“In the old time Pallas [Athena] heaved on high Sicily, and on huge Enceladus dashed down the isle, which burns with the burning yet of that immortal giant, as he breathes fire underground.” — Quintus Smyrnaeus, The Fall of Troy

Key takeaways

  • This small moon of Saturn has a subsurface ocean, making it a prime candidate for life.
  • The Cassini spacecraft revealed geysers, organic compounds, and possible hydrothermal vents on Enceladus.
  • Enceladus has water, chemistry, and energy—three key components necessary for life.
  • The moon’s geysers spray organic-rich particles, making it easier to study its ocean without landing.
  • Proposed missions aim to explore Enceladus further, potentially confirming signs of life.

Enceladus, Saturn’s sixth-largest moon, with a diameter of just 310 miles (500 kilometers) and a mass less than 1/50,000 that of Earth. When it comes to searching for life, Enceladus is at the top of the list, and it’s practically in our cosmic backyard.

A little neglected at first

William Herschel, an English astronomer, discovered Enceladus in 1789, but it remained a mystery until the Cassini spacecraft began orbiting Saturn in 2004. Prior to Cassini, Enceladus was mostly ignored. We had no idea liquid water could exist so far out in the solar system, so why would anyone be interested in another boring, dead ball of ice.

That all changed one year later, when Cassini’s magnetometer (a fancy compass) spotted something unusual in Saturn’s magnetic field near Enceladus. This suggested that the moon was active. Subsequent approaches by Enceladus revealed four huge fissures known as “tiger stripes” in a hot region near the south pole. And from those cracks came a tremendous plume of water vapor and ice granules. Enceladus lost the label of being a dead relic of a bygone era and leaped to center stage as a dynamic world with a subsurface ocean.

But was it truly an underground ocean, or was it more like a small southern sea? Thankfully, Cassini could also address this question. The imaging cameras proved that Enceladus’ icy crust is not related to the planet’s rocky core by detecting excess wobble during its orbital cycle. This is only possible if the crust floats in a vast, subterranean liquid-water ocean.

Cassini did not stop there. During numerous flythroughs of the plume, the spacecraft’s mass spectrometers studied the gas and grains. These instruments, the Ion and Neutral Mass Spectrometer (INMS) and the Cosmic Dust Analyzer (CDA), discovered that the plume is primarily water, but it also contains salts, ammonia, carbon dioxide, and small and big organic compounds. These findings help us paint a picture of the world below the ice: a possibly habitable ocean that’s slightly alkaline, with access to chemical energy in the water and geothermal energy at the rocky seafloor.

Plumes spray water ice and vapor from many locations along the so-called “tiger stripes” crossing Enceladus’ south polar terrain. The four prominent fractures are about 84 miles (135 kilometers) long. This two-image mosaic of the moon shows the curvilinear arrangement of geysers, erupting from the fractures. NASA/JPL/Space Science Institute

Possible energy sources

One of the Cassini mission’s most significant legacies is that it established Enceladus as having all three components of life as we know it: water, chemistry, and energy. Water in the ocean: check. Chemistry of the simple and complex organics discovered in the plume: check. These could be used to construct the molecular machinery of life.

Energy requires a bit more explanation.

Enceladus’ seafloor is anticipated to have hydrothermal vents. We know this because of three pieces of evidence. First, INMS discovered methane in the plume at higher quantities than would be expected if it came from clathrates (high-pressure water-ice cages with trapped methane) or other reservoirs in the ice. Methane is a major byproduct of hydrothermal systems.

Second, CDA detected silica nanograins with a specific size and oxidation state that were traced back to the ocean. These could only have evolved when liquid water touches rock at temperatures of at least 194 degrees Fahrenheit (90 degrees Celsius), which is the temperature range of hydrothermal vents such as “white smokers” on Earth.

Third, the INMS team’s recent confirmation of molecular hydrogen in the plume strongly suggests that liquid water has interacted with a rocky core.

Third, the INMS team’s recent confirmation of molecular hydrogen in the plume strongly suggests that liquid water has interacted with a rocky core.
On Earth, hydrothermal vents near the base of the Mid-Atlantic Ridge support thriving organisms that are as far distant from photosynthesis as one can conceive. These ecosystems rely on geothermal and chemical energy. A similar population may exist around a hydrothermal vent on the bottom of Enceladus.

So we’ve got water, chemistry, and energy. Assume they have mixed together long enough for life to emerge. (Your guess is as good as everyone else’s here—estimates range from 100,000 to 25 million years.) How might we detect it?

