Springtime stunners from Saturn

The Cassini orbiter has sent back a spectacular set of pictures taken during Saturn’s equinox, including a moody portrait of the giant planet's rings at their darkest. Taken together, the pictures reveal that Saturn’s rings are bumpier, more active and more complex than previously thought.

An equinox simply marks the precise time in a planet's orbital cycle when day and night are of equal lengths. It's one of the traditional markers of seasonal change. Thus, the photos herald the return of spring to Saturn’s northern hemisphere after almost 30 earthly years (which means fall has begun in the south).

On Earth, this week's equinox puts people in a mind to think of fall leaves (in, say, New England) or spring flowers (in Australia). On Saturn, however, the main significance of the Aug. 11 equinox is that it provided scientists with an unprecedented opportunity to see how the giant planet's rings are structured.

Because Saturn's orbital cycle is nearly 30 Earth years long, Cassini's equinox encounter marked the first time scientists could take a close-up look at the rings under conditions that were ideal for fine-resolution observations.

"It's like putting on 3-D glasses and seeing the third dimension for the first time, " Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory, explained today in a news release. "This is among the most important events Cassini has shown us."

Because Saturn's rings are edge-on with respect to the sun during the equinox, the shadows cast by small features within the rings are grossly exaggerated, like the shadows of trick-or-treaters at sunset. Those bumps, ruffles and peaks go unnoticed when sunlight falls upon the rings on an angle. The peculiar circumstances of the equinox revealed, however, that such features are actually not small at all. Some of them rise as high above the main plane of the rings as the Rocky Mountains on Earth, NASA said.

Ripples and ridges in the rings

That plane is nowhere near as flat as a plain. In the old days, scientists thought the rings were only 30 feet (10 meters) thick in the main rings (designated by the letters A through D). Cassini's observations confirm that the rings actually ripple up and down in vertical formations spanning as much as 500 miles (800 kilometers). Scientists don't yet know what caused those ripples.

"It looks like something happened in the early 1980s to get this pattern going, but we are still trying to figure out what could have disturbed such a large part of the rings," said Cornell University's Matthew Hedman, a Cassini imaging team associate.

There's a lot going on within those rippling rings: Cassini's shadow pictures helped astronomers spot a wall of ring particles that rose as high as 2.5 miles (4 kilometers), whipped up by the gravitational influence of the Saturnian moon Daphnis.

"We thought the plane of the rings was no taller than two stories of a modern-day building, and instead we've come across walls more than two miles high," said Carolyn Porco, a planetary scientist at the Colorado-based Space Science Institute who leads the Cassini imaging team. "Isn't that the most outrageous thing you could imagine? It truly is like something out of science fiction."

Cassini also spotted streaky clouds of tiny particles floating above the ring plane. That phenomenon suggests that yard-wide bits of interplanetary debris are continuing to rain down upon the rings, throwing up those clouds and contributing to the evolution of the rings. There are also larger-sized "moonlets" that apparently stir up the ring material, creating clumps that whirl like propellers.

Cassini's previous observations confirmed that the rings are made up of chunks of ice and gunk, in varying sizes. During the equinox, those chunks and bits cooled down to new lows, as recorded by the orbiter's Composite Infrared Spectrometer. Temperatures in the A ring, for example, dipped to 382 degrees below zero Fahrenheit (43 Kelvin). That's not quite as chilly as the permanently shadowed craters at our moon's south pole, which last week was dubbed the coldest spot in the solar system. But it's pretty darn close.

See the full show

Our latest Saturn slideshow presents a selection of the equinox pictures, and for more, you can click on over to the Web sites for the Cassini-Huygens mission and the Cassini imaging team. Here's a NASA slideshow complete with spacey audio track, and a video explaining the geometry behind the equinox mission.

I don't think there's any doubt that the highlight of today's selection is the full frontal image of Saturn, its rings and its moons, captured on the day after equinox. In a large-format version of the image, you can spot the moon Janus in the lower left, Epimetheus near the middle bottom, Pandora outside the rings on the right, and Atlas inside Saturn's thin F ring on the right.

Did you spot them all? If so, that's because the picture has been enhanced to make the rings and moons more visible. The bright side of the rings was made 20 times brighter relative to the planet itself, and the dark side's brightness has been bumped up 60 times. The moons have also been brightened by a factor of 30 to 60. Without the brightening, the rings would essentially fade to black, according to the imaging team.

In an e-mail exchange, Porco told me that more equinox pictures will be coming from the Cassini team. "But they will be like the ones we captured as the sun was setting, probably more moon shadows, and images taken here and there," she said.

Porco said she was deeply satisfied with Cassini's coverage of Saturn's equinox, and noted that "it will be a very long time before any of us sees anything like this again." Almost 15 years, to be precise.

"This has been a moving spectacle to behold, and one that has left us with far greater insight into the workings of Saturn's rings than any of us could have imagined," she said in a news release summarizing the latest findings. "We always knew it would be good. Instead, it's been extraordinary."

Update for 7:25 p.m. ET Sept. 22: Check out Brian Williams' take on the new pictures from Saturn, which aired on "NBC Nightly News." 

More about Saturn:

• Spooky shadows on Saturn
• A mystery glows on Saturn
• Saturn mission goes into overtime
• Slideshow: Cassini's greatest hits

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