Your Cassini awesomeness for today

By Phil Plait | October 13, 2008 5:10 pm


Cassini view of a vortex at Saturn’s south pole

If you’re asking yourself, "What the frak is that?" then join the club. I kinda sorta understand what’s going on, but the details! Yikes!*

What you’re looking at is an incredibly detailed image of Saturn’s south pole (with ten times better resolution than any image taken before), taken by the Cassini spacecraft. It was taken from an angle, but then mathematically "deprojected" to make it look like you are hovering over the pole and looking down.

Previous images showed some sort of weird rotating vortex at the pole. It was thought that the edges were clouds at high altitude (40 – 70 km (25 – 45 miles) above the inner deck) surrounding a region of clear air. But this image shows the central air isn’t clear at all; there are clouds in it, forming a loose inner ring. They are convection clouds; something like cumulonimbus clouds on Earth.

Convection is when hot air rises and cold air sinks. This can happen in cells, self-contained regions in the atmosphere. This occurs pretty much whenever you have a temperature gradient vertically; that is, when it’s hot underneath something cold. Saturn’s interior is hot, so the air rises, gets up to some altitude, cools, and sinks.

In fact, the entire gigantic vortex — it’s 4000 km (2500 miles) across, folks — is a convection feature. The relatively clear center indicates this is warm air, which makes it similar to the eye of a hurricane on Earth. However, on Earth, convection doesn’t occur in the eye, leaving it clear and free of clouds. But the clouds in the middle of this Saturnian region indicate convection is happening on smaller scales there too… meaning the analog of this region to a hurricane only goes so far.

Saturn’s a mess.

Some of the inner clouds are "S"-shaped. They get that way due to Saturn’s rotation making them spin (I’ve seen demos of liquids in spinning bowls that look exactly like this). At the upper left of the inner ring you can see one cloud that looks like an oval; that’s actually the eye of a smaller hurricane-like structure. Its rising air has cleared through Saturn’s haze layer, allowing us to see the circular structure.

The power for all this comes from the heat deep inside Saturn, aided by its rapid 10-hour rotation. Saturn is much larger than Earth, so its rapid rotation means things are spinning around there quickly. Even at Saturn’s poles, unlike Earth’s, there’s enough rotational momentum to keep hurricanes circulating.

So there’s a basic understanding of some of what’s going on in this image, but the details — what’s powering the smaller convection cells; why is one big enough to punch through the lower atmosphere, what’s the chemical composition of these features, how long do they last, how old is this whole vortex thingy — are still head-scratchers.

I imagine images like this will keep planetary atmospheric scientists gleefully arguing with each other for years. That’s one thing I kinda miss about not doing research: the debating over what the heck we’re seeing. When Hubble images would come in, we’d all gather around someone’s computer screen and point and talk and conjecture and speculate, and generally have a blast. It was only later, when we had to dig in and do the dirty work that we started to get our real answers. Sometimes you can see what’s going on just by looking at a picture, and other times — like with this shot of Saturn — it shows us that there’s a lot left to figure out.

*Note: I am trying to puzzle this out after examining the picture and reading the Cassini website. This is all pretty odd stuff, so any mistakes stated here are mine, and not from the Cassini website.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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