Saturn's Day Could Be Shorter Than Previously Thought

We certainly know how long our own day is, and Mars is not much longer (24 hours and 40 min). The closest planet to the Sun, Mercury's own incredibly long day lasts more than 1,407 hours. What's interesting, however, is that astronomers know all this with near certainty. That's not the case for Saturn, on the other hand, whose day length has remained something of an enigma.

But how is this so? Traditionally, experts simply have to find a stationary landmark on a planet or moon's surface and track its rotation to determine how long that heavenly body's full day is. This was easily done with the Moon and Mars, which both have 'seas' of craters or canyons that can't be missed.

For gas giants like Saturn, however, the naked eye and even orbital telescopes can't physically see a surface through the planet's soupy atmosphere. Some gas planets, like Jupiter, still have notable landmarks.

Jupiter's Great Red Spot, for instance, is a fixed mark in the planet's thick atmosphere. By compensating for wind shifts, planetary tilt, and other changes, astronomers are still able to use the spot as a point of reference.

Saturn, however, has no such spot. Instead, experts have relied on assessments of the ringed planet's rotation using radio waves - measurements that have revealed some serious discrepancies between flybys from NASA's Voyager spacecraft 35 years ago, and the more recent Cassini probe.

"At present Saturn's rotation period is thought to be between 10h 32min and 10h 47min, which is unsatisfactory for such a fundamental property," a team of researchers recently wrote in their study of Saturn's rotation.

"Estimates based upon Saturn's measured wind fields have increased the uncertainty even more, giving numbers smaller than the Voyager rotation period," they added. (Scroll to read on...)

Specifically, it was Cassini who set the 10 hour and 47 minute maximum, but Voyager's own minimum of 10 hours, 39 minutes has since been overruled.

Why? In an attempt to resolve the anomolies between the two spacecraft analyses, Ravit Helled, a planetary scientist at Tel Aviv University in Israel, recently led a team in measuring Saturn's rotation in a new way. They did this by paying close attention to how Saturn's equator bulges - a consequence of the planet's spinning gravity field. But measuring these distortions, the researchers were able to come up with a rough approximate of Saturn's rotation speed, which, in turn, allowed them to estimate the day length.

The results were published in the journal Nature.

However, the numbers they got have only complicated things, with an estimated minimum that is between six and seven minutes less than previously estimated.

"Six minutes makes a big difference," Andrew Ingersoll, a planetary scientist at the California Institute of Technology (Caltech), who was not involved in the work, recently told the AAAS.

He explained that if Saturn spins faster than once thought, astronomers' understanding of planetary wind patterns can change with it. That's because scientists measure wind speeds relative to a planet's spin. The Voyager estimates suggested that most winds blew only one way, whereas the new, faster spin estimate means about half of the winds blow east and the other half blow west.

That, Ingersoll says, "seems a little more sensible," because other gas giants such as Jupiter boast a similar pattern.

This may indicate that Helled and his team are closer than ever before in determining Saturn's exact day length. However, it will take a lot more research, and even finer data and instrumentation, to know once and for all how long a Saturn day really is.

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