For scientists searching for alien lifeforms, the siren song of Mars is climbing toward a crescendo. Multiple recent observations made by rovers on the red planet could bear the signatures of microbes—a possible indication that Earth is not the only refuge for life in the solar system.
One exciting glimmer was announced earlier this month: NASA’s Curiosity rover observed a mixture of carbon isotopes in the rocks of Gale crater that, if seen on Earth, would be a sign of life. The rover has also witnessed both random and seasonal surges of methane, a gas on Earth that is predominantly produced biologically.
About 2,300 miles away in Jezero crater, NASA’s Perseverance rover has spied strange purple coatings on the crater floor’s rocks. These coatings are widespread and resemble desert varnishes on Earth that grow in the presence of microbes.
For now, though, scientists aren’t ready to conclude that our vermillion neighbor was once inhabited. Just about every alluring hint of biology could also be explained by some as-yet unfamiliar aspect of Mars’s geology or chemistry—there’s just so much we don’t know about how the planet works, and how nonliving phenomena could be masquerading as life’s fingerprints.
“This is an alien world that we’re looking at, and so who knows what we haven’t even thought of,” says Curiosity deputy project scientist Abigail Fraeman of NASA’s Jet Propulsion Laboratory.
Scientists say the next step in probing Mars for life is bringing bits of the planet back to labs on Earth, where the sharpest instruments available can search for answers to one of humanity’s oldest questions. The Perseverance rover is already busy collecting the first set of samples, which could contain evidence that microorganisms lived in Jezero crater billions of years ago.
No matter the answer, it will tell us something profound about the origins of life on our own planet.
“So much of [the two planets’] ancient history is similar, and it’s so intriguing that in our planetary evolution, those pathways have diverged so greatly,” says astrobiologist Amy Williams of the University of Florida. “If there isn’t life on Mars, why not? What changed? What happened? Why wouldn’t it be there? And if it took hold, what happened to it?”
Is there life on Mars?
In our fantasies, Mars has almost always been inhabited—if not by aliens, then at least by our future selves. But spacecraft observations quickly snuffed out dreams of advanced civilizations, seasonally flourishing vegetation, or even benign, gelatinous vegetarians.
“We don’t have anything glowing, we don’t have anything saying hello, we had no ray-guns when we landed there,” says Andrew Steele of the Carnegie Institution for Science.
Instead, images from orbit and experiments conducted by NASA’s Viking landers on the planet’s surface made it clear that Mars was not a world awash in easily detectable life. “That kind of kicked a hole in Mars research for a very long time,” Steele says.
In 1996, scientists announced that a Martian meteorite recovered from Antarctica’s Allan Hills region appeared to contain microfossils—tiny, worm-shaped, mineralized signs that life had crawled across the planet’s surface some 4.1 billion years ago. Those observations were ambiguous and extremely divisive, provoking debates that persist to this day. But there was an upside.
“The Allan Hills controversy has really fueled so much of the astrobiology field,” says astrobiologist Kennda Lynch of the Lunar and Planetary Institute. “I feel so grateful to that rock, because it’s made us really, really think about what we know about life.”
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 gouge is home to a large mountain containing many layers of sediments that preserve a record of the Martian past. Curiosity’s primary goal is to search for signs of past habitability, such as water, organic compounds, and an energy source—the ingredients necessary for life as we know it.
Finding evidence of water was easy; after all, scientists already suspected the crater had once been filled by a deep lake. Curiosity almost immediately identified a swath of rocks that can form only when water is present.
The rest hasn’t been so simple.
Over the years, Curiosity has uncovered evidence in the crater for numerous organic molecules —the chemical building blocks for carbon-based lifeforms. And it has spotted signs of ancient hydrothermal activity, where heat and chemical compounds mixed with flowing water, creating possible energy sources.
