Are we alone in the solar system?
Astronomers and planetary scientists are racing to discover whether alien life is widespread among the worlds in our cosmic neighbourhood
The NASA-led research that has provided a surprising answer
It’s not too long ago that scientists assumed our planet was the only place in the Solar System with the right conditions for life, but a series of stunning discoveries have recently shown that’s far from the case. Instead of having to search for telltale hints in the light of distant planets orbiting other stars, perhaps alien life (at least in the most simplest form) is waiting to be found on our cosmic doorstep.
Ask a dozen biologists for a definition of life and you’re likely to get a dozen different answers – life is one of those things that is hard to pin down, though you know it when you see it. Even the most open minded of biologists, however, tend to agree that two of the key requirements for life, which guide our chances of finding it elsewhere in the Solar System, are abundant carbon and a plentiful solvent, most likely liquid water.
Carbon is important because, of all the elements, it is the one best suited to building the hugely complex, self-replicating molecules required by most living processes. Fortunately it’s one of the most common elements in our galaxy, generated in huge quantities by nuclear fusion processes inside stars and scattered through interstellar space when they die, for incorporation into later generations of stars and planets.
Water, meanwhile, is needed for the most basic of reasons: in order for the complex chemical precursors of life to arise, simpler chemicals must first encounter one another and go through chemical reactions. This means they must be able to move around, something that’s most likely to happen when they’re dissolved in a fluid solvent. The unique chemistry of water makes it the most effective solvent among all liquids that commonly occur in nature, and once again we’re fortunate that it seems to be widespread in our galaxy.
So what can the requirement for these two basic ingredients tell us about the possibilities for life in our Solar System? While carbon is commonplace across all the Solar System’s planets and moons, liquid water seems at first glance to be a much trickier requirement – Earth is the only planet with abundant surface water, thanks to its position in the Solar System’s ‘Goldilocks zone’, where temperatures are neither so hot that the oceans boil away into the atmosphere, nor so cold that they freeze solid. Up until the dawn of the space age, many astronomers suspected that our neighbouring planets, Venus and Mars, might also have liquid water on their surfaces, but the first spaceprobe flybys put an end to these hopes, revealing Venus as a toxic, roasting hellhole and Mars as a frozen, arid desert.
Fortunately it’s now clear that the Goldilocks zone isn’t the be-all and end-all of possibilities for life. Planetary scientists have discovered evidence
for liquid water in surprising places across the
Solar System – for example hidden beneath the icy crusts of moons whose interiors are heated by the strong tidal forces of their parent planets, or perhaps kept liquid even at sub-zero temperatures by the presence of other chemicals such as salts or ammonia. Meanwhile, in the past few decades, biologists have also found that life on Earth is able to thrive in extremes of acid, alkali, heat, cold and darkness very different from those we normally experience. The discovery of these ‘extremophile’ organisms has opened up a whole range of new habitats where life might exist beyond Earth.
When it comes to the basic materials for life in the Solar System, it now seems that all bets are off – so where should we look, and what might we find?
At first glance, Mars remains the most obvious candidate as an environment for life. Since those early disappointments, photographs and other data from orbiting spaceprobes, along with soil analysis by surface rovers, have revealed there’s much more to the Red Planet than arid desert. The surface soil is mixed with large amounts of ice to form permafrost, and in some places even flows to create glacier-like features. Ancient features also show that liquid water flowed freely on the surface in the distant past, when the Martian atmosphere was thicker and its orbit was perhaps different. Mars almost certainly had the right conditions for life to gain a foothold billions of years ago – but is there any chance it could still cling on today?
So far the only experiments to deliberately search for life on the Martian surface were carried aboard the Viking missions of the 1970s. These robot landers exposed soil samples to a series of chemical reactions and looked for signs of living metabolic processes. They produced inconclusive results and have never been properly repeated – the British-built Beagle 2 Lander, designed to continue the direct search for life, sadly ended up wrecked on the Martian surface during its 2003 landing.
Perhaps the most controversial evidence for life, however, comes from a meteorite called ALH84001 – a fragment of 4.5-billion-year-old Martian rock that was blasted off the planet in a meteorite impact and fell to Earth in Antarctica about 13,000 years ago. In 1996, a team of NASA scientists claimed to have discovered chemical biomarkers (molecules created by biological activity) and microscopic fossil-like structures within it. They suspected the action of primitive ‘nanobacteria’, similar to (though much
“Discovery of these ‘extremophile’ organisms has opened up a whole range of new habitats where life might exist”
smaller than) some of Earth’s own ‘extremophile’ bacteria. The claim remains hugely controversial, however – other scientists have proposed ways for the molecules and ‘fossils’ to have arisen without the need for life, and the matter probably won’t be settled for good until scientists have more samples of Martian rock to examine.
But is today’s Mars suitable for life? Conclusive evidence of liquid water on the surface today (perhaps seeping from underground water tables) remains frustratingly elusive, and while a lack of liquid water on Mars today wouldn’t entirely rule out specially adapted microbes, it would seem to make it far less likely.
Balanced against this, the most intriguing evidence for possible Martian life so far comes from the detection of methane gas. The first traces of methane (a few parts per billion in the atmosphere) were discovered from Earth-based telescopes and orbiting spaceprobes in the early 2000s, and have since been confirmed by rovers such as NASA’s Curiosity. The gas is puzzling because it is unstable in Martian conditions – fierce ultraviolet radiation should rapidly break its molecules apart – so for methane to persist, something must be constantly producing it.
