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

Why galaxies are flat (and Earth isn’t)

The universe teems with flat stuff. Most galaxies, including the Milky Way, are quite flat and (relatively) thin pancakes of stars. All planets of the solar system (real planets, not Pluto) orbit pretty much on the same plane. Unsurprisingly, it’s no coincidence.

The plane along which all (real) planets orbit around the Sun. credit: pics-about-space.com

The plane along which all (real) planets orbit around the Sun. credit: pics-about-space.com

Galaxies and star systems form the same way: coagulating clouds of gas—though at obviously different scales.

Imagine throwing a plume of gas or atoms in space. Push them in random directions: some one way, some another, some up, some down. Unless you cheated, they bump into each other and, because of gravity, clump together. When the atoms didn’t collide head-on (ie, most of the times), these clumps spin. Clumps themselves attract each other and collide into bigger spinning blobs.

After each collision, the atoms and chunks of atoms align, canceling out all of their opposing motion, but keep spinning (in fancy physics terms, it’s called angular momentum conservation). You can see the blobs in the video up here as a forming galaxy seen from “above”.

Slowly but surely, the whole cloud flattens to a plane. If it’s a galaxy, it forms stars on that plane, whereas in the Solar System it became that begot the orbital plane.

Other planetary systems and galaxies spin too, but each inclined its own way, because they formed from different clouds of gas.

A lot of galaxies, photographed by the Hubble Space Telescope. They’re spinning in every which way. Credit: NASA/wikimedia

But if planets and stars also form by congealing gas, why aren’t they flat as well?

The reason is that planets and stars are much denser than galaxies. Being closer to each other, their clumps of gas feel a stronger gravitational pull to the center of the blob, that wins over the mechanism that would keep them flat. So planets and stars become spheres.

Saturn formed across all the stages: most matter coalesced in the huge (clearly spherical) planet, but a little formed some of its many more or less round moons, finally the last faint leftovers ended up as the iconic, extremely flat rings.

Round, flat, round: Saturn, its rings, and four of its many moons. Credit: NASA/wikimedia

If you want more
  • A long but excellent post on planets, galaxies, and roundness by the great Neil DeGrasse Tyson
  • Minutephysics has a cool video that explains this more technically, and shows why it can only happen in a 3D universe


Cover photo: CC0 WikiImages/pixabay