The twentieth century dreamers who imagined Venus covered in a thick ocean may have been partly right, in a strange sort of way. However, the latest explanation for the planet's extraordinary evolution conceives of seas very different from anything we know.
Venus's atmosphere is today made up largely of carbon dioxide, but it is thought at at one time it had plenty of H2O, enough, by some estimates, to cover the surface meters deep.
It is unlikely this ever occurred however, since even in its early days the planet may have been too hot for liquid water. Eventually so much water was lost that now just 0.002% of the atmosphere is water vapor. Yet the surface has features that look like river valleys and flood plains. The question of what could have formed these is even more puzzling than equivalent features on Mars.
In the Journal of Physical Chemistry Letters Cornell's Dr Dima Bolmatov makes the case that the ancient Venusian seas were formed from supercritical carbon dioxide.
CO2 forms more than 96% of Venus's atmosphere, and with a density 93 times that of Earth that makes for a lot of the gas, with a runaway Greenhouse Effect that turned Venus into the hellhouse it is.
Under Earth's atmospheric conditions carbon dioxide becomes a gas at low temperatures, and a liquid not at all. Things are different, however, under the intense atmospheric pressure of other worlds. There are thoughts that a planet-wide ocean of supercritical fluids exist on exoplanet 55 Cancri e.
We use supercritical carbon dioxide to produce pharmaceuticals and decaffeinate coffee beans but “theoretical understanding of the supercritical state is lacking,” Bolmatov notes, given the wide range of possible conditions. Replicating the environment of an early Venus in the lab is not easy, but Bolmatov's computer simulations of 14000 atoms contained a surprise. Rather than a gradual shift between largely gaseous and liquid behavior, Bolmatov concluded there are points of sudden shifts.
If, as has been previously postulated, Venus' atmosphere was several times denser than it is today during its first 100-200million years, the carbon dioxide may have had very liquid-like characteristics near the planet's surface
"This in turn makes it plausible that geological features on Venus like rift valleys, riverlike beds, and plains are the fingerprints of near-surface activity of liquidlike supercritical carbon dioxide," Bolmatov told Space.com.
The whole planet may not have been covered with an ocean, but Bolmatov but raises the possibility of things that, “Looked like soap bubbles.... A bubble of gas that is covered by a thick layer of liquid.” If these flowed over the landscape they could have left markings reminiscent of water, which may have survived until the present day.
Exploration of Venus is not easy, for all NASA's plans, but the authors say, “Our results call for experimental observation” of supercritical carbon dioxide to see whether it might have behaved as required under possible early Venusian conditions.