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Register a new userEvolution of Earth-like extrasolar planetary atmospheres: Assessing the atmospheres and biospheres of early Earth analog planets with a coupled atmosphere biogeochemical model
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Understanding the evolution of Earth and potentially habitable Earth-like worlds is essential to fathom our origin in the Universe. The search for Earth-like planets in the habitable zone and investigation of their atmospheres with climate and photochemical models is a central focus in exoplanetary science. Taking the evolution of Earth as a reference for Earth-like planets, a central scientific goal is to understand what the interactions were between atmosphere, geology, and biology on early Earth. The Great Oxidation Event (GOE) in Earths history was certainly caused by their interplay, but the origin and controlling processes of this occurrence are not well understood, the study of which will require interdisciplinary, coupled models. In this work, we present results from our newly developed Coupled Atmosphere Biogeochemistry model in which atmospheric O$_2$ concentrations are fixed to values inferred by geological evidence. Applying a unique tool, ours is the first quantitative analysis of catalytic cycles that governed O$_2$ in early Earths atmosphere near the GOE. Complicated oxidation pathways play a key role in destroying O$_2$, whereas in the upper atmosphere, most O$_2$ is formed abiotically via CO$_2$ photolysis. The O$_2$ bistability found by Goldblatt et al. (2006) is not observed in our calculations likely due to our detailed CH$_4$ oxidation scheme. We calculate increased CH$_4$ with increasing O$_2$ during the GOE. For a given atmospheric surface flux, different atmospheric states are possible; however, the net primary productivity (NPP) of the biosphere that produces O$_2$ is unique. Mixing, CH$_4$ fluxes, ocean solubility, and mantle/crust properties strongly affect NPP and surface O$_2$ fluxes. Regarding exoplanets, different states of O$_2$ could exist for similar biomass output. Strong geological activity could lead to false negatives for life.
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Earth-like planets orbiting M-dwarfs are prominent future targets when searching for life outside the solar system. We apply our newly developed Coupled Atmosphere Biogeochemistry model to investigate the coupling between the biosphere, geosphere and atmosphere to gain deeper insight into the atmospheric evolution of Earth-like planets orbiting M-dwarfs. Our main goal is to understand better atmospheric processes affecting biosignatures and climate on such worlds. Furthermore, this is the first study to our knowledge which applies an automated chemical pathway analysis quantifying the production and destruction pathways of O$_2$ for an Earth-like planet with an Archean O$_2$ abundance orbiting in the habitable zone of the M-dwarf AD Leo. Results suggest that the main production arises in the upper atmosphere from CO$_2$ photolysis followed by catalytic HO$_x$ reactions. The strongest destruction does not take place in the troposphere, as was the case in Gebauer et al. (2017) for an early-Earth analog planet around the Sun, but instead in the middle atmosphere where H$_2$O photolysis is the strongest. This result was driven by the strong Lyman-$alpha$-radiation output of AD Leo, which efficiently photolyzes H$_2$O. Results further suggest that early Earth-like atmospheres of planets orbiting an M-dwarf like AD Leo are in absolute terms less destructive for atmospheric O$_2$ than for early-Earth analog planets around the Sun despite higher concentrations of reduced gases such as e.g. H$_2$, CH$_4$ and CO. Hence the net primary productivity (NPP) required to produce the same amount of atmospheric O$_2$ at the surface is reduced. This implies that a possible Great Oxidation event, analogous to that on Earth, would have occurred earlier in time in analog atmospheres around M-dwarfs.
