Enceladus: a song of ice and tides

An artist impression of Cassini diving into Enceladus water plumes. credit: NASA/JPL

Cassini will terminate its 20-odd-years-long mission in September. But it’s determined to go out with a bang. In yesterday’s press conference, NASA announced that the probe, during a 2015 flyby of Saturn’s moon Enceladus, found clues that the ocean within the icy moon has almost all we think it needs to spark life.

Enceladus is a fascinating world, with an ice version of Earth’s tectonic activity. Like Earth, Enceladus has volcanoes on its surface, but they spew water, which is what Cassini investigated. Instead of magma, in fact, its surface floats on a gigantic salty ocean. This ocean, NASA announced, seems now the place to go look for life in our solar system.

An illustration of the interior of Enceladus: its icy crust, rocky core and liquid ocean in between. credit: NASA/JPL-Caltech

But that is way off the habitable zone! Shouldn’t Enceladus and all the other ocean worlds be frozen solid all the way through?

They might just escape an icy death using an unusual tool: tides. For astronomers, tides are the difference between gravitational pulls on different sides of a planet or moon. For example, one side of Enceladus is closer to Saturn than the other, so it feels a little more gravity. Since it pulls more on one side than the other, the tidal force stretches Enceladus.

As Enceladus moves around, it gets stretched and pulled in ever-changing ways. So its crust and its interior have to rearrange themselves all the time under this force, parts move and slip on each other. The friction warms the planet up, in a process called tidal heating.

It happens to Earth as well, of course, with tidal forces from the Moon and the Sun. But our planet has an enormous amount of heat left over from its formation, enough to melt rock into magma. Tidal heating doesn’t do much here.

To keep this heat in, Enceladus’ ocean has another unusual ally: the kilometers-thick ice crust over it. Ice is a pretty good thermal insuator, and acts like a giant blanket around the ocean, keeping the freezing void out and precious heat in.

We always focus on what sort of atmosphere planets must have to harbor life, or how far they have to be from cold, dim stars. But it might turn out that a big fat ice cover and powerful tides might go quite some way.

If you want more
  • NASA, as usual, put together a great package with al lot of info on Enceladus and other ocean worlds
  • While sufficient to keep oceans all over some of Jupiter’s moons, tidal heating doesn’t seem to be enough to maintain an ocean around all of Enceladus

Cover photo: CC0 Tilgnerpictures/pixabay

Mercurial sunset

Nothing’s more naturally predictable than the progression of a day, right? The Sun comes up in the East, rises through the sky, then sets in the West. That’s part of the fascination of places where it isn’t quite so.


However, one of the places with the craziest days is rather unaccessible: Mercury.

It’s the closest planet to the Sun, so it feels the star’s gravitational pull the most. This formidable force put Mercury in what’s called a tidal lock. Think of the Moon: it turns around itself as fast as it revolves around the Earth, showing us always the same face.

Mercury’s situation is similar, but more interesting: the ratio between orbit time and rotation, instead of 1:1 (as it is for the Moon), is 3:2. In other words, the planet turns one and a half times per (local) year, and the Sun takes two (local) years to do one full round through the sky.

To complicate matters, Mercury speeds up and down a lot orbiting the Sun—because its trajectory isn’t very round—but rotates on its axis always at the same time. As a consequence, sometimes the rotation drives the Sun’s apparent motion (as it is on Earth), sometimes it’s orbital motion. Which, needless to say, is weird.

This makes for very peculiar (and rather long) days. If we stood on Mercury, we’d see the Sun rise through the sky as usual. But after a while, once the orbital speed gets high enough, we’d see the Sun stop, and even go back for a while! Then, as rotation goes back in the driver’s seat, the Sun would resume its previous route and set normally.

There’s a cool animation of the day on Mercury in this episodes of Crash Course Astronomy.

Everywhere in the universe there is a kind of magic.


Cover photo: Wind in the Desert With Sun Flare, CC-BY-NC-ND Bill Gracey, via Flickr. Some rights reserved