Although at first glance the answer to whether Mars harbors life may seem obvious, the evidence is far from settled.
Dredging the bottom of Scottish lochs looking for plesiosaurs, or scouring the Himalayas in search of hominids is a lost cause. There is no Big Foot, no Yeti, or Nessie, but when it comes to Mars there is enough evidence to suggest we need to look closer for the possibility of life existing there now.
There’s no doubt humanity will reach Mars and eventually settle there, but what will we find?
In my latest novel, Retrograde, I explore what life would be like living in a colony beneath the surface of Mars. Lava tubes would provide a ready-made shelter from the harsh cosmic radiation lashing the frozen surface of the planet. Having stood for hundreds of millions of years, lava tubes would be ideal to protect us, but perhaps they protect native life as well.
Let’s take a look at the evidence for the possibility of life on Mars.
Earth has been shaped by hydrology—water locked in a cycle. Water evaporates from the oceans, streams and lakes, condenses in the air, and falls as rain, only to repeat that cycle over and over again, providing the natural environment in which the chemistry for life can thrive.
In some cases, the hydrological cycle is rapid, occurring in days. But sometimes water can become locked in subsurface caverns, or beneath thick layers of ice in Antartica for millions of years. In all cases, water sustains life, even when trapped and cut off from the outside world for untold millennia.
How can water exist in lakes beneath the ice for millions of years? Wouldn’t it freeze? Good question. The answer is, intense pressure, salinity, and in some cases geothermal warming keep these hidden lakes in liquid form beneath miles of glacial ice.
Lake Vostok in Antarctica has been sealed off from the outside world for at least 15 million years. The DNA in the bacteria thriving there differ from their surface counterparts by up to 86%, branching in their own distinct evolutionary direction.
On Earth, everywhere we’ve found water, we’ve found life.
We’ve found microbes in gold mines, upwards of two miles beneath the surface, having branched away from their surface counterparts some 25 million years ago, and thriving at scorching temperatures of 140F/60C.
There’s sufficient evidence that Mars was once hospitable, with liquid water flowing on its surface. But what about now? Is there evidence of water flowing on Mars today? Is there any evidence of fluids capable of supporting life there now? Yes. Mars is the only other planet in our solar system that has a hydrological cycle. There’s no rain, but we have observed conditions that would allow water vapor to condense and cling to dust particles.
Each summer, orbiting satellites like MRO (Mars Reconnaissance Orbiter) record an unusual phenomena known as RSL or recurring slope lineae, streaks that appear on the sides of craters, gullies and canyons as the planet warms.
There’s some discussion about whether these recurring slope lineae (RSL) are caused by subsurface frozen salt brines melting in the Martian summer, or some other process, such as frozen carbon dioxide (dry ice) sublimating and dragging dark (but dry) dust down the crater slopes.
As NASA’s Phoenix Lander found water ice beneath a few inches of soil (the tiny fragments in the shadow at the bottom left), mixed in with dry ice (the large regions of white in the centre), RSL could easily be the result of both. Summer temperatures can reach as high as 70F/20C so liquid water is entirely possible, even if it is only fleeting on the surface due to the low atmospheric pressure.
That Mars was once warm and wet is beyond dispute given the evidence gathered by NASA’s rovers. Not only are there ancient lake beds, the humidity and temperatures observed by the NASA Curiosity rover would allow a liquid brine to form even during winter.
To our untrained eyes, Mars appears as a bleak desert, but dig a trench and the soil and the bottom will be slightly darker due to the moisture content. It may not contain enough water to sustain us, but there’s enough to allow microbes to survive if they can tolerate chemicals like perchlorates.
On Earth, the temperature increases by roughly 25C for every kilometer you go beneath the surface, and life can exist several miles down. On Mars, this same effect is probably only 6 degrees per kilometer, but it’s enough that liquid water could exist at depths that support life on Earth.
Closer to the Martian surface, scientists have found frozen reservoirs holding as much water as Lake Superior in North America. Buried beneath anywhere from a few feet to thirty feet of rock, it calls to mind the glaciers of Antarctica that support life in hidden pockets beneath the ice.
Over the past decade, research has revealed that ancient Mars may have held an ocean to rival that of the Arctic on Earth.
That Mars could have supported life in the past is beyond doubt, but what about now?
Methane has been detected in the Martian atmosphere, which is highly unusual as methane is short-lived, and generally produced only by geothermal activity or life.
