An orbiting snowball
Enceladus is the 14th moon outward from Saturn, in the outermost E ring that it is responsible for replenishing. Although it is only 500 kilometres in diameter (10 times smaller than Titan and almost 7 times smaller than our Moon), it attracted the attention of the mission’s scientists—who had planned several flybys with Cassini—from the outset.
“We already had some images from the Voyager mission in 1981 that revealed some quite big surprises,” recalls Gabriel Tobie. “Parts of the moon were studded with craters, showing that its surface had preserved the traces of past impacts and was therefore not very active. Conversely, other regions exhibited signs of tectonics and a geologically active interior. And strangely, its surface was very bright, in fact the most reflective object in the solar system, as if it was covered in pure white snow, even on its cratered side, which suggested that a powdery material was being deposited.”
Ice geysers at Enceladus’ south pole. Credits: NASA/JPL-Caltech/Space Science Institute
The mystery was solved in July 2005 when Cassini flew over the moon’s previously unobserved south pole: the backlit view revealed water vapour and ice grains being spewed continually into space through the icy surface crust.
“We observed variations in the plume’s intensity in step with the tidal forces exerted by Saturn on Enceladus every 33 hours. There was a dip between 2012 and 2017 which we don’t yet fully understand, but the jets are a permanent feature.”
During Cassini’s nominal mission (2004-2008), 5 close flybys of Enceladus were already planned, but after the discovery of the geysers in July 2005, 22 more were accomplished during its extended mission. The probe was thus able to fly through and ‘taste’ the ice plumes with its instruments on multiple occasions. The Cosmic Dust Analyzer (CDA) identified salt-bearing ice grains with the same composition as the E ring, while the Ion and Neutral Mass Spectrometer (INMS) detected carbon dioxide, ammonia, methane and hydrogen mixed with water vapour.
“Around the time of the Voyager missions, the scientific community was already hypothesizing that Saturn’s E ring might be made of dust ejected by Enceladus. But we needed to observe these geysers up close and analyse them to establish a link with the ring where the moon orbits the planet. Some fairly complex organic molecules were even detected in water vapour, but Cassini flew through the plumes too fast for us to obtain anything other than fragments. We’ll have to go back there to learn more.”
A subsurface ocean
So where is all that water in the geysers coming from? It took 10 years from the time of the geysers’ discovery for scientists to determine whether they were coming from a reservoir of water, pockets under the ice or a subsurface ocean.
“It was Cassini’s Imaging Science Subsystem (ISS) that identified Enceladus’ libration motions in 2015,” explains Gabriel Tobie. “The moon’s wobble can only be explained by an inner layer of liquid water separating the surface and the core, which means there is indeed a subsurface ocean covering all of Enceladus. And we’ve even been able to deduce how thick the layer of ice is: between 20 and 25 kilometres on average, and thinner at the poles, probably less than 5 kilometres.”
However, the temperature on the moon’s surface is still an icy –200°C, and for the water to remain liquid it must reach at least 0°C. How can such a large swing be possible in such a thin layer? Where is the heat coming from? The answer lies in Enceladus’ core.
“Enceladus’ radioactivity is comparable to Earth’s, but its core is just too small to generate enough energy to maintain an internal temperature above 0°C. There’s only one way of increasing the temperature inside a planetary body, and that’s tidal forces. Heat comes from deformations in the core caused by Saturn’s gravity. The friction forces in the rock core, which we think is porous and filled with water, dissipate heat and raise the temperature locally to above 100°C. This circulation of hot water from the porous core is thus able to maintain an internal ocean at a temperature of 0 to 2°C on average. In comparison, the mean temperature of Earth’s deep oceans is 4°C.”
Another clue to this internal source of energy came from Cassini’s instruments, which detected nanometric-size grains of silicon that suggest hot water is venting somewhere, a hypothesis confirmed in April 2017 when hydrogen was detected.
“Hydrogen is a highly volatile gas that should have disappeared a long time ago,” says Gabriel Tobie. “That means there’s an internal source, and hot water interacting with rock seems the most likely explanation. It’s the serpentinization reaction that breaks the water molecule and produces the hydrogen escaping from the geysers. For a long time we thought tidal forces were being exerted mostly on the icy layer, but these results show there must be deformations in the core and an internal circulation that are stabilizing the hot water and creating all the ingredients needed for life.”
Ideal place to look for life
Enceladus thus has three of the key ingredients needed for life: large amounts of liquid water, a source of heat and complex organic molecules.
“Many space exploration missions are looking for habitable environments. We’re still looking on Mars, but we’ve already found one on Enceladus! All the ingredients are there for life to take hold. Now, we need to conduct analyses with more-precise instruments, because Cassini was conceived in the 1990s and wasn’t designed to do this job. Today, we have the capability to send fully-fledged portable laboratories weighing no more than a few kilograms.”
NASA has proposed two projects aimed at studying the geysers and analysing the ice grains and water vapour, but as no lander mission is currently envisioned they would only be able to detect signs of possible life in the form of molecules.
“We now need to go back with a follow-on mission to Cassini and find out if biological activity could have developed there,” concludes Gabriel Tobie. “Hopefully within 10 to 15 years.”
- Francis Rocard, head of CNES’s solar system programme : francis.rocard at cnes.fr
- Gabriel Tobie, research scientist at the LPGN planetology and geodynamics laboratory, a joint research unit of the French national scientific research centre CNRS, the University of Nantes and the University of Angers : gabriel.tobie at univ-nantes.fr