Imagine the bottom of an ocean. The water is icy cold and impenetrably dark, without even a glimmer of sunlight. But in one patch of the ocean floor, a jet of hot water spurts upwards out of a rocky vent. This water is laced with life-giving chemicals, and around it a collection of strange microorganisms has gathered.

These tiny organisms are like nothing anyone has ever seen before. But that's no surprise, because we're not on Earth. We're 630 million km from Earth, on Europa, one of the moons of Jupiter.

Antarctica's lakes are the best analogue we have for Europa's hidden ocean

Or at least, we could be. Europa is one of the few places in our solar system where life could exist. It's believed to have an ocean up to 100km deep, ten times as deep as any ocean on Earth, which could contain twice as much water as our entire planet. All the conditions for life may be met, somewhere in those dark waters.

So far we haven't looked, so we don't know if anything is alive down there. But there are clues to be found much closer to home, in an environment that's almost as extreme.

Deep under the thick ice of Antarctica, there are hundreds of hidden lakes. Some of them have been isolated for millions of years. But in the last few years, scientists have started to explore them, in the hope of finding life.

Antarctica's lakes are the best analogue we have for Europa's hidden ocean. So if life can endure the conditions under the Antarctic ice, the odds of finding it on Europa will improve. What's more, knowing how life survives in these hidden lakes will tell scientists what to look for when they seek it on other worlds.

Europa has held out the promise of alien life since 1979, when Nasa's Voyager 2 spacecraft flew past it. Voyager revealed that Europa's surface is a thick layer of ice. But there are vivid lines and streaks on this icy crust, suggesting that it has been penetrated by minerals from below the surface.

JUICE won't specifically look for life, but it will try to find out what Europa's ocean is like

Most scientists now believe there is an ocean of salty water flowing under Europa's ice. In this water, single-celled microorganisms could have evolved, if the conditions are right.

In 2022, the European Space Agency will launch a space probe called the JUpiter ICy moons Explorer (JUICE) to study three of Jupiter's moons, including Europa.

"The mission is going to do a fantastic job of expanding our understanding of the entire Jovian system," says Kevin Hand, an astrobiologist at Nasa's Jet Propulsion Laboratory in Pasadena, California.

JUICE won't specifically look for life, but it will try to find out what Europa's ocean is like. The first step is to find out how deeply it is buried.

The answer may be: not very deep at all. An analysis by Nasa researchers in 2011 suggested that there is only about 3km of ice overlaying the ocean. That is thinner than many areas of the Antarctic ice sheet. JUICE will use radar to find out if this is true, and map out Europa's internal structure.

JUICE will also investigate what is going on at the bottom of the ocean. A key question will be whether there are hydrothermal vents: jets of hot chemical-rich water shooting out of the sea floor. These vents could supply the energy for life in the ocean, just like they do in the depths of the sea on Earth.

So far there is no direct evidence that Europa has hydrothermal vents, but JUICE might be able to help. It could find some crucial evidence drifting above Europa's surface – if, that is, Europa is anything like Enceladus.

Enceladus is a moon of Saturn, and in many respects it could be Europa's twin. The surface is a thick layer of ice, and it looks like there is liquid water trapped underneath it. That would explain why Enceladus has geysers.

The Cassini spacecraft has found dozens of water jets spurting out of Enceladus, and has even managed to take samples. A study published in March 2015 reports that the jets contain grains of sand, suggesting that the hidden ocean has a floor of rock and sand, and hinting at the presence of deep sea vents.

Life doesn't just need water. It needs other chemicals

Hand says Europa might well have similar plumes, and JUICE should be able to spot them. On Enceladus they erupt as high as 500km, which makes them easily visible. Europa's will be less obvious, says Hand. Its gravity is stronger so the plumes would not rise higher than about 100km. JUICE won't be able to sample them – but a follow-up mission might.

Finally, JUICE will try to find out if Europa has the right chemicals to support life. It won't land, but it can study the makeup of Europa's thin atmosphere, says team member Andrew Coates of University College London in the UK. This should reveal something about the makeup of the ocean. "We expect to be able to see water, but the non-water composition will of be of particular interest."

That's because life doesn't just need water. It needs other chemicals, such as carbon and nitrogen, and an energy source to keep it going. Sensors called infrared spectrometers will tell us whether these chemicals are present, says Coates.