Assuming an energy-limited environment (a suitable analog is Lake Vostok, a body of water in Antarctica that has been encased in ice for the last 35 million years), we can expect cell densities ranging from 100 to 1,000 cells per milliliter of ocean water. For comparison, the Earth’s oceans contain approximately 1 million cells or more per milliliter.

We anticipate that this life would use readily available building ingredients, such as amino acids, which are abundant in carbonaceous chondrites and likely present all throughout the Saturnian system — in numbers on par with Earth-based life.

This assumption is reasonable because life needs chemical complexity to carry out the reactions that keep cells functional. Then we are looking at concentrations of biomarkers on the order of less than 1 part per billion. That’s tough for current instruments to achieve, without some kind of concentration step.

Does this mean we have to wait for more advanced instruments before we search for life? Nope.

Organic enrichment in the plume

A percentage of the ice grains observed by the CDA instrument contained a high concentration of organic molecules, which the CDA team refers to as high mass organic cations (HMOC). While the equipment was unable to properly identify the HMOC structures, a thorough study resulted in some informed predictions, such as aromatics (carbon-containing ringed structures) and oxygen- and nitrogen-bearing species. Within Enceladus’ ocean, there could be a complex organic soup of chemicals.

The most plausible explanation for how these organic-rich ice grains formed is “bubbles bursting.” The grains were not only organically rich, but also salt-free, indicating that they came from an organic layer near the ice-ocean boundary.

On Earth, something comparable floats at the surface of our oceans. It’s a film known as a “organic microlayer,” because it’s not very thick and is often composed of organics derived from biological activity (i.e., fragments of cells) as well as other sources.

Organic molecules want to hang out together and are not fond of salts or water, so they push them out of the microlayer. Then, wave activity causes bubbles in this microlayer to break, releasing organic-rich, salt-poor aerosols.

A similar mechanism may be occurring on Enceladus. Organic molecules in the ocean may cluster at the ocean-ice boundary, forcing water and salts out of the film, just as they do on Earth. As the liquid surface at the base of the plume boils into vacuum, bubbles may rupture and disseminate the organic coating, resulting in grains with a high organic content but minimal salt.

What’s the end result of all this? Enceladus may be contributing to the concentration of organic compounds, which astrobiologists are particularly interested in studying.

Earth’s aerosols include organic compounds that are hundreds to thousands of times richer than those found in the water. If we collect samples by landing on the surface or flying through the plume, we may have a greater chance of detecting evidence of life on Enceladus, if it exists.

Future mission concepts

Enceladus has fascinated us and provided more than enough reasons to return. Many prospective missions would suffice, and a few have been proposed in the post-Cassini period, though NASA has yet to approve them.

Some would do what Cassini did: fly through the plume and investigate the gas and grains, but with updated instruments capable of considerably more sensitive and effective life tests. Others would land on Enceladus’ south polar region to sample fresh snow deposited on the surface by the plume.

Even more ambitious suggestions include a sample return mission (albeit with a 14-year round-trip time, we’d have to wait a lot for that sample) or various climbing or melting robots to descend the 1.2 to 6.2 miles (2 to 10 km) through the ice shell and reach the ocean itself.

Whatever we send, the next mission to Enceladus — if astrobiology is the primary goal — will require a well-designed suite of equipment capable of looking for several, independent lines of evidence for life. Our understanding of life’s traits has evolved significantly since the Viking period, when NASA explicitly announced that the hunt for life was their major mission.

When the two Viking landers landed on Mars in 1976, for example, we only knew about two of the three types of life. (Archaea, the third and most primordial branch of the tree of life, was found in 1977.) The Viking landers conducted three biological experiments to look for life in the martian regolith. One test result was affirmative, one was negative, and another was equivocal. Since then, we’ve learnt a lot about how to construct trials so that an ambiguous outcome is far less likely.

We’re also getting better at looking for biosignatures that are as unrelated to Earth life as feasible. For example, a future mission to Enceladus might not look for DNA, which is specific to Earth-life, but rather for a molecule that could serve the same function for alien life: a large molecule with repeating subunits (similar to an alphabet) capable of storing information, such as blueprints for an alien cell. If such a molecule is discovered, combined with the positive identification of a number of other biosignatures, a compelling case might be made for the first detection of life on another planet.

Active, accessible, and relevant

Enceladus is not the only place that potentially support life. Europa has a much larger liquid water reserve, while Titan’s ocean may contain impossibly rich organic chemistry.

However, Enceladus is the only area where researchers can be guaranteed that they will be able to retrieve material from the ocean without having to dig, drill, or land. We can utilize current technology to test the hypothesis that life may exist elsewhere in the solar system.

Enceladus may be a small moon, but excellent things may come in small packaging. It is now time to answer the central question that has propelled humankind since we first glanced up: Are we alone?

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