Enceladus, Saturn’s tiny moon, is a beacon for planetary scientists.

Forty years ago, NASA scientists got their first hint that something very odd was happening on the sixth moon of Saturn.

During their grand tour of the outer solar system, twin Voyager spacecraft flew by the small moon, Enceladus. Scientists had expected to see a planetary surface covered with craters, like our own moon. Instead, smooth barren planes sliced across the rough badlands. “Something is erasing the craters there,” an astronomer commented. In fact, the surface was so smooth and white that Enceladus stood out as the most reflective object in the solar system.

Scientists saw the luminous surface as evidence that Enceladus was a geologically active world that possessed some sort of heat engine. Their theories were later confirmed—and then some—when, in 2005, NASA’s Cassini spacecraft obtained astonishing close-up images of the moon that revealed enormous geysers of icy particles erupting from four, 80-mile-long crevices at the south pole, which scientists nicknamed “tiger stripes.”

Emily Martin, a research geologist at the National Air and Space Museum’s Center for Earth and Planetary Studies, recalls when, as an undergrad, she first saw the images of the geysers. “I can’t remember if I gasped, but you should imagine that I did,” she says. “You only see this kind of geology in action on the Earth.”

The eruptions on Enceladus appear to be continuous, spewing particles at approximately 800 mph. Some of the particles are ejected into space, where they form Saturn’s 186,000-mile-wide E ring. Most of the particles fall like snow back onto the fractured exterior of Enceladus, creating a landscape that would look like a “scary wintery wonderland” to anyone on the surface, quips Martin.

But what has scientists like Martin most excited is what’s beneath the surface of Enceladus. Subsequent research and examination of Cassini’s data indicates the existence of a vast subsurface ocean. What’s more, the moon’s plume contains organic compounds, as well as rocky particles that suggest the existence of hydrothermal vents like those that support vibrant ecosystems in the darkest depths of Earth’s oceans.

Put it all together, and tiny Enceladus—which could fit within the borders of Arizona—emerges as one of the best candidates for sustaining extraterrestrial life in our solar system. And, whereas other potentially habitable worlds, such as Jupiter’s moon Europa, would require us to dig deep below the surface to find signs of life, Enceladus’ geysers are giving away samples of its subsurface ocean, beckoning us to mount an expedition across 790 million miles of space to borrow a cup of frozen alien water.

Habitable zones

If Enceladus seems like an odd destination for seeking out alien life, consider that astronomers as far back as the 19th century thought we might find habitable planets in the frigid outer reaches of our solar system. Their optimism was driven, in large part, by life’s stubborn determination to eke out an existence in even the most extreme environments on Earth. French astronomer Camille Flammarion declared, in his characteristic grandiloquent prose: “From the high regions of the air, where the winds carry the germs, to the oceanic depths, where they undergo a pressure equal to several hundred atmospheres, and where the most complete night extends its eternal sovereignty; from the burning climate of the equator and the hot sources of volcanic regions to the icy regions and the solid seas of the polar circle, life extends its empire like an immense network, surrounding the whole Earth.”

Flammarion and other astronomers were confident this network of life must surely encompass other worlds. And some of them offered elaborate theories to explain precisely how that might happen. British astronomer Robert Anthony Proctor, for instance, suggested that Jupiter was still cooling down from its fiery creation and was therefore generating enough heat to warm its surrounding moons. “If Jupiter be still in a sense a sun, not indeed resplendent like the great center of the planetary scheme, but still a source of heat, is there not excellent reason for believing that the system which circles around him consists of four worlds where life—even such forms of life as we are familiar with—may still exist?”

Their theories were fanciful, but those early astronomers were onto something. Throughout much of the 20th century, the dominant scientific belief had been that life would be found only on planets whose orbits were within the so-called Goldilocks Zone: not too hot, not too cold, and just the right distance from a star to melt ice into liquid water, which acts as a universal solvent for dissolving molecules, making it a vital ingredient for the chemistry of life.

Jupiter and Saturn aren’t emitting heat in the way that Proctor envisioned, but the two gas giants do play a role in keeping their icy moons toasty through a process called tidal heating. In the case of Enceladus, the gravity from another moon, Dione, stretches Enceladus’ orbit around Saturn into an elliptical shape. The gravity that Saturn exerts on Enceladus increases and decreases as the moon moves closer and farther from the planet. The result is friction within Enceladus’ rocky core and along the faults of its icy shell that, according to one study, could generate 10 to 30 billion watts of heat.