The JUICE mission is still a long way off. But fortunately, we don't need to rely on missions into space to understand an environment like Europa. Astrobiologists are paying close attention to the search for extreme life closer to home, in the lakes under Antarctica.

Places like Lake Vostok can tell us whether to expect life in places like Europa

The environments are eerily similar. The Antarctic lakes are dark and cold, trapped under a crushing weight of ice. And just like Europa's ocean, many of them have been locked away from the outside world for millions of years.

The largest subglacial lake is Lake Vostok in east Antarctica. The water there is beneath 4km of ice and is up to 800m deep. Yet many scientists believe there is life down there, life that has adapted to seemingly lethal conditions.

"These life forms in extreme environments on Earth help us develop a framework for habitability [of other planets]," says Hand. Places like Lake Vostok can tell us whether to expect life in places like Europa.

No two lakes are alike. Some have been trapped for much longer than others, depending when their part of Antarctica froze over. Different lakes also have different flows of chemicals coming in and out. So they are a kind of natural laboratory for discovering the conditions necessary for life. "It's only in this coupled context that we'll understand the limits and habitats for life," says Hand.

Antarctica's subglacial lakes were first identified in the late 1960s using radio echo sounding.  High energy signals were fired into the ice and the resulting reflections give insight into what lies beneath.

However, these discoveries were "quickly forgotten about because nobody was interested", says glaciologist Martin Siegert of Bristol University in the UK, who has spent over two decades analysing the lakes. That's because no one thought that these extreme environments could harbour life.

The lake's water has probably been trapped for 14-15 million years

The turning point came in 1993, when Siegert and his colleagues discovered Lake Vostok. It had actually been spotted by radio echo sounding in the 1970s, but they were able to use satellite data to confirm not only its existence, but its size: 10,000 sq km, making it the largest subglacial lake in Antarctica.

This time, "folk got instantly excited by lakes at the base of Antarctica," says Siegert. Scientists were on the hunt for "extremophiles": organisms that could survive in seemingly-lethal conditions. "[Vostok] became thought about as a potential viable habitat for microbial life."

We now know of almost 400 such lakes, but Vostok remains the most alluring. That is partly due to its age. The lake's water has probably been trapped for 14-15 million years, since Antarctica's ice sheet formed. If there is life there, it is ancient. The challenge is getting to it.

Even before Lake Vostok was discovered, a Russian team of scientists was drilling down towards it. They began in 1990, before the satellite studies, says Sergey Bulat of the Petersburg Nuclear Physics Institute in Russia. Knowing nothing of the hidden lake, they were drilling for ancient ice that carries records of past climates.

Bulat says he is confident that one of the bacteria from the sample is from the lake

In 1998, the Russian drilling team almost reached the lake, but technical difficulties meant they had to stop drilling at a depth of 3,600m. After that the project stalled, until in 2010 another attempt to sample the lake water got the go-ahead.

In 2012, after over 20 years of drilling, the Russian team finally breached the surface of the lake. A year later, Bulat's team announced that they had discovered a new type of bacterial life.  However, the water they took from the lake was contaminated with drilling fluid, so it's unclear whether the "new life" was truly from Lake Vostok or simply from the drill.

It's a tricky problem. The DNA sequences Bulat found have never been seen before, but that doesn't mean much because most bacteria have never had their DNA sequenced. Nevertheless, Bulat says he is confident that one of the bacteria from the sample is from the lake, while the other 48 species were from the drilling fluid.

For now his team has stopped working on samples from this water. They cannot fully sequence the bacteria because of the contamination. But there's another way to get DNA from the lake.

The ice that covers Lake Vostok is actually a thick glacier, which moves across the lake over a period of about 10,000 years. As it does so, water from the lake freezes onto the glacier's underside, forming "accretion ice". This ice eventually gets carried away from the lake.

Rogers has confirmed about 40 different species of fungi and bacteria

Scott Rogers of Bowling Green State University in Ohio and his colleagues have obtained samples of accretion ice, and scoured it for DNA.

They have found evidence of abundant life. So far Rogers has confirmed about 40 different species of fungi and bacteria that he could grow in his lab. Additional genetic traces of life in the ice hint at 3500 more.

