FROM A DISTANCE, Saturn’s moon Enceladus looks like an overgrown cue ball. White, round and icy, it’s a 500km sphere that could snuggle nicely between Melbourne and Sydney with a couple of hundred kilometres to spare. It’s named after a giant from Greek mythology, but it isn’t really all that big. Saturn alone has five larger moons. But maybe there’s more to the name than simply size.
In Greek lore, the giant Enceladus was buried under Italy’s Mount Etna, one of the world’s most active volcanoes. In some versions of the story, his breath fuels the mountain’s fires. While nobody could have foreseen it when British astronomer William Herschel first spotted Enceladus in 1789, it turns out that the not-so-giant moon behaves in some ways like an enormous volcano.
“It’s the only unambiguously cryovolcanically active moon in the Solar System,” says Dennis Matson, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who has spent years looking for volcanoes on the moons of Saturn and Jupiter.
Not that this means it’s an extraterrestrial Etna. As the name suggests, cryovolcanoes don’t produce fiery magmas incinerating everything in their paths. If they do erupt magmas (and the ones on Enceladus do not) they produce cooler lavas that solidify into materials we’d see as ice, not rock. Instead of blasting out ash and superheated steam, Enceladus produces geyser-like jets of ice spicules and chilly vapour – a bit like in America’s Yellowstone National Park, except deep-frozen.
The discoveries on Enceladus have been astounding for astronomers seeking extraterrestrial life. Forget Mars – Enceladus’s chilly breath might well be the Solar System’s best place to look for aliens.
NOT MUCH WAS known about Enceladus until 1980 and 1981, when NASA’s two Voyager spacecraft passed within 200,000km of it on their grand tour of the outer Solar System. Even then, the space cameras of that era weren’t good enough to reveal much more than the basics.
One of these was that the surface, while not quite cue-ball smooth, was unusually free of craters – a sign of recent geological activity. Also, the Voyagers found that Enceladus orbits in the heart of Saturn’s tenuous E ring (so faint it wasn’t even seen until 1967), raising the possibility that material escaping from the cue-ball moon’s surface might somehow have created it. But it wasn’t until NASA’s Cassini spacecraft began a series of close fly-bys in 2005 that Enceladus emerged as one of the Solar System’s most exciting worlds.
One of the first discoveries was of strange, bluish bands near the south pole, quickly dubbed ‘tiger stripes’. There were four major ones, now named after cities in The Arabian Nights: Alexandria, Cairo, Baghdad and Damascus.
A couple of months later, a subsequent fly-by revealed that the tiger stripes weren’t just 130km-long cracks in Enceladus’s icy facade; they were hot. Or at least hot by the standards of an airless moon, 1.4 billion km from the Sun, where the average temperature is somewhere around –200°C. In places, the tiger stripes have now been measured with temperatures as high as –95°C, strong signs of an internal heat source.
The biggest find came later that year, when Cassini scientists released a photo showing a cloud of backlit dust jetting from Enceladus’s south polar regions, right above the tiger stripes. It almost appeared as though the moon was rocket propelled, blasting northward out of its orbit. That’s not actually the case, of course. Rather, the jets are Matson’s cryovolcano, geysers now known to be spewing 200kg of water vapour and ice particles into space each second – enough to fill an Olympic swimming pool every few hours.
Enceladus had hit the astronomical big time because the instant question was: if these jets are composed of water and ice, does that mean there is liquid water beneath the icy moon’s surface? And if so, might there even be life?
SINCE 2005, CASSINI has made a dozen more fly-bys, with a total of 28 scheduled before the mission ends in 2017. Some have dipped as close as 21km to the surface, flying right through the heart of the plume. Others have photographed the surrounding terrain, made increasingly detailed thermal maps or used spectroscopes to analyse the composition, speed and density of the plume’s dust grains and gas.
In the process, the scientists have found that the tiger stripes are emitting an enormous amount of energy – about 16 gigawatts: equivalent to several large nuclear reactors. They’ve also detected a whole stew of intriguing compounds in the plume: ammonia, methane, carbon dioxide, hydrogen cyanide, formaldehyde, acetylene and other hydrocarbons.
“[Enceladus] has liquid water, organic carbon, nitrogen [in the form of ammonia] and an energy source,” NASA astrobiologist Chris McKay said in May 2011 in Mountain View, California, at a meeting of a small team of scientists called the Enceladus Focus Group. Except for Earth, he added, “there is no other environment in the Solar System where we can make all those claims”.
