The Hubble Space Telescope observations of Jupiter's icy moon Europa have revealed the presence of persistent water vapour, suggesting potential conditions hospitable for life, according to NASA.
Hubble is an international cooperation between the US space agency and the European Space Agency.
Astronomers at the KTH Royal Institute of Technology, Space and Plasma Physics, Sweden found that Europa harbours a vast ocean underneath its icy surface, but, mysteriously, only in one hemisphere.
Previous observations of water vapour on Europa have been associated with plumes erupting through the ice, as photographed by Hubble in 2013. They extend more than 60 miles high and produce transient blobs of water vapour in the moon's atmosphere, which is only one-billionth the surface pressure of Earth's atmosphere, NASA said.
The new results, published in the journal Geophysical Research Letters, show similar amounts of water vapour spread over a larger area of Europa in Hubble observations spanning from 1999 to 2015.
This suggests a long-term presence of a water vapour atmosphere only in Europa's trailing hemisphere -- that portion of the moon that is always opposite its direction of motion along its orbit. The cause of this asymmetry between the leading and trailing hemisphere is not fully understood.
To make this discovery, KTH's Lorenz Roth delved into archival Hubble datasets, selecting ultraviolet observations of Europa from 1999, 2012, 2014 and 2015 while the moon was at various orbital positions.
The observations were all taken with Hubble's Space Telescope Imaging Spectrograph (STIS). The ultraviolet STIS observations allowed Roth to determine the abundance of oxygen -- one of the constituents of water -- in Europa's atmosphere, and by interpreting the strength of emission at different wavelengths he was able to infer the presence of water vapour.
The same technique recently resulted in the discovery of water vapour in the atmosphere of Jupiter's moon Ganymede.
"The observation of water vapour on Ganymede, and on the trailing side of Europa, advances our understanding of the atmospheres of icy moons.
"However, the detection of a stable water abundance on Europa is a bit more surprising than on Ganymede because Europa's surface temperatures are lower than Ganymede's," Roth said.
Europa reflects more sunlight than Ganymede, keeping the surface 60 degrees Fahrenheit cooler than Ganymede. The daytime high on Europa is a frigid minus 260 degrees Fahrenheit.
Yet, even at the lower temperature, the new observations suggest water ice is sublimating - that is, transforming directly from solid to vapour without a liquid phase - off Europa's surface, just like on Ganymede, the team said.
The findings helps lay the groundwork for planned science missions to the Jovian system to, in part, explore whether an environment half-a-billion miles from the Sun could support life.