By Hailey Reissman
Right here in our solar system, three small ocean-containing moons are intriguing researchers, who think they might just harbor the conditions for existence to begin.
Draw a Venn diagram with two overlapping circles, label one “oceans” and the other “planetary moons,” and look at where the two spheres intersect — that’s where our search for alien life is focused right now. Saturn’s moons Enceladus and Titan, and Jupiter’s moon Europa, extraterrestrial bodies confirmed to contain liquid oceans and referred to as ocean worlds, are three targets of keen interest and of future exploration.
Drawing conclusions from Earth, scientists have settled on a shortlist of the ingredients needed for life as we know it: liquid water; an energy source for metabolism; and a chemical cocktail of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Data from space missions has revealed the likely presence of these components on Enceladus, Titan and Europa — along with other unique elements — which raises the possibility they may have evolved their own ways of sustaining life. Planetary scientist Alexander Hayes, a professor at Cornell University and one of the researchers engaged in the hunt for life, explains why our hopes currently rest on these three moons.
Why these three moons? Earth’s oceans have led us there. In 2000, oceanographers probing the depths of the Atlantic Ocean came across masses of hydrothermal vent systems — essentially pools of heated water — in a spot near Ecuador where they’d never expected to discover them. Until then, such systems had been found only above underwater volcanoes. But these vents, known as the Lost City Hydrothermal Field, were different — their heat was created by chemical reactions between seawater and rock rather than from magma. From this process a unique biome had arisen. High amounts of methane and hydrogen gas came out of the vents, and the microbial world in the Lost City fed on it. Just like plants use water and sunlight to grow, the microorganisms relied on this gas. “Life was found literally running off the energy provided by chemical reactions between rock and ocean water,” says Hayes. Scientists wondered if the bottoms of extraterrestrial oceans could support similar biomes.
Enceladus: a moon with intriguing jets of ice and water. Enceladus is the sixth-largest moon of Saturn, with a diameter of roughly 310 kilometers — “it would fit nicely between Los Angeles and San Francisco,” says Hayes. This ocean world has been the subject of several fly-by investigations from NASA’s Cassini spacecraft. In 2005, Cassini data showed dozens of jets of ice and water particles coming out of Enceladus, breaking through the moon’s icy shell. “Something was generating heat,” Hayes says. “Icy water particles were coming out of more than 100 jets.” A Cassini sample of the particles also revealed they contained organics, Hayes says, “and about a year later, the team found salts in particles from the jets, which meant they must have come from a saline body of water and froze in a process like a sea spray.” An ocean was hiding under the moon’s surface.
The jets provide clues that hint at vents with a striking similarity to those of the Lost City Hydrothermal Field. In 2015, the Cassini spacecraft took a very fast (speeds reached 19,014 mph) and risky dive into one of the jets, which gave scientists a wealth of new samples to analyze. The team, led by NASA’s J. Hunter Waite, discovered molecular hydrogen in the samples, posing major implications in the hunt for life there. If Enceladus creates its own molecular hydrogen — and hydrogen is what feeds the microorganisms living in the Lost City vents — it’s another reason that life could exist there.
Could an expedition to Enceladus be next? Hayes’s Cornell University colleague Jonathan Lunine has proposed the Enceladus Life Finder (ELF), a mission to fly a specially designed probe through the jets and collect more samples and data to learn what’s going on. Cassini did sample the jets, but Lunine and his team explain that its instruments are much less advanced than the technology available now (Cassini launched back in October 1997, a lifetime ago in tech terms). “Enceladus is particularly attractive [for a mission] because liquid water from its deep interior is actively erupting into space, making sampling of the interior straightforward,” Lunine and team wrote in their proposal. ELF’s instruments could look at the jets’ solids and gases to search out life-building amino and fatty acids and look for more indications of conditions suitable for life.
Titan, Saturn’s largest moon, appears most like Earth, with some notable distinctions. Titan is the only body besides Earth known to have stable bodies of liquid on its surface. It harbors lakes of methane and seas of hydrocarbon, along with an underground ocean of an unknown liquid. It contains mountains and dunes, but the dunes are made of solid hydrocarbons, a plasticlike substance. It also has rain storms and canyons that flood. “Titan has the possibility for life as we know it — and perhaps for life as we don’t know it,” says Hayes. “Whether or not you need liquid water is an interesting question, and you cannot help but ask whether or not some exotic form of life may have developed in these hydrocarbon lakes and seas.”
Ideas to explore Titan: a boat? an orbiter? a balloon-boat orbiter combo? Eight years ago, an international team designed the Titan Saturn System Mission, a joint NASA/European Space Agency (ESA) initiative to send a balloon-boat orbiter combo to the ocean world. With goals of taking high-resolution photos of Titan from above and extracting samples from the atmosphere and one of its seas, the mission was floated for a 2020 launch. Since then, the Titan Mare Explorer (a boat mission to Titan’s Ligeia Mare), the Oceanus orbiter, and the Dragonfly drone have been proposed. All these potential ventures share the same essential objective: to uncover more information about Titan and gather samples that could reveal details on its lakes and seas, the currents within them, and the chemical composition of its dunes, rain and atmosphere.
On Europa, a search for hydrothermal vents. Europa, a moon of Jupiter that’s slightly smaller than our moon, has been found to have a curious, time-varying magnetic field, which scientists say indicates it harbors a salty, underground ocean ten times deeper than the deepest area of Earth’s oceans. Thermal vents like the ones on our planet, which are found up to 1.5 miles underwater, might exist on Europa, too, according to information gathered by the Voyager and Galileo spacecrafts. Images show “the surface [of Europa] is covered by fracture patterns that crisscross all over the surface,” Hayes says. “They’ve been formed as Europa is being pushed and pulled by Jupiter in an eccentric orbit.” This constant pushing and pulling creates heat. And if Europa has a stable source of heat beneath its global ocean — which appears to have three times more water than all of Earth’s oceans combined — it could contain yet-uncovered pockets of life.
On the horizon: the Europa Clipper mission. This NASA mission is intended to attain a better picture of Europa’s landscape and gather reconnaissance for possible future landings … and maybe even human habitability. Hayes is one of the scientists working to develop the project. “We’re running all of these models right now to say, ‘These are the kind of images we can acquire from this orbital trajectory,’ and then taking the data we think we can acquire based on the position of the spacecraft and doing a tour assessment,” he says.
With so many efforts under way, Hayes is optimistic about what researchers could find. “I think it’s safe to say that we live in truly extraordinary times,” he says. “We have the capability to look for signs of life in the ocean worlds of the outer solar system; all we need is the will to move forward and progress past these recent discoveries and move on to other ones. We live in an era where we could see evidence in our own lifetimes for a second genesis of life in our solar system.”