Monday, November 08, 2010

Moon of the Month: Miranda



In astronomy, Uranus is perpetually the butt of jokes. The name is funny, but to compound this, the planet is made of methane, the primary substance of farts. Yes, Uranus really is a big, smelly, butt planet.

Astronomers don't feel much pity for the planet either. As Voyager 2 cruised through the solar system, scientists were delighted by beautiful images of Jupiter and Saturn--bands of color, complex ring systems. Then they arrive at Uranus to see this:



Sure it's serene and pretty, but there's no reason to take close-ups. However, the visit wasn't a complete disappointment; Uranus' saving grace is its moon system. It's got 27 of them, five of which are "major" moons. These were the first moons for whom the Greco-Roman naming trend was broken; they have the names of Shakespearean characters.

The first of Uranus' moons were discovered by astronomy's number one badass--William Herschel. He thought he'd discovered six moons, though only two of them bore properties that matched later observations. Miranda itself was discovered by Gerard P. Kuiper; he has the entire belt of debris in the outer solar system named after him.

Most of Uranus' moons are relatively plain looking--smooth balls with craters. Voyager 2 snapped a few pictures of them, and that's about it. That's all we know about them.

Then there's Miranda, the bad nut. Voyager snapped pictures of it too, revealing giant regions of scarps bisecting the moon's surface. Some of these scarps are estimated to be 3 miles high, such as Verona Rupes:



It's on the bottom left of the image, the lighter part. That is a vertical outcrop that ascends three miles. It sounds like a lovely place for basejumping, except that without an atmosphere, a parachute would be worthless.

Scarps are usually caused by some form of cooling. When most materials cool, they shrink. Because the inside of the body shrinks but the outside is already solid, the surface cracks and forms ridges.

This brings back a familiar question--where did the heat come from and why did it go away? It's suspected that Miranda used to be in an orbital resonance with another one of Uranus' moons--that is, they both interacted with each other gravitationally. This forced Miranda to have a more eccentric orbit than it presently does, inducing tidal heating.

Since then, Miranda lost its resonance. Its orbit became less eccentric, and without a source of heat, it cooled down, forming its present strange shape.

Other theories suggest that an earlier incarnation of Miranda was destroyed and reformed with its current shape. The truth may be some combination of both theories; such an impact may have heated Miranda as well.

As for knowing more, it will be quite some time. There are no plans in place to further explore Uranus. Although politicians and scientists may have a private desire to do so, there are places that are easier to reach and are arguably equally or more interesting. As a result, the Uranian system will remain unpenetrated for sometime more.

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