Elusive answer to length of Saturn’s day reached with TAU, Weizmann team

Team finds the length of day of Saturn to be 10 hours, 32 minutes and 45 seconds.

By JUDY SIEGEL-ITZKOVICH
Saturn. (photo credit: TEL AVIV UNIVERSITY)
Saturn.
(photo credit: TEL AVIV UNIVERSITY)
Israeli scientists have solved the longtime puzzle of when the sun rises on the planet Saturn.
Planetary sciences experts at Tel Aviv University and the Weizmann Institute of Science in Rehovot published a paper on the matter in the prestigious journal Nature on Wednesday night.
To the layman, it would seem that the length of the day on a planet can be obtained from a clear and unmistakable physical measurement. But it turns out that for Saturn, the situation is different. Even today, in 2015, scientists lack the certain and final information on how long a day is.
But young scientists at the Tel Aviv and Rehovot institutes of higher learning say they have now managed to develop an accurate solution to the puzzle that is a key to additional puzzles in our solar system and throughout the galaxy. Dr. Ravit Helled, a senior lecturer in planetary sciences at TAU’s geosciences department, and Dr. Eli Galanti and Dr. Yohai Caspi of Weizmann published their Nature article.
Saturn is a monster of gases covered by clouds, said Helled, who is involved in space assignments of the US National Aeronautics and Space Administration and the European Space Agency that include the Cassini Solstice Mission revolving around Saturn, NASA’s Juno solar-powered mission to Jupiter in 2016 and the JUICE (JUpiter ICy moons Explorer) planned for launch in 2022.
“Since Saturn is composed of gas, it does not have a stable surface, so one can’t determine the time of its rotation in the same way solid planets are measured, explained Helled. “The conventional technique for solid planets is to choose an identifying sign on the surface and measure the amount of time that passes until it is seen again. In addition, the gases are always covered by a layer of clouds which makes the measurement even more difficult.”
Because of these difficulties, scientists over the years have tried to estimate the length of the Saturn day using different methods based on a variety of phenomena that can be viewed and measured – such as the magnetic field, radio wave radiation, clouds and winds. But the results of these studies were not uniform. There were major gaps among the estimates of some 15 minutes, which is a long time in terms of research into the solar system.
In 1977, NASA dispatched the Voyager 2 spacecraft, which produced a great deal of valuable information about the four outer planets in our solar system – Saturn, Jupiter, Uranus and Neptune, Helled said. “According to Voyager 2 observations, the time it takes for Saturn to make a complete rotation is 10 hours, 39 minutes and 22 seconds, and this has been regarded as accurate for the last three decades – until the measurements from the Cassini spacecraft, which entered the path around Saturn in 2004 and found that the measurement technique of Voyager 2 was not accurate. The puzzle of Saturn’s day was reopened.”
The Israeli team chose to attack the puzzle using a technique called “statistical optimization” in which they estimated the speed of the planet’s rotation according to its gravitational field, the density of the material and its flattened shape. This way, they were able to calculate the length of the day at 10 hours, 32 minutes and 45 seconds. To test the accuracy of the technique, they also calculated the time it took Jupiter to rotate (an established figure) and reached the correct result.
“Our findings have broad significance, way beyond the solution of an interesting puzzle,” said Helled. “First, the rotation time has a strong effect on winds and the weather over Saturn. And maybe more important, in our previous study, we find a gap of seven minutes in the rotation that has major implications on the study of the internal structure of the planet. One can reach important conclusions on how Saturn was formed, as well as other giant masses of gas, and the conditions that existed there when it was formed. This information adds an important pillar to research into the development of our solar system and other systems in the galaxy.”
In future studies, Helled and her team will use the technique they developed to study Uranus and Neptune, which are also gas giants, and for the study of planets in other solar systems.


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