We describe how environmental context can help determine whether oxygen (O
) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, ...interdisciplinary example of O
biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O
is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O
faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O
without life being present. Consequently, our ability to interpret both the presence and absence of O
in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O
release into the atmosphere for several billion years, producing a false negative for biologically generated O
. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O
, planetary processes that may generate abundant atmospheric O
without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O
and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O
is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.
ABSTRACT
Atmospheric temperatures are to be estimated from thermal emission spectra of Earth-like exoplanets orbiting M-stars as observed by current and future planned missions. To this end, a ...line-by-line radiative transfer code is used to generate synthetic thermal infrared (TIR) observations. The range of ‘observed’ intensities provides a rough hint of the atmospheric temperature range without any a priori knowledge. The equivalent brightness temperature (related to intensities by Planck’s function) at certain wavenumbers can be used to estimate the atmospheric temperature at corresponding altitudes. To exploit the full information provided by the measurement we generalize Chahine’s original approach and infer atmospheric temperatures from all spectral data using the wavenumber-to-altitude mapping defined by the weighting functions. Chahine relaxation allows an iterative refinement of this ‘first guess’. Analysis of the 4.3 and $15\rm \, \mu m$ carbon dioxide TIR bands enables an estimate of atmospheric temperatures for rocky exoplanets even for low signal-to-noise ratios of 10 and medium resolution. Inference of Trappist-1e temperatures is, however, more challenging especially for CO2 dominated atmospheres: the ‘standard’ 4.3 and $15\rm \, \mu m$ regions are optically thick and an extension of the spectral range towards atmospheric window regions is important. If atmospheric composition (essentially CO2 concentration) is known temperatures can be estimated remarkably well; quality measures such as the residual norm provide hints on incorrect abundances. In conclusion, temperature in the mid atmosphere of Earth-like planets orbiting cooler stars can be quickly estimated from thermal IR emission spectra with moderate resolution.
•Determination of all significant O3 producing and consuming pathways and quantification of their contributions in the Martian atmosphere with help of an automated computer algorithm.•O3 production ...results from CO2 and O2 photolysis.•O3 is consumed by catalytic cycles involving HOx (= H + OH + HO2).•The Martian atmosphere can be divided into two chemically distinct re- gions according to the O(3P):O3 ratio.•Vertical transport of O(3P) from upper layers downwards into the O3 layer at around 50 km altitude provides an additional source of Ox (= O + O3), which is pivotal to the formation of the Martian O3 volume mixing ratio maximum.
Atmospheric chemical composition is crucial in determining a planet’s atmospheric structure, stability, and evolution. Attaining a quantitative understanding of the essential chemical mechanisms governing atmospheric composition is nontrivial due to complex interactions between chemical species. Trace species, for example, can participate in catalytic cycles – affecting the abundance of major and other trace gas species. Specifically, for Mars, such cycles dictate the abundance of its primary atmospheric constituent, carbon dioxide (CO2), but also for one of its trace gases, ozone (O3). The identification of chemical pathways/cycles by hand is extremely demanding; hence, the application of numerical methods, such as the Pathway Analysis Program (PAP), is crucial to analyze and quantitatively exemplify chemical reaction networks. Here, we carry out the first automated quantitative chemical pathway analysis of Mars’ atmosphere with respect to O3. PAP was applied to JPL/Caltech’s 1-D updated photochemical Mars model’s output data. We determine all significant chemical pathways and their contribution to O3 production and consumption (up to 80 km) in order to investigate the mechanisms causing the characteristic shape of the O3 volume mixing ratio profile, i.e. a ground layer maximum and an ozone layer at ∼50 km. These pathways explain why an O3 layer is present, why it is located at that particular altitude and what the different processes forming the near-surface and middle atmosphere O3 maxima are. Furthermore, we show that the Martian atmosphere can be divided into two chemically distinct regions according to the O(3P):O3 ratio. In the lower region (below approximately 24 km altitude) O3 is the most abundant Ox (= O3 + O(3P)) species. In the upper region (above approximately 24 km altitude), where the O3 layer is located, O(3P) is the most abundant Ox species. Earlier results concerning the formation of O3 on Mars can now be explained with the help of chemical pathways leading to a better understanding of the vertical O3 profile.