At less than 1% in concentration it’s minuscule, but given there’s no tectonic or volcanic activity on Mars, it raises the possibility of being produced by microbes. If methane was seasonal, produced by some repeating physical process, we’d expect to see it at regular intervals. Instead, it comes and goes sporadically. At one point, NASA’s Curiosity rover observed it continuously for 60 days, and then it disappeared.
ESA’s ExoMars satellite arrives in orbit around Mars in 2021 to better understand this chemical phenomenon and help isolate its origin.
Searching for life
Microbes are small—insanely small. A single strand of hair is about 75 microns thick (with a micron being a millionth of a meter). Bacterium can be as smalls as 0.2 of a micron, making them rather difficult to detect at a distance of 30 million miles even with remote-control robots.
Even if we could get an electron scanning microscope to Mars, we could stare at bacterium like this and still be unable to distinguish it from dust particles.
The NASA Viking missions back in the 1970s are the ONLY missions to specifically look for life on Mars, and they did it using an ingenious method. Rather than looking for life, they looked for evidence of metabolism. They were designed to tease out respiration from microbes hungry for nutrients, and the results?
Out of the three experiments, only one produced positive results, so it was largely ignored at the time, but a recent review of the evidence has reached a different conclusion.
The Labelled Release apparatus (LR) was designed to release nutrients into one of a number of samples taken from the Martian surface. By spiking the nutrients with radioactive carbon, scientists were confident they could measure any gas released by microbes (either as carbon dioxide or methane). Immediately, measurements spiked from the background levels of 50/60 counts to over 10,000. Something had happened, but what?
The other samples were used as controls, and subject to darkness for a month to kill any microbes relying on photosynthesis, or other microbes that might prey on them, the idea being, if the first results were a simple chemical reaction, they would be repeated in 30 days. If the first run had revealed life, and that life died in the darkness, there wouldn’t be any second result. Remarkably, there was no second reaction. It’s circumstantial evidence, for sure, but it suggests Viking may have found life.
Just a few years ago, Joseph Miller from the University of Southern California noticed something rather startling when he overlaid the Martian temperature fluctuations from one day to the next with the results from Viking—he saw a circadian rhythm.
On Earth, the metabolic activity of every life-form from microbes to Blue whales follows some kind of day/night cycle.
The red line in the chart above shows the temperature in the sample chamber during the experiment, fluctuating from one day to the next. The sample was protected from the harsh variations in temperature that occur over night on Mars by the internal heaters on Viking itself, but there was still some minor fluctuations, and surprisingly, this coincided with significant variations in the amount of radiation detected. Although it’s tempting to think these variations were abiotic and simply heat related, such a minor temperature variation of only 4 degrees can’t account for the increased activity, and there’s a delay. Each day, the spike in radioactivity is delayed by approximately two hours, something that’s inconsistent with the effect simply coming about from a slight increase in heat. In addition to this, the baseline increases each day, meaning each peak and trough is successively higher, so for the same temperature variation from one day to the next, NASA observed a steady increase in the amount of radiation being released at each rest and peak period, something that is consistent with microbial growth.
During this same period, Viking 2 also observed water frost on the ground in the morning, with the ice sublimating during the day, demonstrating an active, albeit modest hydrological cycle.
Although it’s not conclusive, the Viking results make a strong case for the possibility of life on Mars.
Scientists at NASA have proposed a Mars Sample Return mission to allow for the direct examination of martian soil, but it doesn’t have funding yet. Even if it is funded, the selection of the sample site is challenging. Imagine trying to understand life in the ocean from just a teaspoon of water. Ideally, the Viking experiments should be extended further and repeated. If we continue to see anomalous results like these that suggest a biological mechanism, then we have identified an ideal site for a sample return.
There’s one other compelling piece of the puzzle suggesting Mars may still harbor life—homeostasis.
In biology, homeostasis is “the tendency towards a relatively stable equilibrium between interdependent elements,” and is a characteristic of all life. Humans, for example, maintain a body temperature around 98.6 degrees Fahrenheit. Vary too far outside that, and death comes quickly.
When it comes to the search for life beyond Earth, the point is often made that our planet is ideal for life. We look at the hellish domains of Mercury and Venus, or the frozen wastelands of Mars and Titan, and they’re inhospitable. We even call the region of space we inhabit the Goldilocks Zone (not too hot, not too cold), but this is somewhat of a misnomer. Earth doesn’t house life because it’s ideal for life. On the contrary, Life tamed the planet.
Life first arose when Earth was hellish.
