Astronomy

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the Voyager imaging team. For her, some
of the greatest insights from the mission
involve revelations about Saturn’s vast ring
system, made possible through a combi-
nation of high-resolution imaging and
occultations of the rings observed in radio,
ultraviolet, and infrared wavelengths.
“Between those and our images,” Porco
says, “we’ve got a very nice picture.... We
know that the particles behave coherently
for longer than we thought they would.
They seem to clot in ways that we didn’t
think they would. That’s the kind of detail
we wanted.”
Porco asserts that the insights provided
by ring research will shed light on the pri-
mordial debris disks, known as “accretion
disks,” that lead to the formation of

planets. “The rings are a planetary forma-

tion process; we get to see the initial stages.

You can predict how smaller ring particles

will behave, and you can predict how

coherently a bunch of these particles will

behave,” she says. “It’s these insights into

the fine-scale behaviors that we’ve gotten

from Cassini.”

The new orbits also enable Cassini to

obtain improved images of the moons

nearest the rings. Pandora, Prometheus,

and Epimetheus display bizarre grooves

and shallow craters filled with material

that has slumped down the crater walls,

revealing bright outcrops along their rims.

Landslides mark long streams of material

that has flowed even in these tiny satellites’

low gravity. This low-gravity transport of

material has researchers reaching for their

keyboards to come up with new models.

Other moons shepherd material within

the ring system itself. Tiny Daphnis, a

favorite of Spilker’s, orbits in the Keeler

Gap. “It’s making waves on the edge of the

gap, and it has a little tendril [of ring mate-

rial] wrapping around it and a ridge around

the equator,” Spilker says. These ripples had

previously led scientists to infer the pres-

ence of a moon in the gap, but it was

Cassini that first directly spotted Daphnis

on May 1, 2005.

Teasing out answers

On its initial passes through the ring plane,

Cassini f lew with its antenna facing for-

ward in the direction of f light, using the

During its ring-grazing orbits, Cassini imaged
Saturn’s B ring at twice the resolution of
previous images. Such shots reveal features
such as “straw,” visible as streaks near the edge
of the ring on the left, in greater detail. These
structures are actually clumps of ring particles
that continued to move during the long exposure,
smearing out into elongated shapes that shed
light on the motion of ring particles.

Saturn’s atmosphere boasts high winds and monstrous storms, including
this planet-encircling system photographed February 25, 2011. By the time
Cassini took this image, the storm had been raging for 12 weeks.

First observed by Voyager in the 1980s, these

radial “spokes” in Saturn’s B ring were spotted

again by Cassini. This picture, dated February 2,

2009, is just one of many images Cassini took of

these features, which are thought to arise from

interactions between dust particles and the

planet’s magnetic field. NASA/JPL/SPACE SCIENCE INSTITUTE

The tiny shepherd moon Daphnis, just 4.8 miles

(7.7km) across, creates waves in the edge of the

A ring as it orbits within the Keeler Gap. These

ripples are visible as shadows across Saturn’s

face when the Sun and the ring plane align.

During its Grand Finale, Cassini slips between Saturn and the inner edge of

its rings in this artist’s concept. This unique vantage point allows the space-

craft to study Saturn’s atmosphere, magnetic field, and more with greater

precision than previously possible. Not only will these orbits help to pin

down the length of Saturn’s day, they will also better constrain the mass —

and possibly the age — of the rings. NASA JET PROPULSION LABORATORY

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE