A generalised microbial cell model for methane biosignature predictions
Department of Physics and Astronomy, University of Exeter（埃克塞特大学物理与天文学系）
Arwen E. Nicholson, Nathan J. Mayne
备注：
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review
The majority of potentially habitable planets detected to date are likely quite different to Earth, for example, being larger in radius and mass, differing rotation rates and with host star spectra unlike the Sun. Therefore the first alien life detected will potentially be living in conditions not found on our planet. This necessitates a generalised approach to modelling biology that can be applied to numerous planetary scenarios, built on fundamental knowledge of life on Earth, but not limited by it. Here, we explore a generalised model of a microbial cell, whose metabolic rate is governed by thermodynamics and substrate diffusion across its cell wall. We model a single-species biosphere consisting of methane producing microbes and determine how changing the cell size, cell death rate and biomass synthesis cost influence the biosignature on the planet - in this case methane. We discuss approaches to predicting upper estimates for the biosignature gas abundance and the applicability of the model to other metabolisms. This tool adds to the body of work attempting to grapple with the complexity of potential alien biospheres.
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 …
The disequilibrium combination of abundant methane and carbon dioxide has been proposed as a promising exoplanet biosignature that is readily detectable with upcoming telescopes such as the James Webb Space Telescope. However, few studies have explored the possibility of non-biological CH4 and CO2 and related contextual clues. Here, we investigate whether magmatic volcanic outgassing on terrestrial planets can produce atmospheric CH4 and CO2 with a thermodynamic model. Our model suggests that volcanoes are unlikely to produce CH4 fluxes comparable to biological fluxes. Improbable cases where v…
The James Webb Space Telescope (JWST) may be capable of finding biogenic gases in the atmospheres of habitable exoplanets around low mass stars. Considerable attention has been given to the detectability of biogenic oxygen, which could be found using an ozone proxy, but ozone detection with JWST will be extremely challenging, even for the most favorable targets. Here, we investigate the detectability of biosignatures in anoxic atmospheres analogous to those that likely existed on the early Earth. Arguably, such anoxic biosignatures could be more prevalent than oxygen biosignatures if life exis…
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…
Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy. Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life. However, how this disequilibrium changed over time and, in particular, the biogenic disequilibria maintained in the anoxic Archean or less oxic Proterozoic eons are unknown. We calculate the atmosphere-ocean disequilibrium in the Precambrian using conservative proxy- and model-based estima…
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,…
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…
The search for life on exoplanets is motivated by the universal ways in which life could modify its planetary environment. Atmospheric gases such as oxygen and methane are promising candidates for such environmental modification due to the evolutionary benefits their production would confer. However, confirming that these gases are produced by life, rather than by geochemical or astrophysical processes, will require a thorough understanding of planetary context, including the expected counterfactual atmospheric evolution for lifeless planets. Here, we evaluate current understanding of planetar…
With the characterisations of potentially habitable planetary atmospheres on the horizon, the search for biosignatures is set to become a major area of research in the coming decades. To understand the atmospheric characteristics that might indicate alien life we must understand the abiotic characteristics of a planet and how life interacts with its environment. In the field of biogeochemistry, sophisticated models of life-environment coupled systems demonstrate that many assumptions specific to Earth-based life, e.g. specific ATP maintenance costs, are unnecessary to accurately model a biosph…
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…
The biosignatures of life on Earth do not remain static, but change considerably over the planet's habitable lifetime. Earth's future biosphere, much like that of the early Earth, will consist of predominantly unicellular microorganisms due to the increased hostility of environmental conditions caused by the Sun as it enters the late stage of its main sequence evolution. Building on previous work, the productivity of the biosphere is evaluated during different stages of biosphere decline between 1 Gyr and 2.8 Gyr from present. A simple atmosphere-biosphere interaction model is used to estimate…
Early Earth may have hosted a biologically-mediated global organic haze during the Archean eon (3.8-2.5 billion years ago). This haze would have significantly impacted multiple aspects of our planet, including its potential for habitability and its spectral appearance. Here, we model worlds with Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf (GJ 876) and show that organic haze formation requires methane fluxes consistent with estimated Earth-like biological production rates. On planets with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and CH3S…
