Scientists have found signs of potentially life-supporting chemical energy in a plume of liquid erupting from the surface of one of Saturn's moons and, for a second time, have also spotted a similar, intriguing fountain on one of Jupiter's moons, NASA announced Thursday.
The latter discovery, on the moon Europa, was led by a Baltimore astronomer using the Hubble Space Telescope.
Hubble previously spied a plume erupting from near the equator of Europa, the smallest of Jupiter's four large moons, in 2014. That prompted scientists to wonder if it was liquid water venting from beneath the body's icy crust.
At a briefing Thursday at NASA Headquarters in Washington, they said they saw it again last year — and realized it was in a spot that earlier observations of Europa had shown to be unusually warm.
"That was a, 'you've got to be kidding me' kind of moment," said Bill Sparks, the astronomer at the Space Telescope Science Institute in Baltimore who served as principal investigator on the Europa research.
Neither the Europa mission nor Cassini — the spacecraft that has been exploring Saturn and its moons for more than a decade — are designed to detect life itself. But progress in exploring the relatively near worlds has scientists hopeful they are closer to finding biological activity beyond Earth.
The findings on Saturn's moon Enceladus are more solid, scientifically speaking. The Cassini spacecraft detected hydrogen emanating from the moon while passing through that body's plume on several fly-by maneuvers.
That suggests water beneath Enceladus' ice crust is interacting chemically with the moon's rocky core, NASA officials said. Such chemistry is a source of energy, independent of sunlight, that feeds life in Earth's oceans — and scientists wonder whether it could do the same on Enceladus.
NASA officials said Thursday that beneath its crust, that moon contains an ocean twice the size of Earth's, from which 60-mile-high plumes are escaping through long, bluish parallel cracks in the surface scientists have dubbed "tiger stripes."
Previous findings showed those plumes contained about 98 percent water, plus ammonia, carbon dioxide, methane and some organics, said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
"The part that had been elusive before had been the hydrogen," Spilker said. "We now know that Enceladus has almost all of the elements you need to support life as we know it on Earth."
Astronomers are reluctant to say anything conclusive about the observations of Europa's plume, but Sparks said it "at least makes it an interesting place to look" for more surprises.
He said the intermittent geyser could be coming through a relatively thin portion of Europa's icy shell — "thin" meaning up to two miles thick — and could give an opportunity to study the moon's ocean without drilling down to reach it.
While the photographs of Europa are less definitive than the Cassini's findings on Enceladus, the older age of Jupiter and its moons make life more likely to have evolved there than on the younger Saturn moon, according to Mary Voytek, astrobiology senior scientist at NASA Headquarters.
"My money, for the moment, is still on Europa," she said.
As for whether any such life found is more likely to be bacterial or more advanced, Voytek said, astronomers are setting their expectations low.
"Most of us would be excited by any life," she said.
Europa is about the same size as Earth's moon, but holds more water than Earth, Sparks said. It's not yet clear what causes the coloration around those cracks in Europa's crust, but salt and iron deposits are a few possibilities, he said.
"It's an ongoing study of research," Sparks said. "It's obviously a very interesting question. It'll be very nice to know the answer."
The discovery heightens hope for what another mission — planned for the early 2020s — might uncover. Engineers at the Johns Hopkins Applied Physics Laboratory in Laurel are designing two instruments for that mission: one to gather data from Europa's magnetic field to learn more about the thickness of its icy crust and the depth of its oceans, and another to capture images of its cracked and ridged surface.
The unmanned, solar-powered Juno spacecraft launched in 2011 and entered Jupiter's orbit last summer. It carries a camera capable of taking high-definition color photographs, but its mission is to survey the giant planet, not its moons, and it won't be diverted to Europa, NASA officials said.
The discoveries on both moons represent "truly a really exciting time" in the search for life in space, said Jim Green, director of the Planetary Science Division at NASA Headquarters.
"These observations are really informing us of major things happening in these ocean worlds right now," Green said. "The science is proceeding so rapidly we can hardly keep up with these discoveries."
"These ocean worlds have just been discovered," he said. "They're in our solar system, and we need to probe them, because they're one of the best locations, we believe, that may be able to harbor life today."