The habitable zone (HZ) describes the range of orbital distances around a star where the existence of liquid water on the surface of an Earth-like planet is in principle possible. While 3D climate studies can calculate the water vapor, ice albedo, and cloud feedback self-consistently and therefore allow for a deeper understanding and the identification of relevant climate processes, 1D model studies rely on fewer model assumptions and can be more easily applied to the large parameter space possible for exoplanets. We evaluate the applicability of 1D climate models to estimate the potential habitability of Earth-like exoplanets by comparing our 1D model results to those of 3D climate studies in the literature. We applied a cloud-free 1D radiative-convective climate model to calculate the climate of Earth-like planets around different types of main-sequence stars with varying surface albedo and relative humidity profile. These parameters depend on climate feedbacks that are not treated self-consistently in most 1D models. We compared the results to those of 3D model calculations in the literature and investigated to what extent the 1D model can approximate the surface temperatures calculated by the 3D models. The 1D parameter study results in a large range of climates possible for an Earth-sized planet with an Earth-like atmosphere and water reservoir at a certain stellar insolation. At some stellar insolations the full spectrum of climate states could be realized, i.e., uninhabitable conditions as well as habitable surface conditions, depending only on the relative humidity and surface albedo assumed. When treating the surface albedo and the relative humidity profile as parameters in 1D model studies and using the habitability constraints found by recent 3D modeling studies, the same conclusions about the potential habitability of a planet can be drawn as from 3D model calculations.
The origin of life on Earth seems to demand a highly reduced early atmosphere, rich in CH4, H2, and NH3, but geological evidence suggests that Earths mantle has always been relatively oxidized and its emissions dominated by CO2 H2O, and N2. The paradox can be resolved by exploiting the reducing power inherent in the late veneer, i.e., material accreted by Earth after the Moon-forming impact. Isotopic evidence indicates that the late veneer consisted of extremely dry, highly reduced inner solar system materials, suggesting that Earths oceans were already present when the late veneer came. The major primary product of reaction between the late veneers iron and Earths water was H2. Ocean vaporizing impacts generate high pressures and long cooling times that favor CH4 and NH3. Impacts too small to vaporize the oceans are much less productive of CH4 and NH3, unless (i) catalysts were available to speed their formation, or (ii) additional reducing power was extracted from pre-existing crustal or mantle materials. The transient H2-CH4 atmospheres evolve photochemically to generate nitrogenated hydrocarbons at rates determined by solar radiation and hydrogen escape, on timescales ranging up to tens of millions of years and with cumulative organic production ranging up to half a kilometer. Roughly one ocean of hydrogen escapes. The atmosphere after the methanes gone is typically H2 and CO rich, with eventual oxidation to CO2 rate-limited by water photolysis and hydrogen escape.
A radiative-convective climate model is used to calculate stratospheric temperatures and water vapor concentrations for ozone-free atmospheres warmer than that of modern Earth. Cold, dry stratospheres are predicted at low surface temperatures, in agreement with recent 3-D calculations. However, at surface temperatures above 350 K, the stratosphere warms and water vapor becomes a major upper atmospheric constituent, allowing water to be lost by photodissociation and hydrogen escape. Hence, a moist greenhouse explanation for loss of water from Venus, or some exoplanet receiving a comparable amount of stellar radiation, remains a viable hypothesis. Temperatures in the upper parts of such atmospheres are well below those estimated for a gray atmosphere, and this factor should be taken into account when performing inverse climate calculations to determine habitable zone boundaries using 1-D models.
We study the influence of low-level water and high-level ice clouds on low-resolution reflection spectra and planetary albedos of Earth-like planets orbiting different types of stars in both the visible and near infrared wavelength range. We use a one-dimensional radiative-convective steady-state atmospheric model coupled with a parametric cloud model, based on observations in the Earths atmosphere to study the effect of both cloud types on the reflection spectra and albedos of Earth-like extrasolar planets at low resolution for various types of central stars. We find that the high scattering efficiency of clouds substantially causes both the amount of reflected light and the related depths of the absorption bands to be substantially larger than in comparison to the respective clear sky conditions. Low-level clouds have a stronger impact on the spectra than the high-level clouds because of their much larger scattering optical depth. The detectability of molecular features in near the UV - near IR wavelength range is strongly enhanced by the presence of clouds. However, the detectability of various chemical species in low-resolution reflection spectra depends strongly on the spectral energy distribution of the incident stellar radiation. In contrast to the reflection spectra the spectral planetary albedos enable molecular features to be detected without a direct influence of the spectral energy distribution of the stellar radiation. Here, clouds increase the contrast between the radiation fluxes of the planets and the respective central star by about one order of magnitude, but the resulting contrast values are still too low to be observable with the current generation of telescopes.
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