"The fact that there are living organisms in the accretion ice can only mean that there are living organisms in the lake," says Rogers. The most logical conclusion, he says, is that organisms are growing and multiplying in the lake, with some being trapped in the accretion ice.

The organisms Rogers found are highly adaptable. They can grow in temperatures of up to 22 °C, but also survive below 0 °C.

Some of the bacteria were similar to those that live in hot hydrothermal vents on the sea floor, hinting that similar vents might exist on Vostok's bottom. If they do, they could supply its inhabitants with nutrients and energy.

If there are fish they must be very tiny

The lake could even support larger organisms. Rogers has found the remnants of bacteria known to live in fish intestines. Does that mean there are fish in Vostok?

Fish need oxygen to breathe, which ought to be in short supply. But Hand says oxygen could be slowly carried into the lake by the glacier. Oxygen could get trapped in surface snow, be buried in the glacier, and finally seep into Lake Vostok when ice melts off the underside of the ice. "You can think about the ice sheet as a conveyer belt," he says.

Still, it would be a tough life. "If there are fish they must be very tiny, as there's not much to live on down there," says Rogers.

We may have definitive answers as to what the lake water contains fairly soon, Russian scientists have again drilled into the lake, reaching the water in January 2015. As of March 2015, a new set of samples are being stored on site, and will be shipped back to Russia in May.

Lake Whillans is quite different to Vostok or Ellsworth

The success of the Vostok drilling mission stands in stark contrast to the other attempt to breach a deep subglacial lake. In 2012, British scientists led by Siegert tried to drill into Lake Ellsworth in west Antarctica. It's a smaller and shallower lake: 160m deep and under 3.2km of ice. Still, the water has been sealed off for thousands of years.

Unfortunately, the drill they had built didn't work, so the project was called off in December 2012. It was intensely frustrating, says Siegert, but he plans to go back within the next five years.

However, less than a month after the Ellsworth failure, a US team successfully drilled into Lake Whillans. This lake is quite different to Vostok or Ellsworth. It is shallower, sitting under just 800m of ice. It is also only a few metres deep, and is part of a hidden network of rivers and streams under the ice. Water readily flows and in and out of the lake.

The team, led by Brent Christner of Louisiana State University in Baton Rouge, published their results in 2014. Unlike the Vostok samples, there were no contaminants. The water from Lake Whillans turned out to be teeming with microbial life, with around 1000 bacteria per cubic millimetre of water. For the first time, a viable ecosystem had been found under the Antarctic ice.

There might be bigger organisms in the larger lakes

While it's not as deep as the other two, Lake Whillans is still an extreme place. It is pitch-black, so the organisms must be getting energy from something other than the Sun.

Christner says they could be eating the sediments ground off the underlying rocks by the movement of the ice. They might not sound appetising, but they contain the chemicals life needs. Christner says life in Whillans is "powered by dark forces".

So far they have only found single-celled organisms. But Christner says there might be bigger organisms in the larger lakes, which could have more sources of energy. "The higher life forms we might encounter are things like [roundworms]," he says.

Lake Whillans is not an ideal guide to conditions on Europa. It is too shallow, and not isolated enough.

Vostok is better, because it is buried so deep. It may not be as cold as Jupiter's moons, says Coates, but the environment is certainly the best analogue on Earth.

Drilling into the icy crust of Europa, by remote control, will be even harder

But that doesn't mean the data from Lake Whillans can't tell us anything. The microbes in the two lakes might be using at least some of the same tricks. Christner's team found several bacteria in Lake Whillans that are similar to those Rogers found in the Vostok accretion ice.

The next step is to get an uncontaminated sample from a deep lake, whether it's Lake Vostok or somewhere else. That won't be easy. It took 20 years to drill into Lake Vostok, and the resulting samples don't look great, while Ellsworth has so far defeated us.

Drilling into the icy crust of Europa, by remote control, will be even harder. But that doesn't mean it's impossible. If Europa does produce plumes of water, getting samples from them would be much easier. Those samples could be the key to discovering life on another planet for the first time.

We could then go even further. There are now almost 2000 known planets outside our solar system. Many of these "exoplanets" could be habitable, but we don't know which ones. So Coates says we should find out where life exists in our own solar system, then use it to guide the hunt for life around other stars.

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