In this series of posts I’ve looked at planetary bodies in our Solar System that could support life, from the moons of Saturn and Jupiter, to the cloud layers of Jupiter itself, to the ephemeral-once-jungles of Venus, I’ve even looked at Earth itself.
Now one of my favourites, Mars, the Red Planet.
Like Venus, both scientists and artists have long thought that Mars could be a potential home for alien life.
In the 1870s an Italian astronomer called Giovanni Schiaparelli examined the surface of the Red Planet with a (then) powerful telescope. He claimed to have found a series of long straight grooves cutting across the planet’s surface that he called canali, as he believed they were formed by running water. In Italian canali literally means groove or channel, but many English-speaking readers mistakenly translated canali as canal, a man-made rather than natural feature, and thus read more into Schiaparelli’s finding than he intended.
One such person was the American astronomer Percival Lowell, who adopted the idea of artificial canals on Mars with great excitement. He began a detailed examination of Mar’s surface from 1894 onwards and reported to have found evidence of a technologically-advanced alien civilization, and was convinced that Schiaparelli’s caneli were Martian-made canals created to transport water from Mars’s polar ice caps to its lower latitudes. Lowell constructed a vision of Mars as a dying planet with an ancient technologically-advanced alien civilisation struggling to keep itself alive.
Unsurprisingly Lowell’s ideas birthed a Sci-Fi obsession. From the 1880s onwards alien life on Mars became a staple of science fiction, from the comic book stories of Dan Dare and Flash Gordon, to one of the greatest works of fiction ever written, H.G. Wells’s War of the Worlds (if you’ve not read this then you really should, Wells dreamed-up a story of technologically-superior Martian invaders striding across London in walking tripods armed with lasers cannons, at a time when humanity was still getting around on the back of horses, cars hadn’t even been invented yet!)
However, the canali seen by Schiaparelli and the other structures observed by Lowell were likely a mix of optical illusion created by the telescopes used at the time, and particularly on Lowell’s part, wishful thinking. The surface of Mars, whilst interesting, shows no evidence of Martian civilisation, not even the “Face of Mars”, which was actually an optical illusion caused by shadows cast by the Sun over rocky terrain. If you look at the exact same spot on Mars from a different angle the “face” vanishes. Don’t take my word for it? You can look it up yourself on the brilliant Google Mars.
In 1965 the NASA spacecraft Mariner 4 arrived at Mars and examined it from orbit. It found no evidence for civilisation, current or ancient. In fact Mariner 4 found that Mars was a cold planet (daily temperatures range from -125°C to 17°C), with no surface liquid water, almost no atmosphere and no magnetic field. This meant that the surface of Mars would be uninhabitable, as life as we know it requires liquid water, and a magnetic field is useful as it shields a planet’s surface from much of the harmful radiation emitted by our Sun. If a little green man was to find himself on the surface of Mars today he’d be quickly be killed by freezing, desiccation and radiation.
Mars is a dead planet, with an inhospitable surface. None of this sounds good for Martian life.
However, remember before I said the surface of Mars is interesting? No? Well it is. It doesn’t show signs of civilisation, but it does display evidence that water may have run over the surface of Mars in the past. Various missions to Mars from the 60s onwards have revealed geological features that look a lot like branching river channels, canyons, valleys, debris fans from floods, and even sedimentary rocks. Mars may be dead today, but earlier in its history it may have been wet and alive.
There are two versions of this idea, the more traditional view is that Mars was once a warm and wet planet, with flowing rivers, lakes and even large oceans of water covering its surface, similar to the early Earth. More recent evidence suggests that Mars may have been wet but cold instead, with conditions more like the Arctic on Earth, with cold oceans surrounded by ice and glaciers. Either way, where there is liquid water there is a chance that life could have existed, and Mars may well have seen the emergence of single-celled life early in its history.
So what happened? How did Mars die?
It’s mostly to do with internal heat. As planets form they trap heat deep inside their cores (more about planet formation here), and Mars was no exception. This internal heat melts part of the core of rocky planets producing an internal layer of liquid metal, and flows in this liquid metal produce a planetary magnetic field.
Internal heat also causes processes like tectonics and volcanism. It’s not clear whether or not Mars had tectonic plates like those on Earth, but it did have volcanoes. The largest volcano in our Solar System is found on Mars, called Olympus Mons, it’s almost 22km tall, that’s about three times as high as Mount Everest!
Volcanism on Mars would have caused gas to escape from the planet’s interior and would have created an atmosphere. Atmospheres are important for two reasons. Firstly, greenhouse gasses in an atmosphere raise a planet’s temperature, such an affect on Mars may have raised the surface temperature to a level that ice could have melted.
