Masked or not, breathe, close our eyes. Imagine the Earth, seen from space, from the Moon. The Earth is blue. If the poet sees it as blue as an orange, our Earth has been blue for more than 4 billion years, because there is liquid water on its surface.
We only know of one planet with liquid water on its surface: ours. We now know that it is the interaction of the water cycle with plate tectonics and the greenhouse effect, as well as the geometry of the solar system, that ensure the perennial presence of water on the Earth’s surface. . Geosciences allow us to reconstruct this history during which the Earth became, and remained, blue: it was never too cold or too hot, with very rare exceptions
At sea level, water freezes below 0 ° C and boils above 100 ° C. The surface temperature on Earth today is around 15 ° C. By comparison, the average temperature on Venus is above 450 ° C while it is approximately -65 °C on Mars.
The Earth’s surface is therefore maintained in a temperature window that may appear wide (between 0 and 100 ° C), but which is in reality extremely narrow, and has been for more than 4 billion years.
The greenhouse effect is essential for the presence of liquid water on Earth
This window is narrow, because the average temperature at the surface of a planet results from three parameters which can vary greatly from one planet to another. First, the energy received from the Sun – which depends on its luminosity and the distance between it and the planet; then albedo – that is, the fraction of the energy received which is in fact reflected by the surface of the planet; and finally, the greenhouse effect – certain gases present in the atmosphere return heat to the surface. Without the greenhouse effect, the average temperature of the Earth would be – 15 ° C: there would therefore probably be no oceans.
Thus, since the oceans were formed, the interactions between these three parameters have maintained a relatively constant energy balance. However, the energy received from the Sun has changed since the formation of the Earth. The young Sun was less bright and the energy received by the Earth was therefore less. However, the levels of greenhouse gases in the atmosphere, in particular CO2, were higher than today and kept temperatures high enough for the water to be liquid.
The greenhouse effect linked to CO2 could decrease because the latter is continuously pumped out of the atmosphere by two processes. On the one hand, surface rocks are “dissolved” by the acidifying action of CO2, which releases the calcium. CO2 is thus extracted from the atmosphere into the ocean, in the form of a carbonate ion, where it combines with calcium to form limestone rocks. This is a carbon sink.
On the other hand, once life appeared, organisms capable of photosynthesis began to take CO2 in the atmosphere. It is then exported to the sediments as organic matter – this is another carbon sink.
While these carbon sinks subtract CO2 of the atmosphere, other mechanisms bring CO2 in the atmosphere: these are volcanoes and ocean ridges, kept alive by plate tectonics. By participating in the carbon cycle and therefore in the greenhouse effect, plate tectonics contribute, over very large time scales, to maintaining the temperature within a fairly narrow range. It is therefore essential for the presence of liquid water on Earth, the only known planet with real active tectonics.
Without tectonics, no oceans. Without oceans, no tectonics
Liquid water and plate tectonics are therefore intimately linked. The oceanic plates move, carried along by the subduction which makes them plunge towards the Earth’s mantle.
This plunge is itself allowed by the hydration of the rocks and minerals of the plate, because the presence of liquid water modifies their mechanical properties. As it plunges, the plate becomes dehydrated, which allows the formation of the granites which constitute the base of the continents: without water, there would therefore be neither tectonics nor continents as we know them.
Since once old, the plate plunges towards the earth’s mantle during subduction, it is necessary to come up with something to replace it. This ascent takes place at the level of the ocean ridges, these chains of submarine volcanoes that crisscross our planet. On its way up, this mantle material, hot, cools down and degasses CO2, which makes it possible to maintain the greenhouse effect. Liquid water can therefore be maintained, our planet remains blue, and the circle has been closed for several billion years.
Since when is Earth blue?
The loop is complete, but it had to start: liquid water was not present on the surface of the Earth when it formed, it was surely too hot there.
The first indisputable evidence of the presence of oceans dates back 3.8 billion years. The oldest marine sediments are found at Isua and Akilia in Greenland and at Nuvvuagittuq in Canada, as well as “cushion” basalts, spectacular balls of rocks that result from the solidification of lava in contact with water.
It was long thought that water had been brought on Earth after its formation, by bodies rich in water from the outer solar system. A study by our laboratory published in August 2020 questions this hypothesis and suggests that the constituents of water — oxygen and hydrogen — may have been brought in by the rocks that accreted to form the young Earth. In this case, “water” could have been present in the rocks that formed the Earth from the beginning, but it is not known if there were oceans.
Why is there water on Earth?
In addition, if there were oceans, they were instantly vaporized during thegiant impact between the young Earth and a planetary body (no doubt as big as Mars) which melted the surface of our planet and gave birth to the Moon there is 4.4 billion years.
Gradually, the magma formed during the impact cools and solidifies. Earth is then probably only one barren surface of dark basalt rocks, where temperatures are higher than today. As these rocks form, elements like hydrogen, oxygen and carbon escape as different molecules, including H20, water, but also gases such as methane or carbon dioxide. The first oceans could closely follow the giant impact. Indeed, the oldest known earth minerals contain “signatures”, that is to say indirect traces, of the presence of liquid water during their formation. The Earth has been blue for almost 4.4 billion years.
Whether 3.8 billion or 4.4 billion years old, the history of the oceans is inextricably linked with that of the Earth and of life. Today the oceans acidify) and heat up under the impact of CO emissions2 that disrupt natural cycles. The oceans will not disappear, but the life they currently support is seriously threatened. Our societies depend on subtle balances of mechanisms on the surface and within the Earth. It would seem that what took place over thousands, millions, even billions of years, could be drastically upset in a few decades by human activities.