After Earth's origin, our host star, the Sun, was shining 20-25% less brightly than today. Without greenhouse-like conditions to warm the atmosphere, our early planet would have been an ice ball, and ...life may never have evolved. But life did evolve, which indicates that greenhouse gases must have been present on early Earth to warm the planet. Evidence from the geological record indicates an abundance of the greenhouse gas CO(2). CH(4) was probably present as well; and, in this regard, methanogenic bacteria, which belong to a diverse group of anaerobic prokaryotes that ferment CO(2) plus H(2) to CH(4), may have contributed to modification of the early atmosphere. Molecular oxygen was not present, as is indicated by the study of rocks from that era, which contain iron carbonate rather than iron oxide. Multicellular organisms originated as cells within colonies that became increasingly specialized. The development of photosynthesis allowed the Sun's energy to be harvested directly by life-forms. The resultant oxygen accumulated in the atmosphere and formed the ozone layer in the upper atmosphere. Aided by the absorption of harmful UV radiation in the ozone layer, life colonized Earth's surface. Our own planet is a very good example of how life-forms modified the atmosphere over the planets' lifetime. We show that these facts have to be taken into account when we discover and characterize atmospheres of Earth-like exoplanets. If life has originated and evolved on a planet, then it should be expected that a strong co-evolution occurred between life and the atmosphere, the result of which is the planet's climate.
► The first automated quantified chemical pathway analysis of the martian atmosphere with respect to CO2 is presented. ► All dominant pathways related to CO2-production have been quantified as a ...function of altitude. ► Their contributions to the atmospheric CO2 abundance of individual pathways vary considerably with altitude. ► Results endorse the importance of transport processes in governing the stability of CO2 in the martian atmosphere. ► An unknown chemical pathway contributing approximately 8% to global CO2-production has been identified.
The chemical composition of a planetary atmosphere plays an important role for atmospheric structure, stability, and evolution. Potentially complex interactions between chemical species do not often allow for an easy understanding of the underlying chemical mechanisms governing the atmospheric composition. In particular, trace species can affect the abundance of major species by acting in catalytic cycles. On Mars, such cycles even control the abundance of its main atmospheric constituent CO2. The identification of catalytic cycles (or more generally chemical pathways) by hand is quite demanding. Hence, the application of computer algorithms is beneficial in order to analyze complex chemical reaction networks. Here, we have performed the first automated quantified chemical pathways analysis of the Martian atmosphere with respect to CO2-production in a given reaction system. For this, we applied the Pathway Analysis Program (PAP) to output data from the Caltech/JPL photochemical Mars model. All dominant chemical pathways directly related to the global CO2-production have been quantified as a function of height up to 86km. We quantitatively show that CO2-production is dominated by chemical pathways involving HOx and Ox. In addition, we find that NOx in combination with HOx and Ox exhibits a non-negligible contribution to CO2-production, especially in Mars’ lower atmosphere. This study reveals that only a small number of chemical pathways contribute significantly to the atmospheric abundance of CO2 on Mars; their contributions to CO2-production vary considerably with altitude. This analysis also endorses the importance of transport processes in governing CO2-stability in the Martian atmosphere. Lastly, we identify a previously unknown chemical pathway involving HOx, Ox, and HO2-photodissociation, contributing 8% towards global CO2-production by chemical pathways using recommended up-to-date values for reaction rate coefficients.
