The solar wind disrupts the timing of radio pulses emitted by Saturn, new measurements suggest. The finding helps explain why scientists have been frustrated in their attempts to determine a precise rotation rate for Saturn by using the pulses as a metronome.
Measuring the rotation rate of a planet with no solid surface is difficult, but it could be important in determining which of several competing ideas describe how gas giant planets like Saturn form. That's because a planet's rotation rate, along with its gravity field, sheds light on the size of any rocky core it may have, a point on which different formation scenarios disagree in their predictions.
In the 1980s, NASA's Voyager spacecraft measured regular pulses of radio waves coming from Saturn every 10 hours and 39 minutes. That was close to the rough rotation rate scientists have obtained by tracking the motion of Saturn's clouds, suggesting the pulse period is tied to the planet's rotation rate.
Listen to the pulses, as measured by the Cassini spacecraft, and watch an animation illustrating their creation and detection.
Scientists disagree about exactly what causes the pulses, but one possibility is that they are due to a "bump" in Saturn's magnetic field that is carried around with the planet's rotation. Once per rotation, the bump meets a spot in Saturn's upper atmosphere where auroras are intensely active, leading to a burst of radio waves.
But the Cassini spacecraft recently showed the period of the pulses varies by several minutes on a timescale of months and years. Saturn is too massive an object for its spin rate to change so quickly, so this suggests the pulses are not precisely tied to the planet's rotation rate after all.
Now, scientists led by Philippe Zarka of the Observatoire de Paris in Meudon, France, think they know why.
They used measurements of the solar wind – a stream of charged particles from the Sun – taken near Earth's orbit by the ACE and WIND spacecraft to estimate the speed of the solar wind at Saturn.
They found that the time between pulses tended to increase when the solar wind was slower, and shortened when the solar wind sped up.
The researchers suspect that the solar wind causes the spot of intense aurora activity in Saturn's atmosphere to move around. That changes the timing of the collision with the magnetic field bump, leading to fluctuations in the time period between pulses.
With this connection revealed, it may still be possible to use the radio period to measure the planet's true rotation rate, Zarka says. If scientists measure the movement of the auroral spot at the same time they measure fluctuations in the period of the pulses, they can "then subtract the two to get the precise interior rotation period," he told New Scientist.
The Cassini spacecraft has the ability to make radio images, which should allow the motion of the spot to be studied, although it is a complex task, he says.
But Margaret Galland Kivelson of the University of California in Los Angeles, US, who was not involved in the study, is doubtful. "I do not think that radio periodicity will give us the internal rotation period," she told New Scientist. She adds that other methods, such as one involving measurements of the planet's gravity field, are being pursued.
As for the connection between solar wind speed and pulse timing proposed by Zarka's team, she says time will tell if the evidence holds up. "Then we will need to understand far better than we do today just how the solar wind effect leads to period variations," she says.
Cassini: Mission to Saturn - Learn more in our continually updated special report.
Journal reference: Nature (vol 450, p 265)
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20:00 03 July 2008
