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How much closer to the Sun is Venus?

Can this explain most of its differences with Earth?

--Answered by Dr. Colin Wilson, EnVision's lead on Atmospheric Processes

Yes, a lot of the difference between how Venus evolved and how Earth evolved is due to their different distances to the Sun.

Interestingly, though, Venus today absorbs less sunlight than does the Earth. Sunlight is about twice as bright at Venus, due to how close it is the Sun, but 70% of this is reflected away by the highly reflective clouds which envelope Venus. Earth receives less sunlight than Venus, but reflects only 30% away, absorbing the other 70%. Therefore, the stratospheric temperatures at Venus are actually colder at Venus than at Earth. The high surface temperature at Venus is due not to solar heating, because only ~2% of incident sunlight reaches the surface. Rather, the high surface temperature at Venus is due to an enormous greenhouse effect due mainly to the 92 bar of carbon dioxide in Venus’ atmosphere, but also to the clouds, and water vapour and other gases.

Figure 1 : Artist impression of the Venus surface under 92 bar of carbon dioxide. Credit: ESA


So why, then, is Venus different from Earth? Early on in their formation, when Venus and Earth were still hot enough to have molten magma oceans, any water had to be in vapour form so the planets would have had dense atmospheres containing large abundances of carbon dioxide and water vapour. As Venus was closer to the sun, the water vapour in its atmosphere could more easily become photodissociated and lost to space. On Earth, temperatures were cooler and water managed to condense into liquid form. Over time and through interaction with liquid water the carbon dioxide in Earth’s atmosphere was mostly locked up in carbonate rocks, leaving us with the low carbon dioxide concentrations we have today on Earth – and a much lower abundance of greenhouse gases than Earth initially had, leading to a cooler planet. One of the fundamental questions about Venus evolution is: did Venus ever had liquid water on its surface, or whether water was lost before the water could condense? If early Venus had a liquid water phase, then this would have presented a habitable environment similar to that found on early Earth, a result which could have important implications for the development of life in the Solar System…

How can we find out whether there was ever liquid water on the surface of Venus? With Envision we will study the tessera highlands of Venus, thought to be the most ancient terrain on Venus, to see if there any morphological or compositional signs of a watery past. But even this terrain may not date back to a liquid water era. Measurements of noble gas isotopic abundances from within the atmosphere of Venus would also help to constrain evolutionary scenarios for early Venus.

There is a lot of new interest in this question because, as we discover planets around other stars, we’re very interested to know how close one can be to a star before the temperatures get too hot. Study of Venus and its history is helping us to answer that question...

This is a very abbreviated summary of a lot of fascinating and ongoing modelling work about the history of the terrestrial planets. For more info, see works by David Grinspoon, Eric Chassefiere, Ravi Kumar Kopparapu, and many others …

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