The Whole Enceladus
A new place to search for life in the outer solar system
Step aside, Europa. Make way, Titan. Saturn’s small moon Enceladus is becoming one of the hottest places to look for signs of life in the chilly outer solar system. NASA’s Cassini spacecraft recently discovered that a giant plume of water vapor, dust, and small ice crystals shoots out from a crack-lined region in the southern hemisphere of this 500-kilometer-wide moon. Observations of the plume and surrounding material on the moon’s surface suggest that Enceladus harbors the basic ingredients necessary for life as we know it.
An internal heat source probably drives the geyser, which looks like Yellowstone’s Old Faithful. The source might heat pockets of liquid water at the bottoms of the cracks, driving it out as hot water or steam. The mix of inorganic compounds and hydrocarbons found in the plume, as well as organic compounds detected on nearby regions of the moon, suggest that a rich, warm organic soup lies beneath the surface. Such a soup would be a prime place for finding amino acids, building blocks of life, says Cassini scientist Dennis Matson of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
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He and his colleagues report the basic findings about Enceladus in nine articles in the March 10 Science.
A water-bearing zone on Enceladus might be more easily explored than other promising sites in the outer solar system could, some planetary scientists now argue. Although Jupiter’s moon Europa may harbor a vast, briny ocean, it would lie beneath an icy shell estimated to be tens of km thick. And if Titan, the largest Saturnian moon, contains liquid water, the reserves are probably well beneath its hydrocarbon-shrouded surface, which is cold enough to freeze even methane. In contrast, the exposed crevasses on Enceladus that may hold liquid water are only about a half-kilometer deep.
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“I’m not saying we’re going to find bugs on Enceladus, but I am saying that whatever the incipient conditions are for life, they are readily accessible there,” says Cassini researcher Christopher Parkinson of the University of Michigan in Ann Arbor. “And it’s worth taking a look.”
Enceladus, a tiny moon whose area is smaller than that of New Mexico, first captured the attention of planetary scientists a quarter-century ago. That’s when the two Voyager spacecraft revealed ancient, pockmarked terrain on the moon lying adjacent to much younger, smoother regions. The smooth areas suggested that parts of the moon had recently undergone a facelift in which upheavals erased old craters.
Those and later Earth-based observations showed a diffuse ring of ice particles residing around the moon, another indication that Enceladus is geologically active and venting material.
Researchers planning the Cassini mission, which settled into orbit around Saturn in 2004, hoped that the robotic craft might get lucky and record a snapshot of the moon during an eruption. The scientists got more than they bargained for.
During the first two Cassini passages of Enceladus, in February and March 2005, the craft’s magnetometer detected radio-wave oscillations at the exact frequency expected when ionized water molecules gyrate along magnetic field lines. The ions were probably created when sunlight struck water vapor emanating from the moon, researchers concluded (SN: 4/16/05, p. 253).
Then, on July 14, Cassini swooped in for a closer look, coming within 172 km of the moon. That’s the nearest that the craft has come to a satellite of Saturn. Images revealed a terrain of faults, folds, and ridges devoid of craters, further evidence that Enceladus is one of the most geologically active places in the solar system. The images also showed fresh-looking deposits of amorphous and crystalline ice that could be just hours to decades old.
Then, to the astonishment of planetary scientists, “Cassini saw this great big, wonking thing,” recalls Parkinson. A giant geyser of water vapor was erupting from the vicinity of 100-meter-wide linear cracks, dubbed tiger stripes, at the south pole. The plume soared 175 km above the moon (SN: 1/7/06, p. 13).
“This was a heart stopper,” says Carolyn Porco, Cassini imaging scientist at the Space Science Institute in Boulder, Colo.
Spectra revealed that the geyser contained compounds tentatively identified as carbon dioxide, methane, propane, acetylene, and molecular nitrogen. Molecular nitrogen requires a relatively warm temperature, notes Matson. For a crucial 24 seconds during this flyby, instruments on Cassini observed the southern hemisphere of Enceladus while a background star, Bellatrix, dipped behind the moon. Just before the star vanished, some of its light filtered through the tenuous gases surrounding Enceladus.
Viewing this stellar occultation, an ultraviolet detector on Cassini measured the amount of water vapor expelled by the plume. It also revealed in the plume and on the nearby surface, a mixture of organic and inorganic compounds that might support life. Because that material probably came from the cracks, the vents would seem to be a hospitable place for life, says Parkinson.
Completing the picture, Cassini revealed an infrared glow—a hint of heat. Though it’s still a frigid 157 kelvins, the region around the plume is 25 kelvins warmer than other areas on the moon.
Solar heating isn’t enough to explain the high temperature, Parkinson says. Instead, the moon must have some inner source of heat. If the temperature at the base of the plume at the bottom of the crack is higher than that of the moon’s surface, as several models suggest, it means that the crack may hold liquid water.
Other evidence hints that Enceladus’ plume isn’t a one-shot event but has persisted for years. The longer the moon has been belching water vapor, the more likely it is to have retained liquid water beneath its surface, says Parkinson. Assuming that the geyser continues erupting, Cassini might get another chance to study it when the craft passes the moon again in spring 2008.
Enceladus was first detected in 1789. In the early 1980s, ground-based observations revealed that the moon inhabits an interesting neighborhood. It lies in the middle of Saturn’s E ring, a tenuous outer circle of fine ice particles. Scientists immediately suspected that Enceladus both created and replenishes the ring.
Computer models indicate that the largest chunks of ice in the plumes probably follow paths that send them hurtling back toward Enceladus. But some of the smaller plume particles disperse into orbits that make up the E ring.
