In 1877, Italian astronomer Giovanni Schiaparelli turned his 21.8-centimetre telescope – one of the finest of the time – on the enigmatic disk of Mars.

Scientists had long known that rather than simply being a point of light in the sky Mars was an entire world unto itself, but Schiaparelli was the first to attempt to map it in detail.

He observed dark areas, which he presumed to be seas, connected by linear features hundreds of kilometres long. He dubbed the latter canali, a term that technically means channels, but was translated into English as “canals.”

In the 1870s and ’80s, Schiaparelli mapped Mars again and again, convincing himself that the canal system was rapidly expanding – much as if an advanced civilisation were desperately trying to preserve its water supply in the face of drought.

Even at the time, many of Schiaparelli’s colleagues were dubious, wondering, in the words of US astronomer David Weintraub in his 2018 book Life on Mars (Princeton University Press), whether these features were simply “the result either of bad optics in Schiaparelli’s telescope or in his own head.”

But Schiaparelli’s vision captured the public imagination. Others would even suggest that the Red Planet’s colour was due to ruddy vegetation, much as if it were covered in Japanese maples. In 1938, Orson Welles’ radio adaptation of War of the Worlds panicked hundreds of thousands of listeners, convincing them that death-dealing Martian “tripods” were on the verge of showing up at their doorsteps.

In 1976, when NASA’s Viking 1 orbiter provided the first good images of Mars, one, dubbed the “Face on Mars”, entered tabloid infamy as proof that humanoid aliens once existed on our planetary neighbour, creating giant structures that would put the ancient Egyptians to shame.

We now know that the Face on Mars, like the canals, was a trick of light and shadow. But the search for life on the planet continues to tantalise. Orbiting spacecraft and landers have proven that Mars was once remarkably Earthlike, with oceans, lakes and rivers, plus an atmosphere considerably denser than the thin film it has today.

The Red Planet’s earliest epoch is now officially dubbed the Noachian – a term designed to conjure images of vast amounts of water.

Today, the burning question isn’t whether Mars might once have been habitable – at various times in its distant past, it most certainly was – but whether it might have developed life before its climate became too cold and dry. If so, that would be evidence of what astrobiologists call a “second genesis” of life (the first being our own).

Even if that second genesis never developed beyond single-celled microorganisms, it would mean that life arose at least twice in our own solar system. And if that happened here, how often might it have occurred on the thousands of planets astronomers are finding, circling distant stars? And, how often might some of those microorganisms evolved into creatures like us?

The easiest way to find life on Mars would be if a multi-tentacled something from a science-fiction writer’s dream jumped out from behind a rock and waved to us: “Welcome, Earthings, here I am!” Second best would be if a rover were to scoop up a soil sample and see a bunch of wriggling microorganisms.

But the surface of Mars is an extremely harsh environment, and signs of life, if it exists or ever existed, could be hard to detect. But that doesn’t mean there aren’t a number of well-thought-out ways to hunt for it.

1 | LOOKING FOR STRUCTURES IN ROCKS

On Earth, this means fossils. “Dinosaur bones,” says Jorge Vago, project scientist for the European Space Agency’s (ESA) ExoMars project. “If you see something like that, you can tell it was alive.”

But sadly, that won’t apply to microorganisms. “You would need an electron microscope to see them,” Vago says, “and you can’t fly that to Mars.” Even if you could, “they are little rods and spheres and there are all kinds of processes that have nothing to do with life that can produce rods and spheres”.

Intensive monitoring of Mars has so far failed to provide evidence of life, either now or in the past. NASA

That was exactly the problem in 1984, when scientists found a 1.9-kilogram meteorite in the Allan Hills region of Antarctica: a meteorite that proved to be a chip blasted off the surface of Mars by an ancient asteroid impact.

When electron microscope images showed rod-shaped structures that looked a lot like fossilised microbes, scientific excitement was so intense that even US President Bill Clinton spoke about it in a White House briefing. Then it all went bust.

“It was pretty quickly shown to be something not related to Mars life,” says Abigail Allwood, an Australian geologist and astrobiologist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “It was either terrestrial contamination of the rock, or not biological.”

