Ů��С��Ƶ

Jupiter’s icy moon Europa repeatedly spews plumes of water into space, and spacecraft set to visit the moon in a few years might be able to take a taste. In a press conference today, NASA announced that the Hubble Space Telescope has glimpsed the plumes for a second time, which suggests Europa has an active ocean underneath its frozen crust – and cements Europa’s status as one of the best places in the solar system to look for life beyond Earth.

“If there are plumes emerging from Europa, it is significant because it means we may be able to explore that ocean for organic chemicals or even signs of life without having to drill through unknown miles of ice,” said at the Space Telescope Science Institute in Baltimore, Maryland, which manages Hubble.

Observations over the past few decades have hinted that Europa has an ocean of water underneath its pale, streaked crust. In 2012, Hubble strengthened those hints when it spotted plumes of water vapour spurting from the moon’s south pole. But a year later, the plumes had vanished. Old data from the Cassini orbiter, which flew past the Jupiter system in 2001 on its way to Saturn, also showed no evidence for plumes. That suggested if there is plume activity, it is intermittent. It could be seasonal, or related to stretching and pulling by Jupiter’s gravity.

Now, we’ve seen them again (see image below), using a technique borrowed from exoplanet studies. When an exoplanet passes in front of its star, or transits, we can see light filtering through its atmosphere and figure out what it’s made of.

Europa orbits Jupiter every three and a half days, meaning Sparks and colleagues could use Jupiter’s light to catch plumes venting off Europa’s surface. In 2014, Sparks’s team watched Europa transit across Jupiter and analysed ultraviolet images to infer the presence of the plumes.

The plumes are estimated to rise about 200 kilometres above Europa before raining onto the moon’s surface. While this announcement does not hint at Europan aliens, the presence of a liquid water ocean and a repeating cycle of geysers is a major step toward looking for life elsewhere in the solar system. As far as we know, life needs water to survive, and Europa has plenty of it: the 2012 observations suggested Europa is spewing an Olympic swimming pool’s worth of water every eight minutes.

Even better, plumes mean that a future spacecraft will be able to directly sample that ocean and see if anything does live inside.

Grab a geyser

“For a long time, humanity has been wondering whether there is life beyond Earth. We are lucky to live in an era when we can address questions like that scientifically,” said Paul Hertz, director of NASA’s astrophysics division. “On Earth, life is found wherever there is energy, water and nutrients, so we have a special interest in any place that might possess those characteristics. And Europa might be such a place.”

NASA and the European Space Agency have both started working on probes that will visit Europa in the next few years. ESA’s (Juice) is planned for launch in 2022. NASA is planning the , occasionally referred to by the nickname Europa Clipper, which should launch sometime later in the 2020s.

Both spacecraft will be state-of-the-art plume hunters, said NASA’s Curt Niebur, who is working on both missions. They will be able to detect hotspots in Europa’s “chaos terrain“, the striped areas where the plumes originate.

The Europa Clipper in particular will be able to chase a plume quickly if one opens up. “If we find a feature like a plume or something else we find interesting, we can, within a week or two, adjust the trajectory so we can potentially fly right over that,” Niebur says.

Although the Juno probe is orbiting Jupiter right now, it will not venture too close to the moon, in part because scientists don’t want to contaminate the oceans. To prevent any Earth stowaways from reaching Europa, Juno will make a suicide dive into the gas giant sometime in 2018.

Even when the new probes arrive, they probably won’t look for direct evidence of life. Instead they will study whether the moon could be habitable, said Niebur.

“We know how to measure habitability. We have a lot of experience with that. We have a lot of instruments that are very good at doing that,” he says. “When it comes to finding life, we don’t have as much experience. We have an ongoing and vigorous debate in the scientific community about the best way of going about detecting life on a mission like this.”

Hubble can’t directly detect the chemicals in the plume, but it’s possible the material might have the chemicals necessary for life, said at Georgia Institute of Technology in Atlanta.

“The jury is out,” she says.