Revelations in Saturn's rings continue as equinox approaches
The equinox 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.
Provided by the Space Science Institute, Boulder, Colorado
August 10, 2009
Thanks to a special play of sunlight and shadow as Saturn continues its march towards its August 11 equinox, recent images captured by NASA's Cassini spacecraft are revealing new three-dimensional objects and structures in the planet's otherwise flat rings.
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 occurs every half-Saturn-year, or about 15 Earth years. It 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 wide, but the main rings — D, C, B and A rings (working outward from the planet) — are only about 30 feet (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 become available only during the period of equinox when features protruding above and below the rings can cast shadows.
The new moonlet in the B ring, situated about 300 miles (480 km), inward from the outer edge of the B ring, was found because of a shadow 25 miles (41 km) long that it throws on the rings. The shadow length implies the moonlet protrudes about 660 feet (200 meters) above the ring plane. If the moonlet orbits in the same plane as the ring material surrounding it, which is likely, it must be about 1,300 feet (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 that 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.