The methane cycle is a key component of the Earth system that links planetary climate, biological metabolism, and the global biogeochemical cycles of carbon, oxygen, sulfur, and hydrogen. However, currently lacking is a numerical model capable of simulating a diversity of environments in the ocean where methane can be produced and destroyed, and with the flexibility to be able to explore not only relatively recent perturbations to Earth's methane cycle but also to probe methane cycling and associated climate impacts under the reducing conditions characteristic of most of Earth history and like…
Understanding the possible climatic conditions on rocky extrasolar planets, and thereby their potential habitability, is one of the major subjects of exoplanet research. Determining how the climate, as well as potential atmospheric biosignatures, change under different conditions is a key aspect when studying Earth-like exoplanets. One important property is the atmospheric mass hence pressure and its influence on the climatic conditions. Therefore, the aim of the present study is to understand the influence of atmospheric mass on climate, hence habitability, and the spectral appearance of plan…
When Carl Sagan observed the Earth during a Gallileo fly-by in 1993, he found a widely distributed surface pigment with a sharp reflection edge in the red part of the spectrum, which, together with the abundance of gaseous oxygen and methane in extreme thermodynamic disequilibrium, were strongly suggestive of the presence of life on Earth. This widespread pigmentation that could not be explained by geological processes alone, is caused by the cellular structure of vegetation - a mechanism for potentially limiting damage to chlorophyll and/or limiting water loss. The distinctive increase in the…
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）
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…
Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes through its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptical biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, …
Bayesian Analysis for Remote Biosignature Identification on exoEarths (BARBIE) IV: Analyzing CO2 Detections in the Near-IR to Determine the Long-Wavelength Cut-off for the Habitable Worlds Observatory Coronagraph
NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；Sellers Exoplanets Environment Collaboration（塞勒斯系外行星环境合作组织）；Southeastern Universities Research Association（东南大学研究协会）；George Mason University（乔治梅森大学）；Morgan State University（摩根州立大学）
Photosynthesis is an ancient metabolic process that began on the early Earth, offering plentiful energy to organisms that utilize it, to the extent that they can achieve global significance. The potential exists for similar processes to operate on habitable exoplanets and result in observable biosignatures. Prior to the advent of oxygenic photosynthesis, the most primitive phototrophs, anoxygenic phototrophs, dominated surface environments on the planet. Here, we characterize surface polarization biosignatures associated with a diverse sample of anoxygenic phototrophs and cyanobacteria, examin…
The ancestor philosophers' dream of thousands of new worlds is finally realised: about 3500 extrasolar planets have been discovered in the neighborhood of our Sun. Most of them are very different from those we used to know in our Solar System. Others orbit their parent star inside the belt known as Habitable Zone where a rocky planet with the appropriate climate could have the availability of liquid water on its surface. Those planets, in HZ or not, will be the object of observation that will be performed by new space-/ground-based instrumentation. Space missions, such as JWST and the very rec…
Atmospheric temperature and mixing ratio profiles of terrestrial planets vary with the spectral energy flux distribution for different types of M-dwarf stars and the planetary gravity. We investigate the resulting effects on the spectral appearance of molecular absorption bands, that are relevant as indicators for potential planetary habitability during primary and secondary eclipse for transiting terrestrial planets with Earth-like biomass emissions. Atmospheric profiles are computed using a plane-parallel, 1D climate model coupled with a chemistry model. We then calculate simulated spectra u…
The history of the Earth has been marked by major ecological transitions, driven by metabolic innovation, that radically reshaped the composition of the oceans and atmosphere. The nature and magnitude of the earliest transitions, hundreds of million years before photosynthesis evolved, remain poorly understood. Using a novel ecosystem-planetary model, we find that pre-photosynthetic methane-cycling microbial ecosystems are much less productive than previously thought. In spite of their low productivity, the evolution of methanogenic metabolisms strongly modifies the atmospheric composition, le…
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）
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 investigate atmospheric responses of modeled hypothetical Earth-like planets in the habitable zone of the M-dwarf AD Leonis to reduced oxygen (O2), removed biomass (dead Earth), varying carbon dioxide (CO2) and surface relative humidity (sRH). Results suggest large O2 differences between the reduced O2 and dead scenarios in the lower but not the upper atmosphere. Ozone (O3) and nitrous oxide (N2O) also show this behavior. Methane depends on hydroxyl (OH), its main sink. Abiotic production of N2O occurs in the upper layers. Chloromethane (CH3Cl) decreases everywhere on decreasing biomass. Ch…
