Modern Venus Could Have Been Earth-like Several Hundred Million Years Ago

A major topic in planetary habitability is why, when, and how Venus and Earth diverged in their climate conditions. One hypothesis holds that Venus has been in a post-runaway greenhouse state since the solidification of its magma ocean. Another is that Venus was once Earth-like, with liquid water stable at its surface. Under the latter scenario, Venus could have remained habitable for billions of years until some mechanism triggered a runaway greenhouse—possibly the coeval eruption of two or more large igneous provinces that injected sufficient CO₂ to overwhelm a putative carbon–silicate cycle.

The average model age for the Venus surface is about ~700 Myr, although this value is subject to considerable uncertainty. Even so, the question arises: could Venus have plausibly undergone a transition from an Earth-like state to its present climate within that ~700 Myr timeframe? The answer is yes.

To illustrate the plausibility of such a transition, which is characterized by the loss of oceans and the emplacement of expansive volcanic plains, consider the volume of modern Earth’s ocean basins of 1.34×10⁹ km³. There are numerous estimates for Earth’s present eruptive flux, ranging from a low of 3.4 km³/yr to 22.5 km³/yr. These eruptive values bracket the time needed to entirely fill Earth’s ocean basins with volcanic products to as long as 390 Myr to as rapidly as 60 Myr. Modern Venus has a surface occupied by about 72% volcanic plains. Were such plains occupying former ocean basins of an average depth comparable to that of Earth of 4 km, and under the assumption that Venus has a comparable eruptive flux to Earth, the fill times are virtually identical: 59–390 Myr.

An important constraint on any scenario under which modern Venus transitioned from an Earth-like state is the fate of O₂. With the onset of a runaway greenhouse, any oceans present would have evaporated and eventually be photodissociated by solar UV radiation into its component hydrogen and oxygen. The H was likely subject to thermal or non-thermal escape mechanisms, but the O₂ would have been too massive to be lost to space. A likely repository of photodissociated O₂, then, would have been the planet’s crustal rocks.

The oxidation state of Venus’ crust is poorly known, as is the ability of the crust to serve as an oxygen sink (which depends on lithology and oxygen fugacity). Nevertheless, should a clement Venus have had a comparable water ocean volume to Earth, the photodissociated O₂ could have been sequestered in an oxidized layer of basalt 50 km thick. Again, given the effusive flux of modern Earth, a layer of this thickness would be emplaced in 740 Myr—remarkably similar to the current estimated average model age for the planet surface. And, if hydration reactions were possible (akin to water–rock reactions on Earth), an oxidized layer as thin as 10 km is required, which could have been emplaced in as little as 147 Myr.

None of these calculations establishes that Venus was once Earth-like, with a comparable hypsometry and ocean volume. Instead, they demonstrate that Venus could have been Earth-like within the Phanerozoic, acquiring vastly different climate, volcanic, tectonic, and geodynamic properties within a relatively short period of geological time. Future remote-sensing and in situ missions to Venus are necessary to fully establish the climatic and geological history of the planet. Of note, Earth’s climate may enter a runaway greenhouse under increasing solar luminosity within perhaps the next billion years or so. Assuming its eruptive flux remains relatively constant, the calculations here show that Earth could acquire the major planetary properties of Venus within only a few hundred million years, too.