Carbonate-Silicate Cycle Predictions of Earth-like Planetary Climates and Testing the Habitable Zone Concept
Owen R. Lehmer, David C. Catling, Joshua Krissansen-Totton
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Published 12/1/2020 in Nature Communications. The supplemental data and code referenced in this work are available from the Nature Communications website (see DOI below) or by email from the authors
In the conventional habitable zone (HZ) concept, a CO$_{2}$-H$_2$O greenhouse maintains surface liquid water. Through the water-mediated carbonate-silicate weathering cycle, atmospheric CO$_{2}$ partial pressure (pCO$_{2}$) responds to changes in surface temperature, stabilizing the climate over geologic timescales. We show that this weathering feedback ought to produce a log-linear relationship between pCO$_{2}$ and incident flux on Earth-like planets in the HZ. However, this trend has scatter because geophysical and physicochemical parameters can vary, such as land area for weathering and CO$_2$ outgassing fluxes. Using a coupled climate and carbonate-silicate weathering model, we quantify the likely scatter in pCO$_2$ with orbital distance throughout the HZ. From this dispersion, we predict a two-dimensional relationship between incident flux and pCO$_2$ in the HZ and show that it could be detected from at least 83 ($2σ$) Earth-like exoplanet observations. If fewer Earth-like exoplanets are observed, testing the HZ hypothesis from this relationship could be difficult.
We have produced synthetic analogues of cosmic silicates using the Sol Gel method, producing amorphous silicates of composition Mg(x)Ca(1-x)SiO3. Using synchrotron X-ray powder diffraction on Beamline I11 at the Diamond Light Source, together with a newly-commissioned gas cell, real-time powder diffraction scans have been taken of a range of silicates exposed to CO2 under non-ambient conditions. The SXPD is complemented by other techniques including Raman and Infrared Spectroscopy and SEM imaging.
The habitable zone for exomoons with Earth-like properties is a non-trivial manifold, compared to that of Earth-like exoplanets. The presence of tidal heating, eclipses and planetary illumination in the exomoon energy budget combine to produce both circumstellar and circumplanetary habitable regions. Analytical calculations suggest that the circumplanetary habitable region is defined only by an inner edge (with its outer limits determined by orbital stability). Subsequent calculations using 1D latitudinal climate models indicated that the combined effect of eclipses and ice-albedo feedback can…
For decades, scientists have tried to explain the evidence for fluvial activity on early Mars, but a consensus has yet to emerge regarding the mechanism for producing it. One hypothesis suggests early Mars was warmed by a thick greenhouse atmosphere. Another suggests that early Mars was generally cold but was warmed occasionally by impacts or by episodes of enhanced volcanism. These latter hypotheses struggle to produce the amounts of rainfall needed to form the martian valleys, but are consistent with inferred low rates of weathering compared to Earth. Here, we provide a geophysical mechanism…
Little is known about the early evolution of Venus and a potential habitable period during the first one billion years. In particular, it remains unclear whether or not plate tectonics and an active carbonate-silicate cycle were present. In the presence of liquid water but without plate tectonics, weathering would have been limited to freshly produced basaltic crust, with an early carbon cycle restricted to the crust and atmosphere. With the evaporation of surface water, weathering would cease. With ongoing volcanism, carbonate sediments would be buried and sink downwards. Thereby, carbonates …
The potential habitability of an exoplanet is traditionally assessed by determining if its orbit falls within the circumstellar `habitable zone' of its star, defined as the distance at which water could be liquid on the surface of a planet (Kopparapu et al., 2013). Traditionally, these limits are determined by radiative-convective climate models, which are used to predict surface temperatures at user-specified levels of greenhouse gases. This approach ignores the vital question of the (bio)geochemical plausibility of the proposed chemical abundances. Carbon dioxide is the most important greenh…
The carbon-silicate cycle regulates the atmospheric $CO_2$ content of terrestrial planets on geological timescales through a balance between the rates of $CO_2$ volcanic outgassing and planetary intake from rock weathering. It is thought to act as an efficient climatic thermostat on Earth and, by extension, on other habitable planets. If, however, the weathering rate increases with the atmospheric $CO_2$ content, as expected on planets lacking land vascular plants, the carbon-silicate cycle feedback can become severely limited. Here we show that Earth-like planets receiving less sunlight than …