Saturn’s grip on Enceladus can be seen when observing the moon’s tiger stripes. “The brightness of the plume changes, getting brighter when Enceladus is closer to Saturn and getting a little bit dimmer when it’s further away,” says Martin. “The more Saturn is pulling on Enceladus, the more the tiger stripes—these big fractures at the south pole—open up, allowing more material to escape.”

And that’s not all. Scientists have found evidence that points to the existence of hydrothermal vents—like those found on Earth—that form when ocean water seeps deep below the sea floor, where it is heated, and then erupts back into the ocean, where it resembles smoke because it’s thick with dissolved minerals and particles. On Earth, that superheated water can reach temperatures over 700 degrees Fahrenheit.

Scientists first suspected the presence of hydrothermal​ vents when the Cassini spacecraft’s ion and neutral mass spectrometer detected excessive amounts of methane in the plume material shooting from Enceladus’ tiger stripes. That was a provocative clue since, on Earth, hundreds of deep-sea vents along the western seaboard of the United States churn out methane.

Other vents on Earth produce molecular hydrogen at an extraordinary rate. It’s a difficult molecule to detect, but Cassini finally found some in Enceladus’ plume during a very close flyby, just 30 miles above the moon’s surface, in 2015.

The strongest evidence to date was provided by Cassini’s cosmic dust analyzer, which found nanograins of rock embedded within the frozen plume particles. The rock grains were later determined to be silica crystals—a form of silicon found in quartz sand on Earth. Laboratory experiments and computer modeling indicated that silica grains of that size could be created only under very specific conditions: when minerals dissolved in very hot water come in contact with cold water. (It’s similar to the method for making rock candy, when sugar-saturated water is cooled to form crystals.) Hydrothermal vents would be the most likely source of heat for such a chemical reaction.

If, in fact, there’s life on Enceladus, it might owe its existence to those hydrothermal vents. The Cassini data revealed that the plume water contains organic compounds similar to those on Earth that are part of the chemical reactions that produce amino acids, which are the building blocks of life. Enceladus’ hydrothermal vents might provide the energy that leads to the production of amino acids.

“Energy and liquid water plus the right chemistry are the three things you need in order to create a habitable environment,” says Martin. We’ve seen firsthand how oceanic vents can sustain a microbial ecosystem. When scientists discovered hydrothermal vents on Earth in the 1970s, they were shocked to discover a vast community of organisms clustered there. Without sunlight and photosynthesis, biologists thought there couldn’t possibly be enough food on the deep ocean floor to sustain life. But bacteria living at the vents possess the capability to convert minerals and other chemicals in the water into energy, forming the basis of a food web that sustains over 300 animal species.

“Based on life on Earth, we guess that life on Enceladus would be mostly associated with the hydrothermal vents and will be microbial, anaerobic,” says NASA astrobiologist Christopher McKay. Anaerobic refers to species that exist without oxygen. Some of them produce methane gas, suggesting that life could be among the sources of the methane detected on Enceladus. McKay notes, however, that the apparent lack of oxygen molecules “suggests that large animals are not present.”

Could microbial life exist on Enceladus, clustered near hydrothermal vents at the bottom of a dark ocean nearly 900 million miles away from the sun? A proposed $4.9 billion NASA mission could find out.

The Orbilander

The Galileo spacecraft, which arrived at Jupiter in 1995, sent images and data back to Earth that confirmed earlier observations—made by the Voyager spacecraft—that the massive planet appeared to have three ocean-bearing moons: Ganymede, Callisto, and Europa. “That really threw the doors open for us to consider this new class of planetary bodies in the outer solar system called Ocean Worlds,” says Martin. The exploration of those outer worlds is picking up pace. In April, the European Space Agency launched the JUpiter ICy moons Explorer (JUICE), which will orbit Ganymede. And, in October 2024, NASA will launch the Europa Clipper, which will make repeated flybys of Europa while orbiting Jupiter.

The scientific community is now also eager to send a mission to search for life on Enceladus, especially since the plume provides comparatively easy access to the subsurface oceans. “We don’t have to drill through tens of kilometers of ice, which is hard to do here on Earth, let alone if you had to launch your whole rig system on a rocket for many years into space and then land on a moon and start a drilling campaign,” says Shannon MacKenzie, a planetary scientist at the Johns Hopkins University Applied Physics Lab.