In the Hadean era, shortly after the planet formed, temperatures were as high as 400F/220C and entire oceans boiled as Earth was pummeled by asteroids and comets in the Late Heavy Bombardment.
Life arose during the Archean almost four billion years ago. Temperatures reached 185F/85C with a toxic, acidic atmosphere choking the planet. Early microbial life forms were able to exploit an atmosphere that would have killed us, and they thrived in the rich CO2 environment.
Earth was unrecognizable during this time, appearing more like Venus, or the Saturnian moon Titan, than the beautiful Blue Marble we live on today.
Life transformed our planet beyond recognition.
The first life appeared ~3.8 billion years ago and used CO2 for its metabolism, burping out O2, which is how O2 started building up in the atmosphere. As O2 is toxic to bacteria, a lot of bacterial species died out, while those that could tolerate O2 thrived.
2.3 billion years ago, the Great Oxygenation Event was caused by microbial action changing the planet, and releasing vast amounts of oxygen into the atmosphere. Since O2 is very efficient for metabolism, there was a spike when natural selection harnessed oxygen for the first time. The levels of O2 continued to rise until ~600 million years ago when they reached roughly the present level.
During the Proterozoic era, the seas were home to colonies of algae easily visible from space, and a supercontinent called Rodinia emerged, but as best we understand it, Rodinia was devoid of life. It would be another billion years before complex life would emerge from the oceans.
Even when a Snowball Earth emerged for hundreds of millions of years, life would not be deterred, and fought for planetary homeostasis, creating the moderate world we inhabit today.
Earth has a CO2 cycle that operates over hundreds of millions, stretching into billions of years, and this has helped keep the planet’s atmosphere stable.
The fact that life started in the ocean helped it survive the Snowball Earth (water has unique properties: ice expands which means it floats, and thus was able to create an isolation blanket over the oceans, while the hydrothermal vents provided heat).
If you had a time machine and could visit these various epochs, Earth would appear lifeless to the untrained eye. Not only has Natural Selection had the time and ability to modify species, along with their associated allied and predatory species, but even their environments have changed. The planet itself has been shaped by evolution. When viewed over eons, the entire planet has been transformed. Life competes to live. Life seeks out niche environments, and by its presence, invariably modifies those environments. Given 3.8 billion years, Life has fought against natural geological processes to modify Earth’s climate to become temperate. It’s astonishing to realize microbes tamed and transformed an entire planet, and that brings us to Mars.
There’s something rather unusual about Mars.
Mars is the only other planet in the solar system where water can exist in three states simultaneously—as water, ice and vapor—but only just. The Triple-Point of Water describes the combination of temperature and atmospheric pressure that allows H2O to be stable as water, ice and vapor simultaneously. Mars sits right on the cusp of this range.
The existence of H2O as a liquid is entirely dependent on the relationship between temperature and pressure. Too much temperature and water becomes a vapor. Too little and it’s ice. Pressure changes that equation. On Mt. Everest, water boils at 160F or a mere 70C.
All life on Earth requires water. Chemical reactions need a medium in which to occur, and water is uniquely suited to that, meaning, more than likely, life elsewhere will also utilize water. But the existence of liquid water is finely balanced by temperature and pressure.
Mars is slipping away from the Triple Point of Water.
It could be a coincidence that Mars is so close to the Triple Point, but it could also be that we’re observing a dying planet hanging onto the last vestiges of microbial life.
Given everything else we’ve observed, it might be that this frail combination of temperature and pressure on Mars, allowing for minuscule amounts of liquid water, is the last remnants of planetary homeostasis. We may be indirectly observing Martian microbes succumbing to defeat as their environment fades. This could well be the dying gasps of life on another planet.
There are extremophiles here on Earth that can survive in Mars-like conditions, so perhaps there is still life beneath the surface of the red planet.
It’s speculation, of course, and at best merely circumstantial, but given the clear hydrological history of Mars, the unexplained presence of trace amounts of methane in the atmosphere, the circadian rhythm in the results of the NASA Viking experiments, and the finely balanced Martian environment, hovering around the Triple Point of Water, there’s a strong case to be made that we should be looking for microbial life on Mars now in subsurface pockets of water.
The ESA ExoMars mission will help bring some clarity to the question of how methane arises on Mars, but it seems only a return mission with samples will allow us to know for sure if there is life on the fourth planet in our solar system.
Peter Cawdron is a science enthusiast and science fiction writer. His latest novel, RETROGRADE, explores at the complexity of colonizing Mars and is available in hardback, paperback and electronic formats.