Catalytic cycles and other chemical pathways affecting ozone are normally estimated empirically in atmospheric models. In this work we have automatically quantified such processes by applying a newly ...developed analysis package called the “Pathway Analysis Program” (PAP). It used modeled chemical rates and concentrations as input. These were supplied by the “Module Efficiently Calculating the Chemistry of the Atmosphere” MECCA box model, itself initialized by the Free University of Berlin Climate Middle Atmosphere Model with Chemistry. We analyzed equatorial, midlatitude and high‐latitude locations over 24‐hour periods during spring in both hemispheres. We present results for locations in the lower stratosphere, upper stratosphere and midmesosphere. Oxygen photolysis dominated (>99%) in situ ozone production in the equatorial lower stratosphere, in the upper stratosphere and in the mesosphere. In the lower stratosphere midlatitudes the “ozone smog cycle” (already established in the troposphere) rivaled oxygen photolysis as an in situ ozone source in both hemispheres. However, absolute ozone production rates in midlatitudes were rather slow compared with at the equator, typically 16–50 ppt ozone/day. In the equatorial lower stratosphere, five catalytic sinks were important (each contributing at least 5% to chemical ozone loss): a HOx cycle, a HOBr cycle and its HOCl analog, a water‐HOx cycle and a mixed chlorine‐bromine cycle. Important in midlatitudes were the HOx cycle, a NOx cycle, the HOBr cycle and the mixed chlorine‐bromine cycle. In lower‐stratosphere high latitudes, the chlorine dimer cycle and the mixed chlorine‐bromine cycle dominated in both hemispheres. A variant on the latter, involving BrCl formation, also featured. In the upper stratosphere high latitudes (where strong negative ozone trends are observed), a nitrogen cycle, a chlorine cycle, and a mixed chlorine‐nitrogen cycle were found. In the mesosphere, three closely related HOx cycles dominated ozone loss.
The search for life beyond the Solar System is a major activity in exoplanet science. However, even if an Earth-like planet were to be found, it is unlikely to be at a similar stage of evolution as ...the modern Earth. It is therefore of interest to investigate the sensitivity of biomarker signals for life as we know it for an Earth-like planet but at earlier stages of evolution. Here, we assess biomarkers, i.e. species almost exclusively associated with life, in present-day and in 10% present atmospheric level oxygen atmospheres corresponding to the Earth’s Proterozoic period. We investigate the impact of proposed enhanced microbial emissions of the biomarker nitrous oxide, which photolyses to form nitrogen oxides which can destroy the biomarker ozone. A major result of our work is regardless of the microbial activity producing nitrous oxide in the early anoxic ocean, a certain minimum ozone column can be expected to persist in Proterozoic-type atmospheres due to a stabilising feedback loop between ozone, nitrous oxide and the ultraviolet radiation field. Atmospheric nitrous oxide columns were enhanced by a factor of 51 for the Proterozoic “Canfield ocean” scenario with 100 times increased nitrous oxide surface emissions. In such a scenario nitrous oxide displays prominent spectral features, so may be more important as a biomarker than previously considered in such cases. The run with “Canfield ocean” nitrous oxide emissions enhanced by a factor of 100 also featured additional surface warming of 3.5
K. Our results suggest that the Proterozoic ozone layer mostly survives the changes in composition which implies that it is indeed a good atmospheric biomarker.
Context. In recent years, several potentially habitable, probably terrestrial exoplanets and exoplanet candidates have been discovered. The amount of CO2 in their atmosphere is of great importance ...for surface conditions and habitability. In the absence of detailed information on the geochemistry of the planet, this amount could be considered as a free parameter. Aims. Up to now, CO2 partial pressures for terrestrial planets have been obtained assuming an available volatile reservoir and outgassing scenarios. This study aims at calculating the allowed maximum CO2 pressure at the surface of terrestrial exoplanets orbiting near the outer boundary of the habitable zone by coupling the radiative effects of the CO2 and its condensation at the surface. These constraints might limit the permitted amount of atmospheric CO2, independent of the planetary reservoir. Methods. A 1D radiative-convective cloud-free atmospheric model was used to calculate surface conditions for hypothetical terrestrial exoplanets. CO2 partial pressures are fixed according to surface temperature and vapor pressure curve. Considered scenarios cover a wide range of parameters, such as gravity, central star type and orbital distance, atmospheric N2 content and surface albedo. Results. Results show that for planets in the habitable zone around K-, G-, and F-type stars the allowed CO2 pressure is limited by the vapor pressure curve and not by the planetary reservoir. The maximum CO2 pressure lies below the CO2 vapor pressure at the critical point of pcrit = 73.8 bar. For M-type stars, due to the stellar spectrum being shifted to the near-IR, CO2 pressures above pcrit are possible for almost all scenarios considered across the habitable zone. This implies that determining CO2 partial pressures for terrestrial planets by using only geological models is probably too simplified and might over-estimate atmospheric CO2 towards the outer edge of the habitable zone.
