Geysers spew water from the tiger-striped south pole of Jupiter's ice-covered moon Europa, suggest observations from NASA's Hubble space telescope.
If the existence of the plumes can be confirmed, they will perhaps allow future space missions to examine the inner composition of the Jovian moon and look for signs of alien life there. (See "Could Jupiter Moon Harbor Fish-Size Life?")
Analysis of Hubble ultraviolet-light images of the moon from 1999 and 2012 suggest that subsurface water sprays out of the moon's south pole in two geysers that are 124 miles (200 kilometers) high, according to the study led by planetary scientist Lorenz Roth of the Southwest Research Institute (SWRI) in San Antonio, Texas, and released Thursday by the journal Science.
"We can potentially fly through it, analyze the composition, and understand directly what's the chemistry of the subsurface," said planetary scientist Robert Pappalardo, of NASA's Jet Propulsion Laboratory, who was not involved in the study. "It's extremely exciting in terms of being able to sample Europa's material."
Europa has long been considered a potential home for otherworldly life living in the ocean beneath its icy crust, both in science and in science-fiction novels such as Arthur C. Clarke's 2010: Odyssey Two, which was made into a film in 1984.
The moon, some 1,900 miles wide (3,100 kilometers), possesses only a tenuous oxygen atmosphere, and gets blasted by severe radiation and charged particles from Jupiter. So if life does exist on Europa, it would have to live beneath its 12-mile (20-kilometer) thick shell of ice, where Jupiter's gravitational pull potentially generates enough heat to create a 99-mile (160-kilometer) deep ocean.
Search for Life
Drilling down to reach that ocean would be a formidable challenge for anyone who'd like to assess Europa's habitability. But the new study results open the door to a cheaper and simpler orbiting spacecraft sampling the geysers in a space flyby.
The exact source of the plumes is still unknown, however, said Roth. The water vapor seems to be shooting through a narrow fissure in the ice at about 2,300 feet (700 meters) per second, but the team cannot yet determine whether the water comes from the subsurface ocean, a lake within the ice, or from ice that melts as tidal forces stress the cracks on Europa's surface.
Scientists have been speculating about water plumes on Europa for decades. In 1979, the Voyager flyby returned photos of a bright spot that some scientists thought might be a plume, but follow-up studies couldn't confirm its existence. Scientists later concluded the bright spot was a camera artifact.
The 2005 discovery of an icy plume ejecting from Saturn's moon Enceladus, however, encouraged scientists to keep searching on Europa. Now, armed with the ultraviolet-light data from Hubble, Roth's team said that they're 95 percent sure that the plumes do exist.
"It sure looks like they have a very strong case," said Pappalardo.
The geysers evaded detection for so long because they apparently turn on when Europa is at its farthest point from Jupiter, then turn off when the two draw nearer to each other. That's in line with observations from Enceladus, which has plumes that brighten and dim depending on its distance from Saturn. It is thought that as Enceladus approaches Saturn, the planet's gravity squeezes the moon's surface fractures closed so that the vapor can't escape. As it moves farther away, the surface relaxes, opening its vents.
If other groups can confirm the plumes' existence, Roth said he hopes it will be possible to resolve their size, timing, and composition. Fortunately, Europa is only half as far away as Enceladus, so it may be possible to examine the plumes from Earth-based telescopes using radio astronomy and infrared observations.
Several missions could potentially get a closer look at Europa's plumes. The European Space Agency's Jupiter Icy Moon Explorer launches in 2022 and will make several flybys of Europa, where it will search for chemistry essential to life.
A NASA mission, called Juno, intended to explore Jupiter's atmosphere and magnetic field is headed for a 2016 arrival there. "Juno is not designed to investigate Europa, but we may be able to look for water with our remote sensing instruments," says SWRI's Scott Bolton, Juno's principal investigator, by email. However, he cautions that more analysis would be required to be certain.
The space agency hasn't set plans yet to directly visit Europa, but the moon is consistently rated as a high priority for exploration. A proposed orbiter called the Europa Clipper would investigate whether the moon might harbor conditions suitable to life. The mission would perform about 45 flybys, said Pappalardo, who is lead scientist on the concept. "Some of those could be targeted to fly right through a plume. And if there were organics coming out, we could measure them directly."
Pappalardo added that he thinks the existence of the plumes, if confirmed, will elevate the priority of future missions to Europa.
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