Earth went through at least two periods of global glaciation (i.e., “Snowball Earth” states) during the Neoproterozoic, the shortest of which (the Marinoan) may not have lasted sufficiently long for ...its termination to be explained by the gradual volcanic build‐up of greenhouse gases in the atmosphere. Large asteroid impacts and supervolcanic eruptions have been suggested as stochastic geological events that could cause a sudden end to global glaciation via a runaway melting process. Here, we employ an energy balance climate model to simulate the evolution of Snowball Earth's surface temperature after such events. We find that even a large impactor (diameters of d ∼ 100 km) and the supervolcanic Toba eruption (74 Kyr ago), are insufficient to terminate a Snowball state unless background CO2 has already been driven to high levels by long‐term outgassing. We suggest, according to our modeling framework, that Earth's Snowball states would have been resilient to termination by stochastic events.
Plain Language Summary
The terminations of Earth's longest periods of global glaciation are commonly understood to have occurred due to the gradual build‐up of greenhouse gases in the atmosphere from volcanism. However, the sudden ends of Earth's shorter global glaciation periods likely cannot be explained by the same mechanisms. Large asteroid impacts and supervolcanic eruptions have been suggested as geophysical phenomena that could cause abrupt ends to global glaciation periods. Here, we model the evolution of the planet's surface temperature in the aftermath of such events. Impacts and eruptions open up gaps in the global ice sheet, and also partially cover the ice in far‐spreading dust and ash, both of which increase the amount of solar radiation that is absorbed by the planet comparing to the highly reflective surface of ice and snow. Greater absorption of radiation leads to higher surface temperatures, which increases ice melting, and generates a feedback loop that can melt the entire planet surface. However, we find that the scales of impact or eruption required to produce global melting are too great to have likely occurred at the times of Earth's global glaciations. Other mechanisms must, therefore, be explored to explain Earth's short glaciation periods.
Key Points
We use an Energy Balance Model (EBM) to simulate the response of Snowball climate after stochastic events
Impact simulations and estimates of ash dispersion are used to inform initial conditions
Earth's Snowball states seem resilient to termination by stochastic events within our modeling framework
While their detections remain challenging at present, observations of small terrestrial planets will become easier in a near future thanks to continuous improvements of detection and characterisation ...instruments. In this quest, climate modeling is a key step to understanding their characteristics, atmospheric composition, and possible histories. If a surface water reservoir is present on such a terrestrial planet, an increase in insolation may lead to a dramatic positive feedback induced by water evaporation: the runaway greenhouse. The resulting rise in the global surface temperature leads to the evaporation of the entire water reservoir, separating two very different population of planets: 1) temperate planets with a surface water ocean and 2) hot planets with a puffed atmosphere dominated by water vapor. Therefore, the understanding of the runaway greenhouse is pivotal to assess the different evolution of Venus and the Earth, as well as every similar terrestrial exoplanet. In this work, we use a 3D General Circulation Model (GCM), the Generic-PCM, to study the runaway greenhouse transition, linking temperate and post-runaway states. Our simulations were comprised of two phases. First, assuming initially a liquid surface ocean, there is an evaporation phase, which enriches the atmosphere with water vapor. Second, when the ocean is considered to be entirely evaporated, there is a dry transition phase for which the surface temperature increases dramatically. Finally, the evolution ends with a hot and stable post-runaway state. By describing in detail the evolution of the climate over these two steps, we show a rapid transition of the cloud coverage and of the wind circulation from the troposphere to the stratosphere. By comparing our result to previous studies using 1D models, we discuss the effect of intrinsically 3D processes such as the global dynamics and the clouds, which are key to understanding the runaway greenhouse. We also explore the potential reversibility of the runaway greenhouse that is limited by its radiative unbalance.
Earth has had oceans for nearly four billion years
and Mars had lakes and rivers 3.5-3.8 billion years ago
. However, it is still unknown whether water has ever condensed on the surface of Venus
...because the planet-now completely dry
-has undergone global resurfacing events that obscure most of its history
. The conditions required for water to have initially condensed on the surface of Solar System terrestrial planets are highly uncertain, as they have so far only been studied with one-dimensional numerical climate models
that cannot account for the effects of atmospheric circulation and clouds, which are key climate stabilizers. Here we show using three-dimensional global climate model simulations of early Venus and Earth that water clouds-which preferentially form on the nightside, owing to the strong subsolar water vapour absorption-have a strong net warming effect that inhibits surface water condensation even at modest insolations (down to 325 watts per square metre, that is, 0.95 times the Earth solar constant). This shows that water never condensed and that, consequently, oceans never formed on the surface of Venus. Furthermore, this shows that the formation of Earth's oceans required much lower insolation than today, which was made possible by the faint young Sun. This also implies the existence of another stability state for present-day Earth: the 'steam Earth', with all the water from the oceans evaporated into the atmosphere.
