What is the significance of this storm?

Normally, when you look at Saturn, it's serene and still. You don't see same swirling clouds and storms like you do in Jupiter. Since 1876, there have only been five planet-wide storms [not including this latest one]. These occurred during summertime in Saturn's northern hemisphere, which happens roughly once every 30 years. It's not a precise science, I'm afraid, because a lot of these really old observations are literally guys with telescopes who sketched, by hand, images of these storms. But we didn't expect to see any big storms until about 2020, well after Cassini will be done. That means we've been extremely lucky to see it, lucky that Cassini is still there.

Two things in particular have allowed us to know more about this storm then ever before. First is having Cassini up there in orbit. Second is that we have extremely sophisticated infrared detectors [at] the giant observatories around the world. The combination of those two things has allowed us to study the temperature of the atmosphere. That's revealed some pretty significant surprises, I can tell you, things we didn't know about Saturn.

What sort of surprises?

The main one is the [high-energy, high-temperature] stratospheric beacons. The stratosphere is high up above the cloud tops. We call these things beacons because they are like the light beam of a lighthouse. As Saturn rotates, this hot beacon comes into view once every 10 hours [the length of a day on Saturn], and then you see a massive spike in the amount of emissions coming from the planet, something we've never seen before. We think these beacons reach about 200 to 300 kilometers [124 to 186 miles] higher up than visual clouds. You've got these churning thunderstorms deep within the clouds generating effects hundreds of kilometers higherit'd be like a storm here on Earth affecting something as high up as the orbit of the space station.

Also, we typically only see storms during summertime in Saturn's northern hemisphere. We're seeing them earlier than ever before. We don't have a direct explanation for that yet, but we've got some hypotheses. One is that as spring progresses, the atmosphere warms up, just like on Earth. There's something unique, something mysterious about springtime conditions that has allowed this storm to generate to such an awesome size.

Just how big is it?

Horizontallyin the north-south direction, it's about 10,000 km [6213 miles] long. The storm started as a small thundercloud. Saturn has these really strong windsthey've taken that storm cloud material and spread it east and west Now we're seeing effects of this storm all the way around the planet.

Vertically, the storm reaches 200 to 300 km above the cloud tops, and we believe it is generated another 200 to 300 km under the clouds. Compare that to Earth's own atmosphere and you can see the scales we're talking about are beyond human experience.

What does the storm consist of?

Our best guess right now is that deep down in the water clouds, there's a generation of convective columnslike a bubble of hot gas [that] brings material up with itthings like ammonia, ammonium hydrogen sulfide When they are visible to us from [Earth] and Cassini, they appear as white cloudy material. Ice particlesnot ice of water, but ice of ammonia.

How do you know that these different elements are there?

From the Composite Infrared Spectrometer (CIRS) on Cassini. What CIRS does is measure the spectrum of light coming from Saturn. And in that spectrum we have fingerprints of different gases that make up Saturn's atmosphere.

What does all this tell us about Saturn's storms that we didn't know before?

They aren't restricted to cloud tops the weather layer, where all meteorology is generally thought to take place. The storm itself has a dramatic effect on regions of atmosphere high in the stratosphere, normally thought of as stable.

Does that ever happen on Earth? Storms extending into the stratosphere?

Not that I'm aware of. If you've ever been in a plane, you fly high up, into the lower stratosphere, to avoid any storms.

So does the storm share any features with storms on Earth?

You've got thunderclouds, big columns of moist clouds (except moist with ammonia, instead of water), and within these clouds, you have extremely powerful lightning flashes. In fact, Cassini measurements taken using a device that basically listens to radio waves could hear crackling of lightning flashes taking place. That's how we first knew something big was happeninglistening to lightning crashesin December of last year.

Could a storm like this ever occur on Earth?

Not that we know of. What makes this storm so interesting is its sheer scale; it's wrapped its way around the whole planet. On Earth, we've got continents, mountain ranges, valleys, all sorts of things that would get in way of that sort of huge planetary-scale disturbance. Now, that's not to say that sort of thing can't happen, but conditions on this gas giant are such that there are no obstacles. In fact, it's like the perfect experiment from the point of view of a planetary scientist like myself. You get to see how a storm evolves without a solid surface and continents getting in the way.

Can this teach us anything about our own atmosphere or the atmospheres of other planetsor storms in general?

When we're looking at a storm on Saturn or Jupiter, we're probing those same physical principles and fundamental properties. So, yes, that can teach us about evolving storm systems on our own planet. In fact, what we're doing, to turn the question on its head, is taking all the expertise we've built up over years of research on Earth and applying the same physics to this storm on Saturn to see what we can learn.

How long do you think this storm will last?

We know that it's still raging today. Previous experience has shown it might last to [later this year] or early next year. But since we've never seen a storm like this before, we don't have an answer yet.

What are the broader implications of studying this storm?

Gas giant planets (like Saturn) are important templates for our understanding of planets around our galaxy. Most of the exoplanets we've seen so far are giants with gassy atmospheres, like Saturn and Jupiter. When studying a storm on Saturn or Jupiter, we might gain insights about what's going on elsewhere in our galaxy. Obviously, that will help in interpreting images and spectrum [data] we get from planets and stars in coming years.

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