July 7, 2011 | 4
The Cassini spacecraft is zooming around Saturn as I type, currently in between two flybys of Saturn’s moon Titan – one was in June, the next will be September. It was supposed to explore Saturn and its moons for only four years between 2004 and 2008. But after two mission extensions it is still going strong today, providing scientists with lots of data that they have turned into fascinating results.
A few years ago, the subject of Cassini’s interest was another of Saturn’s moons, Enceladus. The moon was named after a god from Greek mythology, but didn’t appear to be living up to its namesake. Legend has it that the god Enceladus was killed in battle and buried under Mount Etna, and is said to be the cause of subsequent volcanic activity and earthquakes in the region. Enceladus, the moon, was identified by William Herschel in 1789 and looked quite ordinary. Little was known about it until nearly 200 years later when Voyager 2 passed. Voyager saw heavily cratered areas alongside smooth plains, a tell-tale sign that a lot of geological activity was happening on the moon. Due to the close proximity of Enceladus to one of Saturn’s outermost rings, Voyager scientists also suspected that the moon was the source of particles in the ring.
Then, nothing. Scientists had to wait nearly 20 years for a followup mission that might be able to confirm their suspicions.
Cassini has been well worth the wait. Since it got to Saturn in 2004, Cassini has filled in many of the gaps in our knowledge of Enceladus. We now know the source of the smooth expanses of ice. Enceladus is rich with cryovolcanism, or volcanoes that spew out water, ammonia or methane instead of lava. These are what replenishes the moons icy surface, covering the scars of meteorite bombardment. Near its south pole, Enceladus has four long cracks in its surface, affectionately known as tiger stripes. The stripes provide a site for giant plumes of water vapour and ice to escape Enceladus. Particles from the plumes are thought to fill Saturn’s outermost ring, known as the E-ring, much like Voyager scientists suspected. These features were all discovered during the main Cassini mission that ended in 2008. The first extension, known as the Cassini Equinox Mission, was completed in 2010 and the spacecraft is now in the middle of its second extension, the Cassini Solstice Mission.
Cassini has given us even more Enceladus discoveries this year. In April, scientists showed that electrons from Enceladus’ south pole are the source of auroral activity on Saturn. Less than two months ago it was named as the “sweetest spot” for alien life in the solar system.
Some of Cassini’s discoveries throw up more questions than they answer. For example, what is the source of particles for the giant plumes erupting from the tiger stripes? At first scientists thought that the surface ice on Enceladus might provide ice particles for the giant plumes. However, when water freezes, any salt that it contains is squeezed out and should not be present in the resulting ice. Analysis of Saturn’s E-ring in 2009 showed that there is salt present in its ice particles. If Enceladus really was feeding the E-ring, where were these salty ice particles coming from?
That salt is usually present in only large bodies of water adds another restriction to the origin of the ice particles. The discovery in 2009 seemed to suggest that the water from the plumes was coming not from the ice, but from an ocean. Scientists have been speculating for a while that there may be liquid water beneath the surface of Enceladus. Evidence for an ocean exists, but scientists have not yet been able to rule out competing suggestions. Since the salty ice crystals discovered in 2009 were only found in the E-ring and samples were not taken from one of the giant plumes itself, the mechanism driving plume emission was still a hotly debated topic.
A paper published in Nature a few weeks ago may contain the crucial bit of evidence that could push the ocean hypothesis further into the realms of theory. It will certainly be a blow for anyone still in the no-ocean camp.
Cassini scientists have now directly sampled the icy plumes emanating from the tiger stripes on Enceladus and found they contained lots of salt water. This was the first time that samples had been taken from so close to the tiger stripes themselves. Frank Postberg and colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and the University of Heidelberg carried out the research.
Postberg and his colleagues took samples of ice crystals from close to the tiger stripes during three close flybys of Enceladus in 2008 and 2009, and analysed them using the Cosmic Dust Analyser instrument on Cassini. At closest approach the spacecraft was only 21km away from the surface of Enceladus. They found salt-rich ice crystals and even some organic compounds in the denser parts of the plumes. The largest salt-rich ice crystals were found closer to the plumes, whereas particles further away were smaller and contained less salt, much like the particles in Saturn’s E-ring. Large, salt-rich crystals made up around 70% of all particles ejected from the plumes, but, because of their size, this means they make up over 99% of the total ejected mass.
The particles found close to the source of the plumes look a lot like they come from an ocean. In their Nature paper, Postberg and his colleagues conclude that the particles come from a large body of salt-rich water, with a surface from which water vapour can evaporate before it is ejected from the tiger stripes. This reservoir, or ocean, must exist between the rocky core of the moon and its icy surface. Such an ocean can be kept liquid by heat generated by tidal forces when Enceladus is pushed and pulled around by Saturn’s gravity.
These results eliminate several models describing the formation of the plumes. Up until now some non-ocean models were still in the running. Now, all models that do not involve liquid water will now have to be carefully reconsidered or thrown out entirely.
Postberg F, Schmidt J, Hillier J, Kempf S, & Srama R (2011). A salt-water reservoir as the source of a compositionally stratified plume on Enceladus. Nature, 474 (7353), 620-2 PMID: 21697830