As the insolation of an Earth-like (exo)planet with a large amount of water increases, its surface and atmospheric temperatures also increase, eventually leading to a catastrophic runaway greenhouse ...transition. While some studies have shown that the onset of the runaway greenhouse may be delayed due to an overshoot of the outgoing longwave radiation (OLR) – compared to the Simpson-Nakajima threshold – by radiatively inactive gases, there is still no consensus on whether this is occurring and why. Here, we used a suite of 1D radiative-convective models to study the runaway greenhouse transition, with particular emphasis on taking into account the radical change in the amount of water vapour (from trace gas to dominant gas). The aim of this work is twofold: first, to determine the most important physical processes and parametrisations affecting the OLR; and second, to propose reference OLR curves for N
2
+H
2
O atmospheres. Through multiple sensitivity tests, we list and select the main important physical processes and parametrisations that need to be accounted for in 1D radiative-convective models to compute an accurate estimate of the OLR for N
2
+H
2
O atmospheres. The reference OLR curve is computed with a 1D model built according to the sensitivity tests. These tests also allow us to interpret the diversity of results already published in the literature. Moreover, we provide a correlated-k table able to reproduce line-by-line calculations with high accuracy. We find that the transition between an N
2
-dominated atmosphere and an H
2
O-dominated atmosphere induces an overshoot of the OLR compared to the (pure H
2
O) Simpson–Nakajima asymptotic limit. This overshoot is first due to a transition between foreign and self-broadening of the water absorption lines, and second to a transition between dry and moist adiabatic lapse rates.
Water vapour atmospheres with content equivalent to the Earth's oceans, resulting from impacts
or a high insolation
, were found to yield a surface magma ocean
. This was, however, a consequence of ...assuming a fully convective structure
. Here, we report using a consistent climate model that pure steam atmospheres are commonly shaped by radiative layers, making their thermal structure strongly dependent on the stellar spectrum and internal heat flow. The surface is cooler when an adiabatic profile is not imposed; melting Earth's crust requires an insolation several times higher than today, which will not happen during the main sequence of the Sun. Venus's surface can solidify before the steam atmosphere escapes, which is the opposite of previous works
. Around the reddest stars (T
< 3,000 K), surface magma oceans cannot form by stellar forcing alone, whatever the water content. These findings affect observable signatures of steam atmospheres and exoplanet mass-radius relationships, drastically changing current constraints on the water content of TRAPPIST-1 planets. Unlike adiabatic structures, radiative-convective profiles are sensitive to opacities. New measurements of poorly constrained high-pressure opacities, in particular far from the H
O absorption bands, are thus necessary to refine models of steam atmospheres, which are important stages in terrestrial planet evolution.
Understanding the set of conditions that allow rocky planets to have liquid water on their surface, in the form of lakes, seas, or oceans, is a major scientific step in determining the fraction of ...planets potentially suitable for the emergence and development of life as we know it on Earth. This effort is also necessary to define and refine what is known as the habitable zone (HZ) in order to guide the search for exoplanets likely to harbor remotely detectable life forms. To date, most numerical climate studies on this topic have focused on the conditions necessary to maintain oceans, but not to form them in the first place. Here we use the three-dimensional Generic Planetary Climate Model, historically known as the LMD generic global climate model, to simulate water-dominated planetary atmospheres around different types of main sequence stars. The simulations are designed to reproduce the conditions of early ocean formation on rocky planets due to the condensation of the primordial water reservoir at the end of the magma ocean phase. We show that the incoming stellar radiation (ISR) required to form oceans by condensation is always drastically lower than that required to vaporize oceans. We introduce a water condensation limit, which lies at significantly lower ISR than the inner edge of the HZ calculated with three-dimensional numerical climate simulations. This difference is due to a behavior change of water clouds, from low-altitude dayside convective clouds to high-altitude nightside stratospheric clouds. Finally, we calculated the transit spectra, emission spectra, and thermal phase curves of TRAPPIST-1b, c, and d with H
2
O-rich atmospheres, and compared them to CO
2
atmospheres and bare rock simulations. We show using these observables that JWST has the capability to probe steam atmospheres on low-mass planets, and could possibly test the existence of nightside water clouds.
Context.
Ultra-hot Jupiters (UHJs), rendering the hottest planetary atmospheres, offer great opportunities of detailed characterisation with high-resolution spectroscopy. MASCARA-4 b is a recently ...discovered close-in gas giant belonging to this category.
Aims.
We aim to characterise MASCARA-4 b, search for chemical species in its atmosphere, and put these in the context of the growing knowledge on the atmospheric properties of UHJs.
