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One of Saturn’s many moons, Enceladus seems to have a large body of water hiding under its icy crust. NASA, CC BY

Waterworld? Cassini spots the motion of Enceladus’s ocean

An ocean of water has been found underneath the icy crust of Enceladus, Saturn’s sixth largest moon, according to observations of the Cassini spacecraft published in Science today.

This result has come from painstaking charting of the movements of the spacecraft, monitored here on Earth. From measuring small changes in the flight path as Cassini sailed by Enceladus, the scientists here detected a large gravity anomaly close to its southern pole.

Similar to how the gravity fields of Earth and the moon have been measured, the spacecraft would move slightly closer to Enceladus when flying over a denser area, and slightly further away when the ground underneath was less dense.

Coupled with the information that has been gathered about the rest of this body, a model has now been constructed of the moon’s interior that includes a large ocean in line with this anomaly.

This is another significant piece in the puzzle of this strange icy moon. Enceladus is barely the size of the UK but, on examination by the Cassini spacecraft, has been shown to be an active little world.

Enceladus’s water vapour jets, emitted from the southern polar region. NASA/JPL-Caltech and Space Science Institute

Water water everywhere

We’ve suspected there is something going on in that southern pole region for some time. One of the first major results of the Cassini mission was the observation of “tiger stripes” – four long and recently active fissures that Cassini has spent time studying.

The stripes (which, for your daily dose of trivia, are known as Baghdad, Damascus, Cairo and Alexandria after cities in the book One Thousand and One Nights) are darker compared to the rest of Enceladus’s surface, thought to be because of the larger ice crystals formed there.

As well as the stripes close to the southern pole, the region puts out a lot of heat. Cassini’s measurements with its infrared spectrometer instrument show that up 15.8GW (gigawatts) of power is flowing from its icy crust here.

This amount of heat is thought to be too much to be sustainable though – so we could be observing a current event.

The possible interior of Enceladus based on Cassini gravity investigation, which suggests an ice outer shell and a low density, rocky core with a regional water ocean sandwiched at high southern latitudes. Image courtesy of NASA/JPL-Caltech

Enceladus is the latest in a series of icy moons that we’ve suspect are hiding more beneath their surfaces. There’s evidence from magnetic measurements for there to be a global ocean under the ice of Jupiter’s moon Europa, and similar large regional oceans are suspected for Ganymede and Titan.

Why is this observation so interesting? In the case of Europa, and now Enceladus the models suggest that their oceans extend down to the rocks below, such as the buried Antarctic seas of Vostok and Ellesworth.

This is pretty crucial as that means it will be warm down there, and the oceans will have easy access to a heap of minerals and nutrients from the rocks. Who knows what could be living off these nutrients.

Hence, the evidence adds up to a large and active body of water under Enceladus’s southern polar region. But it is going to be a long time before we can get out there and verify if this ocean is there, if ever.

Retirement is nigh

Cassini took seven years to travel to the giant ringed planet, arriving in 2004. I know what you’re all thinking; Cassini has been touring around Saturn and its moons for 10 years now – so how come it’s not seen this gravity anomaly before?

That answer, in part, has been that there’s been just too much for Cassini to study! The exploration of Saturn and its moons has turned out to be more akin to a massive sweet shop than many had expected. Scientists working on the mission really didn’t know what to scoff first.

From the seasons and lakes on Titan, to the new moons and dynamics of Saturn’s rings this mission has given endless delights and surprises to the Cassini scientists.

The key difficulty with the gravity measurements of Enceladus is that when flying past the moon, Cassini’s antenna has to be pointed at Earth to track its movements.

One of NASA’s Deep Space Network antennas used in Enceladus’s gravity field determination. NASA/JPL-Caltech

To do this most of the other instruments on board, such as the infrared spectrometer and high-resolution camera, don’t point at the surface – so other discoveries could be missed. There have only been three flybys where gravity measurements could be made, the last one was in March 2012.

Cassini’s mission has been in its solstice phase since 2008, after the primary mission expired. Its plutonium power source still has enough energy to power the craft until 2017, but there are worries that the Cassini mission may be brought to an abrupt halt before then.

NASA (who runs the mission, which was a joint NASA, European Space Agency and Italian Space Agency venture) is looking to make cuts, and there’s concern that Cassini will lose out to the Mars science laboratory, Curiosity.

Whether it happens sooner, rather than later, the fate of Cassini is sealed. Plans are afoot to direct it to plummet into Saturn, to prevent it crashing into the potential enclaves of life on Titan and Enceladus.

We can only hope that decisions here on Earth means it can be left to make many more discoveries before that fateful day.

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