Revelations in Saturn's rings continue as equinox approaches

Through the detections of shadows cast upon the rings, a moonlet has been spotted for the first time in Saturn's dense B ring and narrow vertical structures are seen soaring upward from Saturn's intricate F ring.

The search for three-dimensional structures in Saturn's rings has been a major goal of the imaging team during Cassini's "Equinox Mission," the 27-month-long period containing exact equinox -- that moment when the sun is seen directly overhead at noon at the planet's equator. This novel illumination geometry, which occurs every half-Saturn-year, or about 15 Earth years, lowers the sun's angle to the ring plane and causes out-of-plane structures to cast long shadows across the rings' broad expanse, making them easy to detect.

Saturn's rings are hundreds of thousands of miles or kilometers wide, but the main rings -- D, C, B and A rings (working outward from the planet) -- are only about 30 feet, or 10 meters, thick. These main rings lie inside the relatively narrow F ring. The thinness of the rings -- well below the resolving power of the spacecraft's cameras -- makes the determination of vertical deviations from them difficult through routine imaging. Solid evidence of these newly seen structures and others like them becomes available only during the period of equinox when features protruding above and below the rings can cast shadows.

The Cassini spacecraft captured this image of a small object in the outer portion of Saturn's B ring casting a shadow on the rings as Saturn approaches its August 2009 equinox.

This new moonlet, situated about 300 miles, or 480 kilometers, inward from the outer edge of the B ring, was found by detection of its shadow which stretches 25 miles, or 41 kilometers, across the rings. The shadow length implies the moonlet is protruding about 660 feet, or 200 meters, above the ring plane. If the moonlet is orbiting in the same plane as the ring material surrounding it, which is likely, it must be about 1,300 feet, or 400 meters, across. Credit: NASA/JPL/Space Science Institute




This image, taken as Saturn approaches its August 2009 equinox, shows a shadow being cast by a narrow, vertically extended feature in the F ring.

Imaging scientists are working to understand the origin of structures such as this one, but they think this image may show the shadow of an object on an inclined orbit which has punched through the F ring and dragged material along in its path.

The second (bottom) version of the image has been brightened to enhance the visibility of the ring and shadow. Background stars appear elongated in the image because of the camera's exposure time. Credit: NASA/JPL/Space Science Institute




Cassini spies a shadow being cast by a vertically extended structure or object in the F ring in this image taken as Saturn approaches its August 2009 equinox.

The structure can be seen as a bulge near the bright core of the ring on the right of the image. Imaging scientists are working to understand the origin of structures such as this one, but they think this image shows the shadow of what appears to be a vertically extended object in the core of the F ring.

The second (bottom) version of the image has been brightened to enhance the visibility of the ring and shadow. Background stars appear elongated in the image because of the camera's exposure time. Credit: NASA/JPL/Space Science Institute




The revelations of equinox continue as a vertically extended structure or object in Saturn's F ring casts a shadow long enough to reach the A ring in this Cassini image taken just days before planet's August 2009 equinox.

The structure can be seen as a bulge within the bright core of the F ring on the left of the image. The structure rises far enough above the ringplane for the shadow to be cast across the Roche Division and onto the A ring. The shadow is barely visible stretching across the top right quadrant of the image. The shadow appears very faint here because this view looks toward the unlit side of the rings. Credit: NASA/JPL/Space Science Institute

The new moonlet in the B ring, situated about 300 miles, or 480 kilometers, inward from the outer edge of the B ring, was found because of a shadow 25 miles, or 41 kilometers, long that it throws on the rings. The shadow length implies the moonlet is protruding about 660 feet, or 200 meters, above the ring plane. If the moonlet is orbiting in the same plane as the ring material surrounding it, which is likely, it must be about 1,300 feet, or 400 meters, across. Unlike the band of moonlets discovered in Saturn's A ring earlier by Cassini, this object is not attended by a propeller feature. The A ring moonlets, which were not imaged directly, were found because of the propeller-like narrow gaps on either side of them that they create as they orbit within the rings. The absence of a propeller feature surrounding the new moonlet is likely because the B ring is denser and the ring material in a dense ring would be expected to fill in any gaps more quickly than in a less dense region like the mid-A ring. Also, it may simply be harder in the first place for a moonlet to create propeller-like gaps in a dense ring.

In recent weeks scientists also have collected a series of images of shadows being cast by vertically extended structures or objects in the F ring. One image shows the shadow of what appears to be a vertically extended object in the core of the F ring, while another image may show the shadow of an object on an inclined orbit which has punched through the F ring and dragged material along in its path. A third image shows an F-ring structure casting a shadow long enough to reach across the wide Roche Division and appear on the A ring. Imaging scientists are working to understand the origin of these structures.

New sights such as these -- and the questions they raise and the insights they may provide -- will continue in the coming days of Saturn's equinox.

Source: Space Science Institute
	More on • Cassini • Saturn