Further evidence has been obtained to show that Jupiter’s icy moon Europa throws jets of water out into space.
Scientists first reported the behaviour in 2013 using the Hubble telescope, but have now made a follow-up sighting.
It is significant because Europa, with its huge subsurface ocean of liquid water, is one of the most likely places to find microbial life beyond Earth.
Flying through the jets with an instrumented spacecraft would be an effective way to test the possibility.
One could even attempt to capture a sample of ejected material and bring it back to Earth for more detailed biological analysis.
The alternative – of trying to land on the moon and drill through perhaps tens of kilometres of ice to examine the ocean’s water – would be immensely challenging.
Hubble made its latest identification by studying Europa as it passed in front of Jupiter.
The telescope looked in ultraviolet wavelengths to see if the giant planet’s light was in any way being absorbed by material emanating from the moon’s surface.
Ten times Hubble looked and on three of those occasions it spied what appeared to be dark “fingers” on the edge of Europa.
What is more, the location for these prominences looks very similar to the region where Hubble two years ago detected an excess of oxygen and hydrogen – the component parts of water.
Taken together, the new work and the earlier observations make a compelling case that H20 is being hurled – if only sporadically – into space from cracks in Europa’s surface.
The suggestion is that the jets reach several hundred kilometres in height before then falling back on to Europa.
A similar phenomenon has already been seen up close at Enceladus, an icy moon of Saturn.
It has vast plumes of water vapour emanating from a series of fissures at its south pole.
The Cassini spacecraft, in orbit currently at Saturn, has even dived through the emissions to “taste” some of their chemistry. But the probe’s instrumentation is not designed to detect the presence or activity of microbes. That would require a mission dedicated to the task.
The US space agency (Nasa) has just sent a satellite to Jupiter called Juno, but again this has no life-detection equipment onboard and, in any case, is not going anywhere near Europa in the course of its work.
Both Nasa and the European space agency do however have future missions in the planning stage that will visit Europa to make repeated flybys, and the determination that the moon has water jets will surely factor into the organisations’ thinking.
Hubble has been working at the limit of its capabilities to see the moon’s jets.
We should get further information when its bigger, more capable successor, the James Webb Space Telescope, is launched in 2018.