Saturn’s Equinox Arrives

After a successful four-year mission studying the ringed planet, the Cassini probe was still orbiting Saturn in near perfect health in June 2008. So NASA dug deep and found the funding to keep Cassini gainfully employed.

The extension, dubbed the Equinox Mission, is primarily focused on changes wrought on Saturn by the onset of equinox, when the sun shines directly on the gas giant’s equator, which happens just once every 15 years.

In the past Saturn, its rings, and its moons were all illuminated from the south. But tomorrow the equinox comes, and afterward the sunlight will glide over to Saturn’s northern face.

Over the long term, Cassini will be able to watch the planet’s seasonal changes—at least until the currently funded mission ends in September 2010.

But the time immediately around the equinox is especially exciting, because changes in the planet’s position combined with light coming in at different angles are exposing all sorts of 3-D effects in the normally “two-dimensional” rings.

Last week, for example, Cassini images revealed a new itteh bitteh moon hovering just outside Saturn’s B ring.


—Image courtesy NASA/JPL/Space Science Institute, little white arrow courtesy moi

The small, light-colored object is so close to the dense rings—the scientists guess it’s a mere 660 feet (200 meters) above—that it was effectively hidden until now.

The unique interplay of light brought on by the upcoming equinox caused the little moon to cast a long shadow on the rings. In general, Saturn’s moons cast shadows on the rings only before and after an equinox, so pictures like this are incredibly rare.

That means it’s a treat even when bigger, known moons decorate the rings with their own shadowy dances, since the shadows allow Cassini scientists to create unprecedented images with scientific punch.

Here, the moon Tethys casts its spiky shadow across the A and B rings in a mosaic of 17 pictures taken about 2 minutes, 17 seconds apart.


—Image courtesy NASA/JPL/Space Science Institute

The changing degrees of light and dark in the same parts of each shadow can tell scientists just how dense the planet’s rings are in certain regions.

Saturn’s moons also sometimes interact directly with the rings, as seen in this picture of the moon Prometheus creating dark “steamers” through the thin outer F ring.


—Image courtesy NASA/JPL/Space Science Institute

The 53-mile-wide (86-kilometer-wide) moon dips into the F ring when its orbit takes it farthest from the planet. The moon’s gravity then pulls material out of the rings as it moves closer to Saturn. (Watch a movie of Prometheus “bouncing” off the F ring.)

So far we’ve only seen this phenomenon lit from the south, but it’s due to happen again late this fall, after the equinox. Scientists hope light coming in from the north will, pardon the cliché, literally shed new light on the moon’s effects on the F ring.

As for the equinox itself, Cassini will likely have its eye trained on Saturn to see the gas giant make its roughly 170,000-mile-wide (273,000-kilometer-wide) rings … disappear!

As wide as the rings are, they’re just 30 feet (9 meters) thick. As the planet turns on its axis during the equinox, the edge of the rings will line up with the light from the sun.

It’s like turning a piece of white paper edge-on against a mostly white wall and then shining a light directly at it. For all you can see, the paper will seem to vanish.


—Image courtesy NASA/Hubble Heritage

Astronomer Galileo Galilei saw this happen in December 1612 (although through his low-power ‘scope, he thought the rings were actually two moons on either side of the planet!).

He was appropriately baffled at the vanishing act, writing in a letter: “I do not know what to say in a case so surprising, so unlooked for and so novel.”

Sadly the folks at home won’t be able to witness the spectacle this year, as Saturn is also in solar conjunction—basically behind the sun as seen from Earth.

So let’s all hope Cassini keeps working overtime and catches the equinox in action!

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