Why earth is called blue planet?

Why earth is called blue planet? What is the origin of water on Earth? Was it present in quantity since its formation, 4.5 billion years ago? Or did she arrive later, trapped in the meteorites that heavily bombarded our planet in her youth? It is by studying Venus that Belgian astronomers from the ULB, the Royal Observatory of Belgium, but also the University of Louvain, gathered within the EOS ET-HoME project , are trying to answer this question.

Why Venus? Because it’s a planet that looks a lot like Earth. “It is telluric, it has an atmosphere and it displays the same waist size as our planet”, indicates Dr Cedrick Gillmann, paleologist at the G-time laboratory , of the Free University of Brussels.

The Venusian Trail

On the other hand, there is no or more liquid water on its surface. “Venus has a surface that is fairly uniform. We have no clue that shows that there was a lot of liquid water on the surface, as can be found on the surface of the Earth, ”says the researcher. “But we find traces of water in its atmosphere.”

In the absence of Venusian soil samples to analyze on Earth, the researchers opted for numerical simulations in order to try to better understand its evolution and the history of its water.

The interior of the planet interacts with its atmosphere. Its atmosphere interacts with the solid part. By combining these mechanisms, we are trying to reconstruct the evolution of the atmosphere of Venus. And we compare these results with what we observe today, in order to determine whether these mechanisms are working, ”says Dr. Gillmann.

Doubts about the hypothesis of the impacts of wet meteorites

By reconstructing the evolution of these gases in its atmosphere, we can determine which scenarios are compatible with current observations. And it turns out that in the scenario where we suddenly add a lot of water to the planet, for example following the multiple impacts of a meteor shower, this does not allow us to arrive at the composition of the current atmosphere of Venus. Otherwise, it should be much richer in water and oxygen, ”he says.

“Therefore, we believe that the meteorites that crashed into Venus were mostly dry chondrites. This also means that the water present on the planet had to be brought from the very formation of the planet. “

Water was present since the formation of the planet

“All this shows us that it was ultimately during the formation of the planets that water accumulated. Subsequent meteorite bombardments were of little consequence. They only added a very small amount of water, which was subsequently lost in the history of the planet, ”says the researcher.

But why did water remain on the surface of the Earth, and not on Venus? “What maintains water on the surface of the Earth results from a set of parameters that can only be found on our planet”, recalls the ULB scientist.

First of all, our planet is massive enough to limit the escape of water. It is a matter of gravity. The water is too heavy to easily escape from the Earth. On Mars, which is much smaller, the gravity is lower. The water was therefore able to escape more easily ”.

In theory, for Venus, the situation had to be identical to the terrestrial situation. But if the water remained on the Earth, and not on Venus, it is because of another phenomenon. “A part of the original water of the Earth remained on (in) the planet, because it was constantly” recycled “, sheltered in the earth’s crust, via the movements generated by the tectonics of the plates”, said the researcher. “These surface conditions present on Earth do not exist on Venus. The water could not therefore be recycled and “hidden” inside the planet by the movements of its crust, “explains Dr. Gillmann. This explains why the earth’s mantle is rather humid.

Finally, on Earth, another phenomenon of water protection is also present. It is located in its atmosphere. We find there what we call a cold trap: a particularly cold atmospheric zone which prevents water from diffusing towards space ”, concludes Cedrick Gillmann.