Cassini data sheds light on Saturn formation and Enceladus habitability

by Chris Gebhardt

While its mission ended just over three years ago, data from NASA’s Cassini spacecraft continues to help scientists unlock the mysteries of the Saturnian system that contains more than 80 identified moons. 

In particular, new and updated research has helped determine the drift rate of Titan away from Saturn, in turn leading to a better understanding of how the ringed planet system formed, while a completely different set of analyses revealed the surface ages of Enceladus and provided key insights into the moon’s potential to harbor life.

Drift, drift, drift away

Moons drift away from their host planets.  This is a fact of life for various bodies in the solar system.  Earth’s Moon drifts away from it at roughly 3.8 cm (1.5 inches) per year. 

Using models to estimate how fast a moon drifts from its planet, the theories assumed that in systems such as Saturn’s, with dozens of moons, the outer moons like Titan migrated outward more slowly than moons closer in because they are farther from their host planet’s gravity.  Therefore, scientists calculated the rate at which they believed Saturn’s moon Titan was moving away from it, a calculation that came in at 0.1 cm (0.04 inches) per year.

However, data from Cassini — which ended its mission in September 2017 — reveals that Titan is moving away from Saturn at a rate of 10.16 cm (4 inches) per year, one hundred times faster than previously predicted.

“This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturn system and how its moons formed,” said Dr. Valery Lainey, lead author of the study published earlier this year in Nature Astronomy.

Titan’s new drift rate was calculated using radio data obtained by Cassini over a 10-year period from 10 close flybys between 2006 and 2016.  Corroborating data was compiled using star maps in the background of Cassini images as the craft tracked Titan’s position.

Titan, backdropped by Saturn on 6 May 2012. (Credit: NASA/JPL-Caltech/Space Science Institute)

“By using two completely different datasets, we obtained results that are in full agreement, and also in agreement with Jim Fuller’s theory, which predicted a much faster migration of Titan,” said Dr. Paolo Tortora, of Italy’s University of Bologna — co-author of the study.

Given the new understanding of Titan’s drift from Saturn and the fact that it is currently 1.2 million kilometers (759,000 miles) from the planet, this would indicate the moon formed much closer to Saturn than originally believed and that the entire system of moons around the planet expanded outward more quickly than previously predicted.

Moreso, the updated drift rate provides confirmation to a new, four year old theory from theoretical astrophysicist Dr. Jim Fuller who proposed that the outer moons of multiple-moon planetary systems can drift at the faster rates of the more inner moons because of a “different orbit pattern that link to the particular wobble of its host planet.”

As a multi-moon planet is tugged and stretched and relaxed by the gravitational pull of their moons, the planet can “wobble” — with some of that energy transferred to the outer moons (like Titan) of the system.

“The new measurements imply that these kind of planet-moon interactions can be more prominent than prior expectations and that they can apply to many systems, such as other planetary moon systems, exoplanets — those outside our solar system — and even binary star systems, where stars orbit each other,” said Dr. Fuller, another co-author of the study.

Enceladus’ young surface age and the search for life

Just as with Titan and numerous other elements of the Saturnian system, data from Cassini’s 13 year mission at the planet continues to reveal new and intriguing elements about its moons, in this case Enceladus.

Shortly after its arrival in the system, Cassini imaged large geyser-like ejections from the southern hemisphere of Enceladus, later confirmed to be ice particles and vapor from a subterranean ocean.

The discovery quickly catapulted Enceladus to near the top of the list of places in the solar system where life might exist other than Earth and made the moon a prime target for follow-up investigations by Cassini as well as proposed, future missions from NASA and other space agencies.

Now, using data collected from Cassini’s multiple encounters with Enceladus, scientists have produced a composite, detailed global infrared map that has revealed strong evidence for geologic activity in the northern hemisphere. 

The newly compiled infrared map reveals signatures strongly suggesting new geologic activity and a younger surface near the south pole’s tiger stripe gashes — as expected given the resurfacing events that routinely occurred due to the geyser-like ejected material falling back to the moon’s surface. 

Intriguingly though, the same spectral data revealed strong evidence of young surface features in the northern hemisphere, suggesting fresh ice coverage and recent geologic activity of some kind on the opposite side of Enceladus. 

“The infrared shows us that the surface of the south pole is young, which is not a surprise because we knew about the jets that blast icy material there,” said Gabriel Tobie, Visible and Infrared Mapping Spectrometer (Cassini instrument) scientist with the University of Nantes in France and co-author of the new study published in the journal Icarus.

“Now, thanks to these infrared eyes, you can go back in time and say that one large region in the northern hemisphere appears also young and was probably active not that long ago, in geologic timelines.”

This new understanding of the geologic age of the northern sections of Enceladus could prove useful not just in understanding the moon’s evolution but also in providing options for potential future missions that would seek to categorize the nature of the organic compounds Cassini discovered in the ice particles being ejected from the moon. 

The plumes of Enceladus. (Credit: NASA/JPL-Caltech)

Last year, scientists discovered new kinds of organic compounds — potential precursors to amino acids, one of the building blocks of life — in the ejecta material being thrown from Enceladus’ interior.

“If the conditions are right, these molecules coming from the deep ocean of Enceladus could be on the same reaction pathway as we see here on Earth,” said Dr. Nozair Khawaja, who led the research at the Free University of Berlin.  “We don’t yet know if amino acids are needed for life beyond Earth, but finding the molecules that form amino acids is an important piece of the puzzle.”

The results showed that the organics first dissolved in the ocean of Enceladus, then evaporated from the water surface and condensed and froze into ice grains inside the fractures in the moon’s crust before being blown out into space by an ejecta plume.

“This work shows that Enceladus’ ocean has reactive building blocks in abundance, and it’s another green light in the investigation of the habitability of Enceladus,” said co-author Dr. Frank Postberg.

Related Articles