The planet Venus is often regarded as Earth’s evil twin. It is the second largest of the four terrestrial planets and lies closer to the Sun than the Earth. The surface bears the scars of millions of years of vulcanism, crushing pressure, furnace temperatures and sulphuric acid rains that fall from clouds that permanently obscure the surface from those watching the planet from the ground. Glowing benignly in the west after sunset, showing graceful phases like the Moon, the current evenning star hides the secret of its raging violence below this sulphuric shroud.
But was it always like this? Was there once a time when a younger Venus held a surface on which waves gently lapped an alien beach? The planet is smaller than Earth and so should’ve cooled quicker after all, although it still absorbed a higher amount of radiation directly from the Sun.
However, this dream of a benign youth choked out and turned on to a lifetime of anguished heat has had cold water poured onto it by an atmospheric physicist whose model of the Venusian atmosphere through time sees little scope for a stable ocean at any time.
Eric Chassefière, Université Paris-Sud, France, suggests that water would only exist in large quantities in the atmosphere and that by the time the surface was cooling to the point that it had a surface that could hold water, the molecule was already being destroyed by sunlight. The ESA probe Venus Express has measured the escape of oxygen and hydrogen atoms in a roughly 1:2 ratio, suggesting they are the result of water molecules being dissociated – ripped apart – by high energy radiation from the Sun striking the upper atmosphere. As water is lost from above, water from below moves up to try and equalise the partial pressure, exposing it to dissociation. By measuring the rate of escape now, looking at the solar emission spectrum and working out the rate of dissociation through the ages, the amount of water at earlier times in the atmosphere can be extrapolated. Deuterium, which is a heavier form of hydrogen, is slowly being enriched – its presence relative to the amount of hydrogen is increasing – due to the extra energy needed to boost a deuterium atom out of the atmosphere compared to that needed to extract a hydrogen one.
The model suggests that the time at which there was lots of water in the atmosphere, the greenhouse effect from the water and the insulation it provided the molten planet actually prevented it from cooling and forming a solid surface. It wasn’t until the loss mechanisms kicked in that the surface could finally crystalise. The loss mechanisms started working on all atmospheric species, reducing Venus to an atmosphere not unlike Mars, but as the volcanoes started up, a layer of sulphur and other gases from within Venus built up and produced the raging fury that passes as a gentle breeze on the Venusian precipices today.
However, the model leaves many questions open – such as the amount of standing water that could be produced by comet impacts on the planet after the surface had cooled. Far more extensive modelling, including a number of different scenarios, is required in order to produce a menu of different atmospheric recipes. Data from samples taken by Venus Express and other probes over the years and in the future can then be used to distinguish between the various scenarios.