NASA has discovered more evidence that water erupts from beneath the surface of Europa, one of the many moons of Jupiter. The new findings lend more legitimacy to observations from 2013, and further tease the possibility that Europa contains a subsurface ocean. A team from the Space Telescope Science Institute (STScI) made the observations using the Hubble Space Telescope, and the agency announced the findings during a teleconference this afternoon. The full findings will be published in the Astrophysical Journal on September 29th.

The STScI team watched Europa travel across the face of Jupiter on 10 different occasions over a period of 15 months, starting in December of 2013. During three of those transits, the team captured what appeared to be plumes of water vapor erupting from near the icy moon’s south pole. William Sparks, a STScI astronomer who led the research team, called the findings "statistically significant" — even if they aren’t proof. "We do not claim to have proven the existence of plumes, but rather to have contributed evidence that such activity may be present," Sparks said.

It's not proof, but it's a great sign

NASA has found evidence of water all over our Solar System, but the agency has only directly spotted geysers on one body — Enceladus, a moon of Saturn. Scientists have long proposed that Europa is home to a subsurface ocean, but it took until 2012 to find evidence of water plumes erupting from that ocean. That year, a different NASA research team spotted aurorae in the southern region of Europa. Aurorae are caused by charged particles interacting with an atmosphere — we see them on Earth (you may know them as the Northern and Southern lights) thanks to the Sun’s solar wind. The light show on Europa has a different cause — hydrogen and oxygen, scientists argued. But the indirect connection wasn’t enough to confirm the existence of water plumes.

Sparks’ team spotted the evidence in a more direct way. The team was using Hubble to measure Europa’s atmosphere as the moon transited across the bright background of Jupiter. This technique is similar to what planetary scientists use when they try to determine the atmospheric qualities of exoplanets. The light coming from the bigger, brighter object in the background makes it easier to see features at the edges of smaller, more dimly lit objects in the foreground.

What Sparks’ team saw was one step closer to direct images of water plumes. Dark splotches appeared in the same southern region of Europa every few months. This is a hint that Europa’s surface breaks up now and then due to the massive gravity of Jupiter. When the surface cracks, the rifts sometimes run deep enough to release water into the atmosphere from the ocean below. The team estimates that these plumes shoot out about 125 miles (200 kilometers) above Europa before they fall back down to the surface, similar to estimates from the 2012 observations.

NASA believes Europa’s ocean could have Earth-like qualities that we know to be beneficial to life. But Europa’s surface is thought to be miles thick, which would make it hard to directly measure that ocean. The possibility of plumes on Europa is tantalizing because it offers a new (and easier) way to make those measurements.

NASA already plans to send a spacecraft — known as the Europa Clipper — to the icy moon in the 2020s. The agency has said it won’t send the Clipper through these plumes like the Cassini spacecraft did when it studied Enceladus. But a proposed lander, or even a second orbiter (which would fly closer to Europa than the Clipper), could tag along and directly measure these plumes for evidence of life.

The Clipper mission is a long way out, though. More observations can be done from Earth before then, and Sparks’ team has already started on these. But he was also careful to note that, like the team in 2012, they’re already stretching the limits of the 26-year-old Hubble. More detailed observations — and actual proof of water plumes — might have to wait until Hubble’s successor, the James Webb Space Telescope, launches in 2018.

Keeping the Solar System clean to find alien life