The Curious Case of Europa's Missing Geysers

The huge geysers on Jupiter's icy moon Europa have gone underground.

The huge geysers on Jupiter's icy moon Europa have gone underground.

Late last year, scientists announced that NASA's Hubble Space Telescope had detected plumes of water vapor spewing about 120 miles (200 kilometers) into space from Europa's south pole in December 2012. The news was met with a great deal of excitement, as it suggested that a robotic probe may be able to sample Europa's possibly life-supporting subsurface ocean without touching down.

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The researchers have trained Hubble on Europa repeatedly since then, trying to confirm and characterize the plumes during observations in January, February, November and December of this year. But they've come up empty. [Photos: Europa, Mysterious Icy Moon of Jupiter]

"We have not yet found any signals of waper vapor in the new images so far," team member Lorenz Roth, of the Southwest Research Institute in San Antonio, said Dec. 19 during a talk here at the annual fall meeting of the American Geophysical Union (AGU).

Other research teams have also failed to confirm the plumes. For example, a recent re-analysis of images gathered by NASA's Galileo probe, which studied the Jupiter system up close from 1995 through 2003, turned up no evidence of their existence, said Cynthia Phillips of the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California.

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Europa's plumes are thus unlikely to resemble the famous powerhouse geysers that erupt continuously from the south pole of Saturn's icy moon Enceladus, which also harbors an ocean of liquid water beneath its icy shell, she said. (The two moons' oceans are kept liquid by heat-generating tidal forces, the same mechanism thought to power the geysers.)

"I find it hard to believe that if a plume that was similar to the plumes we see on Enceladus had been going off on Europa during the Galileo era - I find it really unlikely that we would have missed it," Phillips said during her talk at AGU on Dec. 19. "I think we would have seen that thing."

Further, researchers announced on Dec. 18 at AGU that NASA's Cassini spacecraft, which flew by Jupiter in 2001 on its way to Saturn, also didn't see any plume activity at Europa at the time.

"We found no evidence for water near Europa, even though we have readily detected it as it erupts in the plumes of Enceladus," Larry Esposito of the University of Colorado at Boulder, team lead for Cassini's ultraviolet imaging spectrograph insturment (UVIS), said in a NASA statement.

UVIS measurements also suggest that most of the hot gas surrounding the satellite originates from the neighboring volcanic moon Io, not Europa, and that Europa's wispy atmosphere is 100 times less dense than thought, the study found.

However, none of this necessarily means that Europa's geysers don't exist.

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"It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus," Cassini UVIS team member and study co-author Amanda Hendrix, of the Planetary Science Institute in Tucson, Arizona, said in the NASA statement. "If eruptive activity was occurring at the time of Cassini's flyby, it was at a level too low to be detectable by UVIS."

Indeed, the plume's discoverers had no expectation of constant and intense activity; Hubble observations in October 1999 and November 2012 did not detect any geysers, Roth said.

"It was clear from the beginning that this is a transient phenomenon," he said.

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Roth counseled patience, describing the Hubble plume hunt as a work in progress. (The current search campaign should continue through April 2015.). Phillips voiced similar sentiments, saying that weak and/or intermittent plumes could have gone undetected by Galileo.

"The end result here is, stay tuned," Phillips said.

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Artistic illustration of Europa's icy surface with a water jet in the foreground and Jupiter and the sun in the background.

On August 4, the much-anticipated "Wonders of the Solar System" documentary will premier on the Science Channel.

Presenter and physicist Prof. Brian Cox will show you the hidden mysteries of our interplanetary neighborhood, as well as breathtaking sights of the planets, moons and the sun. Cox will also examine some of Earth's extreme environments to see how life has adapted, perhaps helping us understand whether life can exist elsewhere in our solar system. To provide a taster of what you can expect from "Wonders," Discovery News has gathered some facts, figures and the best images of our solar system to assemble a special Wide Angle supporting this groundbreaking documentary. Let's begin, in the orbit of the solar system's biggest planet, Jupiter...

