate the reliability of O3 as a proxy for O2, we used Atmos, a 1D coupled climate/phot…
With the upcoming launch of space telescopes dedicated to the study of exoplanets, the \textit{Atmospheric Remote-Sensing Infrared Exoplanet Large-survey} (ARIEL) and the \textit{James Webb Space Telescope} (JWST), a new era is opening in the exoplanetary atmospheric explorations. However, especially in relatively cold planets around later-type stars, photochemical hazes and clouds may mask the composition of the lower part of the atmosphere, making it difficult to detect any chemical species in the troposphere or to understand whether there is a surface or not. This issue is particularly exac…
Science-Driven Performance Requirements for UV Coronagraph Design: Implications for O$_3$ Detection in Earth-like Atmospheres
Mary W. Jackson NASA Headquarters（NASA 总部）；NASA Goddard Space Flight Center（NASA 戈达德太空飞行中心）；Habitability, Atmospheres, and Biosignatures Laboratory, University of Arizona（亚利桑那大学宜居性、大气与生物标志物实验室）；NASA Nexus for Exoplanet System Science Virtual Planetary Laboratory, SETI Institute（SETI 研究所系外行星系统科学虚拟行星实验室）；Lunar & Planetary Laboratory, University of Arizona（亚利桑那大学月球与行星实验室）；Department of Astronomy & Astrophysics, The Pennsylvania State University（宾夕法尼亚州立大学天文学与天体物理学系）；Center for Exoplanets and Habitable Worlds, The Pennsylvania State University（宾夕法尼亚州立大学系外行星与宜居世界中心）；Institute for Computational and Data Science, The Pennsylvania State University（宾夕法尼亚州立大学计算与数据科学研究院）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）
A number of transiting, potentially habitable Earth-sized exoplanets have recently been detected around several nearby M dwarf stars. These worlds represent important targets for atmospheric characterization for the upcoming NASA James Webb Space Telescope. Given that available time for exoplanet characterization will be limited, it is critically important to first understand the capabilities and limitations of JWST when attempting to detect atmospheric constituents for potentially Earth-like worlds orbiting cool stars. Here, we explore coupled climate-chemistry atmospheric models for Earth-li…
