Why signs of life on Mars remain so mysterious

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A new era of Mars exploration began in 2012 when NASA’s six-wheeled Curiosity rover landed in Gale Crater. Today, the 96-mile-wide gulf is home to a large mountain containing many layers of sediment that preserve a record of the Martian past. Curiosity’s main goal is to search for signs of past habitability, such as water, organic compounds and a source of energy, the ingredients necessary for life as we know it.

Finding evidence of water was easy; after all, scientists already suspected that the crater had once been filled by a deep lake. Curiosity almost immediately identified a band of rocks that can only form in the presence of water.

The rest was not so simple.

Over the years, Curiosity has discovered evidence in the crater of many organic molecules – the chemical building blocks of carbon-based lifeforms. And he spotted signs of ancient hydrothermal activity, where heat and chemicals mixed with running water, creating possible energy sources.

The rover also determined that methane gas in the crater rises and falls with the seasons, and it observed occasional massive pulses of the gas, confirming observations from Earth that have defied explanation for more than a decade. Such a fluctuation on Earth would be a strong sign of beings with active metabolisms.

However, none of these observations have so far been linked to biology, and there’s always a chance that processes we don’t fully understand are mimicking the signatures of life.

“Most carbon-related processes on the Earth’s surface are biological, so trying to change our mindset and think about a world where that might not be true is really challenging,” says astrobiologist Christopher House. from Pennsylvania State University. “Once you get out of the Earth-centered mindset, you can start thinking about these other ways Mars might behave.”

The curious case of Martian carbon

Curiosity’s weirdest and most tantalizing sighting only recently emerged. In several rock samples from various locations in the crater, the rover found organic compounds containing odd ratios of carbon isotopes, or atoms of the same element that contain different numbers of neutrons in their nuclei.

On Earth, organisms prefer to use the lighter form of carbon in metabolic or photosynthetic reactions, leading to a biased ratio in which the lightest the form is much more plentiful than the heaviest form.

And at five locations in Gale Crater, scientists found exactly the same thing: lighter carbon isotopes were far more abundant than their heavier cousins, compared to what scientists have seen in the Martian atmosphere and in meteorites. The observations resemble carbon ratios collected in Australia’s Tumbiana Formation, a 2.7 billion-year-old outcrop that contains the carbon signatures of ancient methane-metabolizing microbes.

“These really depleted carbon isotope results are so intriguing. So compelling. On Earth, the only way to do that is with biology,” Williams says.

But House, who led the analysis, says the story is far from clear. He and his colleagues offered three possible explanations for the imbalance.

The first is that the signature does come from ancient microbes. Another possibility is that the solar system sailed long ago through an interstellar dust cloud with a particular carbon isotope ratio – such clouds are known to exist – and left its mark on Mars. And a third possible explanation is that ultraviolet light interacting with Mars’ carbon dioxide atmosphere produced the strange signature.

“We don’t know the answer,” House said. “It may be organic, and it may not be organic. All three explanations match the data.

A mysterious coating on the rocks

NASA’s Perseverance rover arrived at Jezero Crater on Mars last year, and it’s also looking for signs of ancient life.

During his travels through Jezero, Perseverance saw many rocks with an iron-rich purple coating. Bradley Garczynski of Purdue University, who studies the coating, says it’s unlike anything rovers have spotted on Mars before, although rocks with different coatings have been seen on other parts of the planet. planet.

On Earth, such coatings are often seen in deserts, where conglomerates of rock-munching microbes thrive.

“They’re really intriguing, and they’re certainly on Earth of biological interest, so by translation, they’re then of great astrobiological interest to us when we see them forming on other worlds,” Williams says.

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