Melissa Brobby interviews Martin Turbet
Research into whether Venus had liquid oceans in its past is giving more insight into how Earth has so far avoided a runaway greenhouse effect
Considering how molten Venus’s surface was in its past, how might it have formed a liquid water ocean?
When we think about the evolution of planets, we envision them beginning their life in a hot and molten state, and as they evolve through time, they emit radiation into space and cool down. So, the key question is whether or not by cooling down, Venus was able to condense the water it had in its atmosphere onto the surface. If the water did not condense, then it is unlikely that liquid water oceans would have formed. The current thinking is liquid water oceans might have formed from steam at an early point during Venus’s evolution, the steam condensing on the planet’s surface and turning from a vapour to a liquid. This is probably what also happened on Earth in its past.
What methods were used to find this out?
I have been using a Three-Dimensional Global Climate Model designed to represent all the dimensions of a planet, and the physical processes that are at play in planetary atmospheres. We simulated the way the gas and clouds interact with light coming from the Sun, and the way the water vapour in particular will condense and evaporate from clouds. We also simulated the way the atmosphere circulates. All these equations are combined in the same model to represent in the best way possible how an atmosphere will behave. This type of model is similar to the one that colleagues in my research group have been using to evaluate global warming on Earth.
According to your simulations how long would these oceans have existed on Venus and when would they have begun to disappear?
The simulations give an atmosphere that is cloudless in regions on Venus where the Sun reaches the zenith; clouds are formed mainly at the poles and on the night side of the planet. This peculiar distribution of clouds would produce strong heating by the greenhouse effect, preventing liquid water from forming on Venus’s surface, which is likely to have been the case since the early stages of the planet’s development.
What could the study of Venus’s history tell us about climate change happening on Earth and what the future could hold for our planet?
If at some point Earth gets warm enough, it will trigger a runaway greenhouse effect, which will result in oceans evaporating from the surface and into the atmosphere, and this may be irreversible. However, this would happen over a long period of time during Earth’s evolution.
What prevented Earth from suffering the same fate as Venus in its earlier years?
What we’ve found is that Earth’s insolation – the amount of solar radiation a planet receives on its surface – was probably the key, and maybe the reason for why it escaped a similar fate. This was how it was able to condense its oceans in the first place. The only way water could condense on Earth’s surface would be to decrease the amount of solar radiation that the planet received. On Earth, four billion years ago, this was sufficiently low because the luminosity of the Sun was lower in the past, which is known as the ‘faint young Sun paradox’. What we are seeing from our simulation results is that this is a key element to consider in the evolution of Earth, and how immensely important this part of the planet’s history is.
Are there plans to apply this model to any future exoplanet missions?
A mission we may see in the coming years will give us the opportunity to probe exoplanets with steam atmospheres, and use those planets to better understand what lies beneath, and ask how their clouds are forming. The best telescope we may have for answering these questions is the James Webb Space Telescope (JWST).
Martin Turbet is a CNRS research scientist at the University of Geneva