In Memory of Cassini

September 20, 2017

Last week, NASA’s Cassini Mission came to an end when the spacecraft crashed into the planet Saturn. This was, of course, a planned event: a way for the mission to end in a blaze of glory, collect a little extra data about Saturn’s atmosphere, and also protect Saturn’s potentially habitable moons (Titan, Enceladus, and possibly also Dione) from microorganisms that may have hitched a ride from Earth aboard the spacecraft.

Cassini’s last few days were an oddly emotional time, at least for me. Somehow knowing that the end was coming, that everything was proceeding according to schedule, made it a little harder to bear. When the words “data downlink ended” started appearing in my Twitter feed, I got a little misty eyes and had to walk away from the computer for a while.

This despite the fact that I never got to know Cassini all that well. I never really followed the Cassini Mission closely (especially compared the way I follow Juno). Looking back through my old posts, it seems Cassini only ever appeared on this blog twice. Once for that time it spotted sunlight glinting off the surface of Titan’s methane lakes…

… and once more for the time it used precise measurements of Enceladus’s librations to determine that Enceladus does indeed have an ocean of water beneath its crust.

So today I thought I’d turn the floor over to several of the moons of Saturn and also Saturn herself. They’re the ones who got to know Cassini well. Not me. It’s right that they get the chance to give Cassini’s eulogy.

Meet a Moon: Dione

October 24, 2016

Regarding Greek mythology, it seems no one’s really sure who Dione was. Ancient sources contradict each other, and modern scholars think there may have actually been more than one mythical woman who went by that name. But I was able to find out this much: according to Wikipedia, at least one of these Diones was “sometimes associated with water or the sea.”

What that in mind, I’d like to introduce you to Dione, one of Saturn’s moons.


You sure are, Dione. In fact, I can’t think of a better description for you.

The Waters of Enceladus

Over the last decade or so, one of Saturn’s other moons has become famous for having an ocean of liquid water beneath its surface. That moon is called Enceladus. We know about Enceladus’s water for two reasons:

  • Geysers: Enceladus has a series of cracks (called tiger stripes) in its south polar region, and saltwater shoots out of these cracks at regular intervals.
  • Libration: Enceladus wobbles in place (librates) more than it should. This is best explained by the presence of a layer of liquid separating the moon’s crust from its core.

It’s still a mystery how Enceladus generates enough heat to keep its liquid water from freezing, but at this point, it’s pretty clear the water is there.

The Waters of Dione

Dione doesn’t librate the way Enceladus does, and we haven’t noticed any saltwater geysers, but a recent paper in Geophysical Research Letters says Dione might have a subsurface ocean too.

The authors of the paper created a new theoretical model for icy moons, a model which fits precisely with observations of Enceladus. Then they applied this new model to Dione and concluded that Dione should have a subsurface ocean.

This raises two questions that are fairly easily answered.

  • Where are Dione’s geysers?: Dione may not spew saltwater (anymore), but it does have cracks and fissures in its surface, suggesting that it may have had active “tiger stripe” geysers in the past.
  • What about Dione’s libration?: The new model suggests that Dione should librate, but not as much as Enceladus does. The Cassini spacecraft (currently orbiting Saturn) does not have instruments sensitive enough to detect the predicted libration.

So there you have it. According to at least one theoretical model, Dione should have a subsurface ocean, but we cannot yet confirm that it does. And it’ll probably be awhile before we can send a new spacecraft to Saturn to find out one way or another.

But hey, how appropriate is it that we named this moon, which might have a subsurface ocean, depending on your theoretical model, after a mythical figure that might sometimes have been associated with water, depending on which ancient sources your reading!