Coming just after the news of the ringed asteroid and new dwarf planet, some more exciting news from the outer Solar System was announced yesterday, and this will be of particular interest to those hoping to find evidence of alien life elsewhere in our Solar System. Saturn’s tiny moon Enceladus, famous for its geysers of water vapor spewing out into space, has long been suspected of harboring an internal ocean, just like Jupiter’s moon Europa (and possibly others). Now it seems that scientists have the evidence they’ve been looking for, thanks to new findings based on data returned by the Cassini spacecraft, still in orbit around Saturn.
The water vapor plumes were a big surprise when first discovered in 2005, since Enceladus is so small, only about 310 miles (500 kilometres) in diameter, and covered by a shell of ice. How could such a world have water geysers, especially in this region of the Solar System which is so far from the Sun and bitterly cold? It was reasoned that either the plumes were being generated in a manner similar to comets, as in ice being warmed somehow and then being ejected into space, or there was a reservoir of water beneath the surface. The latter prospect, needless to say, was the more exciting of the two possibilities.
The Cassini spacecraft has even flown directly through the plumes more than once and analysed them with its instruments. Water vapor, ice particles, salts, and organic compounds have all been found in the plumes so far. Continued studies of the data suggested that there likely was water below the surface, and that it was salty like Earth’s oceans. The big question became whether the water was in the form of a global ocean, like on Europa, or if it was smaller like a sea and more regional to the south pole, where the plumes emanate from.
The new results support the existence of a regional sea below the south pole, about 18-25 miles (30-40 kilometres) below the surface. It is thought to extend at least to 50˚S latitude, and possibly farther, however. The information was obtained by analysing tiny perturbations in the motion of the spacecraft, caused by Enceladus’ gravity, as Cassini passed within 62 miles (100 kilometres) of the moon’s surface on three different occasions.
As Luciano Iess, lead author of the new paper published in Science, explained:
“By analysing the spacecraft’s motion in this way, and taking into account the topography of the moon we see with Cassini’s cameras, we are given a window into the internal structure of Enceladus. The perturbations in the spacecraft’s motion can be most simply explained by the moon having an asymmetric internal structure, such that an ice shell overlies liquid water at a depth of around 30-40 km in the southern hemisphere.”
It isn’t known yet if the water vapor plumes are actually directly connected to the water below, but it seems reasonable that they are. Further studies should help to establish this if so. They are already known to erupt from long, deep fissures in the surface called “tiger stripes” which are much warmer than the surrounding ice. If these cracks do extend down to the water below, then this makes the prospect of looking for signs of life both exciting and easier. Instead of having to drill through the ice, a new mission could re-sample the plumes directly just like Cassini has. Cassini, unfortunately though, isn’t equipped to detect actual biological organisms, only simpler organic matter.
This makes Enceladus a priority location in the search for extraterrestrial life in the Solar System. Even at great, dark depths, Earth’s oceans are still teeming with life, including under the ice at the poles. The water in Enceladus is thought to be in contact with the rocky core, like oceans on Earth are in contact with the rocky mantle, providing a source of organic nutrients. Might there also be hydrothermal vents on Enceladus’ ocean bottom as well? With water, heat, and organics, the basic ingredients for life are there. What may be waiting to be discovered inside Enceladus?
The paper in Science is available here (although behind a pay wall). See also the new article in Astrobiology by Carolyn Porco, Chris McKay, Ariel Anbar, and Peter Tsou called “Follow the Plume: The Habitability of Enceladus.”
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