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…
Recently, phosphine was discovered in the atmosphere of Venus as a potential biosignature. This raises the question: if Venusian life exists, could it be related to terrestrial life? Based on the known rate of meteoroid impacts on Earth, we show that at least $\sim 6 \times 10^5$ asteroids have grazed Earth's atmosphere without being significantly heated and later impacted Venus, and a similar number have grazed Venus's atmosphere and later impacted the Earth, both within a period of $\sim 10^5$ years during which microbes could survive in space. Although the abundance of terrestrial life in t…
Motility is a ubiquitous feature of microbial life on Earth, and is widely regarded as a promising biosignature candidate. In the search for motile organisms, it is therefore valuable to have rough estimates for the number of such microbes that one may expect to find in a given area or volume. In this work, we explore this question by employing a simple theoretical model that takes into account the amount of free energy available in a given environment and the energetic cost of motility. We present heuristic upper bounds for the average biomass density and the number density of motile lifeform…
We use the critical step model to study the major transitions in evolution on Earth. We find that a total of five steps represents the most plausible estimate, in agreement with previous studies, and use the fossil record to identify the potential candidates. We apply the model to Earth-analogs around stars of different masses by incorporating the constraints on habitability set by stellar physics including the habitable zone lifetime, availability of ultraviolet radiation for prebiotic chemistry, and atmospheric escape. The critical step model suggests that the habitability of Earth-analogs a…
The search for signs of life through the detection of exoplanet atmosphere biosignature gases is gaining momentum. Yet, only a handful of rocky exoplanet atmospheres are suitable for observation with planned next-generation telescopes. To broaden prospects, we describe the possibilities for an aerial, liquid water cloud-based biosphere in the atmospheres of sub Neptune-sized temperate exoplanets, those receiving Earth-like irradiation from their host stars. One such planet is known (K2-18b) and other candidates are being followed up. Sub Neptunes are common and easier to study observationally …
Theory and observation for the search for life on exoplanets via atmospheric "biosignature gases" is accelerating, motivated by the capabilities of the next generation of space- and ground-based telescopes. The most observationally accessible rocky planet atmospheres are those dominated by molecular hydrogen gas, because the low density of H$_2$-gas leads to an expansive atmosphere. The capability of life to withstand such exotic environments, however, has not been tested in this context. We demonstrate that single-celled microorganisms ($\textit{E. coli}$ and yeast) that normally do not inhab…
The current explosion in detection and characterization of thousands of extrasolar planets from the Kepler mission, the Hubble Space Telescope, and large ground-based telescopes opens a new era in searches for Earth-analog exoplanets with conditions suitable for sustaining life. As more Earth-sized exoplanets are detected in the near future, we will soon have an opportunity to identify habitable worlds. Which atmospheric biosignature gases from habitable planets can be detected with our current capabilities? The detection of the common biosignatures from nitrogen-oxygen rich terrestrial-type e…
M-dwarf stars are generally considered favourable for rocky planet detection. However, such planets may be subject to extreme conditions due to possible high stellar activity. The goal of this work is to determine the potential effect of stellar cosmic rays on key atmospheric species of Earth-like planets orbiting in the habitable zone of M-dwarf stars and show corresponding changes in the planetary spectra. We build upon the cosmic rays model scheme of Grenfell et al. (2012), who considered cosmic ray induced NOx production, by adding further cosmic ray induced production mechanisms (e.g. for…
In the modern search for life elsewhere in the Universe, we are broadly looking for the following: the planets similar to Earth - physical indicators of habitability, and the manifestation of life - the biological signatures. A biosignature is a measured parameter that has a high probability of being caused by the living organisms, either atmospheric gas species or some surface features. Therefore, the focus of a search is on a product or phenomena produced by the living systems, mostly by microorganisms as these are the most abundant on our planet like, say, methane. However, we may need to d…
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 s…
Context. The temperate Earth-mass planet Proxima b is the closest exoplanet to Earth and represents what may be our best ever opportunity to search for life outside the Solar System. Aims. We aim at directly detecting Proxima b and characterizing its atmosphere by spatially resolving the planet and obtaining high-resolution reflected-light spectra. Methods. We propose to develop a coupling interface between the SPHERE high-contrast imager and the new ESPRESSO spectrograph, both installed at ESO VLT. The angular separation of 37 mas between Proxima b and its host star requires the use of visibl…