Secondly, an atmosphere provides pressure. Mars is a much smaller planet than Earth, it has less mass and so gravity is not as strong on Mars. Today if ice melts on Mars it sublimes straight from a solid to a gas as there is not enough atmospheric pressure to allow water to exist as a liquid. This sounds pretty crazy but its true, without enough pressure, any substance that melts from a solid will change straight into a gas.
A short video on sublimation as its quite a cool concept. Iodine sublimes from a solid to a gas on Earth because the atmospheric pressure of the Earth isn’t strong enough to allow it to become a liquid.
So the heat within Mars created volcanic outgassing, which allowed the planet to maintain an atmosphere that warmed the surface of Mars melting surface ice, and the pressure from this atmosphere also allowed the ice to melt to liquid water rather than vapour. The internal heat also produced a molten outer-core, creating a magnetic field that will have shielded the planet’s surface from harmful radiation.
Unfortunately Mars is a much smaller planet than Earth though, and it lost its internal heat much faster.
The smaller a sphere is, the larger its surface area to volume ratio. Basically, for every square meter of surface area, Mars has less volume of material under that square meter than the Earth does. This meant that Mars lost its internal heat much more quickly than the Earth will. As this heat was lost, volcanism will have slowed to a stop and gasses would no loner be vented into Mars’s atmosphere.
Being small has another drawback too. Gravity on Mars is not strong enough to stop most gas molecules in its atmosphere from escaping into space. So as outgassing stopped, its atmosphere will have slowly eroded and escaped into space, stripping the planet of its warm greenhouse blanket.
Mars will have cooled and lost its atmospheric pressure, causing liquid surface water to freeze at the poles, and sublime to gas and escape into space at lower latitudes. As its core solidified, Mars will have also lost most of its magnetic field, causing its surface to be bathed in sterilizing radiation.
So Mars may once have been a living planet, but as the planet died internally this surface life was likely extinguished in turn.
One hope of planetary scientists and palaeontologists is that fossils of ancient Martian organisms will be found by future expeditions to Mars. Such life would have long since turned to stone, but would still be an immensely important find as we would have proof of life emerging on our nearest planetary neighbour.
Another intriguing idea is that life may have emerged first on Mars and was then transported to Earth, so that all life on Earth may be Martian; you, your mum, and your cat. (The idea that life can spread between planets is called panspermia; you can read more about here if you want).
This may sound like madness, but we know that rocks from Mars’s surface have travelled to Earth in the past. If Mars is hit by an object with sufficient force, such as an asteroid or a comet, parts of the Red Planet’s surface can be ejected into space, and some of this rock will fall to Earth. Some scientists believe that single-celled life may be able to survive the harsh conditions of space and the fall to Earth protected inside such rock, and could then emerge on the Earth’s surface and prosper.
In August 1996 NASA held a press conference in which they announced that a team of scientists had found fossilised microbial life inside a meteorite that had fallen to Earth from Mars, giving support to both the idea of Martian life and infection by panspermia. Sadly this evidence has since been discounted, and the consensus opinion is that the shapes are not fossils but are the result of non-living geological processes.
Another exciting, and equally controversial idea is that life could still exist on Mars today.
From studying extremophlile life on Earth (single-celled life that lives in extreme conditions like in acid or in boiling water, more about extremophiles here) scientists now know that life can survive in some pretty harsh environments, including in areas of water deep within the Earth’s crust and in ice. And in 2002 the NASA Mars Global Surveyor spacecraft began imaging Mars’s surface in great detail and found evidence that liquid water may still exist, trapped underground, but close to the planet’s surface. The best evidence came from images of erosion channels cut into the side of craters, which were probably formed by floods of running water. Incredibly, some of these erosion channels were seen on images taken in 2006, but not in images of the same craters taken in 2002, suggesting they had formed in the time after Surveyor began orbiting Mars.
Its therefore possible that areas of liquid water still exist below Mars’s surface, kept liquid by a high salt content (salty water is harder to freeze than fresh water), and prevented from vaporising by being trapped underground. This water may periodically break through the side of a crater, creating the channels seen by Surveyor, and once exposed on the surface of Mars will quickly sublime to gas, but still, pockets of underground salty water could provide a habitat for the last remaining Martian life to survive in the form of extremophile microorganisms. This may be a long shot, but it means that life on Mars today is not totally impossible.
Bacterial life may be less awe-inspiring than Lowell’s ancient advanced Martian civilisations, but any alien life on Mars would be a momentous discovery.
So there you have it. An intelligent civilization does not live on Mars today, and likely never did, but life may have once emerged on Mars and may have left fossil evidence. Martian life may still even survive today, trapped in underground water caches, or may live on as life on Earth. Come to think of it, the panspermia hypothesis may explain Bowie…