Carbonate precipitation in oceans is essential for the carbonate-silicate cycle (inorganic carbon cycle) to maintain temperate climates. By considering the thermodynamics of carbonate chemistry, we demonstrate that the ocean pH decreases by approximately 0.5 for a factor of 10 increase in the atmospheric carbon dioxide content. The upper and lower limits of ocean pH are within 1-4 of each other, where the upper limit is buffered by carbonate precipitation and defines the ocean pH when the carbon cycle operates. If the carbonate compensation depth (CCD) resides above the ocean floor, then carbo…
The nearly logarithmic radiative impact of CO$_2$ means that planets near the outer edge of the liquid water habitable zone (HZ) require $\sim$10$^6$x more CO$_2$ to maintain temperatures conducive to standing liquid water on the planetary surface than their counterparts near the inner edge. This logarithmic radiative response also means that atmospheric CO$_2$ changes of a given mass will have smaller temperature effects on higher pCO$_2$ planets. Ocean pH is linked to atmospheric pCO$_2$ through seawater carbonate speciation and calcium carbonate dissolution/precipitation, and the response o…
Gl581d has a minimum mass of 7 MEarth and is the first detected potentially habitable rocky Super-Earth. Our models confirm that a habitable atmosphere can exist on Gl581d. We derive spectroscopic features for atmospheres, assuming an Earth-like composition for this planet, from high oxygen atmosphere analogous to Earth's to high CO2 atmospheres with and without biotic oxygen concentrations. We find that a minimum CO2 partial pressure of about 7 bar, in an atmosphere with a total surface pressure of 7.6 bar, are needed to maintain a mean surface temperature above freezing on Gl581d. We model t…
The NASA/Dawn mission has acquired an unprecedented amount of data from the surface of the dwarf planet Ceres, providing a thorough characterization of its surface composition. The current favorite compositional model includes a mixture of ultra-carbonaceous material, phyllosilicates, carbonates, organic, Fe-oxides, and volatiles, as determined by the Dawn/VIR-IR imaging spectrometer, with the constraints on the elemental composition provided by the Dawn/GRAND instrument. The recent calibration of the VIS channel of the VIR spectrometer provided further constraints on the overall compositional…
The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, ar…
The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can maintain above-freezing conditions through regulation by the carbonate-silicate cycle. Recent calculations have suggested that planets in the outer regions of the habitable zone cannot maintain stable, warm climates, but rather should oscillate between long, globally glaciated states and shorter periods of climatic warmth. Such conditions, similar to 'Snowball Earth' episodes experienced on Earth, would be inimical to the development of complex land life, including intelligent life. Here, we b…
Ocean planets are volatile rich planets, not present in our Solar System, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atm…
Planetary rotation rate has a significant effect on atmospheric circulation, where the strength of the Coriolis effect in part determines the efficiency of latitudinal heat transport, altering cloud distributions, surface temperatures, and precipitation patterns. In this study we use the ROCKE-3D dynamic-ocean general circulation model to study the effects of slow rotations and increased insolations on the 'fractional habitability' and silicate weathering rate of an Earth-like world. Defining the fractional habitability f_h to be the percentage of a planet's surface which falls in the 0 <= T <…
The habitable zone concept is important because it focuses the scientific search for extraterrestrial life and aids the planning of future telescopes. Recent work has shown that planets near the outer edge of the habitable zone might not actually be able to stay warm and habitable if CO$_2$ outgassing rates are not large enough to maintain high CO$_2$ partial pressures against removal by silicate weathering. In this paper I use simple equations for the climate and CO$_2$ budget of a planet in the habitable zone that can capture the qualitative behavior of the system. With these equations I der…
It is likely that unambiguous habitable zone terrestrial planets of unknown water content will soon be discovered. Water content helps determine surface land fraction, which influences planetary weathering behavior. This is important because the silicate weathering feedback determines the width of the habitable zone in space and time. Here a low-order model of weathering and climate, useful for gaining qualitative understanding, is developed to examine climate evolution for planets of various land-ocean fractions. It is pointed out that, if seafloor weathering does not depend directly on surfa…
Weathering of silicate rocks at a planetary surface can draw down CO$_2$ from the atmosphere for eventual burial and long-term storage in the planetary interior. This process is thought to provide an essential negative feedback to the carbonate-silicate cycle (carbon cycle) to maintain clement climates on Earth and potentially similar temperate exoplanets. We implement thermodynamics to determine weathering rates as a function of surface lithology (rock type). These rates provide upper limits that allow estimating the maximum rate of weathering in regulating climate. This modeling shows that t…
