People visiting me in the hot dry Turkish summer often find it hard to believe that we get rain here too. Lots of it! It just falls between November and April, and this past winter was one of the wettest that I remember – our roof started leaking everywhere. We earthlings are so used to liquid water falling from the sky and flowing on the surface, we only really take notice when there is too much or to little of it.
But in the wider context of the solar system, this substance, so vital for life as we know it, makes our planet unique . There is plenty of ice out there – many of the satellites of Jupiter, Saturn, Neptune and Uranus are frozen snowballs, and ice has even been observed in dark corners of craters on the moon - but to date, no definite proof of liquid water has been found anywhere outside our world.
That's why astronomers, and in particular planetary scientists, are so excited when they find hints that it may exist elsewhere too.
Underwater ocean
Earlier this year a team from Cologne University announced strong evidence for an ocean of liquid saltwater on Jupiter's largest satellite Ganymede, under its150-km-thick crust of solid ice. Their method was ingenious –they used the Hubble Space Telescope to look indirectly inside a planet, by observing the interlinked movements of the aurorae (northern and southern lights) of both Jupiter and Ganymede. They realised that there was much less movement than would be expected, and concluded that this is due to an underground body of saltwater suppressing the movement of the aurorae.
Ganymede's sister moon, Europa, is also believed to have an underground body of liquid water. Not only that, but water vapour has been observed above its north pole, suggesting water plumes erupting off the surface, a feature that has also been seen at Enceladus, one of Saturn's moons. It's easy to see why Europa is a top candidate the search for life, and therefore for NASA's next mission to Jupiter. |
There may even be liquid water closer to home. Missions to Mars have provided plenty of evidence that there was once flowing water on its surface; now, within the last year there have also been tantalising suggestions of liquid water very near to the surface. Dark seasonal flows have been observed on some slopes, and these features appear to be rich in iron which suggest deposition by flowing water.
Exciting times indeed in the search for extraterrestrial water!
Hubble Sees Potential Underground Ocean on Jupiter's Largest Moon
http://www.nasa.gov/press/2015/march/nasa-s-hubble-observations-suggest-underground-ocean-on-jupiters-largest-moon
Hubble Sees Evidence of Water Vapour at Jupiter Moon
http://www.nasa.gov/content/goddard/hubble-europa-water-vapor
NASA selects science instruments for proposed Europa missions
www.nasaspaceflight.com/2015/05/nasa-selects-science-proposed-Europa/
Clues to possible water flows on Mars
http://www.nasa.gov/jpl/mro/martian-seasonal-flow-20140210
1. NASA/ESA
2. Galileo Project, JPL, NASA
3. NASA/JPL-Caltech/UA/JHU-APL