Where did the lost atmosphere of Mars go? The latest discovery is that it may be "hidden" on Mars

Produced by: Science Popularization China

Author: Earth's Gravity

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

In our current understanding, Mars is a red, desolate planet.

However, as we have been studying Mars more and more in recent years, scientists have realized that Mars was once a planet rich in liquid water . Moreover , the content of liquid water is extremely large, not only forming large-scale rivers on the surface of Mars, but also creating a huge ocean. **As for the evidence of this liquid water, ordinary people like us can directly see it through the photos sent back by the Mars rover.

Remains of rivers on Mars

(Image credit: NASA)

River deltas on Mars

(Image credit: NASA)

What does it mean to discover that Mars was once rich in liquid water?

Since liquid water can exist on the surface of Mars, it means that the temperature on Mars must have been relatively warm at that time, otherwise the liquid water would have been frozen into an ice shell long ago! Further reasoning, in order to keep the surface temperature of Mars suitable, the atmosphere of Mars must also be similar to that of the modern Earth, dense and warm .

Through scientific observations, geologists speculate that the carbon dioxide concentration on Mars at that time was between 0.25 and 4 bar (bar, 1 bar is approximately equal to one atmosphere, which is the normal atmospheric pressure on our surface). At present, the carbon dioxide concentration on Mars is only 0.054 bar. This shows that Mars once experienced a huge gas loss.

The thin atmosphere on Mars

(Image credit: NASA)

Where did the huge amount of gas on Mars go?

Scientists believe that the decrease in carbon dioxide on Mars occurred about 3.5 billion years ago, which is also the time when the water on Mars began to disappear rapidly.

But why did the carbon dioxide disappear from Mars?

Previously, there was a theory that it might be caused by the solar wind constantly stripping away the atmosphere on Mars. However, according to recent research, scientists used the monitoring results of the Martian atmospheric escape data between 2007 and 2017 to calculate and found that the solar wind could only take away 9 millibars of atmosphere at most in the past 4 billion years - this is two orders of magnitude lower than the actual atmospheric loss on Mars.

Therefore, the disappearance of carbon dioxide on Mars remains a mystery.

Now, two MIT geologists have proposed a possible answer: Mars’ missing carbon dioxide may be locked up in layers of clay on the planet’s surface.

The evolution of liquid water on Mars. The numbers 4.0, 3.8, etc. represent 4 billion years, 3.8 billion years, etc.

(Image credit: NASA)

Locking carbon dioxide to generate methane, similar discoveries have also been made on Earth

This statement sounds counterintuitive, but geologists have actually discovered a similar process on Earth: carbon dioxide reacts chemically with certain rocks to form methane .

Although in common sense, methane is closely related to biological processes, either directly formed in organisms, or produced by microbial digestion in an anaerobic environment after the organic matter of the organisms is buried after death. Therefore, methane formed by the latter factor is often the main component of natural gas.

Global methane generation sources and reduction factors in 2017, with sources on the left and reduction factors on the right

(Image source: Wikipedia)

However, in addition to the methane from the above causes, geologists have long ago discovered large amounts of methane in Precambrian non-sedimentary strata in South Africa, Canada and Finland.

The reason why we emphasize non-sedimentary strata is that the fossil fuels such as oil and natural gas that we have discovered so far are all deposited on the bottom of the lake after the death of organisms, and the rocks formed by sediments are therefore called sedimentary rocks. Non-sedimentary strata refer to strata formed by magmatic activity or metamorphism of magmatic rocks (that is, the process of rock changes caused by high temperature and high pressure). Obviously, there are basically no organisms in these strata.

The reason why the Precambrian period is emphasized is that in the era older than the Cambrian period (about 540 million years ago), the life on Earth was relatively rare, and was mainly tiny algae and bacteria.

Therefore, the large amounts of methane found in Precambrian non-sedimentary strata are clearly not of biogenic origin.

It has nothing to do with living things, so how is this methane formed?

In order to find out how these methanes are formed, geologists have conducted long-term research. The results show that the rock formations with a large amount of methane are basically ultramafic rock formations. The so-called ultramafic refers to rocks that contain a large amount of magnesium and iron (olivine is one of the most typical minerals), and the magma that formed these rocks comes from the mantle.