Ongoing and future space missions aim to identify potentially habitable planets in our Solar System and beyond. Planetary habitability is determined not only by a planet's current stellar insolation and atmospheric properties, but also by the evolutionary history of its climate. It has been suggested that icy planets and moons become habitable after their initial ice shield melts as their host stars brighten. Here we show from global climate model simulations that a habitable state is not achieved in the climatic evolution of those icy planets and moons that possess an inactive carbonate-silic…
The long-term carbon cycle is vital for maintaining liquid water oceans on rocky planets due to the negative climate feedbacks involved in silicate weathering. Plate tectonics plays a crucial role in driving the long-term carbon cycle because it is responsible for CO$_2$ degassing at ridges and arcs, the return of CO$_2$ to the mantle through subduction, and supplying fresh, weatherable rock to the surface via uplift and orogeny. However, the presence of plate tectonics itself may depend on climate according to recent geodynamical studies showing that cool surface temperatures are important fo…
Terrestrial planets covered globally with thick oceans (termed ocean planets) in the habitable zone were previously inferred to have extremely hot climates in most cases. This is because ${\rm H_2O}$ high-pressure (HP) ice on the seafloor prevents chemical weathering and, thus, removal of atmospheric CO$_2$. Previous studies, however, ignored melting of the HP ice and horizontal variation in heat flux from oceanic crusts. Here we examine whether high heat fluxes near the mid-ocean ridge melts the HP ice and thereby removes atmospheric ${\rm CO_2}$. We develop integrated climate models of an Ea…
The "liquid water habitable zone" (HZ) concept is predicated on the ability of the silicate weathering feedback to stabilize climate across a wide range of instellations. However, representations of silicate weathering used in current estimates of the effective outer edge of the HZ do not account for the thermodynamic limit on concentration of weathering products in runoff set by clay precipitation, nor for the energetic limit on precipitation set by planetary instellation. We find that when the thermodynamic limit is included in an idealized coupled climate/weathering model, steady-state plan…
From analytical studies of tidal heating, eclipses and planetary illumination, it is clear that the exomoon habitable zone (EHZ) - the set of moon and host planet orbits that permit liquid water on an Earthlike moon's surface - is a manifold of higher dimension than the planetary HZ. This paper outlines the first attempt to produce climate models of exomoons which possess all the above sources and sinks of energy. We expand on our previous 1D latitudinal energy balance models (LEBMs), which follow the evolution of the temperature on an Earthlike moon orbiting a Jupiterlike planet, by adding pl…
Venus's past climate evolution is uncertain. General circulation model simulations permit a habitable climate as late as ~0.7 Ga, and there is suggestive-albeit inconclusive-evidence for previous liquid water from surface geomorphology and mineralogy. However, it is unclear whether a habitable past can be reconciled with Venus's inferred atmospheric evolution. In particular, the lack of leftover atmospheric oxygen argues against recent water loss. Here, we apply a fully coupled model of Venus's atmospheric-interior-climate evolution from post-accretion magma ocean to present. The model self-co…
Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-silicate cycle that regulates CO2 between the atmosphere, surface, and the interior. Such theories have been used to cast doubt on the habitability of ocean worlds. However, Levi et al (2017) have recently proposed a mechanism by which CO2 is mobilized between the atmosphere and the interior of an ocean world. At high enough CO2 pressures, sea ice can become enriched in CO2 clathrates and sink after a threshold density is achieved. The presence of subpolar sea ice is of great importance for habitabi…
The habitable zone is the region around a star where standing bodies of liquid water can be stable on a planetary surface. Its width is often assumed to be dictated by the efficiency of the carbonate-silicate cycle, which has maintained habitable surface conditions on our planet for billions of years. This cycle may be inhibited by surface condensation of significant amounts of $CO_{\mathrm{2}}$ ice, which is likely to occur on distant planets containing high enough levels of atmospheric $CO_{\mathrm{2}}$. Such a process could permanently trap $CO_{\mathrm{2}}$ ice within the planet, threateni…