On Earth, methane is produced by living organisms or geological activity such as active volcanoes. Volcanism or other processes can’t yet be ruled out, but the announcement in early 2018 of a seasonal cycle in which methane levels in the Martian northern hemisphere rise to a peak in later summer adds to the mystery – could it be that methane-producing microbes are stirred into activity by the summer sunshine? The European Space Agency's and Roscosmos' ExoMars Trace Gas Orbiter, due to start work in orbit around the Red Planet, may shed more light on the mystery.
Further out in the Solar System, a good handful of worlds offer tantalising prospects for life. Solid worlds beyond the middle of the asteroid belt – such as dwarf planets, moons, asteroids and comets – are made from rock and ice mixed in varying amounts, and it’s now clear that tidal forces raised on satellites orbiting giant planets, or simply the addition of chemicals that lower the freezing point of water, can be enough to create a deep liquid ocean layer beneath a solid outer crust.
The best known examples of such hidden oceans are Jupiter’s satellite Europa and Saturn’s moon
“Europa and Enceladus are probably the Solar System’s most likely habitats for life – and not just single-celled microbes”
Enceladus. On Europa, a 25-kilometre- (15-mile) thick outer crust of jostling ice plates slowly drifts and rearranges itself on top of a global ocean about 160-kilometres (100-miles) deep. The ocean on Enceladus is shallower, but closer to the surface, with the crust just five-kilometres (three-miles) thick in places. On Enceladus at least, sea-floor hydrothermal vents belch out gas and minerals from deep within the crust. The environment around these vents could provide an ideal oasis for life to arise (indeed, many biologists now suspect that life on Earth got started around similar vents).
Although it’s currently impossible to investigate these hidden oceans directly, both moons release plumes of vapour into space which we can study. The jets above Enceladus have already provided clues to conditions in its ocean – molecules of hydrogen within them have been linked to active undersea vents. Europa’s vapour plumes are thinner and more intermittent – they may not escape directly from the ocean, but might instead be knocked off the moon’s icy surface by radiation from the Sun and Jupiter. But since Europa’s surface ice is itself made up of solidified and recycled ocean ice, even this could offer important clues. NASA’s Europa Clipper mission, planned to launch in the mid-2020s, will aim to find out more.
Together, Europa and Enceladus are probably the Solar System’s most likely habitats for alien life – and perhaps not just single-celled microbes, but more advanced creatures that have evolved to suit their environment. Such organisms might have streamlined shapes similar to fish, or flexible forms that take advantage of buoyant conditions, like squid and other cephalopods. But then again, it’s worth bearing in mind that single-celled life on Earth existed for at least 3 billion years before blossoming into varied, multicellular organisms (for reasons that we don’t yet fully understand).
Aside from these two icy showcases, it’s now clear that many other Solar System bodies could have hidden oceans deep beneath their surfaces. On Jupiter’s giant moons Ganymede and Callisto, that water is sealed off hundreds of kilometres below the surface, and detectable only through interactions with Jupiter’s magnetic field. NASA’s Dawn asteroid probe has revealed signs of a hidden ocean on the largest asteroid, Ceres, and there could even be liquid water on distant Pluto. During its 2015 flyby, the New Horizons mission photographed extraordinary features that many believe could only have been created by the effects of a fluid mantle just beneath the crust. Any of these worlds could have their own hydrothermal vents, and potentially their own life, though it would be far harder for us to detect and investigate.
However, if there’s a prize for the most unlikely potential outpost for life, it must surely go to Titan. Cloaked in an opaque, nitrogen-rich atmosphere, Saturn’s largest moon is one of the coldest worlds in the Solar System, allowing hydrocarbons such as methane to condense into liquid and play a similar role to that of water on Earth. Methane rains from clouds, erodes a landscape covered in oily hydrocarbon ices and, amazingly, collects in lakes near the poles.
In many ways, Titan is a low-temperature version of Earth – so if it has its own surface liquid, could it also have its own life? There’s carbon aplenty, and indeed hydrocarbon molecules are an important first step on the way towards complex biochemistry. But while liquid methane is a less efficient solvent than water, once chemicals are dissolved, they are more likely to persist and remain stable for longer, giving greater potential for lifegiving reactions to arise.
Regardless of the form it takes, if life is found to be widespread across the Solar System, it could have huge implications for our understanding of the wider universe. If, for example, life on other worlds turned out to share basic biological features, it would support the so-called ‘panspermia’ theory that life is carried between worlds, and perhaps even between stars, on comets and meteorites. If different strains of life prove to be entirely independent, it would suggest that life arises naturally wherever conditions are even remotely suitable. In either case, we could expect basic life in our galaxy to be equally commonplace – perhaps even giving rise to intelligent aliens that we might one day contact.
the panspermia theory suggests that comets have seeded many of our Solar System’s worlds with life
naSa’s europa Clipper is a dedicated probe to investigate the icy moon in a series of close flybys due to launch in the 2020s
Strange forms of life such as these giant 'tube worms’ flourish around deep-sea vents on earth – could the same go for enceladus?
electron microscopy of martian asteroid alh84001 revealed tiny microbe-like ‘fossils’ whose true nature is still controversial
any future mission to land on europa would need to be sterilised in order to prevent contamination of the moon’s environment