The big question, of course, is what is producing the plumes. The energy itself probably comes from gravitational flexing, as Enceladus moves through Saturn’s enormous gravity field. That causes the moon’s inner materials to rub against each other, generating heat. But even if you also throw in energy from radioactive decay, there doesn’t seem to be enough heat to drive the plumes indefinitely, said Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, in another talk at the Focus Group meeting.
Many Enceladus scientists therefore believe the plumes don’t operate continuously, but instead follow what engineers call a duty cycle, in which they turn on and off at intervals, storing energy for future eruptions during the intervening quiescent periods.
“The energy output suggests that it cannot be active much more than 10% of the time – at least at the current scale,” said Frank Postberg, a physicist at Heidelberg University, Germany.
Nobody knows how long the ‘on’ cycles might last. All we know for sure is that the geysers have been at full blast ever since Cassini first saw them seven years ago. It’s also a pretty good guess that, since they do indeed appear to feed the E ring, they have been going at least since the ring was first spotted, 55 years ago. In fact, they have presumably been active long enough for the E ring as we know it to have formed – a process Postberg estimates would take somewhere between a few hundred and “maybe” a thousand years.
But maybe there’s another marker, says Paul Schenk, a planetary scientist at the Lunar and Planetary Institute, Houston, Texas. Snow.
When the plumes are operating, Enceladus isn’t just a cold, airless world; it actually has weather – cold and sunny, with a chance of snow flurries. That’s because many of the ice grains ejected from the geysers are moving slower than Enceladus’s 239m-per-second escape velocity. “The vast majority, we think, fall back to the surface,” says Carolyn Porco, head of the Cassini imaging team and a Focus Group leader.
But the snowfall isn’t distributed evenly across the surrounding terrain. Rather, the dual effects of Enceladus’s and Saturn’s gravity concentrates it in two narrow bands, stretching hundreds of kilometres northward from the tiger-stripe zone, a team led by Sascha Kempf of the Max Planck Institute in Germany, calculated in a 2010 study in the journal Icarus.
Based on this, Schenk went looking for snowdrifts. Using the highest-resolution photos available of the fallout zones, he found a 20 x 20 kilometre region where photos had been taken from two angles, allowing him to construct 3-D maps.
What he found was an area whose topography was muted into rounded contours indicative of deep snow, with the ghostly outlines of underlying features peaking through. “It’s different from what you see in other areas that have been photographed at high resolution,” he says. “Those had incredible detail everywhere you look. This is much smoother.”
Better yet, he reported in October 2011 at a joint meeting of the European and American planetary societies in Nantes, France, the area was cut by deep canyons, the biggest of which was 500m deep and 1.5km across. Looking at this, he could find the line where snow lay atop bedrock, allowing him to calculate its depth. “I get a thickness of 125m,” he says. “Give or take 50. It varies.”
That’s a lot of snow, anywhere. But on Enceladus it’s enormous, because Kempf’s models don’t show the snow coming down at blizzard rates. Rather, his team calculated that even in the deepest zones it was accumulating at a rate of only one millimetre every 1,500 years. At that rate, it would take 150 million years to pile up 100m.
Kempf’s accumulation rate was based on the assumption that the material falling back to the surface was compressed ice, which is denser than snow. But even if the snow is a lot fluffier, it would take tens of millions of years to reach the observed depths, Schenk says.
“This is a whole new order of constraints for how long the plumes have been active,” adds Postberg. “It’s a big step from a thousand to 10 million years.”
And the longer the geysers have been active, the greater the chances that life might have had time to evolve.
Life as we know it, however, needs liquid water, not snow, and the mere fact that the tiger stripes are jetting out ice and water vapour doesn’t mean there’s liquid beneath. It’s possible that the geysers are produced by ‘dry’ processes, such as sublimation from the walls of the vents that feed them (a process in which ice evaporates directly into vapour, without ever getting warm enough to melt). Or, it may come from a more exotic process, by decomposition of subsurface layers of clathrate deposits (a type of ice composed of a cage-like lattice, tightly woven enough to trap gas molecules).
Both processes occur on Earth. Fishermen often dredge up methane-containing clathrates (also called methane hydrates) from the Arctic waters, watching them decompose into water and methane. Villagers in Greenland use sublimation to air-dry laundry on cold, sunny days when temperatures are well below freezing.
These processes produce mostly water vapour, not ice crystals, says Andrew Ingersoll, a planetary scientist from California Institute of Technology in Pasadena, California.
It turns out that it’s not easy to get that vapour to re-condense into the roughly 50/50 mix of ice and vapour his studies have found to be jetting from the surface. “It’s hard to get solid-to-gas ratios of more than 1%,” he told the Focus Group meeting.
