Saturn Takes Center Stage

The planet Saturn reached opposition this past weekend, rising as the sun sets and making its closest approach to Earth this year.  The planet saunters across Virgo over the next few weeks: it’s the brightest object between the stars Porrima in Virgo and Denebola in Leo.  Saturn is truly one of the prettiest sights you can see with a telescope.

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As a gas giant and the second largest planet in the solar system, Saturn is much like Jupiter.  It has no solid surface.  It’s made of cold hydrogen and helium gas in its outer layers, with no solid surface.  Like Jupiter, it likely has a solid rocky core that’s 10x Earth’s mass.  And it has a strong magnetic field and a huge collection of 60 moons, almost as many as Jupiter.But Saturn differs from Jupiter and the other gas giants is ways that make it especially fascinating.

Its rings, for example.  While Jupiter and Uranus have faint rings, Saturn has the brightest and most complex ring system in the solar system.  No one knows for sure how or when the rings were formed.  They might have assembled with the planet 4.5 billion years ago.  Or they may have formed just 100 million years ago when a small moon or comet came too close to the planet and was ripped to pieces by tidal forces.  In any case, Saturn’s rings are made mostly from tiny ice particles that extend from 6,600 km to 120,000 km directly above the equator of the planet.  The whole set of rings, despite their extent, are just 10-20 meters thick.

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The gravity of the planet and several embedded “shepherd moons” segment the rings into amazingly complex filamentary structures.The rings are organized into four main sections labelled from A, the outermost ring, to D which is closest to the planet.  On a clear night with a small scope, you can see the A and B rings.  And you can see the narrow gap between these rings.  This gap is the Cassini division.

While it’s a mighty big planet, Saturn is just 30% as massive as Jupiter.  But it spans 84% of Jupiter’s diameter.  That means it’s far less dense than Jupiter, or any other planet in the solar system.  It has just 68% the density of water, which means it would float in the bathtub (if you had a big enough bathtub).

Saturn also rotates quickly… just once every 10 hours… so it’s flattened at the poles more than any other planet.  Although since the surface isn’t solid, it’s a little tricky to get a good estimate of the rotational period; astronomers figured out the true rotation period of the planet by measuring the magnetic field.

The face of Saturn shows a few faint bands, but few other features.  Though there is the matter of the strange white storm that appears on the surface every 30 years or so…

The “Great White Spot”, as it’s called was observed from Earth in 1876, 1903, 1933, and 1960, and 1990.  The spot is presumably a storm that assembles in the atmosphere or bubbles up to the outer layers.  It starts small, then spreads out for several weeks before disappearing.  It seems to correspond to summer solstice in Saturn’s northern hemisphere, and occurs one per revolution of the planet around the sun. The next appearance of this mysterious spot is due around 2020.

As for moons, well, Saturn has a gaggle of them, each as distinct as snowflakes.  Titan is the largest.  It’s bigger than our moon, and is large enough to have an atmosphere and lakes of liquid hydrocarbons.

But Enceladus, Saturn’s sixth largest moon, is perhaps the most intriguing.  Though its surface is cold, Enceladus has liquid water under its surface.  The water is warmed by tidal forces, and sometimes shoots out in plumeswhich were recently imaged by the Cassini space probe.  Warm liquid water suggests the interior of Enceladus is a good place to search for life in the solar system.


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