Moons of Jupiter: Io

Welcome to Jupiter, the solar system's biggest planet and host to more than 63 peculiar moons -- a steady contender for moons compared to satellite-strewn Saturn. From sulfur-spewing furnaces to ice-encrusted oceans, these Jovian satellites are anything but normal! Browse through 10 of our favorites here. If you're looking for a fiery, volcanic spectacle, head to Io, the innermost of Jupiter's "big four" moons. Jupiter's gravity pulls at Io so strongly that the land has tides of up to 300 feet (100 meters). This gravitational tug-of-war produces scorching heat and raises more volcanic activity here than anywhere else in the solar system. Io has a sulfurous surface, and its volcanoes spew silicate magma, causing the hellish moon's surface look like a pizza.

Moons of Jupiter: Metis

While it doesn't look like much in this image (the highest-resolution available!), Metis is Jupiter's closest companion -- for now. It's orbiting the gas giant at a distance of just 75,500 miles (128,000 kilometers) and moving faster than Jupiter spins. Metis is so close to Jupiter's surface, in fact, that it will gradually succumb to the planet's gravity and plunge into its churning clouds. This same principle applies to man-made satellites orbiting the Earth; if their orbit is too low, they'll eventually fall.

Moons of Jupiter: Adrastea

Like Metis, Adrastea is on its way down, eventually: Its orbit is just 1,000 kilometers (621 miles) outside of its fellow doomed moon. The two bodies probably provide lots of the material making up Jupiter's main ring, shown in this Galileo spacecraft image. Adrastea is also tiny at a wee 12 miles (20 kilometers) in diameter.

Moons of Jupiter: Ganymede

If Ganymede orbited the sun instead of Jupiter, it would be a planet of its own -- it's even bigger than Mercury. Its interior is made from layers surrounding a rocky core, and its surface is covered in water ice. Although Ganymede doesn't have much of an atmosphere, it does have some ozone gas near its surface. This gas comes from charged particles in Jupiter's magnetic field smacking into the moon's icy crust.

Moons of Jupiter: Themisto

We're not entirely sure what Themisto looks like, but scientists know it one odd little satellite. Unlike most other moons, Themisto is oblong and doesn't fit into the traditional groups of Jovian moons. Because this body is so small -- just 5 miles (8 kilometers) in diameter -- astronomers in 2000 confused it for a new object when it was originally found in 1975. Shown here is a speculative illustration of how the irregular moon might be shaped.

Moons of Jupiter: Callisto

Along with Io, Ganymede, and Europa (which is yet to come in this slide show), Callisto is one of the four Galilean satellites. These are the moons of Jupiter that Galileo discovered while looking through a telescope in 1610. Callisto is about the same size as Mercury, and it's a heavily cratered moon that has almost no geological activity. Callisto's surface may also be one of the oldest landscapes in the solar system, including Earth's moon -- about 4 billion years old.

Moons of Jupiter: Thebe

Along with Metis and Adrastea, Thebe was discovered by scientists studying images from the Voyager spacecraft in 1979 and 1980. It's closer to Jupiter than Io is, but it's not so close that it's in danger of losing its altitude. The material in Jupiter's Gossamer ring probably comes from Thebe and another moon, Amalthea.

Moons of Jupiter: Amalthea

Speaking of Amalthea, shown here is an artist's rendition of the mysterious moon (inset: our best real view so far). Astronomers don't know much about it, but they do know it's the reddest body in the solar system. Unlike the other three moons that lie within Io's orbit -- Thebe, Metis and Adrastea -- Amalthea wasn't discovered by the Voyager science team. Edward Emerson Barnard discovered Amalthea it in 1892, and it had been almost 300 years since anyone had discovered a new moon orbiting Jupiter. Before that, the last person to discover a Jovian moon was Galileo.

Moons of Jupiter: S/2000 J11

This not-to-be-named moon marks an interesting dividing line in the wealth of satellites orbiting Jupiter. S/2000 J11, named after it was found in 2000, moves in the same direction that Jupiter spins -- as do all the moons that are closer to the planet. Almost every distant moon, however, orbits in the opposite (retrograde) direction, as this diagram shows. The only known exception is Carpo, sometimes known as Karpo, which was discovered in 2003.

Moons of Jupiter: Europa

About two-thirds of the Earth is covered in water, but Europa has about twice as much water as our home planet. Europa's might surface is covered in ice, but astronomers are almost convinced there is an ocean of liquid water underneath -- an environment shielded from Jupiter's intense radiation that might be hospitable to life. A cold, salty planet might not seem like a good place to live, but there are species on Earth that thrive in just those conditions.

Slideshow originally posted Feb. 2009.