Shorter days for Saturn

Using the change of radiation as Saturn’s magnetic field rotates, the Voyager spacecraft measured almost 30 years ago that a day of Saturn lasts 10 hours, 39 minutes, and 24 seconds. However, the same measurements by the Cassini spacecraft suggests that it lasts 10 hours, 47 minutes, and 6 seconds, over seven minutes longer. Such a significant change in the bulk rotation rate of Saturn probably means that Saturn’s magnetic field does not quite rotate in lockstep with the planet itself. Another method might be needed to measure the length of a Saturnian day.

Another way to measure the rotation rate of Saturn has been proposed using an analysis of its winds. Applying this analysis to Jupiter gives a length of day close to what is known, so it might be a better way to measure the length of a Saturnian day than using magnetic fields. The analysis suggests that a day of Saturn lasts 10 hours, 34 minutes, and 13 seconds with an estimated error of 20 seconds, which is five minutes shorter than that measured by the Voyager spacecraft. This is also closer to the value of 10 hours, 32 minutes, and 35 seconds derived from measurements of Saturn’s gravity field by the Cassini spacecraft.

While a five minute difference might not seem much, the difference means about a 300 kilometer per second difference for the estimated equatorial wind speeds on Saturn. That is a big difference in wind speeds, and the new estimated length of day for Saturn brings its weather system more in line with Jupiter’s, where eastward and westward winds have about the same speed.