Hubble spots 'water vapour plumes' erupting from Jupiter's moon Europa
Tantalising new evidence of water vapour plumes erupting from the surface of Jupiter's moon Europa has been uncovered by the Hubble Space Telescope.
If shown to be real, the plumes are likely to originate from the global salty ocean known to lie beneath the moon's icy shell.
They could provide a golden opportunity to investigate the ocean - which many scientists believe has the potential to harbour life - without drilling through miles of ice.
Earlier, the American space agency Nasa sparked a frenzy of social media speculation about aliens after promising news of "surprising activity" on Europa.
Nasa eventually scotched the rumours with a tweet that read: " Monday, we'll announce new findings from Jupiter's moon Europa. Spoiler alert: NOT aliens."
The actual discovery is more prosaic but, for scientists, still highly significant and exciting.
Astronomers used Hubble's ability to observe ultraviolet light to look for signs of water above the surface of Europa as the moon crossed the face of Jupiter.
Light reflected from the planet was absorbed at specific wavelengths consistent with the presence of water vapour or particles of water ice.
The results indicated plumes erupting 125 miles (200 kilometres) into space on three occasions during 10 observations over 15 months. All were clustered around Europa's south pole apart from one which had an equatorial origin.
In 2012 another team of Hubble scientists spotted a plume signature above Europa - in this case, the hydrogen and oxygen components of water. However, no further evidence of plumes has come to light until now.
While the scientists are cautiously stressing they have found evidence but not proof, no other natural phenomenon is known that can explain the results.
Astronomer Dr William Sparks, from the Space Telescope Science Institute in Baltimore, US, said at a Nasa news briefing: " If plumes exist, this is an exciting find as it potentially gives easier access to the ocean below, and would allow us to search for signs of life in the ocean of Europa without needing to drill through miles of ice.
"However, I do want to stress that the observations are at the limit of what Hubble can do ... We remain cautious, because we're working at difficult wavelengths for Hubble."
Powerful tidal forces caused by Jupiter's gravity are believed to keep Europa's hidden ocean from freezing solid.
Europa is roughly the same size as Earth's moon, about 3,000km (1,864 miles) in diameter. Yet scientists estimate that its hidden ocean contains twice as much water as all the oceans on Earth combined. The ocean is believed to be in contact with silicate rock, providing a chemical environment potentially suitable for simple life.
Experts are still not sure how the water can penetrate through a blanket of ice 10 to 15 miles thick. Some computer simulations suggest that heat within the ocean may be focused at lower latitudes, making the ice thinner there.
Geoff Yoder, acting associate administrator for Nasa's Science Mission Directorate in Washington DC, said: "Europa's ocean is considered to be one of the most promising places that could potentially harbour life in the solar system."
Europa is not the first moon in the solar system to yield evidence of water vapour plumes. In 2005, Nasa's Cassini orbiter detected jets of water vapour and dust spewing from the surface of Saturn's moon, Enceladus.
The agency is planning a space mission to Europa, likely to be launched in the 2020s, that could confirm the presence of plumes and study them at close range.
The Hubble team's findings are due to appear in The Astrophysical Journal on Thursday.