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Chris McKay has fallen out of love with Mars. The red, dusty, corroded world no longer holds the allure it once did.

“I was obsessed with life on Mars for many years,” confesses the Nasa planetary scientist, who has spent most of his career searching for signs of life on the red planet.

“It’s seduction at the highest level,” he says. “I’m abandoning my first love and going after this other one that’s shown me what I wanted to see.”

The new object of McKay’s affections is Enceladus, the ice-encrusted moon of Saturn. Investigated by the joint Nasa and European Space Agency (Esa) Cassini space probe, the moon is spewing out plumes of water from its south pole – most likely from a liquid ocean several kilometres beneath the surface. Cassini has found this water contains all the vital ingredients for life as we know it: carbon, nitrogen and a readily available source of energy in the form of hydrogen.

“I think this is it,” says McKay. “From an astrobiology point of view, this is the most interesting story.”

Jupiter's moon Europa is though to be one of the most likely places in the Solar System to harbour life (Credit: Nasa)

But Cassini only has a few weeks left before it plunges to its death in Saturn’s atmosphere. “We should be flying through that plume searching for life,” he says. “We have developed a new mission to do that, a mission that will fly low and slow through the plume, collect a huge sample and search for evidence of life.”

This proposed mission is currently in competition for Nasa funds with five other future missions – to comets, asteroids and planets. “Right now we have an opportunity to compete,” says McKay. “But I think we’ve got a damned good story: we’re going to find life, what are you going to find? I’m optimistic that we’ll win the competition because it’s such a compelling target.”

As far back as the 1960s, astronomers theorised that the moon might harbour life

Enceladus, however, is just one of several ice-covered worlds in the Solar System with liquid water – and possibly microscopic life. Other candidates include three of Jupiter’s moons: Europa, Callisto and Ganymede. Even the distant moon of Neptune, Triton, might be habitable for extreme life.

Europa is perhaps the most well-known target for exploration. As far back as the 1960s, astronomers theorised that the moon might harbour life. Writer of the book 2001, Arthur C Clarke, even imagined giant plants growing beneath the ice. Observations from Nasa’s Galileo probe in the late 1990s proved Europa has an ocean of water some 15-20km (9.3-12.5 miles) under a cracked icy surface crust. There may also be areas where lakes of water are trapped within the ice, only a few kilometres down.

While we may have to wait decades for a return to Enceladus, Europa will soon be studied in detail. Esa is building a spacecraft known as Juice, which stands for Jupiter Icy Moons Explorer – possibly the worst acronym in space exploration (I am reliably informed the name was conceived late at night in a bar and may be changed). Due for launch in 2022, the probe will orbit Jupiter and make detailed studies of Europa, Ganymede and Callisto.

The liquid water will probably be reached by a robot drilling through the layers of ice (Credit: Nasa)

Nasa is also planning its own mid-2020s mission, known as Europa Clipper. This robotic space probe is designed to fly past Europa some 40 times and make a detailed study of its surface.

Meanwhile, at the Jet Propulsion Laboratory (JPL) in Pasadena, engineers are already working on the next step: designing robotic landers and sampling systems for these icy worlds.

“Icy moons are extremely challenging places to operate in,” says robotics engineer, Hari Nayar. “They’re cold, remote, rugged and just getting to the liquid, through several kilometres of ice, is an incredibly difficult challenge.”

Finding any life – however small – on worlds once considered dead moons, would be one of the most fundamental discoveries of all time

Nayar envisages a series of lander missions, culminating in a robot that could drill through the ice to collect samples. “We haven’t completely solved that problem yet,” he admits, “but there’re a lot of smart people at JPL.”

His team has developed a number of concepts, including a Europa rover and an anchoring system that uses heated prongs to lock instruments into the ice. Technologies to take samples from beneath the surface include a nuclear-heated robot that could melt its way through the crust. Another design employs a drill to cut through the ice and shuttle samples back up a tube for analysis.

The Cassini spacecraft could have probed Enceladus's plumes of water - but it will soon crash into Saturn's atmosphere (Credit: Nasa)

At the moment, these devices are only at an early “proof of concept” stage, and possibly best described as makeshift. “We’ve built some prototypes in the lab but a mission is at least 15-20 years away,” says Nayar, “I don’t think we have a solution that I’m sure will work yet, but that gives us time to develop the missions.”

Finding any life – however small – on worlds once considered dead moons, would be one of the most fundamental discoveries of all time. It would suggest that life is likely to be common throughout the Universe.

There is, however, a big problem when it comes to searching for life elsewhere in the galaxy: the people who are searching really, really want to find it.

Extraordinary claims require extraordinary evidence – Chris McKay, Nasa

“It’s intrinsic to the search for life that you want the answer to be yes,” says McKay. “I’ve seen papers published making some extraordinary claims, such as life on Mars or whatever, and they’re based on a very selective or narrow interpretation of the data.”

That means ensuring multiple samples are taken and that the spacecraft is completely free of microbes, so any life sampled comes from the icy moons rather than being introduced from Earth.

“Extraordinary claims require extraordinary evidence,” McKay says. “To my mind there is no more extraordinary claim than we have found life on another world – a second genesis.”

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