Ten years ago almost nobody other than planetary geologists had ever heard of Enceladus. Yet this little satellite of Saturn, less than 500km in diameter, is now considered to be one of the leading candidates in the solar system for hosting extra-terrestial life.
I have been following discoveries at this moon for the last five years and I am fascinated by its weirdness and energy. This is no dead lump of rock or ice, but an active evolving world that throws up new surprises every time it we take a close look at it. Let's have a look at the amazing discoveries that have been made and the evidence for the latest astounding claim.
Enceladus, was first spotted back in 1789 but in the following two centuries little more was learned about it other than that appears to be made of pure white water ice, making it the brightest object in the solar system. However even this limited information was intriguing - such brightness suggests that the ice is smooth and uncratered, and hence unexpectedly young – in other words, some process must be actively renewing the ice.
Enceladus suddenly made front page science news in 2005, when the versatile Cassini spacecraft mission to Saturn and its moons finally allowed the chance to examine it up close. The observations made then, and that have kept coming ever since, have drawn a picture of a fascinating object with features far beyond anything that even a sci-fi writer could imagine.
Spectacular features
Most spectacular are the so-called 'Tiger Stripes', massive parallel cracks in the southern region hundreds of kilometres long, and the huge plumes of water-vapour spraying out of these cracks 500 km into space, like geysers.
In addition, only last month (April 2014) researchers announced evidence that suggests the existence of a large ocean of liquid water deep beneath the icy surface.
Apart from the sheer unexpectedness of such features, why does all this matter? What is fascinating is the implication that Enceladus is a place that could potentially host life. What is the evidence to support this claim?
First, consider the conditions required for life as we know it, namely:
1. Heat: to keep water liquid and to provide energy to fuel chemical reaction
2. Complex organic molecules (ie carbon as building blocks)
3. Protection from ultraviolet radiation: for example, an atmosphere with an ozone layer, or an underground or underwater environment.
4. Liquid water
All of these have been discovered on Enceladus by Cassini's various instruments:
There is clearly a heat source somewhere within the moon - Cassini recorded significantly warmer temperatures over the Tiger Stripes compared to surrounding areas. At less than -90 degrees Centigrade, it's still rather chilly, but warm enough for water mixed with ammonia to be liquid. Tick box 1.
There is more. When Cassini flew directly over the Tiger Stripes it took samples of the dramatic geyser-like jets, and found them to be composed of water vapour laced with simple and complex organic compounds, and various gasses including significant amounts of carbon dioxide, carbon monoxide and methane. Tick box 2.
And finally – extremely precise meaurements by Cassini of the gravitational pull of Enceladus on the spacecraft during several fly-bys found the force of gravity to be greater than expected over the southern hemisphere, which implies the existence of a sub-surface layer of something denser than ice. Liquid water, being 7% denser than ice, would fit the bill and is believed to be the most feasible explanation. Although it has not been directly detected, this is strong evidence for a sub-surface sea in the south, which would also explain the source of the geysers. Tick boxes 3 and 4!
Future missions?
Of course, the discovery of conditions favourable to life is not the same thing as actually finding extra-terrestrial life! More detailed analysis of the organics in the plumes is needed, but unfortunately Cassini's mission is scheduled to end in 2017 so a new mission would be necessary if these hints of the possibility of life are to be investigated further. The feasiibility of a new mission depends ultimately on the politics of space agency funding - it may be a very long wait!
Image credits:
1, 2 and 3: NASA/JPL/Space Science Institute
4: NASA/JPL/GSFC/SwRI/SSI
Further information about Enceladus and Cassini:
http://www.nature.com/news/icy-enceladus-hides-a-watery-ocean-1.14985
http://news.nationalgeographic.com/news/2014/04/140403-liquid-water-saturn-moon-enceladus-science/
http://saturn.jpl.nasa.gov/mission/introduction/
https://royalsociety.org/events/2013/dynamic-atmosphere-enceladus/