Length of Saturn's Day Remains Unknown, But Now We Know Why We Don't Know

Strangely, astronomers don't know how long a day is on Saturn, because they can't get a firm footing on the problem given the giant planet's gaseous nature.

So they have long relied on radio measurements of the ringed planet's magnetic field to help estimate the length of the day. But that doesn't really work either, they realized, so estimates have remained loose. Now the scientists at least have a better handle on this aspect of the problem.

Geyser activity from Saturn's small moon Enceladus weighs down the big planet's magnetic field so much that the field rotates more slowly than Saturn itself, new observations reveal. The moon is a mere 310 miles (500 kilometers) wide.

"No one could have predicted that the little moon Enceladus would have such an influence on the radio technique that has been used for years to determine the length of the Saturn day," said Don Gurnett of the University of Iowa.

Gurnett is the principal investigator on a radio and plasma wave science experiment on NASA's Cassini spacecraft. The idea has been to measure Saturn's rotation by taking its radio pulse. The technique works pretty well on the other giant planets.

But the new observations, reported online this week by the journal Science, show that the invisible magnetic field lines, which emanate from Saturn's poles and radiate out like a giant, skeletal pumpkin, slip in relation to the planet's rotation.

The slip owes to the collective weight of electrically charged particles that originate in Enceladus' remarkable geysers of water vapor and ice. Particles in the geysers encircle Saturn and become electrically charged, forming a disk around the equator of hot gas called plasma.

Meanwhile, measurements revealed last year that Saturn's day has gotten about six or eight minutes longer-now roughly 10 hours and 47 minutes-since the 1980s when measured by the Voyager missions. Nobody suspects the trend to continue forever (meaning the days would just get longer and longer at such a rapid rate), but they also don't know what's going on.

Either the geysers on Enceladus are more active now than in the '80s, the astronomers figure, or perhaps there are seasonal variations as Saturn orbits the Sun, a year that takes more than 29 Earth-years to complete.

"One would predict that when the geysers are very active, the particles load down the magnetic field and increase the slippage of the plasma disk, thereby increasing the radio emission period even more," Gurnett said Thursday. "If the geysers are less active, there would be less of a load on the magnetic field, and therefore less slippage of the plasma disk, and a shorter period."

"The direct link between radio, magnetic field and deep planetary rotation has been taken for granted up to now," said Michele Dougherty, a researcher at Imperial College London and principal investigator on Cassini's magnetometer instrument. "Saturn is showing we need to think further."

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