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Astronomy in the News

Mercury Summer 2009 Table of Contents


The slender shadow of Daphnis (the white blob) is flanked by other shadows.

Credit: NASA / JPL / Space Science Institute.

Vertical Structures in Saturn's Rings

NASA / JPL / Space Science Institute

The search for ring material extending well above and below Saturn's ring plane has been a major goal of the imaging team during Cassini's "Equinox Mission," the two-year period containing exact equinox — that moment when the Sun is seen directly overhead at noon at the planet's equator. This novel illumination geometry 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.

The 5-mile-wide moon Daphnis orbits within the 26-mile-wide Keeler Gap in Saturn's outer A ring, and its gravitational pull perturbs the orbits of the particles forming the gap's edges. The eccentricity, or the elliptical deviation from a circular path, of Daphnis' orbit can bring it very close to the gap edges. There, its gravity causes larger effects on ring particles than when it is not so close. Previous Cassini images have shown that as a consequence, the moon's effects can be time-variable and lead to the waves caused by Daphnis to change in shape with time and with distance from the moon.

However, the new analysis also illustrates that when such a moon has an orbit inclined to the ring plane, as does Daphnis, the time-variable edge waves also have a vertical component to them. This result is backed by spectacular new images taken recently near equinox showing the shadows of the vertical waves created by Daphnis, and cast onto the nearby ring, that match the characteristics predicted by the new research.

Scientists have estimated, from the lengths of the shadows, wave heights that reach enormous distances above Saturn's ring plane -- as large as 1.5 kilometers (1 mile) -- making these waves twice as high as previously known vertical ring structures and as much as 150 times as high as the rings are thick. The main rings -- named A, B and C -- are only about 10 meters (30 feet) thick.

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