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Cassini probes interior of Saturn's moon Dione

Morgan Rehnberg, Correspondent
Jun 19, 2015, 19:15 UTC

Sen—Cassini will never land on any object around Saturn, but that does not mean that it cannot peer deep beneath the surface of these remote worlds. Its target this past week: Dione, the planet’s fourth largest moon.

These observations are Cassini’s fifth encounter with Dione during its 11-year mission.

Although the large icy moons of Saturn appear broadly similar from the outside, their interiors are highly varied. Each contains a different proportion of ice to rock, indicated by its mean density. Tethys seems to be virtually devoid of rock; its density is that of ice. Enceladus, on the other hand, seems to have a relatively large rocky core. Past observations have indicated that Dione falls somewhere in between.

But it is not just how much rock is present, it is also how that material is distributed within the moon. At Enceladus it seems concentrated at the core, while within Rhea the ice and rock are evenly mixed.

In order to evaluate which scenario best describes Dione, astronomers need to peer below its surface. The key is gravity: with different densities, rock and ice do not exert the same amount of gravitational attraction on Cassini. By measuring minute changes in Dione’s gravitational field, scientists can begin to reconstruct the distribution of material within.

Cassini uses a phenomenon known as the Doppler effect to perform these measurements. Ever noticed how a siren changes in pitch as it passes by? That is the Doppler effect at work. As it swings by Dione, Cassini transmits a microwave signal back to Earth. During its traverse of the moon’s uneven gravitational field, the spacecraft’s speed changes slightly, which results in a subtly different frequency reaching Earth. Analyzing these signals enables an estimate of the gravitational field and thus the internal structure.

By better understanding the composition of these bodies, scientists hope to shed light on the formation of Saturn, its moons, and its rings.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.


Dione imaged during a close flyby on June 16, 2015 from a distance of approximately 77,000 km (48,000 miles). Image scale is 1,519 feet (463 meters) per pixel. Image credit: NASA/JPL-Caltech/Space Science Institute