New Pictures of Auroras — on Saturn!

As if having the most impressive rings in the solar system isn’t enough, Saturn also boasts some of the shiniest “footwear”—just check out new shots of the planet’s southern auroras:


—Image courtesy NASA/JPL/University of Arizona/University of Leicester

This quartet of candy-colored pictures comes from NASA’s Cassini orbiter, which carries a nifty tool that can collect data in near-infrared wavelengths of light.

By assigning different visible colors to the otherwise invisible wavelengths, we see auroras in bright green, sunlight reflected off Saturn’s rings and high-altitude haze in shades of blue, and heat emissions from the planet’s interior in deep red.

The pictures were taken in May 2007, but they were released this week as part of a study that compiled data on Saturn’s auroras from the entire catalog, to date, of shots by Cassini’s visual and infrared mapping spectrometer (VIMS).

Here’s a stunning view of the whole planet taken by VIMS in November 2008:


—Image courtesy NASA/JPL/University of Arizona/University of Leicester

In the picture, the ring of green auroras might seem faint, but that curtain of light is shooting up about 600 miles (1,000 kilometers) from the cloud tops of Saturn’s south polar region.

In general, astronomers think auroras on Saturn occur via a process similar to the one that creates Earth’s polar lights.

Charged particles from the sun flow along the planet’s magnetic field lines, hitting the upper atmosphere at the poles. There the particles excite (or transfer energy to) atoms in the atmosphere, and the excited atoms release the excess energy as light.

(See pictures of Earthly auroras generated by a September solar storm.)

In Saturn’s case, auroras can also be sparked by electromagnetic waves generated when the planet’s moons move through the charged gas that fills Saturn’s magnetosphere, the bubble around the planet created by its magnetic field.

The new study, presented today at the European Planetary Science Congress in Rome, used VIMS data to show how Saturn’s auroras change as the planet rotates on its axis. The data reveal that the auroras can vary significantly over the course of a single Saturnian day, which lasts for just under 11 Earth hours.

For instance, the auroras brighten as the planet changes its angle toward the sun, and some parts of the auroras disappear and reappear at the same time and place each day.

“Saturn’s auroras are very complex, and we are only just beginning to understand all the factors involved,” study leader Tom Stallard said in a NASA statement.

“This study will provide a broader view of the wide variety of different auroral features that can be seen, and will allow us to better understand what controls these changes in appearance.”

Of course, understanding auroras on Saturn can also tell us more about how the phenomenon works on other planets, and not just on Earth: Jupiter also puts on an auroral light show writ large, as do Uranus and Neptune.

It’s even been suggested that Venus and Mars, which have no magnetic fields to speak of, still manage to generate a type of light emission that could be called an aurora—not just at their poles but across their whole globes.

The study also shows the power of looking at gassy planets in near-infrared. Adjusting which colors represent which wavelengths, we can see how upper atmosphere haze (green) blocks some heat emissions (red) from deeper in the planet, as well as how cloud systems (dark bands) are structured far below that haze.


—Image courtesy NASA/JPL/University of Arizona

But the buck doesn’t stop at near-infrared.

In a separate study, also being presented at the European meeting, astronomers used three more of Cassini’s instruments to study Saturn’s radio auroras.

Sometimes the auroral curtain can extend far above the visible light show, where it’s made of light in radio wavelengths. The radio emissions come from fast-moving electrons spiraling along the planet’s magnetic field lines.

We’ve seen this phenomenon happening above Earth. But in October 2008, Cassini flew 153 million miles (247 million kilometers) above Saturn’s polar cloud tops, collecting the first direct data on another planet’s radio aurora.

“The instrument that measures radio waves, RPWS, can tell us the direction that each radio wave detected is travelling. By mapping this information onto magnetic field lines, we can work out the location of each radio source,” study leader Laurent Lamy, of France’s Observatoire de Paris, said in a statement.

Tracing the radio sources allowed the team to pick out which magnetic field lines were carrying the radio-emitting electrical currents.

The results showed that the source of the radio emission traces an oval around the planet’s south pole that nicely matches auroral features seen at ultraviolet wavelengths, Lamy said.


Saturn’s auroras in UV light.

—Image courtesy ESA/NASA/Hubble

Cassini had previously crossed the region above Saturn where radio auroras can occur. But the October dive was the first time its instruments had picked up such intense electrical activity at the same time a visible aurora was in action.

The scientists think the intense phenomenon was caused by solar wind squeezing on Saturn’s magnetic field.

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