Since then, Allwood says, other features in the Allan Hills meteorite have been suggested to have had biological origins, but these too have been shot down by arguments that they could be the result of geological processes.

The problem, she says, is that Mars meteorites are simply rocks, ripped out of their geologic settings. “If we had some understanding of the context in which the rocks formed,” she adds, “we would be able to determine whether the biological or abiological hypothesis was correct. The problem with meteorites is we don’t have that context.”

This problem, however, doesn’t apply to rovers operating on the surface of Mars, which might be able to detect imprints left by entire colonies of microbes. “Not one microorganism,” says Vago, “but billions of them.”

Such formations have been found on Earth, in places like the Pilbara Terrane of Western Australia, where a team led by Allwood has detected features known as stromatolites – mound-like structures formed by mats of single-celled organisms – in rocks 3.43 billion years old

Vago suggests that similar formations could be found on Mars, particularly in regions that were

once lake-bottoms, close to ash-spewing volcanoes. “The way the ash settles is different if there is life,” he says. “If there is no life, ash would settle at the bottom and the layers would form roughly horizontal horizons.”

But if there are colonies of microbes on the lake bottom, these microorganisms could wind up trapping sediment grains into stromatolite-like structures, “an imprint that tells you that microbes were there”.

2 | BIOSIGNATURES IN ANCIENT ROCKS

Nearly as good as finding a fossil would be finding rocks containing chemicals related to life.

Not that scientists would be looking for chemicals identical to our own lipids, proteins, and DNA. Rather, they’d be looking for remnants of whatever Mars life might have used in lieu of such chemicals. These remnants, which might be hardy enough to persist billions of years, might have four traits that would make them stand out, Vago and colleagues wrote in 2017, even if they are quite different from the chemical building blocks use by earthly life. These are:

Homochirality

Many organic molecules are asymmetrically shaped, which means they come in “left handed” and “right handed” versions. Abiotic processes tend to produce equal numbers of each. Biological ones only produce one or the other. Organic chemicals have been found on Mars, but the Curiosity rover, which detected them, isn’t equipped to test them for chirality.

“Clustering” of molecular structures and masses

Earth life tends to favour building blocks that fall into limited size ranges. Lipids, for example, tend to cluster in the 14- to 20-carbon range, even though there is no theoretical reason for them not to have more or fewer numbers of carbons. Similarly, the five nucleotide bases used by our DNA and RNA (four for DNA, and another in RNA) have molecular weights between 112 and 151, while the amino acids we use to make proteins range in molecular weight from 75 to 204. “If you find that you have ‘islands’ of compounds,” Vago says, “this clustering is a biosignature.”

Repeating molecular subunits

Life as we know it likes to build chemicals in pieces, adding on sub-units one at a time. We see this in proteins and DNA, but it also shows up in smaller molecules, like lipids, which are assembled in two-carbon units – meaning that they tend to have even numbers of carbons (14, 16, 18 and so on). Isoprenoids – components of essential oils and pigments, including chlorophyll – are assembled in five-carbon subunits. Even if these chemicals have broken down over time, their degradation products retain similar patterns. “This is something that doesn’t happen unless life was involved,” Vago says.

Isotope ratios

Biological processes – at least the ones we know – tend to work slightly differently, with compounds containing different isotopes of important atoms like carbon. Abiotic ones generally have no such preference. On Earth, this is most obviously the case with the two stable isotopes of carbon, 12C and 13C, with the heavier 13C isotope being disfavoured. The effect isn’t huge, but is measurable enough that ratios of these two isotopes can be used to determine if carbon-containing compounds are of biological or abiological origin. It can even be used to determine if steroids and hormones in athletes suspected of being drug cheats are laboratory-synthesised or produced by their own bodies. On Mars, any variation in 12C/13C ratios from background level would be a red flag for the workings of life, not geology.

