NASA Study Suggests Saturn’s Moon Titan May Not Have Global Ocean
A key discovery of NASA’s Cassini mission in 2008 was that Titan, Saturn’s largest moon, could harbor a vast ocean of water beneath its hydrocarbon-rich surface. But a new analysis of the mission’s data suggests a more complex situation: Titan’s interior is more likely composed of ice, with layers of melting snow and small pockets of warm water forming near its rocky core.
Led by researchers at NASA’s Jet Propulsion Laboratory in Southern California and published Wednesday in the journal Nature, the new study could have implications for scientists’ understanding of Titan and other icy moons in our solar system.
“This research highlights the power of planetary science archives. It is important to remember that the data collected by these amazing spacecraft is alive, so discoveries can be made years, or even decades, later, as analysis techniques become more sophisticated,” said Julie Castillo-Rogez, principal research scientist at JPL and co-author of the study. “It’s the gift that keeps on giving.”
To remotely probe planets, moons and asteroids, scientists study radio frequency communications that flow between spacecraft and NASA’s Deep Space Network. It’s a multi-layered process. Since a moon’s body may not have a uniform distribution of its mass, its gravity field will change as a spacecraft passes through it, causing it to speed up or slow down slightly. In turn, these speed variations change the frequency of radio waves entering and exiting the spacecraft – an effect known as Doppler shift. Doppler shift analysis can provide insight into a moon’s gravity field and its shape, which can change over time as it orbits under the gravitational pull of its parent planet.
This change in shape is called tidal bending. In Titan’s case, Saturn’s immense gravitational field squeezes the moon when Titan is closest to the planet during its slightly elliptical orbit, and stretches the moon when it is furthest away. Such bending creates energy which is lost or dissipated in the form of internal heating.
When mission scientists analyzed radio frequency data collected during the now-retired Cassini mission’s 10 close approaches to Titan, they found that the moon was flexing so much that they concluded it must have a liquid interior, since a solid interior would have flexed much less. (Think of a water balloon versus a billiard ball.)
The new research points to another possible explanation for this malleability: an interior made up of layers with a mixture of ice and water that allows the moon to flex. In this scenario, there would be a lag of several hours between Saturn’s tidal pull and when the Moon shows signs of bending – much slower than if the interior were entirely liquid. A muddy interior would also exhibit a stronger energy dissipation signature in the moon’s gravity field than a liquid interior, because these layers of melting snow would generate friction and produce heat as the ice crystals rub against each other. But nothing appeared in the data to suggest that this was happening.
So the study authors, led by Flavio Petricca, a postdoctoral researcher at JPL, took a closer look at the Doppler data to understand why. By applying a new processing technique, they reduced the noise in the data. What emerged was a signature revealing a severe loss of energy deep within Titan. The researchers interpreted this signature as coming from layers of melting snow, covered by a thick shell of solid ice.
Based on this new model of Titan’s interior, the researchers suggest that the only liquid would be in the form of pockets of meltwater. Heated by the dissipation of tidal energy, the pockets of water slowly move toward the frozen layers of ice on the surface. As they rise, they have the potential to create unique environments enriched by organic molecules delivered from below and by materials delivered via meteorite impacts on the surface.
“No one expected very strong energy dissipation inside Titan. But by reducing the noise in the Doppler data, we were able to see these smaller vibrations appear. This was the smoking gun that indicates that Titan’s interior is different from what had been inferred from previous analyses,” Petricca said. “The low viscosity of the melting snow still allows the Moon to expand and compress in response to Saturn’s tides, and to release heat that would otherwise melt the ice and form an ocean.”
“Although Titan does not have a global ocean, that does not preclude its potential to host basic life forms, assuming life can form on Titan. In fact, I think that makes Titan more interesting,” Petricca added. “Our analysis shows that there should be pockets of liquid water, perhaps as hot as 20 degrees Celsius (68 degrees Fahrenheit), circulating nutrients from the moon’s rocky core through layers of melting ice at high pressure to a solid icy shell on the surface.”
More definitive information could come from NASA’s next mission to Saturn. Launching no earlier than 2028, the agency’s Dragonfly mission to the hazy moon could provide the ground truth. The first rotorcraft of its kind will explore the surface of Titan to study the habitability of the Moon. Equipped with a seismometer, the mission could provide key measurements to probe Titan’s interior, based on seismic events that occur while it is on the surface.
The Cassini-Huygens mission was a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. A division of Caltech in Pasadena, JPL managed the mission on behalf of NASA’s Space Mission Directorate in Washington and designed, developed and assembled the Cassini orbiter.
To learn more about NASA’s Cassini mission, visit:
https://science.nasa.gov/mission/cassini/
Contacts with news media
Ian J. O’Neill
Jet Propulsion Laboratory, Pasadena, California.
818-354-2649
ian.j.oneill@jpl.nasa.gov
Karen Fox / Alana Johnson
NASA Headquarters, Washington
202-358-1600 / 202-358-1501
karen.c.fox@nasa.gov / alana.r.johnson@nasa.gov
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