Atmospheric oxidation and the creation of modern Mars

Like Earth, Mars formed about 4.5 billion years ago, but its initial surface was very different from today. The surface of Mars then experienced high rates of meteorite and asteroid impacts dating from the period known as the Late Heavy Bombardment. But while today’s Mars is cold, arid, has two icy poles and an oxidizing atmosphere (where, for example, iron-rich materials develop rust), early Mars was characterized by icy highlands, episodic heat and a reducing atmosphere.

The cause of the climate transition is unclear. Now a Chinese research team is discovering evidence that atmospheric oxidation made Mars cold and bipolar early in its history. Their work is published in Natural communications.

The Noachian is an early period on Mars with high rates of asteroid impacts and possibly the presence of an abundance of surface water. The exact interval is uncertain, but the Noachian era was probably between 4.1 and 3.7 billion years ago. It was followed by the Hesperian Era, 3.7 to 3.0 billion years ago.

Evidence emerging in recent years suggests that early Mars contained carbon dioxide (CO₂) dominant atmosphere with reducing gases such as hydrogen. In a reducing atmosphere, oxidation is prevented by the absence of oxygen and other oxidizing gases, and a reducing gas such as hydrogen, carbon monoxide and methane will readily capture any oxygen and, in the case hydrogen, will transform into a water phase. .

A reducing atmosphere can create a strong greenhouse effect, which may have played an important role in the warming of early Mars. As the intensity of greenhouse warming is closely linked to the presence of reducing gases, the oxidation of the atmosphere would then have led to the cooling observed today. (The greenhouse effect on Mars today is only about 8°C, compared to about 33°C on Earth.)

The Mars Odyssey Gamma Ray Spectrometer is a measuring instrument that resides on the 2001 Mars Odyssey spacecraft, orbiting Mars since 2001, returning data on the geology of the first 30 cm of the surface of Mars, such as l Water location and identification elements.

Previous research found that surface iron abundances in terrains identified as Noachian were relatively low compared to Hesperian terranes (3.7 to 3.0 billion years ago). ) and the Amazonian era (3.0 billion years ago to present), as well as the average abundance of iron on the global surface.

Why was iron abundance in early Mars low compared to the last 3 billion years? The evolution of the earth’s crust does not explain it. This could have been caused by liquid water in early Mars, which could have transported iron below the 30 cm depth range of the spectrometer, because iron mobility is affected by temperature, acidity, water chemistry and the redox state, which is the oxidation state. of an element which determines its chemical behavior. These could have determined the distribution of iron abundance on the surface of Mars, especially if the atmosphere had played a role.

With his colleagues, Jiacheng Liu of the University of Hong Kong examined the distribution of iron on the surface of ancient Mars in space and time, using results from the Mars Odyssey spectrometer. They found that iron abundance decreased with elevation in early Mars, during the older Noachian era, but decreased with latitude in younger Noachian terrains.

Why this difference?

The group used the global geologic gap of Mars to determine the relative surface age in grids arranged on the surface and the relative abundance of iron. These data provided a better understanding of the relationships between redox transitions and climate mode transitions.

“Our finding suggests that the surface temperature of Mars gradually evolved from an altitude-dominant to a latitude-dominant mode, coupled with atmospheric oxidation during the Noachian period,” they wrote.

One of the group’s suggestions is that glacial weathering and “low temperature conditions contributed to surface iron depletion, likely facilitated by anoxic leaching during freeze-thaw cycles under a reducing atmosphere.” .

Leaching is a process widely used in extractive metallurgy, where pure metals are extracted from their natural mineral deposits (ore). (An example is smelting in a blast furnace to produce cast iron, which is used to make steel.)

Further analysis led them to argue that although low pH levels (below 3) of water can mobilize iron, acid leaching cannot fully explain iron depletion across a wide region of the surface of Mars. The decrease in the intensity of iron leaching from the Lower to Upper Noachian suggests progressive atmospheric oxidation; As Mars’ atmosphere oxidized, its greenhouse effect diminished, eventually leading to the cold, dry planet we see today, with ice at both poles.

“Oxidation of Mars’ surface and atmosphere made Mars cold with the migration of ice from the highlands to the polar regions,” Liu said. He points out that some scientists believe the periglacial environment – ​​Mars’ basement beneath the thick cryosphere, which combines long-term, stable liquid water and heat – could be habitable for life.

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