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SYFY WIRE Bad Astronomy

A Saturn Moon Erupts

By Phil Plait

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. Itâs a moon of Saturn, a scant 500 kilometers (310 miles) across, roughly the size of my home state of Colorado. Itâs an icy moon, and itâs 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 moonâs inner structure isnât 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 youâll 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 moonâs south poles, called âtiger stripesâ due to their resemblance to the big catâs camouflage. Their technical name, which strikes me as just as fanciful, is âsulciâ (Latin for âfurrowâ).

In this picture weâre 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.

If youâre wondering why then you can see the moonâs surface at all, itâs because itâs 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 moonâs ice.

The geysers were discovered in Cassini pictures in 2005, shortly after the spacecraft arrived at the ringed planet. Since then weâve 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 isnât 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. Itâs 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 itâs gone to Saturn and stayed there. Over the years itâs amassed a vast amount of information, and weâve been able to adapt the mission to accommodate what weâve 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.

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