With the growing number of spectroscopic observations and observational platforms capable of exoplanet atmospheric characterization, there is a growing need for analysis techniques that can distill information about a large population of exoplanets into a coherent picture of atmospheric trends expressed within the statistical sample. In this work, we develop a Hierarchical Bayesian Atmospheric Retrieval (HBAR) model to infer population-level trends in exoplanet atmospheric characteristics. We demonstrate HBAR on the case of inferring a trend in atmospheric CO2 with incident stellar flux, predi…
We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites and terrestrial planets; we include an updated estimate for the Bulk Silicate Earth (C/N = 49.0 +/- 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macro-molecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical re…
The bulk chemical composition and interior structure of rocky exoplanets are of fundamental importance to understanding their long-term evolution and potential habitability. Observations of the chemical compositions of the solar system rocky bodies and of other planetary systems have increasingly shown a concordant picture that the chemical composition of rocky planets reflects that of their host stars for refractory elements, whereas this expression breaks down for volatiles. This behavior is explained by devolatilization during planetary formation and early evolution. Here, we apply a devola…
In this paper we present a series of models for the deep water cycle on super-Earths experiencing plate tectonics. The deep water cycle can be modeled through parameterized convection models coupled with a volatile recycling model. The convection of the silicate mantle is linked to the volatile cycle through the water-dependent viscosity. Important differences in surface water content are found for different parameterizations of convection. Surface oceans are smaller and more persistent for single layer convection, rather than convection by boundary layer instability. Smaller planets have init…
Although our solar system features predominantly circular orbits, the exoplanets discovered so far indicate that this is the exception rather than the rule. This could have crucial consequences for exoplanet climates, both because eccentric terrestrial exoplanets could have extreme seasonal variation, and because giant planets on eccentric orbits could excite Milankovitch-like variations of a potentially habitable terrestrial planet,Äôs eccentricity, on timescales of thousands-to-millions of years. A particularly interesting implication concerns the fact that the Earth is thought to have gone …
Endogenous or exogenous, dry or wet, various scenarios have been so far depicted for the origin of water on our Solar Systems rocky bodies. Hydrated silicates found in meteorites and in interplanetary dust particles together with observations of abundant water reservoirs in the habitable zone of protoplanetary disks are evidences that support aqueous alteration of silicate dust grains by water vapor condensation in a nebular setting. We investigate the thermodynamics (temperature and pressure dependencies) and kinetics (adsorption rates and energies, surface diffusion and cluster formation) of…
Large terrestrial planets are expected to have muted topography and deep oceans, implying that most super-Earths should be entirely covered in water, so-called waterworlds. This is important because waterworlds lack a silicate weathering thermostat so their climate is predicted to be less stable than that of planets with exposed continents. In other words, the continuously habitable zone for waterworlds is much narrower than for Earth-like planets. A planet's water is partitioned, however, between a surface reservoir, the ocean, and an interior reservoir, the mantle. Plate tectonics transports…
Earth-Like is an interactive website and twitter bot that allows users to explore changes in the average global surface temperature of an Earth-like planet due to variations in the surface oceans and emerged land coverage, rate of volcanism (degassing), and the level of the received solar radiation. The temperature is calculated using a simple carbon-silicate cycle model to change the level of $\rm CO_2$ in the atmosphere based on the chosen parameters. The model can achieve a temperature range exceeding $-100^\circ$C to $100^\circ$C by varying all three parameters, including freeze-thaw cycle…
The geophysics of extrasolar planets is a scientific topic often regarded as standing largely beyond the reach of near-term observations. This reality in no way diminishes the central role of geophysical phenomena in shaping planetary outcomes, from formation, to thermal and chemical evolution, to numerous issues of surface and near-surface habitability. We emphasize that for a balanced understanding of extrasolar planets, it is important to look beyond the natural biases of current observing tools, and actively seek unique pathways to understand exoplanet interiors as best as possible during …
