posted on 30 September 2016
from The Conversation
The Hubble Space Telescope has obtained evidence of Jupiter's moon Europa erupting plumes of water vapour, NASA reports. Hubble first caught a glimpse of the jets in December 2012, but now more plumes have been revealed using a different technique.
The discovery is exciting, as Europa, with its large subsurface ocean, is one of the best candidates for microbial life in the solar system - despite its surface temperature being a frigid -160°C. And plumes mean that samples of microbes could be collected during a flyby, without having to land on the surface.
Europa is Jupiter's innermost large, icy moon (only slightly smaller than our own moon) and has about 100km of ice and liquid water surrounding its rocky interior.
Scientists started getting really interested in Europa following observations by NASA's Galileo probe of Jupiter and its moons from orbit between 1995 and 2003. These showed that Europa's surface ice is marked by multiple generations of parallel ridges and grooves, except where this pattern has been shattered by the surface breaking apart into randomly disposed slabs (known as "chaos regions"). This can be explained if Europa's icy shell floats on a layer of liquid water. Each grooved ridge could mark a crack that opened and closed with the changing tide during Europa's 3.5 day orbit of Jupiter - the ridges being formed from slush that was squeezed to the surface each time the crack closed. The chaos regions would represent slabs of the ice shell that broke apart as a region of the ocean became melted from below, powered by a release of tidal heat through the ocean floor - now frozen into a new configuration.
A 17km wide Galileo image of 'ball of string' terrain on Europa, where multiple generations of cracks in the ice shell have opened and closed. NASA/JPL/ASU
The region of Europa known as Conamara Chaos, imaged by Galileo. NASA/JPL/University of Arizona
Microbes could live near the floor of Europa's internal ocean, feeding off the chemical energy supplied where hot water ("hydrothermal") vents discharge chemicals dissolved out of the tidally-heated rocky interior. This is the kind of setting where life on Earth is thought to have begun. Microbes whose ancestors had developed a taste for sunlight might now be found surviving by photosynthesis (like plants) in the water drawn up and (mostly) forced down again every time a tidal crack opened. However, although Galileo searched as hard as it was able (hampered by a failed antenna) it found no signs of present-day activity, such as water venting to space.
Alien Ocean: NASA's video about Europa's ocean and its habitability.
Europa versus Enceladus
Soon after Galileo's findings, Europa's crown as the most active icy moon was snatched away by Enceladus, a much smaller (500km diameter) moon of Saturn, in 2005. The Cassini probe discovered jets (or plumes) of ice crystals venting to space from cracks near Enceladus's south pole. In the decade that followed, it became clear that Enceladus has a global internal ocean between its icy surface shell and a rocky interior.
Jets erupting from cracks near the south pole of Enceladus. NASA/JPL-Caltech/Space Science Institute
Cassini was even able to fly through the plumes, finding them to be chemically rich and bearing signs of the kind of water-rock interaction which could feed micriobial life. This raised hopes that such fly-throughs could one day find signs of microbial life (including maybe even a few ejected microbes themselves), without the need for a technically-challenging landing on the surface.
Cross section through the outer zone of Europa's south polar region showing plumes, the fractured ice shell, the liquid water ocean and the rocky interior. Edited by the author from a NASA source
It wasn't until December 2012 that the Hubble Space Telescope at last detected evidence for an active plume erupting from Europa, in the form of a faint ultraviolet glow caused by atoms of hydrogen and oxygen rising up to 200km above Europa's south pole. This is probably water molecules broken apart in the harsh space environment.
The location of ionized oxygen and hydrogen (strongly pixellated) detected by the Hubble Space Telescope in December 2012, superimposed on a Galileo image of Europa. Until now, this was the only evidence of active plumes. NASA/ESA/L. Roth/SWRI/University of Cologne
However, subsequent efforts failed to find any repeat evidence, until now. The latest evidence was obtained with an enterprising new technique, using Jupiter as a light source to reveal plumes as Europa passes across its face. The Hubble Space Telescope was used to collect ultraviolet images during ten transits of Europa across Jupiter in 2014, which on three occasions revealed plumes erupting above the edge of Europa's disc. This shows that the plumes are intermittent.
The plumes of Europa seen as it passed across the face of Jupiter on three dates in 2014. NASA
However, sometimes three or four distinct plumes can be made out, and given that the technique can see only those plumes erupting near the visible edge of the disc, there may be many more undetectable plumes in front of or behind the disc.
Hopefully it won't be too long before a mission designed to detect microbial life from the plumes of Europa or Enceladus sets off to find out. It certainly seems like a worthwhile endeavour - especially after the emergence of this latest evidence.
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