Bad Astronomy

A Saturn Moon Erupts

Enceladus and geysers
Cryogeysers erupt from the south pole of the tiny moon Enceladus.

Photo by NASA/JPL-Caltech/Space Science Institute

Over a billion kilometers from Earth, dark forces are at work.

They reach out from deep inside a planet, plunging into and manipulating the heart of another nearby world. Gently, perhaps, on a cosmic scale, but vast and inexorable by that of humans, these forces alternately squeeze and stretch the inky depths. Implacable, they have wrought their damage for eons, until the very body of the world itself could take no more. The stress finally took its toll, and the small world weepingly burst forth with frigid tears.

And the Cassini spacecraft has a front row seat.

That is Enceladus, the world in question. It’s a moon of Saturn, a scant 500 kilometers (310 miles) across, roughly the size of my home state of Colorado. It’s an icy moon, and it’s not solid: The ice is probably a thick shell surrounding a vast undersurface ocean of water, which itself surrounds a small rocky core.

This knowledge of the moon’s inner structure isn’t exactly guesswork. We have some proof in the form of the Cassini image above. The roughly spherical shape of the moon itself is obvious enough, but you’ll note the broad, fan-like splash of light beneath it. Those are geysers, literally, fountains of liquid water erupting out from the surface of Enceladus. They emerge from deep, long cracks at the moon’s south poles, called “tiger stripes” due to their resemblance to the big cat’s camouflage. Their technical name, which strikes me as just as fanciful, is “sulci” (Latin for “furrow”).

In this picture we’re looking down on the night side of Enceladus; the Sun is almost directly opposite, so if you were standing in the middle of Enceladus in this shot it would be midnight. The plumes of water from the pole stretch so far into space that they breach into direct sunlight, illuminated brilliantly compared to the relatively darker surface.

Enceladus close-up
An image of Enceladus by Cassini taken in 2008. In this much higher-res shot you can see the folds and cracks in the surface ice.

Photo by NASA/JPL-Caltech/Space Science Institute

If you’re wondering why then you can see the moon’s surface at all, it’s because it’s being lit softly by Saturn itself! To someone standing on Enceladus in this picture, Saturn would loom hugely in the sky, vastly bright, like the full Moon seen here on Earth but hundreds of times bigger. The reflected sunlight is enough to fill in the shadows for Cassini, and reveal some hint of features in the moon’s ice.

The geysers were discovered in Cassini pictures in 2005, shortly after the spacecraft arrived at the ringed planet. Since then we’ve learned much about them. From afar we could tell they were mostly water, and made of tiny particles about a micron (one-millionth of a meter) in size. But then scientists did something amazing: They flew Cassini into the plume, to directly sample the material. It found that there were organic molecules in them! Mind you, this isn’t life, but the basic carbon-based molecules that are the fundamental building blocks of life: acetylene, hydrogen cyanide, formaldehyde, and more.

As for the cause, the answer is Saturn itself. The orbit of Enceladus around Saturn is slightly elliptical, so the gravity it feels from the planet as it orbits changes. The change in gravity is called a tide, and it flexes the little moon. This can create a huge amount of heat in the interior, enough to melt the ice and to cause cracks in the surface through which the water can spew. It’s a bit weird to think that an invisible force can reach through the surface of Enceladus and modify it so much, but then, that same force is keeping you pinned to the Earth while you read this. Its reach is subtle, but long

Cassini has been so successful in part because it’s gone to Saturn and stayed there. Over the years it’s amassed a vast amount of information, and we’ve been able to adapt the mission to accommodate what we’ve learned, so that we may learn more. To me, that fact in and of itself says as much about us as the geysers do about Enceladus. Our own forces run deep as well.