NASA asked MacKenzie and a team led by the Lab to draft a concept study for a Flagship mission to search for life on Enceladus. (A Flagship-class mission is a category reserved for the costliest and most complex NASA science missions, typically costing between $2 to $5 billion.)

Together, the scientists and engineers considered the pros and cons of various configurations for a mission to Enceladus. Should they send just a lander? Or maybe a lander loaded with scientific instruments accompanied by a simple relay orbiter? Or, inversely, should they put most of the life-detecting instruments aboard an orbiter and utilize a small lander for simpler measurements, like investigating the seismic activity caused by the gravitational pull of Saturn?

Ultimately, MacKenzie says, the mission architecture was determined by the fact that Enceladus is “super tiny.” The diameter of the moon is about the same distance as the tip of  Scotland to London. Meanwhile, Saturn is a hulk of a planet that’s 10 times the size of Earth. “That makes it hard to get a spacecraft into Enceladus’ orbit,” says MacKenzie. “You have to burn a lot of energy to jump from being in orbit around Saturn, which is the body that’s tugging on you the most, to this tiny little body that’s not going to tug on the spacecraft quite as much.”

But once the spacecraft does achieve an orbit around Enceladus, the amount of energy needed to land on the moon—with its very light gravity—is negligible in comparison. So, the Applied Physics Lab team decided it made the most sense to just take the whole spacecraft down to the ground after completing its mission in orbit. And thus, the “Orbilander” concept was born.

After arriving at Enceladus, the Orbilander would spend 1.5 years circling the moon and gathering scientific data, which would include collecting and analyzing samples of particles as it flew through the geysers.

Although the plume of material obtained by Cassini looks dense, it’s more like mist. That’s partly because only small particles can achieve the escape velocity that frees them to reach such a high altitude. To sustain its orbit, the Orbilander would be moving at roughly the same velocity as the particles. “That means you can sample the plume particles at a relatively low speed, and therefore they don’t break up in the same way as if you were zooming by at tens of kilometers per second like Cassini,” says MacKenzie.

The Orbilander would also scout for an appropriate site for the surface phase of the mission, where it would spend two years conducting additional surveys. The heaviest “snowfall” from the plume likely falls closest to the sources in the tiger stripes. Unfortunately, some of the landing spots “look pretty gnarly,” says MacKenzie. “We have enough data from Cassini to suggest that there are better places to land that have smoother surfaces, fewer boulders, and are not on a slope.” That’s the beauty of the Orbilander concept: Mission planners would have ample time and data to select a landing site that would preserve the safety of the spacecraft while also maximizing opportunities for obtaining high-quality samples.

The large plume particles that can’t achieve escape velocity fall back to the surface of Enceladus. “Those larger particles are more likely to have that carbon-bearing material that we would want to be sampling,” says MacKenzie. “And, when you’re on the surface, you have access to a larger reservoir of material, either from falling directly onto you or by scooping up fresh material. That abundance helps eliminate uncertainties like, ‘Did we not see something because we didn’t sample enough?’ If you’re able to do many more experiments because you’re collecting grams per day rather than micrograms, then that’s a whole other ballgame.”

Of course, an ideal scenario for detecting life in Enceladus’ ocean would be to actually find a microscopic organism in one of the samples. But, if the organic matter in the water is sparse, then even multiple samples might be insufficient to enable the Orbilander to see one of the tiny ocean inhabitants with its microscope.

That’s why the Orbilander would also carry instruments capable of detecting additional biosignatures—indicators that life exists in the moon’s ocean even if we don’t see it. The Orbilander team has recommended a suite of six instruments, including a DNA sequencer and mass spectrometers to weigh and analyze molecules.  Some of the experiments will be designed to distinguish the origin of molecules found in living organisms that could have other sources. McKay, who was part of the Orbilander concept team, cites amino acids and lipids as two examples. “Both are present in biology and in meteorites,” he says. “But in biology there is a distinct pattern in the amino acids used in proteins and in the lipids used in cell membranes.”

The Orbilander mission’s authors propose a 2038 launch with an arrival in 2050, when the south pole of Enceladus will be entering its summer season, which means more of it will be illuminated by sunlight as the mission progresses.

That means, if NASA were to greenlight the mission, it could be another three decades before we find out if life exists on a world other than ours. But, given what we’ve seen so far, scientists believe it’s worth the wait.

Mark Strauss is the managing editor at Air & Space Quarterly.

This article is from the Spring issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

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