Astronomers using the Hubble Space Telescope have discovered what appear to be enormous seasonal jets of water vapour venting from Europa – one of the four large moons of Jupiter that are visible from Earth through simple binoculars. If these observations can be confirmed, then Europa will be only the third world known to have water geysers, after Earth, and Saturn’s moon Enceladus. The jet of water vapour appears to shoot up to a height of over two hundred kilometers before falling back down to the icy moon’s surface, and only vents at certain points in its orbit around Jupiter. Researchers are now trying to confirm if they have found a real phenomenon, and that it is indeed a jet of water coming from the surface of Jupiter’s satellite. Europa is a large moon, orbiting the planet Jupiter. Jupiter is so vast that its intense magnetic field scoops up the Sun’s solar wind of charged particles into a dense belt of radiation, causing strong aurorae around Europa’s poles. Joachim Saur of the University of Cologne, and his team of astronomers, were using the Hubble to make spectroscopic observations of these aurorae, but had to when they spotted something unusual: a fluctuating ultraviolet light. Aurorae are caused by the solar wind smashing into an atmosphere, causing individual molecules to break apart and atoms to be knocked into an excited state – the exact same thing that the electrons from your household electrical supply do to the gas inside a flourescent tube. When gases become excited like this, they emit light, and each type of atom and molecule in the gas emits light in a characteristic colour. A spectroscope splits light into its component colours, allowing astronomers and chemists to measure their different intensities and see exactly what the gas is made of.
In Europas case, the fluctuating ultraviolet light is a tell-tale sign of H2O molecules being smashed apart – there is water vapour in Europa’s atmosphere. “We pushed Hubble to its limits to see this very faint emission. These could be stealth plumes, because they might be tenuous and difficult to observe in the visible light,” said Saur. The water vapour is significant because, although Europa has enormous reserves of surface water, the surface temperature is far below zero. Any water on the surface or in the atmosphere would freeze and settle in short order, so if they are seeing it in the air, it must have only recently been put there.
“By far the simplest explanation for this water vapor is that it erupted from plumes on the surface of Europa,” said lead author Lorenz Roth of Southwest Research Institute in San Antonio. “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.”
Europa is an ice moon, with the entire surface covered in a crust of ice that could be tens or even hundreds of kilometers thick. It is marked with long straight cracks known as lineae. Enceladus has similar surface features, and its jets are known to spray out from inside them. It has long been suspected that beneath the icy crust lies a vast ocean of liquid water, and Roth has suggested that this might be the source of the water. Since Europa’s jets are seasonal, it is possible that they are opening and closing as Europa gets squeezed by Jupiter’s enormous tides. When they open, the water is forced out in a powerful jet, but when they close the stream is cut off. “The apparent plume variability supports a key prediction that Europa should tidally flex by a significant amount if it has a subsurface ocean,” said Kurt Retherford, also of Southwest Research Institute.
Interestingly, the plumes of both Enceladus and Europa have very similar amounts of water vapour in them. The jets aren’t the same, though: Enceladus is able to pump its water out into space, where it freezes into ice crystals which likely contribute material to one of Saturn’s rings. Europa, by contrast, is twelve times more massive so that its jets can’t escape its gravity. They make it up to a height of about two hundred kilometers above the surface before falling down again.
“If confirmed, this new observation once again shows the power of the Hubble Space Telescope to explore and opens a new chapter in our search for potentially habitable environments in our solar system,” said John Grunsfeld, a retired astronaut who worked on the Hubble Space Telescope on several maintenance and repair missions. “The effort and risk we took to upgrade and repair Hubble becomes all the more worthwhile when we learn about exciting discoveries like this one from Europa.”
The observation was made in December 2012, and announced in the 12 December 2013 online issue of Science Express.