The rover has also determined that methane gas in the crater rises and falls as the seasons change, and it has observed occasional, massive pulses of the gas, confirming Earth-based observations 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 Earth's surface are biological, so to try and change our mindset around and think about a world where that might not be true is really a challenge,” says astrobiologist Christopher House of Pennsylvania State University. “Once you get out of the Earth-centric mindset, then you can start to think of these other ways in which Mars might behave.“
The curious case of Martian carbon
Curiosity’s weirdest, most tantalizing observation only emerged recently. In multiple 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 skewed ratio in which the lighter form is much more abundant than the heavier form.
And in five locations in Gale Crater, scientists found the exact same thing: lighter carbon isotopes were much more abundant than their heavier cousins, relative to what scientists have seen in the Martian atmosphere and in meteorites. The observations resemble carbon ratios collected from 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 you do this 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 indeed come from ancient microbes. Another possibility is that the solar system long ago sailed through an interstellar dust cloud with a peculiar carbon isotope ratio—such clouds are known to exist—and it left its traces on Mars. And a third possible explanation is that ultraviolet light interacting with Mars’s carbon dioxide atmosphere produced the odd signature.
“We don’t know the answer,” House says. “It may be biological, and it may not be biological. All three explanations fit the data.”
A mysterious coating on the rocks
NASA’s Perseverance rover arrived at Jezero crater on Mars last year, and it’s also on the hunt for signs of ancient life.
During its travels through Jezero, Perseverance spied numerous rocks with a purple, iron-rich coating. Purdue University’s Bradley Garczynski, who is studying the coating, says it’s unlike anything that rovers have spotted on Mars before—even though rocks with different coatings have been seen on other parts of the planet.
On Earth, such coatings are often observed 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 are then of great astrobiological interest to us when we see them forming on other worlds,” Williams says.
Lynch, who studies terrestrial analogs of Martian environments, says it wouldn’t be out of the question to find biosignatures in the Jezero rock varnishes. “Microbes do amazing things. They put coatings and varnishes on the rocks because they like to eat the rock,” she says.
However, scientists have a lot more context about the environments on Earth in which such varnishes form, Lynch says, and that context is crucial for properly interpreting an observation. Even on our own planet, investigators need to rigorously evaluate whether such a material were produced by life or by some other process. That’s a much tougher question to answer from afar.
“It’s a wonderfully complicated and complex system that we’re exploring on Mars,” Fraeman says.
Ambiguity from another world
For now, definitive detection of life requires bringing pieces of Mars back to Earth, where scientists can use the most capable instruments available to scrutinize them. One of Perseverance’s primary tasks is to identify and collect rock samples for a future spacecraft to send home.
“The samples we’re collecting now, they’re being very carefully selected,” Fraeman says. “We know broadly the context that they’re coming from. … That’s going to be key to pulling apart these big questions.”
But even having chunks of Mars in the laboratory isn’t a salve for ambiguity. Scientists are still arguing about what might or might not have lived in ALH84001, that chunk of ancient Martian crust that crashed into Antarctica some 13,000 years ago. Steele, who recently led a fresh analysis of the meteorite, has been studying the rock for 25 years.
“One of the reasons I kept looking at is: If it’s not life, what is it?” he says.
Steele and his colleagues reported earlier this month that complex organics in ALH84001 had been crafted without life’s input, and that ordinary chemical reactions that occur when subterranean fluids interact with rocks and minerals were to blame.
“Does that mean there is no Martian life in that meteorite? Well, no I can’t prove that,” Steele says. “If a Martian organism exists in there, it’s not showing us something that is common to Earth organisms. It’s something totally different, and I’m still on the lookout for it.”
Might such geologic reactions be the source of Martian methane, or the organics that litter the planet, or the rock coatings in Jezero? It’s completely plausible, astrobiologists say. Mars is another world, a place with exotic chemistry and landscapes that, even though they look vaguely familiar, are still otherworldly.
“Time and again, Mars has demonstrated that it is not Earth. It is not an ancient Earth frozen in time,” Williams says. “It is its own, evolving planet, and the processes that are occurring there, some are still Earth-like, and some are very alien.”
Editor's Note: Kennda Lynch's affiliation has been corrected to the Lunar and Planetary Institute.