Natural satellites in our Solar System may be prime targets in the search for life
The satellites in our own Solar System that could host alien life
The search for life in our Solar System has taken many twists and turns over the decades. Once, Mars was deemed the most plausible location for past and present life, while even worlds like Venus bear the hallmarks of having been habitable. In a change of course, much of the focus today is on new locations that hold considerable promise – the moons of the outer planets, where icy surfaces and other features may hide life-harbouring environments. NASA’S Pioneer 10 and 11 spacecraft in the 1970s were the first to return close-up images of Jupiter, Saturn and their moons, setting in motion a thrilling scientific story that is moving forwards at full pace today. Whereas our own Moon seems mostly devoid of life, early spacecraft like these, and the subsequent Voyager spacecraft, showed there were plenty of secrets awaiting our discovery. It wasn’t until NASA’S Galileo spacecraft arrived at Jupiter in 1995, and their Cassini spacecraft at Saturn in 2004, that excitement really started to ramp up. These probes showed those moons were far more exciting than we could have imagined, with evidence mounting that some could harbour oceans beneath their surface. While none seemed to possess any signs of life on their surface, underground – safe from radiation – scientists started to wonder what could be going on. Fast-forward to today, and the moons of Jupiter and Saturn, and perhaps other moons too, like Neptune’s Triton, are looking like the best bet in the search for life in the Solar System. Using radar and other images, we have almost conclusively proven that locations like Europa and Enceladus house oceans beneath their surface. These oceans are thought to be tens of kilometres under the ice of their respective moons, too deep for us to reach with current technology. But Europa and Enceladus in particular seem to be firing plumes from their oceans into space, with some of that material available to study either in space or on the surface. The Cassini spacecraft was actually able to fly through the plumes of Enceladus and thereby sample its interior. When the mission was designed, however, these plumes were not known about, so the instruments available to study them were limited. Future missions could investigate these plumes even further and look for organic compounds or
As these moons are pushed and pulled by the intense gravity of their host planets they experience tidal heating, melting the vast swathes of ice into the liquid we think resides there today. This liquid is thought to be bountiful – some of these living moons are believed to contain more water than there is on Earth. This tidal heating is interesting for another reason too. Life as we know it needs several key ingredients to thrive, including liquid water, heat and energy. It might be that on some of these moons this gravitational effect heats the cores, forming hydrothermal vents on the ocean floors. On Earth such vents provide not only heat for life but energy and sustenance too. Could it be that some of these moons, deep in their interiors, are similar to Earth?
It’s not just the oceans that are interesting, as Saturn’s moon Titan is intriguing for a whole other reason. While we think this moon may have an ocean underground, it’s what is taking place on the surface that has scientists talking. Titan is the only world other than Earth known to have bodies of liquid on its surface – here in the form of liquid methane and ethane, creating a jet fuel-like liquid. Add in its thick atmosphere and a climate system not too dissimilar to our own, and Titan starts to tick a lot of boxes.
If life does exist on the surface of Titan, it is likely life as we don’t know it, relying on processes that we don’t yet understand. Much of the focus on the ocean moons, meanwhile, has been based on life as we do know it, which makes sense; we know life exists on our planet in certain conditions, so why would we not look for those same conditions elsewhere?
To get answers to these questions and more, a number of spacecraft are now being designed that could probe these moons like never before. NASA’S Europa Clipper, for example, will study the moon it is named after in detail. Launching in the mid-2020s, it will try to work out how thick Europa’s icy sheet is and whether there are signs of habitability on it.
To complement this mission, NASA is looking to develop a Europa Lander that could land on the surface of this moon. If the plumes of Europa rain back down on the surface as we expect, then it could be possible to sample this ocean without having to drill through tens of kilometres of ice. Others argue Saturn’s moon Enceladus is a better bet for such a mission, as its plumes are more constant, with more material available to study. Budgets are limited, however, so for now Europa is in the limelight.
Another mission, the European Space Agency’s (ESA) Jupiter Icy Moons Explorer (JUICE), will also be investigating this system. Aside from just Europa, it will study Jupiter’s other interesting moons, including Ganymede and Callisto. The latter may hide an ancient ocean that has existed long enough to allow life to take hold, whereas the former is the only moon in the Solar System known to have a magnetic field. Earth’s magnetic field protects us from radiation; perhaps Ganymede’s makes it similarly suitable for habitation.
The search for life in the universe is progressing steadily, with some scientists favouring a closer look at Mars, which may have once been more like Earth with seas and oceans on its surface. Others favour studying worlds beyond our Solar System, called exoplanets, to look for some that may be similar to our own. But the moons of the outer planets are without doubt among the most promising targets at the present time.
We still know very little about their oceans and potential habitability, but in the next couple of decades we might get closer than ever to finding out if life in our Solar System is limited to our own rocky planet or is spread abundantly on worlds like and unlike Earth.
Future missions could explore the oceans of Europa and elsewhere