Space explorers not only see Mars as a possible haven for other life - liquid oceans, locked under ice ices, are also found on some other moons of the solar system. Chris McKay, for example, has a crush on Mars. This red, dusty, rust-eaten world is no longer as attractive as it once could have been, but still.
“I've been obsessed with life on Mars for many years,” admits a NASA planetary scientist who has spent most of his career looking for signs of life on the Red Planet, in an interview with the BBC. “This is a temptation of the highest level. I give up my first love and go to another, which showed me what I wanted to see."
McKay's newest object of affection is Enceladus, an ice-covered moon of Saturn. Explored by NASA and the European Space Agency's Cassini space probe, the satellite spews water from the south pole - most likely from a liquid ocean several kilometers below the surface. Cassini discovered that this water contains all the vital ingredients: carbon, nitrogen and hydrogen.
“I think this is what we need,” McKay says. "From an astrobiological perspective, this is an extremely entertaining story."
But Cassini has only a few weeks left before it meets its death in the atmosphere of Saturn. “We have to fly through this train in search of life,” he says. "We designed a new mission specifically for this, to fly low and slow through the plume, collect samples and find evidence of life."
The proposed mission currently competes with NASA's five other future missions - to comets, asteroids and planets. “Now we have the opportunity to fight,” says McKay. “But the story is damn good: we're going to find life, are you? I think we will win this competition because the goal is extremely attractive."
Enceladus, however, is just one of several ice-covered worlds in the solar system with liquid water and possibly microscopic life. Other candidates include Jupiter's three moons: Europa, Callisto, and Ganymede. Even the distant moon of Neptune, Triton, may be habitable.
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Europe is perhaps the most famous destination to explore. Back in the 1960s, astronomers suggested there might be life on this moon. Arthur Clarke imagined plants growing on it under the ice. Observations of the Galileo probe in the late 1990s showed that Europa has an ocean of water 15-20 kilometers deep beneath a cracked ice crust on the surface. There may also be areas in which lakes of water are trapped in ice only a few kilometers away.
And while we may wait decades before we reach Enceladus, Europe will soon be thoroughly explored. ESA is building a spacecraft known as JUICE - Jupiter Icy Moon Explorer. In 2022, a probe will be launched to Jupiter to study Europa, Ganymede and Callisto in detail.
NASA also plans to send a Europa Clipper mission in the mid-2020s. This robotic probe will have to fly around Europe about 40 times. Meanwhile, at the Jet Propulsion Laboratory in Pasadena, engineers are already developing the next phase: designing lander and sampling systems on these icy worlds.
It is extremely difficult to work in frozen conditions. Besides remote control, one attempt to break through several kilometers of ice to liquid will be a serious problem. NASA is considering launching landing missions that will eventually drill through ice to collect samples. This problem hasn't been solved yet, but there are a lot of smart people working at JPL.
McKay's team is working on a variety of concepts, including a Euro-propeller and an anchor system that will use heated prongs to keep instruments on ice. Technologies for taking samples from under the surface include a robot on a thermonuclear element that can "seep" through the bark. Another project involves a drill that cuts into ice and returns samples down a pipe for analysis.
At the moment, scientists have some prototypes in the laboratory, but there are still 15-20 years before the launch of the mission, so other proposals will appear. There is no working solution yet, but there is enough time to find them. Finding any life, even the tiniest, on satellites once thought to be dead is definitely going to be one of the biggest discoveries of all time. This will mean that there is a lot of life in the Universe.
"Extraordinary hypotheses require extraordinary evidence," McKay says. "In my opinion, there is no more extraordinary hypothesis than the existence of life in another world - the second genesis."
Ilya Khel