FAR from the warmth of the sun, in a frigid realm of the solar system a billion kilometres beyond Earth, lies a ball of ice called Enceladus. Only 500 kilometres across, it is one of many icy little moons in orbit around Saturn. They are all geologically dead, their ancient cratered surfaces unchanged in aeons – except for Enceladus.
The surface of Enceladus is fresh and new, and the palest of any object in the solar system, reflecting about 90 per cent of the light that falls on it. When the Voyager 1 and 2 probes flew by in the early 1980s, they saw twisted canyons crossing its surface, and craters that seem slumped and softened. Then in 2005, NASA’s Cassini spacecraft discovered that the south pole of Enceladus – a snowball’s Antarctica – is actually far warmer than the rest of the moon. Most startlingly of all, geysers are erupting from the pole, sending vast plumes of vapour out into space.
The discovery of this south-polar activity has solved some old riddles about Enceladus and the Saturn system. Yet it poses one big riddle of its own: what strange subterranean machinery could be pumping out this plume and animating Enceladus?
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“What strange subterranean machinery could be animating Enceladus?”
Only three other objects in the solar system are known to be volcanically active: Earth and the moons Io and Triton, which orbit Jupiter and Neptune respectively. But they are all far larger than Enceladus; the smallest of them, Triton, is 200 times as massive (see Graphic). “There is definitely something unique about Enceladus – it’s so small and so active,” says Susan Kieffer of the University of Illinois at Urbana-Champaign.
The plume wasn’t a complete surprise. Voyager 1 found in 1980 that Enceladus orbits within the densest part of Saturn’s “E ring”, a band of fine ice particles that lies outside the planet’s main ring system. It seemed likely that Enceladus was somehow generating the E ring, and one theory was that it could be from some kind of watery volcanic activity.
Cassini’s clearest confirmation of this idea came in a back-lit image of Enceladus taken in November 2005, showing perhaps a dozen geysers erupting from the pole. Using the amount of reflected light to assess the density of water vapour in the plume, astronomers have calculated that together the geysers are squirting out a few hundred kilograms of material per second. That’s about the same as the average output of Old Faithful in Yellowstone National Park, Wyoming. With only the feeble gravity of Enceladus to slow it down, the plume reaches hundreds of kilometres into space.
Some of it gets even further and replenishes the E ring, but most falls back onto the surface of Enceladus. The ice particles fall as fine snow while the water vapour condenses as frost, painting Enceladus pure white. Other nearby moons are somewhat whitened too, as ice particles of the E ring scour the surfaces of Tethys, Mimas, Dione and Rhea.
The plume might even explain why parts of Enceladus are free of craters. According to Jeff Kargel of the University of Arizona in Tucson, fallout from the plume could be burying craters near the pole, and then as the frost builds up and forms ice, it could slowly flow away like glaciers on Earth, accounting for some of the other odd tectonic features on Enceladus. But Kargel stresses that this is only a hypothesis. “I’ll be the first to say I’m probably wrong.”
A more pressing puzzle is what drives the geysers. They appear to be coming from four long fissures named “tiger stripes” that cut across the south polar region. The tiger stripes are remarkably warm, up to 80 °C warmer than the rest of the moon, whose surface temperature averages around -200 °C.
Perhaps warm ice down in the tiger stripes is simply subliming – turning directly to vapour. Or it could be that the geysers work just like geysers on Earth, fed by a reservoir of warm pressurised water loaded with carbon dioxide: the . If that’s right, there could be liquid water only metres below the surface of Enceladus.
Kieffer has suggested a . The source could be an icy solid called a gas hydrate, or clathrate, in which gases are trapped within a lattice of water molecules. When Cassini flew through the plume in July 2005, it detected nitrogen, methane and other gases that don’t dissolve readily in cold liquid water but can be stored in clathrates. When exposed to space by a new crack in the crust, the clathrates would decompose into water vapour and these gases.
Whatever the immediate cause of the frigid geysers, there is a deeper puzzle. What is generating the heat to power them? “The ultimate mystery is the source of the energy itself,” says Bill McKinnon of Washington University in St Louis, Missouri.
Some heat must be coming from the slow radioactive decay of uranium and thorium within the rocks of Enceladus, but not enough to account for the 3 to 7 gigawatts radiated by the south pole. The bulk of that is almost certainly supplied somehow by Saturn’s gravity. Because the side of Enceladus that faces Saturn is slightly nearer to the planet, it is more strongly attracted than the side facing away. The difference in gravitational forces means that Enceladus is stretched a little. And because the moon’s orbit is slightly elongated the strength of this effect varies, so the moon is stretched and squeezed once in every 1.3-day orbit. This tidal flexing could have created the tiger stripes by cracking the crust, perhaps exposing fresh ice or clathrates. It probably also opens and closes vents within the tiger stripes, making the geysers highly variable.
If you stretch and squeeze a solid object, it warms up due to internal friction. This mechanism, called tidal heating, is widespread in the solar system. It accounts for the gushing volcanoes of , for example, and to planetary scientists there is simply no other plausible source of heat within this small moon. “There are no goblins or demons inside Enceladus,” says McKinnon.
However, there must be something in there to make Enceladus special. Another moon, Mimas, orbits much closer to Saturn and is pummelled by the planet’s gravity 40 times as strongly as Enceladus. Yet there is no sign of warmth or activity: its surface bears huge scars that date from the dawn of the solar system. “Mimas looks super-old and super-cold,” says Julie Castillo of the Jet Propulsion Laboratory (JPL) in Pasadena, California.
