Curiosity detects new organic molecules: a step closer to the discovery of past life on Mars?

Since 2012, the Curiosity rover part of NASA's Mars Science Laboratory mission has been surveying the Martian soil. The aim of this miniaturised, automated, wheel-mounted laboratory is to determine the past and present habitability of Earth's neighbouring planet.

One of its onboard instruments, SAM (Sample Analysis at Mars), partly developed and used by scientists from LATMOS and the Process Engineering and Materials Laboratory (LGPM - Univ. Paris-Saclay/CentraleSupélec), has just made an unprecedented discovery. For the first time, SAM has detected long-chain linear hydrocarbons in a Martian soil sample. These are organic molecules, i.e. they are composed of a succession of carbon atoms - in this case 10, 11 and 12 atoms - bonded to hydrogen atoms.
 

Cumberland, a promising sample

The sample in question was collected from the floor of Gale Crater, where the rover landed over ten years ago. Obtained by drilling, it was analysed using Curiosity's various instruments, which revealed that the soil in this region, known as Cumberland, contains around 20% clay. Clays are well known for their ability to preserve organic molecules. "These are sheet-like structures that leave space between them for water to circulate, when it is present, which was the case on Mars at the time this rock was formed. As the water flows through, it carries molecules that are captured by the clay sheets and remain trapped there when the water evaporates," explains Caroline Freissinet, a researcher at LATMOS. In this way, clays preserve organic molecules over the long term.

In addition to its clayey nature, the Cumberland rock also holds a promising history for potential discoveries. The rock was formed around 3.7 billion years ago. At that time, conditions on Mars were similar to those on Earth, where life was beginning to appear. Subsequently, the rock was buried, protecting it from solar and galactic radiation which, in the absence of a dense atmosphere, would otherwise destroy any organic molecules down to a depth of two metres beneath the Martian surface. The Cumberland rock only resurfaced 80 million years ago, which is a relatively short period on the geological timescale and ensures good preservation of potential organic molecules.

SAM, a miniaturised chemistry laboratory

But how can the presence of organic molecules in a sample of Martian soil be verified without it leaving the Red Planet? That's exactly the role of SAM, a miniaturised, automated chemistry laboratory carried aboard Curiosity. The sample is first heated to 850°C in an oven, in order to volatilise any molecules present, that is, to transform them into a gaseous state. Once in gas form, the molecules pass through a gas chromatography column, a hair-thin tube along which different molecules are retained to varying degrees depending on their properties (mass, charge, polarity, etc.). The instrument separates the various molecules, which are then analysed using mass spectrometry, a technique that identifies them based on their mass-to-charge ratio.

This is how LATMOS scientists detected the presence of small organic molecules in an initial Cumberland sample. At that stage, no larger molecules were observed due to the high oxidation conditions on Mars. Indeed, heating the sample releases oxygen trapped in the rock in the form of perchlorates, which oxidise any organic molecules that may be present.

To overcome this problem, the scientists decided to change the analysis protocol, deoxygenating the sample beforehand. Caroline Freissinet explains: "The experiment was carried out in two stages: a preheat to 475°C to decompose the perchlorates and eliminate the oxygen released on Mars, then the same sample was heated to 850°C. This time, with no oxygen remaining, we saw the molecules we were looking for: linear alkanes with 10 to 12 carbons." The researcher points out that this experiment owes its success to SAM's versatility. When the module was designed more than 20 years ago, scientists had not anticipated the presence of perchlorates on Mars. Ultimately this oversight highlights the importance of designing systems that can be easily adapted to respond to unforeseen circumstances.
 

Laboratory confirmation of the hypotheses

However, the discovery of long-chain linear alkanes using SAM's modified protocol raises questions for the LATMOS researchers: were these alkanes present in the rock in their current form, or are they the result of the degradation of carboxylic acids during the sample heating stage? While the second option seems more likely to scientists, since carboxylic acids are more stable, especially under the oxidising conditions of the Martian soil, they need to verify this hypothesis.

To investigate this, they replicated SAM's experiment on Mars in their laboratory on Earth, in Guyancourt. Using terrestrial clay, they injected a solution of carboxylic acids and subjected it to the same treatment as the Martian clay (double heating, chromatography, mass spectrometry). At the end of the experiment, they detected the same alkanes, confirming that the molecules found on the Red Planet could indeed exist in the form of carboxylic acids. By extension, they hypothesise that these carboxylic acids may themselves come from larger organic molecules. As Caroline Freissinet points out, "Martian soil, which is highly oxidising, tends to break down larger molecules, forming carboxylic acids."
 

A step closer to detecting life on Mars? 

It is not currently possible for scientists to decide on the chemical or biological origin of the alkanes, and by extension the carboxylic acids, discovered in the Cumberland rock sample. While such carboxylic acids exist on Earth, especially in bacterial membranes, other chemical reactions known to scientists may also explain their presence.

Nevertheless, this discovery represents a major step forward in the search for past life on Mars. Indeed, the molecules found in the sample are long and linear, and therefore unstable. Despite this, they have been preserved for 3.7 billion years. "This means that if there was life on Mars at that time, when there was liquid water and an atmosphere on the planet, molecules of biological origin could have been preserved to this day, and we would be able to detect them," enthuses the researcher. In her view, future Mars exploration missions will help to determine whether these molecules are of biological or chemical origin.

These missions include ExoMars, supported by the European Space Agency (ESA) whose Rosalind Franklin rover is scheduled for launch in 2028. This will drill to a depth of two metres, gaining access to rock protected from destructive ionizing radiation. Finally, scientists hope that in a few years' time, samples collected by NASA's Perseverance rover, on Mars since 2021 as part of the Mars 2020 mission, will be brought back to Earth. Caroline Freissinet confirms: "If we had the Cumberland rock on Earth, we could analyse it with our state-of-the-art laboratory instruments, which are more powerful than the miniaturised instruments on board Curiosity. If there had been life on Mars, we would then be able to find out."  Exploration continues, but the answer has never been closer.
 

Référence :

C. Freissinet et al., Long-chain alkanes preserved in a martian mudstone, PNAS, 2025.