Opportunity, the older (and smaller) rover cousin of Curiosity made the headlines last week when it discovered more conditions that could have supported life on Mars in its past. But for our best chance of finding life elsewhere in the Solar System, we may need to look beyond the dusty plains, vast mountains and deep canyons of Mars, and travel past the scattered cosmic debris of the asteroid belt. There, in the shadow of the bloated stripy gasbag Jupiter, could be actual living organisms – not the desiccated and irradiated remnants of long dead microbes that scientists hope to find on the red planet.

Jupiter has just under 70 documented moons, the four largest of which are the Galilean moons Io, Callisto, Ganymede and Europa. Io is a fiery world of volcanoes, molten lava and billowing clouds of toxic sulphur; Callisto, an ancient pockmarked body covered in a crust of icy rock. But beneath the vast cracked pancakes of ice on Ganymede and Europa, there are oceans of water. And where there is water, you have the potential for life.

This intriguing possibility has led to the development of a new internationally supported mission to Jupiter’s Moons, Juice, being put together by the European Space Agency. Juice, rather tenuously, stands for JUpiter ICy moons Explorer – an acronym that, I am told by a reliable source, was arrived at after quite a few late-night drinks.

The probe will set off for Jupiter in 2022. After it arrives in 2030, it will spend at least three years studying the Jovian system, flying past the planet and its icy moons and ultimately ending up in orbit around the massive Ganymede, the largest moon in the Solar System.

“It’s a fabulous mission,” says Andrew Coates from University College London, one of the lead scientists on the project. “It will compare all three [icy] moons, look at the habitability of all three and the formation processes.” By “habitability”, he means the right conditions to find life. Or, at least, life as we know it.

“You need a source of energy, you need liquid water, carbon, nitrogen, oxygen, phosphorous and sulphur,” says Coates. “Then you need enough time for life to develop.”

Both Ganymede and Europa are in with a fighting chance of possessing all these attributes. They certainly have liquid water and could easily have the molecular building blocks for life. When it comes to energy, Europa has traditionally been the favourite among astrobiologists. It is the only one of the Jovian moons likely to have a rocky ocean floor, meaning there could be hydrothermal vents. These underwater chimneys support thriving communities of plants and animals on Earth. With no significant energy from the Sun, hydrothermal vents could provide chemical energy for life.

Weird worlds

As almost everything we know about Europa comes from Nasa’s Galileo mission [LINK TO: http://solarsystem.nasa.gov/galileo/ ] of the mid-1990s, which spent eight years studying Jupiter and its moons, hard evidence for habitability is hard to come by. However, new research adds weight to the claim that Europa is indeed where ET will be hanging out.

Based on what we know about Europa today, we have good reason to believe that it is habitable – by Earth based standards – for life as we know it,” says Kevin Hand, an astrobiologist at Nasa’s Jet Propulsion Laboratory in Pasadena, California.

Using data from Earth-based telescopes, Hand and his colleagues at the California Institute of Technology recently confirmed a widespread abundance of hydrogen peroxide across much of Europa’s surface. This substance could help provide the chemical energy needed for life in the ocean below. And, unlike on Mars, there might be enough energy for larger multi-cellular organisms, rather than just microbes.

“We don’t know that much about the evolution of complex life forms on Earth and so it’s hard to make a prediction about Europa,” Hand explains. “That said, some of our work indicates that compounds like hydrogen peroxide and oxygen, which are produced by the irradiation of Europa’s surface ice, could lead to a chemically and energy rich ocean that might be capable of powering complex organisms.”

So if life is out there, what form is it likely to take?

“Anything is possible when you consider a second, independent origin of life,” Hand says. “I’d bet it would likely be heavily dependent on acoustic and sonar sensory perception, since it would live in a vast ocean and have the ice sheet above cracking and creaking, day in and day out, as the moon is tidally tugged and pulled by Jupiter.”

Although it is exciting to imagine a sea full of fearsome alien fish with giant ears, this is, of course, just speculation. The evidence for habitability is growing but just because somewhere is habitable doesn’t mean that there is life. In fact Coates reckons Ganymede is a better candidate.

“It’s got a magnetic field that helps protect it from radiation,” he says. “One of the problems with Europa, in particular, is that the radiation environment is higher there, so the deflection by the magnetic field of radiation could play a key role in making the chemistry right at Ganymede. I think Ganymede is more interesting than Europa in fact.” The ice covering Ganymede’s liquid ocean also has a thicker crust, which may help with shielding from radiation.

The frustrating thing about all this is that, even if everything goes to plan, we will have to wait at least 17 years until Juice gets to Jupiter, and 19 years until it orbits Ganymede, before we get close to any answers. But Coates looks on the bright side: “It’s remarkable to think that two-year-olds now could potentially be doing their PhDs, just as Juice goes into orbit around Ganymede.”

It is also maddening that there is not enough money in the mission budget to include a lander to investigate the surface of either moon. Nevertheless, if results from Juice produce compelling evidence that these Jovian satellites are indeed habitable, you can bet there will be a scramble to build one.

If it turns out that there is life on such weird worlds as Europa and Ganymede – even in an insignificant solar system in the outer reaches of the galaxy such as ours – then there is a good chance that life is widespread in the universe.

And that is very exciting indeed.

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