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SYFY WIRE Bad Astronomy

Portrait of a Titanic World

By Phil Plait

Speaking of moons of outer planets â¦

The Cassini spacecraft has been orbiting Saturn since 2004. Since that time it has returned hundreds of thousands of images of the ringed world and its myriad moons, each one a work of natural art.

Perhaps the most intriguing of these worlds is Titan, Saturnâs largest moon (and the second largest in the solar system). Titan is fully 5,150 kilometers (3,200 miles) in diameterâbigger than Mercury! It has a thick atmosphere, with a surface pressure half again as high as Earthâs. The air there is mostly nitrogen, with a dash of methane and hydrogen, and itâs cold. Really cold: -180 degrees Celsius, or -290 degrees Fahrenheit.

Despite that, the atmosphere is dynamic and interesting. See for yourself: Here is an image taken by Cassini in late August 2012:

Isnât that spectacular? [Click to encronosenate.] I love the greyscale images from Cassini; the worlds it sees look brooding and more dramatic this way. I love how the moon is half-lit, the day-night border (what astronomers call the terminator) splitting it at an angle. We're seeing Titan from below, so to speak: the spaceraft was flying past the moon over its south pole, so it's like we're underneath it looking up.

This image was made using an infrared filter just a wee bit outside what the human eye can detect. This filter is special: The particular wavelength of light it lets through (938 nanometers if youâre curious) is where methane is transparent. Normally, methane is pretty good at absorbing light, but it lets light at 938 nm right through. Itâs like having a window that lets us see right down to the surface, which is normally shrouded in haze.

At the upper left you can some surface features on Titan, those faint grey streaks. Incredibly, those are sand dunes made of frozen hydrocarbon grains, blown by the Titanian wind, hundreds of meters across and hundreds of kilometers long!

Man, Titan is weird. But then, itâs an alien world.

What really caught my attention, though, was the swirl you can see just over the night side of the moon. This is Titanâs south polar vortex, a very odd wind pattern at the bottom of the moon. Itâs a giant cell of air with rising and falling circulation patterns, set spinning by the moonâs rotation. The whole thing takes about nine hours to rotate once. Hereâs an animation showing the vortexâs motion:

Mesmerizing, isnât it? The vortex actually sits at the top of Titanâs atmosphere, which is why you can see it in the Cassini picture at all: Itâs up high enough to still be lit by the rays of the Sun, when the surface it sits above is shrouded in darkness. Itâs like a mountaintop at sunset, still barely lit as the Sun dips below the horizon.

Images like this one help scientists analyze the goings-on with Titan. Weâve barely scratched the surface of this gigantic place. Why does the vortex exist at all? Whatâs causing it? What are the dunes on the surface of Titan actually composed of? Why have they grown to such a large size?

Cassini has answered many questions we have about Saturn, its rings, and its moons. But itâs also opened up just as many questions. More! But thatâs the beauty and, ultimately, the fun of science. The questions never end! Thereâs always more to explore, always another corner to peek around.

Until Cassini got to Titan, we didnât even know about those dunes or the vortex. What else is there just waiting for us to see?

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