Scientists have discovered water vapour on Jupiter's icy moon Europa, which is believed to be a strong candidate for hosting alien life.
NASA's Hubble Space Telescope has observed water vapour above the frigid south polar region of 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 fully certain whether the detected water vapour is generated by erupting water plumes on the surface, but they are confident this is the most likely explanation.
Should further observations support the finding, this would make Europa the second moon in the solar system known to have water vapour plumes.
"By far the simplest explanation for this water vapour is that it erupted from plumes on the surface of Europa," said lead author Lorenz Roth of Southwest Research Institute.
"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," Roth said.
In 2005, NASA's Cassini orbiter detected jets of water vapour and dust spewing off the surface of Saturn's moon Enceladus. Although ice and dust particles have subsequently been found in the Enceladus plumes, only water vapour gases have been measured at Europa so far.
Hubble's imaging spectrograph enabled the researchers to distinguish between features created by charged particles from Jupiter's magnetic bubble and plumes from Europa's surface, and also to 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.
Excited atomic oxygen and hydrogen produce a variable auroral glow and leave a telltale sign that are the products of water molecules being broken apart by electrons along magnetic field lines.
Roth suggested that long cracks on Europa's surface, known as lineae, might be venting water vapour into space. Cassini has seen similar fissures that host the Enceladus jets.
Active jets have only been seen when the moon is farthest from Jupiter. The researchers could not detect any sign of venting when Europa is closer to Jupiter.
The findings were published in the journal Science Express.