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atmosphere working group. This hexagon 16
was still there decades later, when Cassini
reached Saturn.
And then there are Saturn’s winds them-
selves. “I remember being amazed at how
fast the winds were blowing,” recalls
Ingersoll — although how fast isn’t actually
known yet. That’s because scientists don’t
have a reference against which to measure
the wind speeds, explains Ingersoll, who has
studied planetary atmospheres for decades.
“On Earth, we measure the wind relative to
the continents,” explains Ingersoll, and
Saturn, of course, doesn’t have any conti-
nents. But if scientists could measure how
fast the planet’s solid core rotates, that speed
would serve as the reference.
To get at that rotation rate for a giant
planet, researchers track the planet’s mag-
netic field, which is produced in the solid
core. On Jupiter, the magnetic field’s axis is
tilted in relation to the rotation axis, which questions about the ringed world and its 15. False-color images, such as this one
taken August 23, 1981, from 1.7 million
means as the core rotates, the magnetic system, as well as introduced new mysteries miles (2.7 million km), highlight the
field wobbles. “You see the magnetic field for future spacecraft to resolve. varying chemical composition of the
wobble back and forth like a . . . top, and so This world holds beautiful rings that rings, seen as different colors. The C ring
appears blue in this composite, while the
that tells you how fast the interior of the mimic some characteristics of disks around
B ring appears yellow-orange. NASA SPACE
planet is rotating,” says Ingersoll. young stars, intense atmospheric storms, SCIENCE DATA COORDINATED ARCHIVE
Unfortunately, Saturn’s magnetic field axis and a variety of moons — including one
16. Voyager 1 took this image of Saturn
and rotation axis are too similar to produce with an Earth-like weather system and as it departed, looking back from its
a measurable wobble on the Voyagers’ another with the ingredients of a habitable vantage point of 3.3 million miles
instruments. But in its final mission phase, environment. “The study of Saturn has (5.3 million km) four days after closest
skimming just above the cloud tops, provided scientists the means to study pro- approach. Spokes can be seen as bright
patches in the rings from this distance.
Cassini may finally get close enough to the cesses that are at work all across our solar
NASA SPACE SCIENCE DATA COORDINATED ARCHIVE
planet to better track the wobble. system and scale-invariant across the cos-
On September 15, Cassini will end its mos,” says Porco. “No other planet can
study of Saturn, closing the door on up- claim as much.” Liz Kruesi is an Astronomy contributing
close observations of the ringed planet, just Because of Voyager 1 and 2, we know editor who writes about distant objects
like Voyager’s departure in 1981. Both mis- why the Saturn system continues to tempt from her Earthbound home in Austin,
sions have answered long-standing planetary explorers. Texas.

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