Context. Were a potentially habitable planet to be discovered, the next step would be the search for an atmosphere and its characterization. Eventually, surface conditions, hence habitability, and ...biomarkers as indicators for life would be assessed. Aims. The super-Earth candidate Gliese (GL) 581 d is the first potentially habitable extrasolar planet so far discovered. Therefore, GL 581 d is used to illustrate a hypothetical detailed spectroscopic characterization of such planets. Methods. Atmospheric profiles for a wide range of possible one-dimensional (1D) radiative-convective model scenarios of GL 581 d were used to calculate high-resolution synthetic emission and transmission spectra. Atmospheres were assumed to be composed of N2, CO2, and H2O. From the spectra, signal-to-noise ratios (SNRs) were calculated for a telescope such as the planned James Webb Space Telescope (JWST). Exposure times were set to be equal to the duration of one transit. Results. The presence of the model atmospheres can be clearly inferred from the calculated synthetic spectra thanks to strong water and carbon-dioxide absorption bands. Surface temperatures can be inferred for model scenarios with optically thin spectral windows. Dense, CO2-rich (potentially habitable) scenarios do not enable us to determine the surface temperatures nor assess habitability. Degeneracies between CO2 concentration and surface pressure complicate the interpretation of the calculated spectra, hence the determination of atmospheric conditions. Still, inferring approximative CO2 concentrations and surface pressures is possible. In practice, detecting atmospheric signals is challenging because the calculated SNR values are well below unity in most of the cases. The SNR for a single transit was only barely larger than unity in some near-IR bands for transmission spectroscopy. Most interestingly, the false-positive detection of biomarker candidates such as methane and ozone might be possible in low resolution spectra because CO2 absorption bands overlap biomarker spectral bands. This can be avoided, however, by observing all main CO2 IR bands instead of concentrating on, e.g., the 4.3 or 15 μm bands only. Furthermore, a masking of ozone signatures by CO2 absorption bands is shown to be possible. Simulations imply that such a false-negative detection of ozone would be possible even for rather high ozone concentrations of up to 10-5.
A tropospheric chemistry model has been developed within the Goddard Institute for Space Studies general circulation model (GCM) to study interactions between chemistry and climate change. The model ...uses simplified chemistry based on CO‐NOx‐HOx‐Ox‐CH4 and also includes a parameterization for isoprene emissions, the most important non‐methane hydrocarbon. The model reproduces observed annual cycles and mean distributions of key trace gases fairly well. It simulates preindustrial to present‐day changes similar to those seen in other simulations. For example, the global tropospheric ozone burden increases 45%, within the 25%–57% range of other studies. Annual average zonal mean surface ozone increases more than 125% at northern midlatitudes. Comparison between runs that allow calculated ozone to interact with the GCM and those that do not shows only minor ozone differences. The common usage of non‐interactive ozone seems adequate to simulate ozone distributions. However, use of coupled chemistry does alter the tropospheric oxidation capacity, enlarging the preindustrial to present‐day OH decrease by about 10% (−5.3% global annual average uncoupled, −5.9% coupled). Thus simulation of changes in oxidation capacity may be systematically biased (though a 10% difference is within the uncertainty). Global annual average radiative forcing from preindustrial to present‐day ozone change is 0.32 W m−2. The forcing seems to increase by ∼10% with coupled chemistry. Forcing greater than 0.8 W m−2 is seen over the United States, the Mediterranean area, central Asia, and the Arctic, with values greater than 1.5 W m−2 over parts of these areas during summer. Though there are local differences, the radiative forcing is overall in good agreement with other modeling studies in both magnitude and spatial distribution, demonstrating that the simplified chemistry is adequate for climate studies.
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