The Cassini spacecraft has captured its first snapshots of moon shadows on Saturn's rings. The shadows are a sign that the Sun will soon cross Saturn's equator, bringing spring to the planet's northern hemisphere for the first time in almost 30 years.

Like Earth, Saturn experiences seasons because its equator and rings do not lie in the same plane as its orbit – they are tilted by some 27°. Twice during the planet's 29.5-year orbit, the Sun crosses its equator, illuminating the planet's rings edge-on.

The next such equinox will be on 11 August 2009. But NASA's Cassini spacecraft caught one of the first signs of the coming alignment in January, when it snapped images of moon shadows on the planet's rings. Shadows of Saturn's moons are typically seen on the planet itself.

Cassini's mission was extended to 30 September 2010 in order to watch the equinox, which occurs once every 15 years. The team hopes the change in light will offer new insight into the planet's weather as well as reveal more about Saturn's rings.
Dark silhouettes

Measuring the 3D structure of the planet's main, inner rings is usually difficult. That's because Saturn's F ring, which surrounds them, is thicker in the vertical direction than the inner rings, which are about 10 metres thick, obscuring edge-on views of the inner rings.

During equinoxes, though, when the rings lie edge on as seen from Earth, it is usually difficult to directly image the planet's main, inner rings. That's because Saturn's F ring, which surrounds them, is thicker in the vertical direction than the inner rings, which are about 10 metres thick.

But researchers hope to glean more information about the thickness of the inner rings by observing the shapes that the moon shadows cast on them. Any vertical bumps in the rings should distort the shadows' silhouettes.

"Because we know how big the moons are, and where they are in their orbits around Saturn when they cast these shadows, we have all the information we need to infer any substantial vertical structure that might be present," team member John Weiss of the Space Science Institute in Boulder, Colorado, said in a statement.
Unique perspective

Although Cassini will have a clear view of the equinox, the event will not be visible from the ground. While Earth and Saturn's rings will be in exactly the same plane on 10 August and 4 September 2009, Saturn will be too close to the Sun to be a good target for skywatchers.

"One of the best things about being in orbit around Saturn are those mind-expanding opportunities that arise every now and again to see some celestial phenomenon you couldn't possibly see here on Earth," said Carolyn Porco, leader of the Cassini imaging team in Boulder, Colorado.

by:new scientist