Though it's five times farther from the Sun than Earth, and therefore so cold that ice becomes as hard as rock, Jupiter's moon Europa may be the first place to go to look for extraterrestrial life. Ever since the moon, which superficially resembles a cracked eggshell, was photographed close-up by the Voyager space probe, scientists have been intrigued by its potential as a niche for life.
For over the past 30 years it has been hypothesized that the icy crust covers a subsurface ocean. Where there is water there could be life. Now NASA's Hubble Space Telescope has found something the Jovian probes may have missed, plumes of water vapor leaking off into space near the moon's south pole. Astronomers do not know yet if these gas plumes are connected to subsurface liquid water or not. This venting doesn't seem unique. In 2005, NASA's Cassini orbiter discovered similar water vapor plumes spewing off of the tiny moon Enceladus, 1 billion miles away.
NASA's Hubble Space Telescope has observed water vapor above the frigid south polar region of Jupiter's moon Europa, providing the first strong evidence of water plumes erupting off the moon's surface.
Previous scientific findings from other sources already point to the existence of an ocean located under Europa's icy crust. Researchers are not yet certain whether the detected water vapor is generated by water plumes erupting on the surface, but they are confident this is the most likely explanation.
Should further observations support the finding, it would make Europa the second moon in the solar system known to have water vapor plumes. The findings were published in the Thursday, Dec. 12, online issue of Science Express, and reported at the meeting of the American Geophysical Union in San Francisco.
"By far the simplest explanation for this water vapor is that it erupted from plumes on the surface of Europa," said lead author Lorenz Roth of Southwest Research Institute in San Antonio, Texas. "If those plumes are connected with the subsurface water ocean we are confident exists under Europa's crust, then this means that future investigations can directly investigate the chemical makeup of Europa's potentially habitable environment without drilling through layers of ice. And that is tremendously exciting."
In 2005, NASA's Cassini orbiter detected jets of water vapor and dust spewing off the surface of Saturn's moon Enceladus. Although ice and dust particles subsequently have been found in the Enceladus plumes, only water vapor gases have been measured at Europa so far.
Hubble's spectroscopic observations provided the evidence for Europa plumes in December 2012. Time sampling of auroral emissions measured by Hubble's imaging spectrograph enabled the researchers to distinguish between features created by Jupiter's magnetospheric particles and local enhancements of gas, and to also rule out more exotic explanations such as serendipitously observing a rare meteorite impact. The imaging spectrograph detected faint ultraviolet light from an aurora, powered by Jupiter's intense magnetic field, near the moon's south pole. Atomic oxygen and hydrogen produce a variable auroral glow and leave a telltale sign that they are products of water molecules being broken apart by electrons along magnetic field lines.
"We pushed Hubble to its limits to see this very faint emission. These could be stealth plumes, because they might be tenuous and difficult to observe in the visible light," said Joachim Saur of the University of Cologne in Germany. Saur, who is principal investigator of the Hubble observation campaign, co-wrote the paper with Roth. Roth suggested long cracks on Europa's surface, known as lineae, might be venting water vapor into space. Cassini has seen similar fissures that host Enceladus' jets.
The Hubble team found that the intensity of Europa's plumes, like that Enceladus's plumes, varies with the moon's orbital position. Active jets have been seen only when Europa is farthest from Jupiter. But the researchers could not detect any sign of venting when Europa is closer to Jupiter.
One explanation for the variability is these lineae experience more stress as gravitational tidal forces push and pull on the moon and open vents at larger distances from Jupiter. The vents are narrowed or closed when the moon is closest to the gas giant planet.
"The apparent plume variability supports a key prediction that Europa should tidally flex by a significant amount if it has a subsurface ocean," said Kurt Retherford, also of Southwest Research Institute.
Europa's and Enceladus' plumes have remarkably similar abundances of water vapor. Because Europa has roughly 12 times more gravitational pull than Enceladus, the vapor, whose temperature is measured at minus 40 degrees Celsius, does not escape into space as it does at Enceladus. Instead, it falls back onto the surface after reaching an altitude of 125 miles, according to the Hubble measurements. This could leave bright surface features near the moon's south polar region, the researchers hypothesize.
"If confirmed, this new observation once again shows the power of the Hubble Space Telescope to explore and opens a new chapter in our search for potentially habitable environments in our solar system," said John Grunsfeld, an astronaut who participated in Hubble servicing missions and now serves as NASA's associate administrator for science in Washington, D.C. "The effort and risk we took to upgrade and repair the Hubble becomes all the more worthwhile when we learn about exciting discoveries like this one from Europa."