After the formation of rock layers, they will arrive at the mid-ocean ridge, subduction zone and other areas due to plate movement. These areas are rich in water and the temperature is not particularly high (0~600℃), so the rocks will undergo metamorphism. They will form serpentine through a series of chemical reactions, which is called serpentinization. In this process, the iron compounds contained in the rock will react with water to form hydrogen.

Olivine is not only a gemstone, but also one of the important minerals that make up ultramafic rock. Its green color comes from the iron contained in it.

(Image source: Wikipedia)

One possible reaction to form hydrogen

Once hydrogen is formed, it can form methane through the Sabatier reaction. The Sabatier reaction was discovered by French chemist Paul Sabatier and others in 1897. It is a reaction between hydrogen and carbon dioxide at 300-400°C and high pressure, through a nickel catalyst, to produce methane and water. If a catalyst such as aluminum oxide is added, the reaction rate will be greatly accelerated. Based on this analysis, some geologists believe that the amount of methane formed deep underground is no less than that formed by biological causes.

Chemical equation for the Sabatier reaction

Scientists infer that Martian clay plays the same role

Through Mars exploration, scientists have discovered that there are also a large number of olivine-rich rocks on Mars. If these rocks undergo serpentinization, they will absorb a large amount of water and carbon dioxide. According to calculations, if all rocks within a depth of 2 kilometers on the surface of Mars undergo serpentinization, this will reduce the carbon dioxide in the Martian atmosphere by about 5 bars and generate a large amount of methane.

A large part of this methane can be absorbed by the clay that exists in large quantities on Mars after it is formed. Clays on Earth are of various types and contain a lot of minerals. For example, kaolinite, which we are very familiar with, is a clay mineral. In addition, there are montmorillonite, chlorite, dickite, perlite, saponite, etc. According to research, among the clay minerals on Mars, 62% are montmorillonite and 23% are chlorite.

Scanning electron microscope photo of montmorillonite, showing that it is a flaky mineral

(Image source: Wikipedia)

The atomic structure of montmorillonite, the large number of gaps between atoms is the source of its adsorption force

(Image source: Wikipedia)

Montmorillonite is formed by the weathering of silica-rich minerals in igneous rocks. It is a flaky, porous mineral that can be imagined as a sponge among minerals. And according to research, montmorillonite has very good adsorption properties, and the maximum amount of methane it can adsorb is about 0.6% of its weight, far exceeding other clay minerals such as illite or chlorite.

However, due to the limited exploration of Mars, scientists do not know the thickness of the clay layer and can only estimate and calculate it. They believe that the average equivalent thickness of montmorillonite on Mars is in the reasonable range of 117-1440 meters. If the lower limit of montmorillonite thickness is 117 meters, these montmorillonites can absorb 0.07 bar of carbon dioxide, and if the upper limit is 1440 meters, it can absorb 1.7 bar of carbon dioxide.

Modeling of methane formation on Mars and its adsorption by clay

(Image source: Reference 1)

Based on further research on carbon isotopes and hydrogen isotopes, scientists have inferred that there may be about 0.4 to 1.5 bar of carbon dioxide in the original atmosphere of Mars that was absorbed into clay minerals.

In the future, it is expected that methane will be mined and utilized on Mars

This research is actually of great significance to our future exploration and development of Mars, because on the one hand, it means that we are likely to directly mine fossil fuels (methane) from Mars for use, without having to transport it from the distant Earth . Furthermore, if we consider the news a few years ago that Chinese scientists used carbon dioxide to synthesize starch, then in the future we can even use methane as energy directly and extract the thin carbon dioxide in the Martian atmosphere to make food on Mars .

In addition, methane is a greenhouse gas with a strong greenhouse effect. In a 20-year period, the global warming potential (GWP) of methane is 83 times that of carbon dioxide, which means that in 20 years, the global warming effect of one ton of methane is equivalent to 83 tons of carbon dioxide. This ability of methane is naturally extremely dangerous on Earth, but on Mars it can greatly help us speed up the progress of terraforming on Mars.

References:

1.Murray J, Jagoutz O. Olivine alteration and the loss of Mars' early atmospheric carbon[J]. Science Advances, 2024, 10(39): eadm8443.