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.
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,” said Joachim Saur of the University of Cologne in Germany. “These could be stealth plumes because they might be tenuous and difficult to observe in the visible light.” A team member 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 Enceladus’ 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 that 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 the 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 – 40° Fahrenheit (–40° 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 (200 kilometers) according to the Hubble measurements. This could leave bright surface features near the moon’s south polar region, the researchers hypothesized.
“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 from NASA in Washington, D.C. “The effort and risk we took to upgrade and repair Hubble becomes all the more worthwhile when we learn about exciting discoveries like this one from Europa.”