Space Topics: Saturn
The Year in Pictures: 2007
Cassini: Rings Around Saturn
Saturn’s main rings are shown encircling the planet in a view never
possible from Earth. Cassini has flown to 39 degrees to the north of Saturn’s
ring plane, gaining a top-down perspective
on the rings. From this unique point
of view, Saturn’s A and B rings (the rings most easily seen from Earth
in a small telescope) are visible in their entirety, unobstructed by the planet
(although they are darkened by Saturn’s shadow). Because it is currently
winter in Saturn’s northern hemisphere, the rings are lit from behind,
so Saturn’s most opaque ring—the B ring—appears as a wide,
dark band.
It is usually easy to visually distinguish the A and B rings because
they are separated by a mostly transparent lane called the Cassini division.
From Earth, even a small telescope resolves the Cassini division as a black
band between the A and B rings. In the image above, however, the Cassini division
is far from obvious. Unlike the two gaps near the outer edge of the A ring,
which appear black in this image, the Cassini division is not entirely devoid
of particles. It contains several broad ringlets of fine dust that, when lit
from behind, scatter the sunlight forward to Cassini’s camera like dust
motes in a sunbeam. The Saturn portrait below right is from the sunlit side
of the rings and shows a more classical view, with the bright A and B rings
separated by the dark Cassini division.
This global view of Saturn and its complete ring system was captured
by Cassini on February 20, 2007, and is composed of 15 separate frames.
It was assembled from the raw JPEG images by Ian Regan. The view is
from the south, so the rings are fully lit by the Sun. It contains
nearly all of Saturn's moons from Mimas inward. Clockwise from bottom
center: Mimas, Epimetheus (near the left ansa), Pandora, Janus (near
the top), and Prometheus (near the right ansa). Credit: NASA / JPL
/ SSI / Ian Regan |
This view of Saturn differs from previous views in another
way: the shadows cast by Saturn’s rings on the planet itself have moved
markedly southward since the arrival of Cassini at Saturn more than three years
ago. That’s because winter is drawing to a close in the northern hemisphere;
the vernal equinox comes in 2009. The march of the seasons brings changes to
almost all the planets, but Saturn’s broad ring system makes the arrival
of spring even more significant. From equinox to equinox, one
entire side of Saturn’s rings remains constantly illuminated (except when it passes
through Saturn’s shadow), while the other side of the rings remains constantly
in darkness. The arrival of spring in the northern hemisphere will light up
the north side of the rings for the first time in 15 years. Cassini should
remain functioning through Saturn’s equinox and will be watching the
rings and the planet to see how they respond to the arrival of the Sun.
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