Fundamental Science: When the Earth Turned Blue

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The first photos of space exploration immortalized the Earth as a blue marble. An extraterrestrial approaching it in the distant past, however, would not see the blue seas covered by white clouds. Our planet is about 4.5 billion years old, and today we know that it has always been in continuous change, the chemical elements in permanent exchange between the biosphere, the atmosphere, the lithosphere and the hydrosphere, and the tectonic processes changing all the time. your physiognomy.

What would Earth have been like? For almost half of its life, during the Hadean and Archean, the Earth’s atmosphere was quite different from today’s. Oxygen was practically non-existent, as evidenced by minerals in sedimentary rocks older than 2.5 billion years that are not stable in the presence of this element.

Today our atmosphere is about 21% oxygen. Without it, we wouldn’t be here. Where would he have come from?

Fossiliferous and molecular evidence suggests that between 3.5 and 2.5 billion years ago a radical evolutionary change took place. Simple organisms emerged, the cyanobacteria, which use sunlight as a source of energy using carbon dioxide and water as reactants, in the process known as photosynthesis. A by-product of photosynthesis is the oxygen produced by the breakdown of carbon dioxide and water molecules: for the first time a significant source of oxygen was generated.

One of the first fates of this oxygen was to react with huge amounts of iron and other elements dissolved in seawater. Iron is soluble in oxygen-poor conditions, so the ancient seas were not blue, but had greenish or reddish hues, given the high concentration of this metal. Once oxygen had come into contact with all this iron, the two elements combined to form iron hydroxides, which in turn are insoluble in water. The result was the precipitation of gigantic amounts of iron, which ended up being driven to the bottom of the oceans and transformed into mineral deposits.

Great Oxygenation Event, or GEO, is the name scientists give to this period when the oceans were “cleansed” of iron and other metals like manganese. From that moment on, the atmosphere started to present small amounts of oxygen – oxygenation at levels similar to the current ones happened much later, about 500 million years ago.

Oxygen, however, is an extremely reactive element. No wonder we try to consume products and cosmetics with antioxidants, such as wine and açaí: we want to avoid aging associated with oxygen free radicals. Rust is nothing more than the process of oxidation of metals, and combustion is nothing but rust at ultra-accelerated speed. Most of the simple organisms that existed before the emergence and proliferation of cyanobacteria probably had no protection against this gas – they lived in anoxic environments, without oxygen. GEO may have caused one of the planet’s first mass extinctions, which is why it is sometimes called the “Oxygen Catastrophe”.

What was the atmosphere like before the emergence of cyanobacteria? Geological evidence suggests a planet rich in gases such as CO2 and methane, as well as nitrogen, now the largest component of the atmosphere. In addition to the greenish seas, such a composition of the atmosphere may have generated not white clouds, but orange ones, as on Venus. The extraterrestrial would see a planet with green seas and orange clouds.

In 1972, astronomers Carl Sagan and George Mullen suggested that at the beginning of the planet’s life, sunlight was about 70% of its current strength. Calculations indicated that for much of its history the Earth must have been completely frozen. Geological records, however, indicate the existence of free water on its surface since practically its formation. If sunlight was much weaker, how could water have become a liquid?

A plausible explanation for this “Faint Young Sun Paradox” is due to the atmosphere’s distinct composition: both CO2 and methane are greenhouse gases, and their greater amounts in the ancient atmosphere may have ensured the Earth was warm enough to harbor liquid water. from its beginnings. In these waters, photosynthetic cyanobacteria appeared, which, consuming most of the CO2 in the atmosphere, allowed the increase of O2. And this one, when in contact with methane and other reducing gases, forever changed the composition – and the face! – of our planet.

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Fabrício Caxito is a professor of geology and a student of philosophy at UFMG.

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