A view from NASA’s Cassini spacecraft shows Saturn’s northern hemisphere in 2016 as that part of the planet nears its northern hemisphere summer solstice. A year on Saturn is 29 Earth years; days only last 10:33:38, according to a new analysis of Cassini data. Image Credit: NASA/JPL-Caltech/Space Science Institute

January 20, 2019 – Using new data from NASA’s Cassini spacecraft, researchers believe they have solved a longstanding mystery of solar system science: the length of a day on Saturn. Ten hours, 33 minutes and 38 seconds.

It’s a figure that has eluded planetary scientists for decades,
because the gas giant has no solid surface with landmarks to track as it
rotates, and it has an unusual magnetic field that hides the planet’s
rotation rate.

The answer, it turned out, was hidden in the rings.

During Cassini’s orbits of Saturn, instruments examined the icy,
rocky rings in unprecedented detail. Christopher Mankovich, a graduate
student in astronomy and astrophysics at University of California, Santa
Cruz, used the data to study wave patterns within the rings.

His work determined that the rings respond to vibrations within the
planet itself, acting similarly to the seismometers used to measure
movement caused by earthquakes. The inside of Saturn vibrates at
frequencies that cause variations in its gravitational field. The rings,
in turn, detect those movements in the field.

“Particles throughout the rings can’t help but feel these
oscillations in the gravity field,” Mankovich said. “At specific
locations in the rings these oscillations catch ring particles at just
the right time in their orbits to gradually build up energy, and that
energy gets carried away as an observable wave.”

Mankovich’s research, published Jan. 17xx by Astrophysical Journal, describes how he developed models of Saturn’s internal structure that would match the rings’ waves. That allowed him to track the movements of the interior of the planet – and thus, its rotation.

The rotation rate of 10:33:38 that the analysis yielded is several
minutes faster than previous estimates in 1981, which were based on
radio signals from NASA’s Voyager spacecraft.

The Voyager data, which estimated the day to be 10:39:23, was based
on magnetic field information. Cassini used magnetic field data, too,
but earlier estimates ranged from 10:36 all the way to 10:48.

Scientists often rely on magnetic fields to measure planets’ rotation rates. Jupiter’s magnetic axis, like Earth’s, is not aligned with its rotational axis. So it swings around as the planet rotates, enabling scientists to measure a periodic signal in radio waves to get the rotation rate. However, Saturn is different. Its unique magnetic field is nearly perfectly aligned with its rotational axis.

This is why the ring finding has been key to homing in on the length
of day. Saturn scientists are elated to have the answer to such a
central question about the planet.

“The researchers used waves in the rings to peer into Saturn’s
interior, and out popped this long-sought, fundamental characteristic of
the planet. And it’s a really solid result,” said Cassini Project
Scientist Linda Spilker. “The rings held the answer.”

The idea that Saturn’s rings could be used to study the seismology of
the planet was first suggested in 1982, long before the necessary
observations were possible.

Coauthor Mark Marley, now at NASA’s Ames Research Center in
California’s Silicon Valley, subsequently fleshed out the idea for his
Ph.D. thesis in 1990, showing how the calculations could be done, and
predicted where signatures in Saturn’s rings would be. He also noted
that the Cassini mission, then in the planning stages, would be able to
make the observations needed to test the idea.

“Two decades later, in the final years of the Cassini mission,
scientists analyzed mission data and found ring features at the
locations of Mark’s predictions,” said co-author Jonathan Fortney,
professor of astronomy and astrophysics at UC Santa Cruz and a member of
the Cassini team. “This current work aims to make the most of these

Cassini’s mission ended in September 2017, when, low on fuel, it was
deliberately plunged into Saturn’s atmosphere by the mission team, which
wanted to avoid crashing the craft onto the planet’s moons,
contaminating them.

The Cassini-Huygens mission is a cooperative project of NASA, ESA
(European Space Agency) and the Italian Space Agency. NASA’s Jet
Propulsion Laboratory, a division of Caltech in Pasadena, manages the
mission for NASA’s Science Mission Directorate, Washington. JPL
designed, developed and assembled the Cassini orbiter. The radar
instrument was built by JPL and the Italian Space Agency, working with
team members from the U.S. and several European countries.

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