Mars: we discover that its oxidation tells a terrible story

We learn that Mars is red, because it is oxidized, rusty. It actually has a high level of iron oxide in its soil. But why this oxidation and when did it begin, leading to Mars cooling and making it the planet hostile to life that we observe today?

Of recent studies have shown how our neighbor lost most of its atmosphere, but we are far from understanding everything about the tragedy which led it from an undoubtedly blue planet, in any case equipped with oceans, to a world red, cold and sterile it has become.

What is this climate transition that caused the Martian tragedy?

It is often said that Mars may have supported life in the past. That's true, but that would mean that something caused an extinction of life, causing the greatest living tragedy in the history of the Solar System!

Let's reverse the cosmic clock… The history of the Solar System begins 4.56 billion years ago, when our star was formed in the protosolar nebula under the effect of gravity. Some 10 million years later, the planets are there, and among them are Mars and Earth. This duo constitutes magmatic worlds; as it cools, the lavas release gases and a prebiotic atmosphere is formed.

According to our knowledge, the Earth would then have been orange and not blue, a bit like Titan Today. Then, about 4 billion years ago, the planets underwent the Great late bombardment the consequences of which we can still observe, particularly on the Moon which retains all the scars of its past troubles.

At that time, Mars had an atmosphere different from that of today, oxidizing and which gives it its rusty hue. This planet was less cold with episodes of heat, water in the plains, ice in its heights (not only at the poles) and an atmosphere that chemists describe as “reductive” (remember the courses on L'redox !).

An international study published in July 2024 by Nature Communication is precisely interested in the first Martian geological period, described as Noachien. The core of Mars is still liquid, water flows on its surface and recent giant craters, as well as impact basins, dot its surface…

During the Noachian, Mars' atmosphere consisted mainly of carbon dioxide (CO2) and hydrogen, a reducing gas that prevents oxidation because it traps oxygen and water can exist in the liquid phase. This atmosphere then causes a strong greenhouse effect and Mars is a good place to live – if we put aside the cataclysmic bombings. Research in 2007 showed that the soil of this world was poor in iron at that time.

We think that it is precisely liquid water that would have transported the iron, the latter being mobile depending on conditions of temperature, acidity and chemistry. The group of researchers therefore wanted to determine the different ages of Martian surfaces and their iron abundance to better understand climate transitions.

They thus showed that oxidation occurred during the Noachian era. They also found that iron abundance decreased with elevation (altitude) in early Mars, during the older Noachian era, but decreased with latitude (less iron toward the poles) in the more recent Noachian terrains.

This observation is correlated with that linked to the evolution of Martian temperatures: “Our finding suggests that the surface temperature of Mars gradually evolved from an altitude-dominant mode to a latitude-dominant mode, coupled with atmospheric oxidation during the Noachian.”they write. That is to say that during the Noachian, temperatures were mainly correlated with altitude, then with latitude. “We think atmospheric oxidation led Mars to become cold and bipolar in its history”they conclude.