September 6, 2017

Cassini: Destination Moons

In the space of 13 years, Cassini observed Saturn and devoted a lot of attention to its moons. With the mission now about to end, Patrick Michel, astrophysicist, planetologist and research director at the Côte d’Azur Observatory in Nice, reflects on the discoveries it has made about the planet’s natural satellites.

Planet formation model

Most of Saturn’s moons formed from its icy rings through a process of accretion. Despite this common origin, they are also very diverse. Titan is large enough to have retained an atmosphere, indeed it is the only moon with liquid on its surface in the form of hydrocarbon seas. Enceladus has geysers and a liquid ocean under its thick icy crust. Pan and Atlas stand out due to their equatorial ridge in the plane of the rings, while Iapetus has a bright and a dark side, almost certainly made up of organic molecules.

“The size and shape of each moon give us precious clues,” explains Patrick Michel. “Most medium-sized moons are pretty spherical, so their resistance has to be fairly low for gravity to dominate. For smaller moons, the opposite is true: they are between 200 and 700 kilometres in diameter and have varying but relatively low densities, so their gravity is too weak to overcome their resistance, which explains their very irregular shapes.”


Two perspective views of the tiny moon Pan (34 km in diameter). Credits: NASA/JPL/Space Science Institute

As Sébastien Charnoz explains in this article on Saturn’s rings, one of the reasons for studying Saturn is to find out how moons are still forming today from its rings:
 “The idea is to retrace the history of Saturn and our solar system,” adds Patrick Michel.

The primitive nebula in which our Sun formed contained a cloud of dust from which the planets accreted, while Saturn’s rings and magnetic field perturb this disk, like the Sun. Moons and planets accrete in similar ways, but the analogy of the Sun and its disk isn’t as easy to extrapolate to Saturn and its rings as we might have thought at the start of the mission. If Saturn was born without its disk and its rings appeared later, things might not have happened as they did with the protoplanetary disk. And of course the rings consist of water ice, whereas the protoplanetary disk consisted of gas and dust whose composition depended on its distance from the Sun.”

Iapetus (1,500 km across) and its equatorial ridge. Credits: NASA/JPL/Space Science Institute

Trails in the rings

Saturn currently has 53 confirmed moons and 9 more provisional, as yet unnamed moons, making 62 in total. The 9 provisional moons have been deduced from the waves they produce in the surrounding rings, but not observed directly. In other words, their trail may be visible, but not the moons themselves. 

“If a moon is detected as a result of the influence it exerts on the rings, we can deduce its size but not its physical properties,” explains Patrick Michel. “That’s the difference between extrapolation and actual observation. The criteria for confirming a moon are very specific. In particular, we need to establish a precise orbit. So, we can’t name a moon until we’ve seen it. For this and other reasons, it would be useful to send a return mission to Saturn using the technologies we have today, as with higher resolution we would be able to see smaller objects that might be creating disturbances in the rings.”

The moon Prometheus (135 km in diameter) and the disturbances it creates in the F ring, August 2009. Credits: NASA/JPL/Space Science Institute

Beyond Cassini

In discovering new moons, analysing their composition, dynamics and formation, Cassini has greatly advanced our understanding of Saturn’s natural satellites, particularly its largest moon, Titan, where it landed the Huygens probe in January 2005. 
“Landing on Titan was a huge feat for Europe,” says Patrick Michel.

It was an unknown world. We knew it had an atmosphere, but so thick that we knew nothing about its surface. Entering that atmosphere and seeing Huygens land as if we were on the probe was just astounding. ESA recently achieved the same feat landing Philae on comet Churyumov-Gerasimenko. That’s the advantage of planetology over astrophysics, as we can send missions to observe in situ. But these missions are so challenging and costly that space agencies are increasingly reluctant to take the risks that go with them. It would be a great shame to forgo such fantastic adventures that have the power to inspire youngsters and dare them to pursue their dreams.”

So, what lies ahead for Saturn exploration? Several projects are under study to return to Titan and land on Enceladus. NASA is notably looking at a submarine to explore Titan’s methane lakes. But while no mission has yet been decided to succeed Cassini, the spacecraft’s destruction in Saturn’s atmosphere isn’t the end of its contribution to science.
“There was all the work to build the Cassini spacecraft and send it on its way; now, the data it obtained will be keeping scientists busy for years and continue to fuel new discoveries. And Cassini will continue to support scientists’ careers and inspire future generations, encouraging us to renew such missions.”

More information

Science contacts

  • Francis Rocard, head of CNES’s solar system programme
: francis.rocard at
  • Patrick Michel, astrophysicist, planetologist and research director at the Côte d’Azur Observatory in Nice
: michelp at

Titan (1500 km in diameter) ahead of Saturn as seen by Cassini, May 6th, 2012. Credits: NASA/JPL-Caltech/Space Science Institute/J.Major