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Saturn Equinox - A Rare Occurrence
The most recent Saturn equinox occurred on August 11, 2009. It was then that the Sun was directly over the planet's equator - and its famous rings - edge on, making the rings basically disappear from our perspective. Saturn's rings are very thin compared to their horizontal reach. They're only about as thick as a four story building, which is very thin indeed compared to the width of the rings, which are 250,000 km wide. Because one Saturn year lasts approximately 30 earth years, and there are two equinoxes per year, Saturn equinoxes only happen roughly every 15 Earth years. This time the Cassini space probe was ready, however, and captured some amazing data.
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Saturn's B ring, the third closest ring to the planet after the D and C rings, revealed a moonlet that had not been noticed before. The equinoctial view, edge-on to the rings, makes objects sticking up above the rings much more obvious, similar to how lighting a sheet of paper edge-on reveals any bumps rising out of the plane of the paper. The diameter of the moonlet is approximately 400 meters and can't be seen at other times. Bumps of icy ring material that were kicked up by the moonlet's gravitational disturbance were measured at just over 3 km high - about as high as some of the Rocky mountains.
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During Cassini's orbital insertion, it spotted "propellers." These propeller shaped disturbances indicated the presence of 100-meter sized objects within the rings—smaller than a moonlet, but larger than a ring particle. Cassini discovered that there are many of them, in a range of sizes. The biggest is from Titan, Saturn's largest moon (with a diameter of thousands of kilometers), with medium sized moons hundreds of kilometers in diameter creating propellers. Propellers were also discovered around rocks that are tens of thousands of km in diameter.
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A New Mystery
What scientists once thought to be the result of a comet or asteroid strike in Saturn's D ring (the one closest to the planet) in the early 1980s is now more mysterious than ever. Something happened around that time that tilted a large region of the inner rings. It seems less likely that it was a comet or asteroid hit because the equinoctial images show that the effects of the disturbance spread much farther than originally thought. The total radial distance of the disturbance is about 17,000 km. Scientists are unsure what to make of this wavelike disturbance.
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More to Come
One of the major goals of the Cassini Equinox Mission was to look for three-dimensional structures in the rings, and Cassini certainly recorded a wealth of scientific data that will give scientists years worth of information to analyze. Because Saturn's equinox only happens every 15 years, the two year period containing the exact moment of equinox results in amazing images of structures projecting above the surface of the rings. These images will gradually disappear from view until the next Saturn equinox in 2024
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