For nearly two decades, the prime location for finding life beyond Earth was a truly alien world: Titan, Saturn’s largest moon. Beneath its thick orange atmosphere — comparable to Earth’s — and its lakes and seas of methane, a global ocean of liquid water seemed to lie hidden, buried under kilometers of ice. Now, a new study led by NASA scientists forces us to rethink this image: Titan doesn’t harbor a subsurface ocean, but rather an immense layer of warm, partially melted ice. Contrary to what one might think, the chances of life existing in this environment “multiply.”
“The biggest implication of this finding is the existence of very different environments within extraterrestrial worlds, compared to what we thought a few years ago,” Flavio Petricca, a NASA researcher and lead author of the study, explained to this newspaper.
The conclusion stems from a meticulous re-examination of data from the Cassini probe, which orbited Saturn and flew past Titan multiple times between 2004 and 2017. The gravitational measurements from this robotic spacecraft were interpreted as unmistakable evidence of an ocean beneath the icy surface. The satellite responded excessively to Saturn’s enormous gravitational force, deforming as a sphere with a liquid layer inside it would. But not all the measurements captured by the spacecraft could be true at the same time. “This is the first time we’ve been able to reconcile all the data with an accurate model of Titan’s interior,” Petricca summarizes.
The key to the new study lies in examining how long it takes Titan to respond to Saturn’s gravitational pull. If a global ocean existed, the satellite’s deformation would be almost instantaneous, like the tides that follow the Moon’s passage across Earth’s oceans. However, by applying new processing techniques to Cassini’s radio data, the team has detected a delay.
“If you were standing on the surface of Titan and Saturn passed overhead, the ground beneath your feet would only begin to rise about 15 hours later,” Petricca explains. “This is a strong indication that Titan’s interior is made of a ”slushy high-pressure ice layer” with widely distributed pockets of water, rather than a deep, interconnected global ocean.” This slushy ice would dissipate the energy transmitted by Saturn in precisely the observed amount.
The results paint a picture of Titan’s interior structure radically different from what had been imagined. The ocean is actually a “hydrosphere” some 550 kilometers (340 miles) deep, made mostly of high-pressure ice, and containing countless pockets of liquid water whose interior can reach temperatures of 20 degrees Celsius. Although these pockets do not connect to form a global ocean, the total volume of liquid water within these reservoirs could be comparable to that of the entire Atlantic Ocean, according to NASA researchers.
On Titan’s surface, there are rivers, lakes, and seas of liquid methane and ethane. Its atmosphere features clouds, rain, seasons, and complex chemical processes reminiscent of early Earth. Cassini revealed hydrocarbon dunes, polar seas, and a rich organic chemistry capable of producing molecules that could be precursors to life, and even vesicles that might be the first step toward the formation of living cells, according to recent studies by NASA itself.
For years, the supposed subsurface ocean of saltwater completed that picture: a world with energy, chemistry, and liquid water — the three classic ingredients of habitability. The new study doesn’t eliminate water from the picture, but it redistributes it. Instead of a continuous ocean, it proposes countless isolated niches, somewhat like the ecosystems that thrive in the sea ice of Earth’s polar regions.
These environments “could be especially interesting for astrobiology,” explains Antonio Genova, a researcher at Sapienza University of Rome and co-author of the study, published this Wednesday in Nature. “Pockets of liquid water embedded in the ice can concentrate salts and organic molecules, creating chemically rich liquid solutions. Strong convection could transport these bubbles up and down, connecting the rocky ocean floor” with the organic material abundant in the surface lakes and rivers, he notes. This is something never before seen on a moon in the solar system.
This hypothesis can be tested thanks to the Dragonfly mission, a NASA drone that will explore the surface of Titan in the 2030s. In addition to studying organic chemistry, this spacecraft will carry a seismometer that will observe the moon’s interior. Thanks to missions like this, or the Clipper probe, which will reach Europa, Jupiter’s moon, in 2030, “the next decade will mark a turning point in our search for potentially habitable environments in the solar system,” Petricca ventures.
Rosaly Lopes, a NASA researcher and one of the leading proponents of the ocean theory, highlights the value of the new study, in which she did not participate. “They’ve done a great job,” she says. But she adds an important point: “This analysis focuses on Titan’s present state, but it’s very likely that in the past there was indeed an ocean of liquid water that froze over time due to the low temperatures.” The specialist believes that warm ice “increases the chances of bacteria being able to survive in these environments.” “The explanation is that in an open ocean, organic material would be very diluted, while in these pockets it would be much more concentrated,” she proposes.
Astronomer Noemí Pinilla-Alonso, an expert on icy moons, comments on the impact this study could have on our understanding of these bodies. “Titan is sufficiently different from Enceladus [another of Saturn’s moons] and Europa that this doesn’t call into question whether they are ocean worlds,” says the researcher from the Institute of Space Sciences and Technologies of Asturias, at the University of Oviedo. “Rather, it shows us that there is a threshold between a large icy satellite and an ocean world, and that if Titan, like Ganymede [Jupiter’s moon], once had an ocean, the dissipation of the energy deposited by Saturn is insufficient to prevent its progressive freezing, resulting in the current succession of ice layers near the melting point,” she adds.
Juan Luis Rizos, a researcher at the Institute of Astrophysics of Andalusia, highlights another implication of the study. Titan’s orbit is slightly eccentric, elliptical in shape, but it is becoming increasingly circular as the moon moves away from its planet at a rate of 11 centimeters per year. The presence of “hot” ice and its ability to dissipate Saturn’s gravitational pull means that it will have a perfectly circular trajectory in “about 30 million years,” a mere blink of an eye compared to typical timescales in the solar system.
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