Ingersoll thinks a better model is astronaut “urine dumps” into space. “If you throw a big blob of liquid into vacuum, it’s not going to freeze into a big chunk of ice,” he says. “It’ll break up and explode into a cloud of smaller particles.”
Another blow to the ‘dry’ theories of plume formation comes from the discovery that the ice crystals contain salt – a finding announced by Postberg’s team in June 2011 in Nature, based on spectroscopic analysis of the particles’ impact flashes in Cassini’s cosmic dust analyser, when the spacecraft flew through the plume.
Salt, he says, is very hard to explain from ‘dry’ plume models. Rather, its presence is strong evidence that the ice grains began as frothy bubbles fizzing out of a salty, subsurface ocean, rich in dissolved gases. It’s been called the ‘Perrier ocean’ model, says Matson, who notes that the ocean begins to fizz when something cracks the overlying ice. “When you pop the cap, bubbles come up.”
Matson compares the presence of the salt to the beach on a blustery day. “When the surf is up, there’s a lot of spray,” he says. “You can smell the salt in the air.”
Not all of the evidence points to liquid water, however. Hunter Waite, a space physicist at the Southwest Research Institute, San Antonio, Texas, notes that the plume also contains compounds such as hydrogen cyanide that are inconsistent with a liquid source. That’s because, if these compounds had ever met liquid water, they should have reacted to produce other compounds, not yet found.
Waite is careful to note, however, that he’s not attacking the liquid-water model. “We can’t say that all of the things are consistent with liquid water,” he says. “Some are and some aren’t.” One possibility, he says, is that chemicals in the plumes might come from multiple processes, all operating at once.
Even if Enceladus has liquid water, that’s not proof it has life. To answer that question, says Larry Esposito, a planetary scientist at the University of Colorado, Boulder, scientists need to find biomarkers – chemicals that appear to have biological rather than geophysical origins.
That’s not an easy quest in a world where organisms, if they exist at all, would live in underground lakes, ponds or oceans where sunlight never penetrates and photosynthesis isn’t an option.
One possibility, says McKay, is a ‘methanogen system’ in which microorganisms live by making both energy and biological building blocks by synthesising methane from carbon dioxide. He’s particularly fond of this idea because it might be sustainable over very long time periods if geological processes can recycle the methane produced by the bacteria into zones where temperatures exceed 500°C. “That decomposes the methane,” he says. “You’re turning geochemical heat into chemicals and the organisms are eating the chemicals.”
Ronald Oremland, a microbial biogeochemist with the U.S. Geological Survey’s office in Menlo Park, California, believes an even better food source would be acetylene.
Acetylene-eating organisms exist on Earth, he points out. And, he notes, this chemical (used as welding-torch fuel on Earth) also occurs on comets. If there’s enough on Enceladus, he says, it could be ‘fast food’ for microbes – a primordial food source on which Enceladus bugs might still be nibbling away.
Acetylene has already been reported by Cassini’s instruments. But to link it to acetylene-eating organisms, he says, scientists need to find by-products of acetylene metabolism, such as acetate and acetaldehyde.
Other possible biomarkers are amino acids, especially if they can be tested for chirality – the relative amounts of mirror-imaged shapes known as D and L isomers. Abiotic processes tend to produce an even mix of the two. Biological ones favour one or the other. On Earth the L versions are favoured, but there’s no reason extraterrestrial life couldn’t do the reverse. “If we find amino acids and there’s a strong chiral preference, that’s persuasive evidence for a biological origin,” says McKay.
Another marker, McKay and Oremland agree, would be the ratio of carbon’s two stable isotopes, 12C and 13C, in compounds emitted by the plume. That’s because biological processes produce compounds slightly enriched in 12C compared to non-biological ones. Testing 12C/13C ratios, in fact, is one way sports authorities catch drug-cheating athletes, because synthetic hormones, produced in a lab, have different ratios from those produced by the athlete’s own bodies.
None of this, however, can be done with Cassini’s instruments. Such tests would require a return to Saturn, either with a specialised new Enceladus probe, or one piggybacked on a mission to Saturn’s giant moon Titan, also on the wish-list for a return visit.
“What we’re being handed at Enceladus is a potential gift of looking at life in the outer Solar System,” Oremland says. “What’s appealing about Enceladus is that you have some of the conditions for life. There’s liquid water under the ice. It seems to have been around a long time. How long, nobody knows. One hundred million years? A billion? That’s a long time for life to get going, provided there’s something to eat.”