3 | SNIFFING FOR METHANE

It is possible, of course, that a future rover might scoop up living organisms, rather than degraded chemicals contained in ancient rocks. But that’s no problem, Vago says. “If you have a payload that is designed to detect the much more challenging signs of past life; if you were to pick up a sample containing living microorganisms, it would be a walk in the park to detect the chemical components of those.”

But another way of searching for signs of existing life is by testing the Martian atmosphere for methane. On Earth, methane is mostly produced by biological activity, ranging from cow farts to decomposing plants. But it is also produced by geological processes, such as the interaction of water with a mineral called olivine in a process called serpentisation because it produces the green-coloured rock known as serpentine.

In 2004, ESA’s Mars Express orbiter detected traces of methane at various places around the planet, but this has been frustrating, says JPL scientist Chris Webster, because each was a one-off event, with no discernable pattern.

Then, in 2018, Webster reported that six Earth years of measurements (three Mars years) by the Curiosity rover had found atmospheric levels of methane that peaked in the summer and dropped in autumn and winter – that might or might not suggest the presence of methane-producing microorganisms that wake up in warm weather, then go back into hibernation for the winter. “This is the first time we’ve seen something repeatable in the methane story,” Webster says, “[but] we don’t know if it’s from rock chemistry or microbes.”

There’s just one fly in the ointment. A few months later, at the 2018 annual meeting of the American Geophysical Union, in Washington, DC, Vago’s team reported that ESA’s Trace Gas Orbiter, which has been circling Mars since 2016, has been unable to find measurable amounts of methane anywhere in the Martian atmosphere. This does not mean that there couldn’t be localised puffs of it, such as Curiosity observed in Gale Crater, but it does raise questions about how prominent they might be on a global scale.

4 | DIG, BABY, DIG

One thing scientists agree on is that if there is methane on Mars, it’s probably percolating up from the subsurface, either due to seasonal changes in microbial activity or, more likely, seasonal changes in the ability of the surface to allow gas to escape from deeper down.

We also know that the surface of Mars is extremely inhospitable, thanks to an atmosphere that is too thin to block out harsh radiation and high levels of oxidising chemicals such as perchlorates. “We use [perchlorates] for sterilisation,” says John Moores, a planetary scientist from York University, Ontario, Canada.

What’s needed is to peer beneath the surface, beyond the reach of damaging radiation and oxidants. NASA’s InSight lander, which touched down on 26 November 2018, will begin the process by eavesdropping on the seismic echoes of marsquakes – Martian earthquakes – the vibrations of which can reveal much about the Martian interior. But the results of that will be mostly of interest to deep-interior geophysicists. The next step, says Vlada Stamenković, a planetary scientist and physicist at JPL, is to use remote sensing to look for places that might have water, then drill as deep as we can.

That sounds like an immense task, but it doesn’t actually require carrying tonnes of construction materials to Mars and setting up something akin to an oil derrick. Instead, Stamenković says, it can be done with something called a wireline drill. “You can go as deep as you have wire,” he says. “There are wires where a kilometre weighs less than a kilogram.” Weight can also be saved, he and colleagues wrote this January in Nature Astronomy, by compressing carbon dioxide from the Martian atmosphere and using it in lieu of traditional drilling fluids to flush materials back to the surface.

What might be found down there is anybody’s guess. But in a 2018 paper in Nature Geoscience another team led by Stamenković argued that we might drill into a region capable of supporting not just methane-producing bacteria, but aerobic life.

Currently, oxygen is only 0.145% of the Martian atmosphere (compared to 21% of Earth’s), but under temperature and pressure conditions known to occur near the surface, Stamenković’s team calculated, startlingly large amounts could wind up being dissolved in briny Martian groundwater – far more than needed to support aerobic organisms as complex as earthly sponges.

Not that oxygen is the only thing these organisms would need. “There are many other requirements for aerobic life,” says David Catling, a planetary scientist at the University of Washington, Seattle. But the idea that there could be enough oxygen down there, today, to support a relatively complex ecosystem is nevertheless exciting.

5 | TRACES OF ANCIENT AIR

Whether you’re looking for present-day life or signs of long-gone life, a major question is whether the Martian atmosphere was ever thick enough to heat the planet sufficiently to give it a chance to form.

