Astronomy

32 ASTRONOMY • SEPTEMBER 2017

giant dish as a shield from incoming ring
debris. But once engineers were confident
that ring particles posed little danger to
the craft’s survivability, they were able to
point the antenna back toward Earth. This
was important for many experiments, says
Spilker: “We want to point the high-gain
antenna at Earth so we can get close data,
good measurements of Saturn’s gravity,
and probe how deep the winds might go.”
Researchers also want to determine if the
planet is differentiated — whether it has
a rocky core, potentially the size of Earth.
“All these things you can learn by going
into these close orbits,” Spilker says.
One of the most important numbers
researchers want to constrain is the mass of
Saturn’s rings. Cassini’s decade-plus

reconnaissance of the system has given

researchers an accurate mass for Saturn

plus its rings. “But when you hop across the

rings [between rings and planet], you get

the mass of Saturn alone. You can subtract

the two and what you have left is the mass

of the rings,” Spilker says.

She adds that via indirect measurements

of the rings, “there is some other evidence

... that the B ring may possibly be much
less massive than we were estimating.”
Investigators believe the rings alone have a
mass comparable to Saturn’s moon Mimas
(8.4 × 10^19 pounds [3.8 × 10^19 kilograms]).
But if the rings are far less massive than
this estimate, the data point to young
rings, the kind that might be left behind
after the death of a comet or the

destruction of a moon. Greater mass leaves

open the possibility that the rings formed

at same time as Saturn. “It doesn’t rule out

young rings, but it does open up the pos-

sibility of old ones,” Spilker says. With a

direct measurement of the rings, research-

ers hope to narrow the uncertainty to

about 5 percent.

Yet another mystery may be solved

through close encounters of the Cassini

kind: the length of Saturn’s day. In the case

of gas and ice giants, no solid surface is vis-

ible for plotting a planet’s spin rate. Instead,

investigators clock the passing of the plan-

et’s magnetic fields, slightly offset from the

spin axis, to chart the planet’s daily turn.

But Saturn’s magnetosphere is aligned

almost perfectly to its spin axis, so that its

Cassini captured this

stunning view of a

backlit Saturn over

the course of three

hours on September

15, 2006. Such an

image highlights

the fine detail of the

rings and allowed

Cassini to discover

two new, faint rings

associated with

the moons Janus,

Epimetheus, and

Pallene. NASA/JPL/SPACE

SCIENCE INSTITUTE

The Dragon Storm, imaged in near-infrared wavelengths, first appeared in

2004. Curiously, Cassini saw the feature emit intense radio waves only when
      the planet’s rotation took the storm out of direct sunlight and night fell.
      Planetary scientists believe the emission is linked to electrical activity in the
      storm, likening it to Earth’s thunderstorms. NASA/JPL/SPACE SCIENCE INSTITUTE

This infrared view of the vortex at Saturn’s north pole shows a hurricane

1,250 miles (2,000km) across. The vortex sits at the center of a larger

hexagonal feature that has been present on the planet for decades.

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE/ALEXIS TRANCHANDON/SOLARIS