Length of Saturn's Day Revised
A day on Saturn just got a few minutes shorter, if new calculations are correct.
Using data collected by NASA's Cassini, Pioneer and Voyager spacecraft, scientists have revised the ringed planet's rotation period to 10 hours, 32 minutes and 35 seconds—about 15 minutes shorter than an estimate made only last year.
Those precious minutes could have big implications for how scientists think about Saturn and other gas giants.
"While that may seem like a small uncertainty for the average person, it makes an enormous difference in terms of how we can understand the interior of Saturn," said study team member Gerald Schubert of the University of California, Los Angeles.
If the new rotation rate, detailed in the Sept. 7 issue of the journal Science, is correct, then Saturn's winds blow slower than previously thought and instead of whooshing in only a single direction, can blow both east and west. The finding could also shed light on how gas giant planets in general form.
The problem with gas planets
Schubert and his colleague John Anderson of Global Aerospace in Pasadena, Calif., calculated Saturn's faster rotation using a combination of gravity, wind and deflected radio measurements collected by the three spacecraft.
The spin rate for rocky planets such as Earth can be determined by simply monitoring how fast a particular spot on the planet moves relative to other celestial objects. This doesn't work for gas planets like Jupiter and Saturn, however, because their solid cores are hidden by atmospheric clouds.
Scientists instead measure the rotation periods of the gas giants' magnetic fields, which are assumed to be closely tied to the spin rate of their solid interiors. For this to work, the rotation axis of a gas giant's magnetic field and the rotation axis around which the planet's solid core spins must differ.
But for Saturn, the two axes are nearly identical, so scientists have had to rely on even less direct measures to calculate its rotation rate. Using radio emissions data collected by the Voyager spacecraft in the 1980s, scientists initially determined a Saturnian day to be 10 hours, 39 minutes and 22 seconds.
That estimate was revised in 2004, using Cassini-collected data, to 10 hours, 45 minutes and 45 seconds. It was tweaked yet again last year to 10 hours, 47 minutes and 6 seconds.
Schubert admits that his and Anderson's estimate is only the latest educated guess. "We can't say with absolute certainty that this is Saturn's rotation rate," Schubert said. "At the moment, there's no way that anyone knows of to directly measure Saturn's rotation rate."
Knowing the length of a Saturnian day would help scientists better understand the planet's interior.
For example, if the new spin time is correct, it would change estimates of the planet's wind speed. To calculate wind speed, scientists subtract the speed at which clouds appear to move in the planet's atmosphere from the rotation speed of the planet's solid core.
"When people thought they knew Saturn's rotation rate, they did the subtraction to calculate wind speed and they got a pretty crazy result, huge wind speeds and all the winds were blowing in the same direction," Schubert said.
Those calculations don't match with Jupiter, which has significantly slower winds that blow both east and west.
"Now with this faster rotation rate, the wind speeds actually come down to Jupiter-like values," Schubert said in a telephone interview. "Those few minutes make a world of difference."
Gas planet formation
The new finding might also help scientists distinguish between two competing theories of gas planet formation. According to the "core accretion" model, gas giants form in a similar manner to rocky planets, by gradually accreting rocky debris until they become so large that they draw vast quantities of hydrogen and helium gas unto themselves.
In the competing "disk instability" model, gas planets form when large clumps of gas orbiting in debris disks around young stars collapse under their own gravity to eventually form planets.
Morris Podolak, an astronomer at Tel Aviv University in Israel who was not involved in the study, suggests in a related Science article that if Saturn's rotation rate is indeed faster than previously thought, then its internal core must also be smaller, which could in turn favor the disk instability model.
Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington, and the main proponent of the disk instability model, is more skeptical. "I do not think that this result, while provocative, sheds much light on the debate over planet formation," Boss, who was not involved in the new study, told SPACE.com. "It mainly serves to show how little we can be certain about the interiors of the giant planets."
Schubert is reserving judgment for now, saying that while he thinks knowing Saturn's rotation rate is crucial to understanding the planet's interior, he is unsure whether it will have any bearing on choosing between the two hypotheses.
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