Astronomers have a problem when attempting to determine the length of a day on a gaseous planet. Take Jupiter, for example. It’s surface (if it even has one) is hidden by layer upon layer of clouds. The clouds are arranged in bands, each blowing at a different speed. You could pick one band and say if you were floating there, the day on Jupiter would have such-and-such a length.
Traditionally, astronomers looked to the planet’s magnetic field. Supposedly generated deep within the planet, it has always been presumed that the solid inner core would rotate at a certain rate. Ingenious! The planet’s day is determined, independently of the wind speeds of the clouds.
After its arrival in 2004, Cassini first began measuring Saturn’s day, and mission scientists discovered something startling. Somehow, since the Voyager encounters of 1980-81, Saturn’s “day” had lengthened by six minutes. Back to the drawing board for a while, but a theory was published suggesting a cause: a cloud of ice crystals.
A new study of Cassini data reported this week in the online version of the journal Science determined that Saturn’s magnetic field lines, invisible lines originating from the interior of a magnetized planet, are being forced to slip relative to the rotation of the planet by the weight of electrically charged particles originating from geysers spewing water vapor and ice from Enceladus. These results are based on joint observations by two Cassini instruments, the radio and plasma wave instrument and the magnetometer.
The neutral gas particles ejected from the geysers on Enceladus form a donut-like torus around Saturn. As these particles become electrically charged, they are captured by Saturn’s magnetic field, forming a disk of ionized gas, or plasma, which surrounds the planet near the equator. The particles weigh down the magnetic field so much that the rate of rotation of the plasma disk slows down slightly. This slippage causes the radio period, controlled by the plasma disk rotation, to be longer than the planet’s actual rotation period.
Scientists conclude the period Cassini has been measuring from radio emission is not the length of the Saturn day, but rather the rotation period of the plasma disk. At present, because of Saturn’s cloud motion, no technique is known that can accurately measure the planet’s actual internal rotation.
Two possibilities remain for the variance in the 2004-07 and 1980-81 data. First, if the ice geysers of Enceladus are more active, spewing more particles, that would slow down the magnetic field rotation. The changes (in either the field or on the moon) might also be linked to seasonal changes during Saturn’s 29.5 year revolution around the sun.
Enceladus, only 310 miles across, wouldn’t seem to be a significant player next to Saturn, which has over ten million times the volume of the small moon. But it shows that in space, small and seemingly insignificant substance can sometimes affect workings on a much larger scale.
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