Methods.
In order to refine system and planet parameters, we carried out radial velocity measurements and transit photometry with the CORALIE spectrograph and EulerCam at the Swiss 1.2 m Euler telescope. We observed two transits of MASCARA-4 b with the high-resolution spectrograph ESPRESSO at ESO’s Very Large Telescope. We searched for atomic, ionic, and molecular species via individual absorption lines and cross-correlation techniques. These results are compared to literature studies on UHJs characterised to date.
Results.
With CORALIE and EulerCam observations, we update the mass of MASCARA-4 b (
M
p
= 1.675 ± 0.241
M
Jup
) as well as other system and planet parameters. In the transmission spectrum derived from ESPRESSO observations, we resolve excess absorption by H
α
, H
β
, NaI D1&D2, CaII H&K, and a few strong lines of MgI, FeI, and FeII. We also present the cross-correlation detection of Mg I, CaI, Cr I, Fe I, and Fe II. The absorption strength of Fe II significantly exceeds the prediction from a hydrostatic atmospheric model, as commonly observed in other UHJs. We attribute this to the presence of Fe II in the exosphere due to hydrodynamic outflows. This is further supported by the positive correlation of absorption strengths of Fe II with the Hα line, which is expected to probe the extended upper atmosphere and the mass loss process. Comparing transmission signatures of various species in the UHJ population allows us to disentangle the hydrostatic regime (as traced via the absorption by Mg I and Fe I) from the exospheres (as probed by Hα and Fe II) of the strongly irradiated atmospheres.
Abstract A recent study comparing ozone column depths and methane lifetimes at varied atmospheric O 2 ( p O 2 ) levels calculated in the Kasting‐group 1‐D photochemical model and the Whole Atmosphere ...Community Climate Model version 6 (WACCM6) 3‐D model (Ji, Kasting, et al., 2023; https://doi.org/10.1098/rsos.230056 ) has exposed weaknesses in both models in parameterizing photolysis in the O 2 Schumann‐Runge bands, 175–205 nm. WACCM6 does a good job for Earth's present atmosphere but neglects scattering, which becomes important at low p O 2 . The 1‐D model includes scattering but is based on an out‐of‐date band model, and it neglects the temperature dependence of photolysis at low p O 2 . We have revised and improved the 1‐D photochemical model by replacing the old O 2 photolysis algorithm with a new correlated‐k parameterization, which improves accuracy for all O 2 levels and all temperature profiles. The WACCM6 parameterization was also included in the 1‐D model for comparative purposes. The correlated‐k and WACCM6 photolysis algorithms agree well for both the present atmosphere and for an atmosphere containing 10 −3 times the present O 2 level, but only if multiple scattering is included at low p O 2 . The correlated‐k parameterization will be made available to photochemical modeling groups who might choose to adopt it.
Plain Language Summary In today's atmosphere, the absorption of incoming ultraviolet radiation by O 2 plays a decisive role in creating O atoms that can react to form ozone (O 3 ), as well as shielding other gases from photolysis. However, this absorption is difficult to parameterize in atmospheric models due to its complex structure at wavelengths of 175–205 nm, which we call the Schumann‐Runge bands. Good models for SR absorption in today's atmosphere already exist, but not all of them may be suitable for use in low‐O 2 atmospheres in which multiple scattering is important. Here, we develop a new parameterization for SR absorption by employing a “correlated‐k” approach that has been widely used in climate models at both visible and infrared wavelengths. This approach accounts for the strong temperature dependence of the absorption and agrees well with existing models of O 2 absorption in the present atmosphere. It should also be useful in simulations of low‐O 2 atmospheres on early Earth, for which scattering must be included, as well as for atmospheres of putative Earth‐like exoplanets that astronomers hope to identify over the next several decades.
Key Points To parameterize O 2 absorption, we generated a correlated‐k table based on detailed line‐by‐line calculations at 150, 200, 250, and 300 K The new parameterization matches laboratory measurements and is well suited to include scattering at lower O 2 levels Early earth photochemical models based on the old exponential sum fits should consider the new correlated‐k to achieve more accurate results
Abstract
The era of atmospheric characterization of terrestrial exoplanets is just around the corner. Modeling prior to observations is crucial in order to predict the observational challenges and to ...prepare for the data interpretation. This paper presents the report of the TRAPPIST Habitable Atmosphere Intercomparison workshop (2020 September 14–16). A review of the climate models and parameterizations of the atmospheric processes on terrestrial exoplanets, model advancements, and limitations, as well as direction for future model development, was discussed. We hope that this report will be used as a roadmap for future numerical simulations of exoplanet atmospheres and maintaining strong connections to the astronomical community.
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