New spin on Saturn's peculiar, err, spin

 

Excerpt from spacedaily.com

According to the new method, Saturn’s day is 10 hours, 32 minutes and 44 seconds long.

Tracking the rotation speed of solid planets, like the Earth and Mars,

is a relatively simple task: Just measure the time it takes for a

surface feature to roll into view again. But giant gas planets Jupiter

and Saturn are more problematic for planetary scientists, as they both

lack measureable solid surfaces and are covered by thick layers of

clouds, foiling direct visual measurements by space probes.

Saturn has presented an even greater challenge to scientists, as

different parts of this sweltering ball of hydrogen and helium are known

to rotate at different speeds, whereas its rotation axis and magnetic

pole are aligned.

A new method devised by Tel Aviv University researcher Dr. Ravit Helled,

published recently in Nature, proposes a new determination of Saturn’s

rotation period and offers insight into the internal structure of the

planet, its weather patterns, and the way it formed.

The method, by Dr. Helled of the Department of Geosciences at TAU’s

Raymond and Beverly Sackler Faculty of Exact Sciences and Drs. Eli

Galanti and Yohai Kaspi of the Department of Earth and Planetary

Sciences at the Weizmann Institute of Science, is based on Saturn’s

measured gravitational field and the unique fact that its east-west axis

is shorter than its north-south axis.

According to the new method, Saturn’s day is 10 hours, 32 minutes and 44

seconds long. When the researchers applied their method to Jupiter,

whose rotation period is already well known, the results were identical

to the conventional measurement, reflecting the consistency and accuracy

of the method.

Between sunup and sundown on Saturn



For years, scientists have had difficulty coming up with a precise

measurement of Saturn’s rotation. “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,” said Dr. Helled.

“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.” The Cassini spacecraft had

measured a signal linked to Saturn’s magnetic field with a periodicity

of 10 hours, 47 minutes and 6 seconds long — slower than previous

recordings.

“Since then, there has been this big open question concerning Saturn’s rotation period,” said Dr. Helled.

“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.”

Helled’s method is based on a statistical optimization method that

involved several solutions. First, the solutions had to reproduce

Saturn’s observed properties (within their uncertainties): its mass and

gravitational field. Then the researchers harnessed this information to

search for the rotation period on which the most solutions converged.

Narrowing the margin of error



The derived mass of the planet’s core and the mass of the heavy elements

that make up its composition, such as rocks and water, are affected by

the rotation period of the planet.

“We cannot fully understand Saturn’s internal structure without an

accurate determination of its rotation period,” said Dr. Helled.

Knowledge of Saturn’s composition provides information on giant planet

formation in general and on the physical and chemical properties of the

solar nebula from which the solar system was formed.

“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,”

said Dr. Helled. “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.”

The researchers hope to apply their method to other gaseous planets in

the solar system such as Uranus and Neptune. Their new technique could

also be applied in the future to study gaseous planets orbiting other

stars.

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New spin on Saturn's peculiar, err, spin