Where is the heat engine hidden in Enceladus? The internal structure of the moon is still uncertain, but there is probably a rock core surrounded by a thick mantle of ice. Solid rock is too rigid to be warmed up much by tidal forces; ice is more flexible, but calculations show that a simple ice mantle still doesn’t produce nearly enough heat.
“The resolution has to be some kind of structure within the icy layer,” says Kargel. He thinks there could be many faults in the ice rubbing together. “If you take a block of wood in your hands and try to flex it, you just wear your hands out; but take a block of wood that’s been divided into sheets of paper and the sheets are free to slip, so as you flex it heat is generated.”
That could be happening right at the south pole. Francis Nimmo of the University of California, Santa Cruz, believes that tidal flexing might be generating heat within the tiger stripes. “If the two sides of a tiger stripe are moving backwards and forwards, it’s just like when you put your hands together and rub: you generate heat.” He has found that . If the ice is rooted to a rock core it doesn’t move much, but if it’s floating on an ocean of liquid water it can flex enough to explain the heat and the plume.
One difficulty with this idea is how such an ocean would keep from freezing solid. There must be more widespread heating of Enceladus to do that, although it’s possible that the gentle warmth from radioactivity in the core is enough as long as a clathrate layer is acting as an insulating blanket or if ammonia plays the role of antifreeze.
Then again, today’s activity might merely be a relic of heating in the recent past. The little moon’s orbit is influenced by another moon, Dione, which circles Saturn in exactly twice the time taken by Enceladus. This orbital harmony means that Dione’s gravity gradually tweaks the path of Enceladus – it is why the orbit is elongated today, and it might have had a more powerful effect in the past. If the orbit of Enceladus used to be more elongated, making for more tidal flexing and heat, then we are now seeing a steaming kettle that has just come off the hob.
The heat engine could have been kick-started by an ancient burst of radioactivity, according to Castillo. is that Enceladus once held a lot of short-lived radioactive isotopes such as aluminium-26, emitted either by nearby supernova explosions or by the young sun. Enceladus is much denser than the other small moons of Saturn, implying that it has more rock in it and so would have held more radioisotopes. The decaying isotopes could have melted the moon and created a hot rock core. The core could then absorb tidal heat from Saturn, perhaps enough to maintain a liquid ocean and support Nimmo’s rubbing stripes idea.
To test all these ideas, Cassini needs a closer look. It will revisit Enceladus next March, and mission scientists have decided to adjust its trajectory so that it will skim within 150 kilometres of the south pole, much lower than originally planned. “Once the geyser was discovered, we had to get closer,” says mission scientist Jonathan Lunine of JPL. “That means being in a more hazardous region, but we finally came to the conclusion that this would be a reasonably safe altitude to attempt.”
At that height, Cassini’s instruments will be able to get a better idea of any trace gases in the plume – for example, whether there is any sign of ammonia. They should also see more detail in the thermal emission, which could test the rubbing-hands model of the tiger stripes. “The stripes point in different directions, giving them more or less motion, so we can predict which stripes should be hotter,” says Nimmo.
Another vital measurement will be the proportion of ice particles. If there are lots of particles, that suggests there must be a violent liquid-water geyser or decomposing clathrates to blast the particles into space; if it’s mostly vapour, that could be explained by the gentler sublimation of warm ice.
Meanwhile, ongoing measurements of the shape of Enceladus and its gravitational field might reveal more about the moon’s internal structure. Is there really a global ocean under the ice, for example? “I’m not so sure that an ocean is necessary,” says Caroline Porco of the Space Science Institute in Boulder, Colorado, who leads the Cassini imaging team. “A south-polar sea is probably a better bet – after all, the heat is coming out there.”
If there is any amount of liquid water, the question arises: could there be life? Simple organic chemicals – methane, propane and acetylene – were also found in the plume. “If we’re right about the liquid water and the organics, what you then need is to sustain the environment long enough for life to develop,” says Porco.
Life also needs a source of available energy, however. Jupiter’s moon , one of the best candidates for an alien ecosystem, has a core that might well be warm enough to produce hydrothermal vents similar to the life-sustaining black smokers in Earth’s oceans. McKinnon doubts that the core of Enceladus is warm enough to do the job. “I don’t think the chances for life are as high as on Mars or Europa.”
There is only one way to find out. In a sense, Enceladus might be a better target for an astrobiology mission than Europa. Europa’s ocean is under several kilometres of ice, whereas the waters of Enceladus might be only tens of metres down and much easier to sample. But even without the lure of life, Porco thinks there should be a new mission to Enceladus to land on the surface and take seismic readings. Maybe only then will we know what secret lies within this small snow-white world.
Distant streams
Might other small moons sport plumes of fog similar to Enceladus (left)? There have been recent hints. Streams of ionised gas spotted by Cassini have been traced back to two moons of Saturn, and . Geysers on those moons could be supplying the gas, says Jim Burch of the Southwest Research Institute in San Antonio, Texas. Meanwhile, a team led by Jason Cook at Arizona State University, Tempe, claims that crystalline water ice spotted on Pluto’s moon could have been deposited by geysers on that even chillier world ().
Both claims are uncertain, however. The evidence for geysers on Tethys and Dione is still weak, says planetary scientist Francis Nimmo of the University of California, Santa Cruz, and he is dismissive of the Charon claim. “Nobody that I’ve talked to believes that the crystalline water ice has anything to do with geysers. Crystalline ice is ubiquitous in the outer solar system. It means that there’s something we don’t understand about how ice behaves under these conditions.” The fountains of Enceladus may be unique after all.
