Previous observations point to a liquid ocean underneath the icy surface of Europa. The new findings, which are published in the Dec. 12 online issue of Science Express, point toward erupting plumes of liquid water on the surface, though this is not yet confirmed with certainty.
“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. “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.”
Europa is the second moon in the solar system found to exhibit such behavior; the other is Enceladus, a moon of Saturn. The geysers on Enceladus have also been found to emit ice and dust particles, pointing toward cryovolcanic activity. Thus far, only water vapor has been measured on Europa.
Jupiter's strong magnetic field interfered with Hubble's spectroscopic observations by creating auroral activity on Europa, so it was necessary to use time sampling to distinguish between the auroras and the water plumes. Excited atoms of hydrogen and oxygen can be identified by their characteristic emission spectra, and provide evidence of water molecules being broken apart by the electrons in the aurora.
“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, Germany. Saur, who is principal investigator of the Hubble observation campaign, co-wrote the paper with Roth.
Roth suggests that the lineae on Europa's surface may act like the fissures on Enceladus' surface, venting water vapor into space. Another similarity to the cryovolcanoes on Enceladus is that the plumes on Europa vary with distance from Jupiter, only forming active jets at apojove. It is theorized that the lineae are stressed by tidal forces which open the vents at greater distance from Jupiter and close the vents at lesser distance.
“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.
While the plumes of Europa and Enceladus plumes have similar abundances of water vapor, the plumes behave differently on Europa because it has 444 times the mass and 6.19 times the radius of Enceladus. Thus, Europa has about 11.5 times more gravitational pull at its surface than Enceladus. This causes the plumes on Europa to mostly fall back to the surface, rather than escape into space as they do at Enceladus. The plumes may be responsible for bright features near Europa's south pole.
“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 Hubble servicing missions and now serves as NASA's associate administrator for science in Washington. “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.”
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.