Day–night cloud asymmetry prevents early oceans on Venus but not on Earth

Abstract

Earth has had oceans for nearly four billion years 1 and Mars had lakes and rivers 3.5-3.8 billion years ago 2 . However, it is still unknown whether water has ever condensed on the surface of Venus 3,4 because the planet -now completely dry 5 -has undergone global resurfacing events that obscure most of its history 6,7 . 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 3 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 W/m 2 , 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.

Atmospheric Dynamics of a Near Tidally Locked Earth-Size Planet

“…Before reaching the limit of the radiation tables, the surface temperature reached a maximum value of 270 • C, with a global mean of 112 • C. Similar simulations have been used to suggest that Venus may have experienced a significant period of temperate surface conditions [105], with substantial cloud feedback near the substellar point producing a cool- ing effect [104]. Such scenarios assume that water inventory in the atmosphere was able to condense at the surface to form oceans, which may have been prevented via early magma oceans and/or insolation flux [42,101]. Regardless, the atmospheric dynamics of a nearby, slowly rotating, Earth-size planet, and how it has evolved through time, is a critically important laboratory to test many of these principles.…”

“…These climate simulations are often constructed from parent Earth-based models [103], and require a substantial amount of assumptions regarding intrinsic planetary properties [30]. Furthermore, the effect of tidal locking on potential surface habitability is unresolved, and may accelerate [42,101] or decelerate [105] the transition of terrestrial planet atmospheres into a runaway greenhouse state. It is expected that exoplanet atmospheric com-positional data will provide additional insights that may substantially aid the climate modeling approach [62].…”

The Outer Edge of the Venus Zone Around Main-Sequence Stars

“…It has also been recently suggested that Venus did not enter its current extreme conditions, but rather maintained these conditions for its entire history, and was never habitable. This work questions whether water has ever condensed on the surface, and with a 3D global climate model, shows that water clouds forming on the night side (and maintained due to Venus' slow spin) would have had a warming effect that would have thwarted conditions for liquid water on the surface (Turbet et al 2021). Our assumptions about Venus, and in consequence our assumptions about the evolution of habitability, operate under the condition of once-existing liquid water on the surface of Venus, which may not have been the case.…”

“…In addition to the likelihood of observing a Venus-like planet within a given habitable zone, Venus-like planets, and subsequently the Venus zone, might be crucial to understanding the evolution of Earth analogs themselves. Venus shares many similarities with Earth in terms of mass, radius, overall bulk composition (Goettel et al 1982;Kaula et al 1994;Svedhem et al 2007), and is believed to have had an Earth-like environment in the past (Way et al 2016;Khawja et al 2020;Way & Genio 2020), although it is also hypothesized that Venus could have been like it is today since its beginnings (Hamano et al 2013;Turbet et al 2021). Venus' evolution into a post-runaway greenhouse state may suggest that the Venus-like planets that we observe today may have been Earth-like, or akin to modern Earth conditions, in the past, or that a runaway greenhouse may one day be the fate of our Earth (Ingersoll 1969;Lapôtre et al 2020).…”

Reflected spectroscopy of small exoplanets II: characterization of terrestrial exoplanets

“…CH 4 should react with H 2 O to produce CO 2 , but the optical-only retrieval does not provide any constrain on the mixing ratio of CO 2 , and the lifetime of CH 4 can be long if the atmosphere is massive and has some H 2 in the mixture (e.g., Hu & Seager 2014). To maintain a steam atmosphere would require the planet to be sufficiently irradiated, but for a planet close to the inner edge of the habitable zone, it would be difficult to eliminate this degeneracy solely based on theoretical calculations of the planet's temperature, as those calculations have uncertainties and are sensitive to minor gases in the at- mosphere (e.g., Way & Del Genio 2020;Turbet et al 2021). Similarly, the degenerate solution found in the CO 2 -dominated atmosphere with clouds case involves a dry and massive atmosphere made of N 2 , which would be uninhabitable and cannot be easily discounted from theoretical models (e.g., planetary and atmospheric evolution, thermal structure, and atmospheric chemistry).…”