There is abundant geologic evidence that Mars was once warm enough to have liquid water. But did this occur over a long period of time, or in intermittent epochs? It’s an open question, says Moores.

Enter Mars Atmosphere and Volatile EvolutioN (MAVEN), a NASA spacecraft that has been orbiting the planet since 2014, studying how the Martian atmosphere interacts with interplanetary space. “We’ve been able to determine that a large fraction has been lost,” says its principal investigator, Bruce Jakosky of the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder.

That sounds like evidence for an initially thick atmosphere that might have taken a while to erode by enough to put the planet into the deep freeze. But that’s not necessarily the case. It’s possible that the Martian atmosphere was sometimes thick and sometimes thin, producing the type of intermittent warming and cooling suggested by Moores.

“Think of it as being analogous to the money in your wallet,” Jakosky says. “You can be paying out a lot of money, but that doesn’t determine whether you had a little or a lot at any time. You might be constantly replenishing your wallet from the ATM, with only a few dollars at any time.” It’s a difference that could have been crucial in whether the planet was ever warm enough, for long enough at a stretch, for life to have had a realistic chance of getting started.

6 | PICKING THE RIGHT PLACE TO LOOK

NASA’s next mission, the Mars 2020 rover, is headed for a 45-kilometre-wide basin known as Jezero Crater. It was chosen because it once hosted a lake, with a river draining in from the surrounding highlands to produce a large delta. “A delta is extremely good at preserving biosignatures, [be they] evidence of life that might have existed in the lake water, or at the interface between the sediment and the lake water, or, possibly, things that were swept in by the river and deposited in the delta,” project scientist Ken Farley, said in a late 2018 press conference, according to space.com.

But places like the Jezero delta aren’t the only ones that might preserve signs of life. Martin van Kranendonk, director of the Australian Centre for Astrobiology at the University of New South Wales, suggests that it is also possible to look for life signs at the types of places where life might have originated.

Scientists once believed that these places would have been undersea hydrothermal vents, where important chemicals are expelled up from deep in the crust. But current theory says that hot-spring pools like those in America’s Yellowstone National Park probably make better candidates because, however many interesting chemicals might be emitted by undersea vents, they don’t have much time to form more complex pre-biotics. “They just dissipate and disappear,” van Kranendonk says.

Hot springs pools, on the other hand, present no such problems. They also experience fluctuating water levels that produce altering wet and dry cycles – just the thing, laboratory experiments have shown, needed to cause small molecules to link into ever-larger chains. “They are complexity machines,” van Kranendonk says.

These hot springs also produce the mineral silica, which van Kranendonk describes as “the Egyptian tomb of the geological world. It perfectly preserves features, including signs of life”.

Furthermore, they are known to have existed on Mars, because in 2007 NASA’s Spirit rover found the remnants of one in a location called Home Plate in the Columbia Hills region of Gusev Crater. “We think a second genesis is likely on Mars because it has the right ingredients for what is now thought was the recipe on Earth,” van Kranendonk says.

So was there or wasn’t there a second genesis on Mars? The only evidence we currently have is that we haven’t yet found it.

If life still exists, it’s most likely retreated far enough underground to be invisible to the type of orbiting instruments and rovers we’ve used to date. But that doesn’t mean it doesn’t exist. Nor does the fact that so far we’ve not found any true signatures of ancient life provide much evidence that it didn’t exist. Even on Earth, traces of ancient life are rare and scattered.

If we someday find such traces, one of the mantras of science is that extraordinary claims require extraordinary evidence. In the case of life on Mars, JPL’s Allwood says that this means that “every single biological hypothesis you can come up with” is going to have to be ruled out before it is accepted. No ifs, buts, or maybes. Evidence of life on Mars will need absolute proof.

It’s a tough task, but not, Allwood believes, impossible. “I think the evidence will be there if life was there,” she says. “It’s a matter of how good a job we can do.”

This article appears in Issue 82 of Cosmos magazine. To subscribe, and have the latest science delivered direct to your door or inbox, click here.