Measuring the length of a day on a planet doesn’t seem like it should be particularly hard. If, for example, you were watching Earth from space and saw Los Angeles you could hit the “start” button on your stopwatch. When Los Angeles rolls around again, you hit the “stop” button and that is one day.
Measuring a day in the life of a gas giant, however, is a bit tricker. Jupiter and Saturn lack measurable solid surfaces and are covered by thick layers of swirling, churning clouds.
Saturn, in particular, poses a challenge. Different parts of the planet rotate at different speeds but its axis and poles are aligned.
Given the difficulty level, scientists have done a pretty good job of measuring Saturn’s day based on data from the Voyager 2 and Cassini space probes.
Now, Dr. Ravit Helled of the Department of Geosciences at Tel Aviv University thinks he has come up with an accurate way to measure a day on Saturn. His method, detailed in the journal Nature, is based on Saturn’s gravitational field and the fact that it’s north-south axis is longer than it’s east-west axis.
By applying this new method, Helled along with Drs. Eli Galanti and Yohai Kaspi of the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, measured the day at 10 hours, 32 minutes and 44 seconds.
For added assurance, the researchers applied their method to measuring Jupiter’s day and came up with the known rotation period, or day, of “9 hours and 56 minutes around the poles to 9 hours and 50 minutes close to the equator.”
“In the last two decades, the standard rotation period of Saturn was accepted as that measured by Voyager 2 in the 1980s: 10 hours, 39 minutes, and 22 seconds. But when the Cassini spacecraft arrived at Saturn 30 years later, the rotation period was measured as eight minutes longer. It was then understood that Saturn’s rotation period could not be inferred from the fluctuations in radio radiation measurements linked to Saturn’s magnetic field, and was in fact still unknown,” said Dr Helled in a statement.
“Since then, there has been this big open question concerning Saturn’s rotation period. In the last few years, there have been different theoretical attempts to pin down an answer. We came up with an answer based on the shape and gravitational field of the planet. We were able to look at the big picture, and harness the physical properties of the planet to determine its rotational period,” he added.
The method employed by the researchers is based on a statistical method which first reproduced Saturn’s observed properties, mass and gravitational field and then searching for a rotational period in which the solutions converged.
The mass of Saturn’s core and the heavy elements that make it up are affected by the planet’s rotation.
“We cannot fully understand Saturn’s internal structure without an accurate determination of its rotation period,” said Dr. Helled.
An accurate understanding of Saturn’s composition will help researchers understand the formation of gas giants as well as the composition of elements in the early solar system.
“The rotation period of a giant planet is a fundamental physical property, and its value affects many aspects of the physics of these planets, including their interior structure and atmospheric dynamics. We were determined to make as few assumptions as possible to get the rotational period. If you improve your measurement of Saturn’s gravitational field, you narrow the error margin,” said Dr. Helled.
Next, the researchers will apply the method to Uranis and Neptune to further refine it and then attempt to apply it to gas giants found elsewhere in the galaxy.