As the number of detected rocky extrasolar planets increases, the question of whether their surfaces could be habitable is becoming more pertinent. On Earth, the long-term carbonate silicate cycle is able to regulate surface temperatures over timescales larger than one million years. Elevated temperatures enhance weathering, removing CO$_2$ from the atmosphere, which is subducted into the mantle. At mid-ocean ridges, CO$_2$ is supplied to the atmosphere from the interior. The carbon degassing flux is controlled by the melting depth beneath mid-ocean ridges and the spreading rate, influenced by…
Although the Earth's orbit is never far from circular, terrestrial planets around other stars might experience substantial changes in eccentricity that could lead to climate changes, including possible "phase transitions" such as the snowball transition (or its opposite). There is evidence that Earth has gone through at least one globally frozen, "snowball" state in the last billion years, which it is thought to have exited after several million years because global ice-cover shut off the carbonate-silicate cycle, thereby allowing greenhouse gases to build up to sufficient concentration to mel…
Aims: The long-term carbon cycle for planets with a surface entirely covered by oceans works differently from that of the present-day Earth because inefficient erosion leads to a strong dependence of the weathering rate on the rate of volcanism. In this paper, we investigate the long-term carbon cycle for these planets throughout their evolution. Methods: We built box models of the long-term carbon cycle based on CO$_2$ degassing, seafloor-weathering, metamorphic decarbonation, and ingassing and coupled them with thermal evolution models of stagnant lid and plate tectonics planets. Results: Th…
A sufficient amount of water is required at the surface to develop water oceans. A significant fraction of water, however, remains in the mantle during magma ocean solidification, and thus the existence of water oceans is not guaranteed even for exoplanets located in the habitable zone. To discuss the likelihood of ocean formation, we built two models to predict the rate of mantle degassing during the magma ocean stage and the subsequent solid-state convection stage. We find that planets with low H$_2$O/CO$_2$ ratios would not have a sufficient amount of surface water to develop water oceans i…
Context: The long-term carbonate-silicate cycle plays an important role in the evolution of Earth's climate and, therefore, may also be an important mechanism in the evolution of the climates of Earth-like exoplanets. Aims: We investigate the effects of radiogenic mantle heating, core size, and planetary mass on the evolution of the atmospheric partial $CO_2$ pressure, and the ability of a long-term carbon cycle driven by plate tectonics to control the atmospheric $CO_2$ pressure. Methods: We developed a box-model which connects carbon cycling to parametrized mantle convection. The carbon cycl…
Terrestrial planets in temperate orbits around very low mass stars are likely to have evolved in a very different way than solar system planets, and in particular Earth. However, because these are the first planets that are and will be accessible for in-depth atmosphere, clouds and surface characterizations with existing and forthcoming telescopes, we need to develop the best possible observational strategies to maximize the scientific return from these characterizations. Here I discuss and expand on the recent works of Bean et al. (2017) and Turbet et al. (2019) to show that terrestrial plane…
It is widely believed that the carbonate-silicate cycle is the main agent to trigger deglaciations by CO$_2$ greenhouse warming on Earth and on Earth-like planets when they get in frozen state. Here we use a 3D Global Climate Model to simulate the ability of frozen planets to escape from glaciation by accumulating enough gaseous CO$_2$. We find that Earth-like planets orbiting a Sun-like star may never be able to escape from glaciation if their orbital distance is greater than $\sim$ 1.27 AU (Flux $<$ 847 W m$^{-2}$ or 62$\%$ of the Solar constant), because CO$_2$ would condense at the poles f…
Composition of terrestrial planets records planetary accretion, core-mantle and crust-mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 $\times$ CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile elemen…
The population of planets smaller than approximately $1.7~R_\oplus$ is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths. This picture is largely corroborated by radial-velocity (RV) mass measurements for close-in super-Earths but lacks constraints at lower insolations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate $1.51\pm0.04~R_\oplus$ planet Kepler-138 d ($T_{\mathrm{eq, A_B=0.3}}$~350 K) combined with new Keck/HIRES RV measurements of its host star. We fin…
Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures
ETH Zürich（苏黎世联邦理工学院）；European Synchrotron Radiation Facility（欧洲同步辐射装置）；Université Paris Cité, Institut de Physique du Globe de Paris, CNRS（巴黎西岱大学，巴黎地球物理研究所，法国国家科学研究中心）；Nantes Université, Univ Angers, Le Mans Université, CNRS, Laboratoire de Planétologie et Géosciences, LPG UMR 6112（南特大学，昂热大学，勒芒大学，法国国家科学研究中心，行星学与地质学实验室）；School of Earth, Atmosphere and Environment, Monash University（蒙纳士大学地球、大气与环境学院）；Guangzhou Institute of Geochemistry, Chinese Academy of Sciences（中国科学院广州地球化学研究所）
