This is an artist’s illustration of Europa (foreground), Jupiter (right) and Io (middle). Based on new evidence from Jupiter's moon Europa, astronomers hypothesize that chloride salts bubble up from the moon's global liquid ocean and reach the frozen surface and volcanoes. (NASA/JPL-Caltech)

The search for life in the solar system took a turn Thursday with the announcement that Europa, a moon of Jupiter first discovered by Galileo, shows signs of water geysers erupting from its south pole.

The new observations by the Hubble Space Telescope represent the best evidence yet that Europa, heated internally by the powerful tidal forces generated by Jupiter’s gravity, has a deep sub­surface ocean. The hidden ocean has long been suspected, but scientists have never seen anything as dramatic and overt as plumes of water vapor more than 100 miles high.

If this finding holds up — the Hubble will look again, and scientists are already racing to re­examine data gathered years ago by NASA’s Galileo probe — it could provide a major boost to a much-discussed but still unapproved NASA robotic mission to explore the icy moon that circles Jupiter every 31 / 2 days.

“If there’s a geyser 200 kilometers tall, and you could fly a spacecraft through it and sample the water coming out from Europa, that would be phenomenal. What if there are organics in it? That’s getting to the question of ‘Are we alone in the universe?’ ” said John Grunsfeld, NASA’s top official for space science.

The discovery, detailed in a paper published Thursday online by the journal Science, was the subject of a news conference in San Francisco on Thursday morning, at the fall meeting of the American Geophysical Union.

Since the 1980s, shortly after the Voyager probes first ventured to the outer planets, scientists have suspected that Europa has a global ocean, perhaps with more water than in all the oceans on Earth, hidden beneath a shell of ice. The Galileo probe found the signature of a sub­surface ocean through an analysis of Europa’s magnetic field. The surface ice of the moon has brown stains that could be the result of organic material snowing from the plumes, scientists said Thursday.

Late last year, a team of scientists (the lead author on the new paper is Lorenz Roth of the Southwest Research Institute, San Antonio) used the Hubble, and specifically an instrument repaired by Grunsfeld and fellow astronauts in the final Hubble servicing mission, to take a closer look at Europa. The scientists didn’t see a geyser directly, but rather saw a striking surplus of hydrogen and oxygen appearing in a spatially confined area — a plume, in effect — over roughly seven hours.

The implication is that tidal forces within the moon — created by Jupiter’s immense gravity — cause Europa to contract and expand, a bit like a tennis ball being squeezed and released. The Hubble spotted the signs of plumes when Europa was farthest from Jupiter in its elliptical orbit of the planet. The likely scenario is that, when the crust decompresses slightly, liquid water squeezes up through a crack.

As it hit the vacuum of space, the water would flash freeze and some of it would turn into water vapor. Those water molecules would be split into atomic hydrogen and oxygen in the harsh radiation environment of the Jupiter system. But it wouldn’t just be water in the plume: Whatever else was in that ocean would be squirted into space, too, said James Green, head of NASA’s planetary science division.

“For a planetary scientist, it’s huge,” Green said of the news.

This is not the first moon to show signs of geysers. Another candidate for exploration is Enceladus, a moon of Saturn, which has similar south pole plumes and might have a sub­surface sea, though perhaps not a global ocean as Europa appears to have.

Carolyn Porco, leader of the imaging team for NASA’s spacecraft Cassini, which is exploring the Saturn system, said of the Europa announcement: “If it really is a plume of material coming from the ocean beneath the ice shell, that is truly extraordinary. It would put it in the same league as Enceladus as an accessible target.”

Planetary scientists dream of a mission known as the Europa Clipper, but although it is in the formulation phase at NASA, it has not been fully approved. In recent weeks, budget pressures have made new, expensive NASA robotic missions look increasingly less likely to be funded.

Robert Pappalardo, a senior research scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif., said a confirmation of the Europa plumes would make it even more essential for NASA to send a probe to look for signs of life in that distant ocean.

“The possibility that it’s there means that we have to go search for it,” Pappalardo said. “For the sake of humanity, for the sake of science, for the sake of knowledge.”