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Cassini eyes Saturn's dust streams to clock length of its day

Morgan Rehnberg, Correspondent
Jun 13, 2015, 0:22 UTC

Sen—This week NASA’s Cassini spacecraft, in orbit about Saturn for nearly 11 years, put the seemingly mundane front and center. Dust might not seem like the mission’s most exciting topic, but one of its most remarkable aspects could help solve one of Saturn’s most enduring mysteries—how quickly the planet rotates.

Dust, microscopic flecks of material, is everywhere in space—around 30,000 tons of it strikes the Earth’s atmosphere each year. It is hardly surprising, then, that Cassini’s Cosmic Dust Analyzer has spotted plenty in the vicinity of Saturn. What is more intriguing, though, is where it is going: instead of being pulled in by the gravity of the Solar System’s third largest object, streams of dust appear to be traveling directly away from the planet.

How can this happen? It seems to be directly related to Saturn’s magnetic field. When dust in the Saturn system—much of it ejected from the geysers on Enceladus—picks up an electric charge, it falls under the influence of the magnetic field. As the planet spins, the field rotates with it. But a rotating magnetic field generates an electric field which points directly away from Saturn. This electric field pushes away any charged particles and generates the streams of dust observed by Cassini.

Surprisingly, we still don’t have a precise understanding of how quickly Saturn rotates and thus the length of its day. Lacking a solid surface to measure, the Voyagers and Cassini made radio observations of the planet’s interior, which resulted in periods variant by a handful of minutes. Since the speed of these dust streams depends on the strength of Saturn’s electric field, itself dependent on how fast the planet spins, the measurements made this week may prove crucial to resolving this longstanding mystery.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.