14 DAY TRIAL // A small leak will sink a great ship...
Enceladus, one of the Saturn's small moons, is slowing down the giant planet's magnetic field so much that the field rotates at a lower pace than the planet, making it nearly impossible to determine Saturn's day length employing methods used for other giant planets, like Jupiter, Uranus or Neptune.
"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 Dr. Don Gurnett of the University of Iowa, Iowa City, main investigator on the radio and plasma wave science experiment onboard the Cassini spacecraft.
The radio technique determines the rotation speed of a planet by measuring its radio pulse rate.
But a new research found that Saturn's magnetic field lines, rooting from the core of a magnetized planet, are being forced to remain behind the rotation of the planet due to the electrically charged particles originating in the vapor and ice geysers of Enceladus.
This was detected by two Cassini instruments, the radio and plasma wave instrument and the magnetometer.
The gas particles expelled by the geysers of Enceladus form a torus surrounding Saturn and when they are electrically charged, the particles get attracted into Saturn's magnetic field, creating a disk of ionized gas (plasma) around the planet's equator that weighs down the magnetic field. The induced slippage causes the radio period to be left behind the planet's actual rotation period.
Thus Cassini measured just radio emission period, not Saturn's day, but rather the rotation period of the plasma disk. No current technique can precisely measure the planet's actual internal rotation. The giant gaseous planet presents no surface or fixed point to measure its rotation rate, that's why it is so tricky to do this.
When researchers employed periodic radio signals, they were puzzled as Saturn's radio period looked like a pulsed signal rather than a rotating, lighthouse-like beam and the period appeared to slowly change over months to years.
Cassini found a day length some six minutes longer than that registered by Voyager in the early '80s, a 1 % increase. "We have linked the pulsing radio signal to a rotating magnetic signal. Once each rotation of Saturn's magnetic field, an asymmetry in the field triggers a burst of radio waves. We have then linked both signals to material that has come from Enceladus," said Dr. David Southwood, co-author, Imperial College London, and director of science at the European Space Agency.
Researchers suppose that the geysers on Enceladus may be more active now than 20 years ago or there may be seasonal fluctuations due to Saturn orbiting the sun once every 29 years. "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. 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," said Gurnett. "The direct link between radio, magnetic field and deep planetary rotation has been taken for granted up to now. Saturn is showing we need to think further," said Michele Dougherty, principal investigator on Cassini's magnetometer instrument, Imperial College London.