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Register a new userAbrupt climate transition of icy worlds from snowball to moist or runaway greenhouse
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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 planets 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-silicate cycle and low concentrations of greenhouse gases. Examples for such planetary bodies are the icy moons Europa and Enceladus, and certain icy exoplanets orbiting G and F stars. We find that the stellar fluxes that are required to overcome a planets initial snowball state are so large that they lead to significant water loss and preclude a habitable planet. Specifically, they exceed the moist greenhouse limit, at which water vapour accumulates at high altitudes where it can readily escape, or the runaway greenhouse limit, at which the strength of the greenhouse increases until the oceans boil away. We suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable Earth-like state.
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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.
The runaway greenhouse represents the ultimate climate catastrophe for rocky, Earth-like worlds: when the incoming stellar flux cannot be balanced by radiation to space, the oceans evaporate and exacerbate heating, turning the planet into a hot wasteland with a steam atmosphere overlying a possibly molten magma surface. The equilibrium state beyond the runaway greenhouse instellation limit depends on the radiative properties of the atmosphere and its temperature structure. Here, we use 1-D radiative-convective models of steam atmospheres to explore the transition from the tropospheric radiation limit to the post-runaway climate state. To facilitate eventual simulations with 3-D global circulation models, a computationally efficient band-grey model is developed, which is capable of reproducing the key features of the more comprehensive calculations. We analyze two factors which determine the equilibrated surface temperature of post-runaway planets. The infrared cooling of the planet is strongly enhanced by the penetration of the dry adiabat into the optically thin upper regions of the atmosphere. In addition, thermal emission of both shortwave and near-IR fluxes from the hot lower atmospheric layers, which can radiate through window regions of the spectrum, is quantified. Astronomical surveys of rocky exoplanets in the runaway greenhouse state may discriminate these features using multi-wavelength observations.
Coupled models of mantle thermal evolution, volcanism, outgassing, weathering, and climate evolution for Earth-like (in terms of size and composition) stagnant lid planets are used to assess their prospects for habitability. The results indicate that planetary CO$_2$ budgets ranging from $approx 3$ orders of magnitude lower than Earths to $approx 1$ order of magnitude larger, and radiogenic heating budgets as large or larger than Earths, allow for habitable climates lasting 1-5 Gyrs. The ability of stagnant lid planets to recover from potential snowball states is also explored; recovery is found to depend on whether atmosphere-ocean chemical exchange is possible. For a hard snowball with no exchange, recovery is unlikely, as most CO$_2$ outgassing takes place via metamorphic decarbonation of the crust, which occurs below the ice layer. However, for a soft snowball where there is exchange between atmosphere and ocean, planets can readily recover. For both hard and soft snowball states, there is a minimum CO$_2$ budget needed for recovery; below this limit any snowball state would be permanent. Thus there is the possibility for hysteresis in stagnant lid planet climate evolution, where planets with low CO$_2$ budgets that start off in a snowball climate will be permanently stuck in this state, while otherwise identical planets that start with a temperate climate will be capable of maintaining this climate for 1 Gyrs or more. Finally, the model results have important implications for future exoplanet missions, as they can guide observations to planets most likely to possess habitable climates.
The standard model of planet formation considers an initial phase in which planetesimals form from a dust disk, followed by a phase of mutual planetesimal-planetesimal collisions, leading eventually to the formation of planetary embryos. However, there is a potential transition phase (which we call the snowball phase), between the formation of the first planetesimals and the onset of mutual collisions amongst them, which has often been either ignored or underestimated in previous studies. In this snowball phase, isolated planetesimals move on Keplerian orbits and grow solely via the direct accretion of sub-cm sized dust entrained with the gas in the protoplanetary disk. Using a simplified model in which planetesimals are progressively produced from the dust, we consider the expected sizes to which the planetesimals can grow before mutual collisions commence and derive the dependence of this size on a number of critical parameters, including the degree of disk turbulence, the planetesimal size at birth and the rate of planetesimal creation. For systems in which turbulence is weak and the planetesimals are created at a low rate and with relatively small birth size, we show that the snowball growth phase can be very important, allowing planetesimals to grow by a factor of 10^6 in mass before mutual collisions take over. In such cases, the snowball growth phase can be the dominant mode to transfer mass from the dust to planetesimals. Moreover, such growth can take place within the typical lifetime of a protoplanetary gas disk. A noteworthy result is that ... ...(see the paper). For the specific case of close binaries such as Alpha Centauri ... ... (see the paper). From a more general perspective, these preliminary results suggest that an efficient snowball growth phase provides a large amount of room at the bottom for theories of planet formation.
The structure of the icy shells of ocean worlds is important for understanding the stability of their underlying oceans as it controls the rate at which heat can be transported outward and radiated to space. Future spacecraft exploration of the ocean worlds (e.g., by NASAs Europa Clipper mission) will allow for higher-resolution measurements of gravity and shape than currently available. In this paper, we study the sensitivity of gravity-topography admittance to the structure of icy shells in preparation for future data analysis. An analytical viscous relaxation model is used to predict admittance spectra given different shell structures determined by the temperature-dependent viscosity of a tidally heated, conductive shell. We apply these methods to the ocean worlds of Europa and Enceladus. We find that admittance is sensitive to the mechanisms of topography support at different wavelengths and estimate the required gravity performance to resolve transitions between these mechanisms. We find that the Airy isostatic model is unable to accurately describe admittance universally across all wavelengths when the shell thickness is a significant fraction of bodys radius. Our models suggest that measurements of admittance at low spherical harmonic degrees are more sensitive to thick shells with high tidal dissipation, and may complement ice-penetrating radar measurements in constraining shell thickness. Finally, we find that admittance may be used to constrain the tidal dissipation within the icy shell, which would be complementary to a more demanding measurement of the tidal phase lag.
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