The evolution of a single raindrop falling below a cloud is governed by fluid dynamics and thermodynamics fundamentally transferable to planetary atmospheres beyond modern Earth’s. Here, we show how three properties that characterize falling raindrops — raindrop shape, terminal velocity, and evaporation rate — can be calculated as a function of raindrop size in any planetary atmosphere. We demonstrate that these simple, interrelated characteristics tightly bound the possible size range of raindrops in a given atmosphere, independently of poorly understood growth mechanisms. Starting from the equations governing raindrop falling and evaporation, we demonstrate that raindrop ability to vertically transport latent heat and condensible mass can be well captured by a new dimensionless number. Our results have implications for precipitation efficiency, convective storm dynamics, and rainfall rates, which are properties of interest for understanding planetary radiative balance and (in the case of terrestrial planets) rainfall-driven surface erosion.
Orbiter and rover data have revealed a complex and intermittent hydrological history in Gale Crater on Mars, where habitable environments appear to have endured for at least thousands of years. The intermittency may be the result of a dominantly cold climate punctuated by geologically brief periods of warmth and active hydrology. However, the time required to establish an integrated hydrological cycle in a warming climate is difficult to ascertain and has not been thoroughly investigated. Here we model the transient evolution of groundwater flow and subsurface temperature, the slowest evolving components of the hydrological cycle, during a warm departure from cold conditions. We find that tens of thousands of years are likely required before groundwater could be a meaningful source for large lakes in Gale. With highly favorable conditions, primarily high permeability, significant flow might develop in thousands of years. This implies that surface water dominates during the beginning of a warm phase. Annual mean surface temperatures in Gale below 290?K would likely leave the nearby highlands frozen at the surface. In that case, deep aquifers beneath a highlands permafrost layer could deliver water to Gale, where low temperatures would have reduced evaporation.
A key factor in determining the potential habitability of synchronously rotating planets is the strength of the atmospheric boundary layer inversion between the dark side surface and the free atmosphere. Here we analyze data obtained from polar night measurements at the South Pole and Alert Canada, which are the closest analogs on Earth to conditions on the dark sides of synchronously rotating exoplanets without and with a maritime influence, respectively. On Earth, such inversions rarely exceed 30 K in strength, because of the effect of turbulent mixing induced by phenomena such as so-called “mesoscale slope winds,” which have horizontal scales of 10100 s of km, suggesting a similar constraint to near-surface dark side inversions. We discuss the sensitivity of inversion strength to factors such as orography and the global-scale circulation, and compare them to a simulation of the planet Proxima Centauri b. Our results demonstrate the importance of comparisons with Earth data in exoplanet research, and highlight the need for further studies of the exoplanet atmospheric collapse problem using mesoscale and eddy-resolving models.
We present gray gas general circulation model (GCM) simulations of the tidally locked mini-Neptune GJ 1214b. On timescales of 100010,000 Earth days, our results are comparable to previous studies of the same planet, in the sense that they all exhibit two off-equatorial eastward jets. Over much longer integration times (50,000250,000 Earth days) we find a significantly different circulation and observational features. The zonal-mean flow transitions from two off-equatorial jets to a single wide equatorial jet that has higher velocity and extends deeper. The hot spot location also shifts eastward over the integration time. Our results imply a convergence time far longer than the typical integration time used in previous studies. We demonstrate that this long convergence time is related to the long radiative timescale of the deep atmosphere and can be understood through a series of simple arguments. Our results indicate that particular attention must be paid to model convergence time in exoplanet GCM simulations, and that other results on the circulation of tidally locked exoplanets with thick atmospheres may need to be revisited.
Despite surface liquid water’s importance to habitability, observationally diagnosing its presence or absence on exoplanets is still an open problem. Inspired within the solar system by the differing sulfur cycles on Venus and Earth, we investigate thick sulfate (H2SO4–H2O) aerosol haze and high trace mixing ratios of SO2 gas as observable atmospheric features whose sustained existence is linked to the near absence of surface liquid water. We examine the fundamentals of the sulfur cycle on a rocky planet with an ocean and an atmosphere in which the dominant forms of sulfur are SO2 gas and H2SO4–H2O aerosols (as on Earth and Venus). We build a simple but robust model of the wet, oxidized sulfur cycle to determine the critical amounts of sulfur in the atmosphere–ocean system required for detectable levels of SO2 and a detectable haze layer. We demonstrate that for physically realistic ocean pH values (pH gsim 6) and conservative assumptions on volcanic outgassing, chemistry, and aerosol microphysics, surface liquid water reservoirs with greater than 10−3 Earth oceans are incompatible with a sustained observable H2SO4–H2O haze layer and sustained observable levels of SO2. Thus, we propose the observational detection of an H2SO4–H2O haze layer and of SO2 gas as two new remote indicators that a planet does not host significant surface liquid water.
Atmospheric collapse is likely to be of fundamental importance to tidally locked rocky exoplanets but remains understudied. Here, general results on the heat transport and stability of tidally locked terrestrial-type atmospheres are reported. First, the problem is modeled with an idealized 3D general circulation model (GCM) with gray gas radiative transfer. It is shown that over a wide range of parameters the atmospheric boundary layer, rather than the large-scale circulation, is the key to understanding the planetary energy balance. Through a scaling analysis of the interhemispheric energy transfer, theoretical expressions for the day-night temperature difference and surface wind speed are created that reproduce the GCM results without tuning. Next, the GCM is used with correlated-k radiative transfer to study heat transport for two real gases (CO2 and CO). For CO2, empirical formulae for the collapse pressure as a function of planetary mass and stellar flux are produced, and critical pressures for atmospheric collapse at Earth’s stellar flux are obtained that are around five times higher (0.14 bar) than previous gray gas estimates. These results provide constraints on atmospheric stability that will aid in future interpretation of observations and exoplanet habitability modeling.
We use a 3‐D general circulation model to compare the primitive Martian hydrological cycle in “warm and wet” and “cold and icy” scenarios. In the warm and wet scenario, an anomalously high solar flux or intense greenhouse warming artificially added to the climate model are required to maintain warm conditions and an ice‐free northern ocean. Precipitation shows strong surface variations, with high rates around Hellas basin and west of Tharsis but low rates around Margaritifer Sinus (where the observed valley network drainage density is nonetheless high). In the cold and icy scenario, snow migration is a function of both obliquity and surface pressure, and limited episodic melting is possible through combinations of seasonal, volcanic, and impact forcing. At surface pressures above those required to avoid atmospheric collapse (∼0.5 bar) and moderate to high obliquity, snow is transported to the equatorial highland regions where the concentration of valley networks is highest. Snow accumulation in the Aeolis quadrangle is high, indicating an ice‐free northern ocean is not required to supply water to Gale crater. At lower surface pressures and obliquities, both H2O and CO2 are trapped as ice at the poles and the equatorial regions become extremely dry. The valley network distribution is positively correlated with snow accumulation produced by the cold and icy simulation at 41.8∘ obliquity but uncorrelated with precipitation produced by the warm and wet simulation. Because our simulations make specific predictions for precipitation patterns under different climate scenarios, they motivate future targeted geological studies.