Mars' surface contains more organic matter than previously believed

Mars, a planet often associated with a lifeless, dusty expanse, holds a puzzling amount of organic matter on its surface. Despite the absence of visible signs of life, the Red Planet’s geological features, such as deltas, lakebeds, and river valleys, strongly suggest a past where water once flowed abundantly.

To unravel the mysteries of Mars’ history, scientists have been diligently examining sediments preserved near these formations, hoping to find clues about the early environmental conditions and the processes that shaped the planet over time.

Curiosity rover’s unexpected discovery

A new and intriguing study led by Professor Yuichiro Ueno from Tokyo Institute of Technology and Professor Matthew Johnson from the University of Copenhagen shed light on a peculiar finding made by NASA’s Curiosity rover.

The rover collected sediments from Gale Crater, believed to be an ancient lake formed approximately 3.8 billion years ago due to an asteroid impact.

Analysis of these sediments revealed the presence of organic matter, but with a significantly lower amount of the carbon-13 isotope (¹³C) relative to carbon-12 isotopes (¹²C) compared to what is found on Earth.

“On measuring the stable isotope ratio between ¹³C and ¹²C, the Martian organic matter has a ¹³C abundance of 0.92% to 0.99% of the carbon that makes it up,” explains Ueno.

“This is extremely low compared to Earth’s sedimentary organic matter, which is about 1.04%, and atmospheric CO₂, around 1.07%, both of which are biological remnants, and are not similar to the organic matter in meteorites, which is about 1.05%,” Ueno continued.

Carbon isotope conundrum

To understand the discrepancy in carbon isotope ratios, the researchers conducted laboratory experiments simulating different conditions of the Martian atmosphere’s composition and temperature.

They discovered that when ¹²CO₂ is exposed to solar ultraviolet (UV) light, it preferentially absorbs UV radiation, leading to its dissociation into CO depleted in ¹³C, leaving behind CO₂ enriched in ¹³C.

This phenomenon, known as isotopic fractionation, is also observed in the upper atmospheres of Mars and Earth, where UV irradiation from the sun causes CO2 to dissociate into CO with depleted ¹³C content.

In a reducing Martian atmosphere, CO transforms into simple organic compounds such as formaldehyde and carboxylic acids.

Recreating Mars organic matter in the lab

Using model calculations, the researchers found that in an atmosphere with a CO₂ to CO ratio of 90:10, a 20% conversion of CO₂ to CO would lead to sedimentary organic matter with δ¹³C_VPDB values of -135‰.

Also, the remaining CO₂ would be enriched in ¹³C with δ¹³C_VPDB values of +20‰. 

These values closely match those seen in sediments analyzed by the Curiosity rover and estimated from a Martian meteorite, indicating that the main source of organic matter formation on early Mars was an atmospheric process rather than a biological one.

“If the estimation in this research is correct, there may be an unexpected amount of organic material present in Martian sediments. This suggests that future explorations of Mars might uncover large quantities of organic matter,” says Professor Ueno.

Mars organic matter and future exploration

The findings of this study have significant implications for our understanding of Mars’ past and the potential for future discoveries.

As scientists continue to explore the Red Planet, they may uncover even more evidence of organic matter, shedding light on the complex processes that shaped the Martian environment billions of years ago.

While the presence of organic matter alone does not confirm the existence of past life on Mars, it serves as a tantalizing clue, urging us to delve deeper into the planet’s history.

As we continue to unravel the mysteries of Mars, we inch closer to answering the age-old question: Did life ever exist beyond Earth?

The full study was published in the journal Nature Geoscience.

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