The ring requires constant replenishing because its particles are only loosely bound by gravity and escape after about 1,000 or so years of orbiting Saturn, says Parkinson. If Enceladus weren’t belching new ice particles, the E ring would probably have vanished long ago.
The mix of short-chain organic compounds found in the plumes, Parkinson and his colleagues say, suggests that if there is a heat source beneath the surface of the moon, amino acids could have been synthesized there.
Inside Enceladus, water moving through the rocky material near the moon’s core could have created iron-rich clays. Models indicate that such clays could foster the formation of amino acids and even bacteria, Parkinson says.
Another source of these clays could be micrometeorites, which deliver organic compounds and metals. The micrometeorites would pound the surface and ultimately become incorporated into deeper layers of the moon.
Other features also favor formation of biological materials, Parkinson notes. For instance, spectra of the surface of the moon show several compounds that could dissolve in water and trigger energy-releasing chemical reactions. Known as redox reactions, they break down compounds by adding or taking away electrons—reducing or oxidizing the compounds respectively*, in the chemical parlance. The rusting of metal, which gives Mars its reddish color, is one example of an oxidation reaction.
The geothermal activity, liquid water, and redox reactions “give favorable conditions for life on Enceladus,” says Parkinson.
“These conditions are not duplicated anywhere in our solar system [today] except our planet,” he asserts. Mars might have had flowing water at or just beneath its surface, but only in the distant past. Titan is a frozen repository of chemicals that could become part of a biological brew, but other conditions on that moon don’t appear to be favorable for making life.
Some of the water from Europa’s vast underground ocean could have oozed up through cracks in the overlying ice, refrozen, and then reacted with charged particles from surrounding space. However, Parkinson says, most of it would remain isolated beneath a thick layer of ice. Without contact with an atmosphere, redox reactions are unlikely to have occurred there.
Given these conditions elsewhere, Parkinson concludes, “Enceladus is the most exciting object in the solar system for the search of extant life.”
Some other scientists disagree. For Ralph Lorenz of the University of Arizona in Tucson, Enceladus and Europa are mere “side-shows” compared with the intriguing landscape and potential lessons about the development of life offered by Titan. He says that he’s not all that impressed by the shallow “Perrier water” that Enceladus may harbor.
Although they have their favorites, Parkinson, Lorenz, and other planetary scientists support explorations of all three places. But with limited funding for space missions in general, and astrobiology in particular, “everyone is protecting their own turf,” says Parkinson.
Eighteen months ago, a probe carried by Cassini landed on Titan, revealing a surprisingly Earthlike terrain of shorelines and river valleys. These had been sculpted not by water but by liquid methane or ethane and by moving pebbles apparently rounded by the flow of the same hydrocarbon. Heat stored and gradually released by this mammoth moon, which is nearly as big as Mercury, can account for the complex landscape, notes Jeff Kargel of the University of Arizona in Tucson.
Enceladus, by contrast, remains an enigma. The moon is one of the tiniest of Saturn’s retinue of satellites that orbit the planet in the same direction that the planet orbits the sun. Scientists had expected that a moon this small in the frigid outer solar system would be an inactive ball of ice and rock. Indeed, a similar-size Saturnian moon called Mimas shows no geological activity.
Yet Enceladus’ plume flouts that expectation. Researchers are debating what kind of heat source powers the moon’s geysers.
Theorists generally agree that the energy that generates enough heat to liquefy some of the moon’s ice comes from within the moon. In March, at NASA’s annual astrobiology meeting in Washington, D.C., Julie Castillo of NASA’s JPL presented one of the models for that energy.
The key to Enceladus, her team proposes, is that it has a relatively rocky, much larger core than the inactive Mimas does. Coalescing about 3 million years after the birth of the solar system, the rocky material began to undergo radioactive decay, which generates heat. Short-lived radioisotopes, such as aluminum-26, jump-started the heating. Once these isotopes decayed, long-lived ones, such as uranium and thorium, took over and produced enough heat to melt the core, the team suggests.
A liquid core can easily absorb energy from the gravitational stresses generated by variations in the tug of a neighboring body. As Enceladus travels on its oblong path, which the model suggests was even more elongated in the past, the moon responds to variations in Saturn’s gravitational pull. This produces tidal forces that flex the core of the moon and sustain its interior heat, even after the short-lived isotopes have decayed.
Tidal energy would keep the rocky core molten. Heat transferred from the molten core to surrounding ice could then liquefy water and produce a plume, Castillo’s team says.
While many scientists are enthusiastic about planning a mission to Enceladus, they’ll need much more information from Cassini, other craft, and from ground-based telescopes, says Parkinson. Planetary scientists want to determine whether the plumes are generated steadily or sporadically. They also want to confirm hints in spectra that there are compounds of biological significance, such as ammonia and oxidants, on the moon’s surface and to look for chlorophyll and other harbingers of life.
To encounter liquid water, a probe might not have to descend to the bottom of the cracks. Data in hand indicate that a mist of water may lie just 7 m down the vents.
Cassini is scheduled to fly past Enceladus in 2 years. “If a wet domain exists at the bottom of Enceladus’ icy crust … Cassini may help to confirm it,” says Kargel. But learning whether the moon hosts life is beyond Cassini’s ken, he adds.
In the March 10 Science, he wrote, “Any life that existed could not be luxuriant and would have to deal with low temperatures, feeble metabolic energy, and perhaps a severe chemical environment. Nevertheless, we cannot discount the possibility that Enceladus might be life’s distant outpost.”
*Note: the online version of this passage has been updated to correct an error.