Australian Sky & Telescope — January 01, 2018

28 AUSTRALIAN SKY & TELESCOPE January 2018

CASSINI’S SATURN

NASA / JPL / SPACE SCIENCE INST.

1. STORMY SATURN On February 25, 2011,
   Cassini captured a huge storm churning
   through the atmosphere in Saturn’s northern
   hemisphere. By then the storm had raged
   for about 12 weeks, long enough to become
   quite extended in longitude and to wrap itself
   around the planet. The ring system’s shadow
   has a strong seasonal effect on Saturn’s
   atmosphere, so perhaps this storm is related
   to the change of seasons after the planet’s
   August 2009 equinox. The storm was a
   prodigious source of radio noise, arising from
   lightning deep in the planet’s atmosphere.


2. ALIEN LANDSCAPE Hyperion has an unusual
   appearance that probably results from its low
   density, only a bit more than half that of water
   ice. The low density means that Hyperion must
   be highly porous, causing craters to form in
   a different way than they would as impacts
   onto a solid surface. This composite, recorded
   September 26, 2005, includes images taken
   through ultraviolet, green and near-infrared filters.


3. TWO-FACED MOON Iapetus is the most
   distant of Saturn’s ‘regular’ moons and the third
   largest. Most of the hemisphere shown here is

very dark, probably because of infall of material

from Phoebe and other irregular moons orbiting

farther out. The other half is bright white. An

equatorial ridge, standing up to 13 km above

its surroundings, gives Iapetus the appearance

of a walnut. The ridge probably arose early in

the moon’s history when a ring system that

surrounded Iapetus after a giant cometary

impact, fell back onto the surface and piled up.

4. CROSSING OVER Cassini crossed from the
   sunlit to the unlit face of the rings on January 17,


5. In the top two images, the broad, bright B
   ring (right half) dominates. The dark — but not
   empty — Cassini Division separates it from the
   dimmer A ring (at left), which sports the Encke
   Gap near its outer edge. Enceladus appears
   at upper left in the second frame. The bottom
   two images show sunlight filtering through the
   rings to their unlit side. Here appearances are
   reversed: The translucent A ring looks brightest,
   while the more opaque B ring is dark because
   very little light filters through.


6. SUNNY GLINT The bright spot in the B ring
   represents the location of opposition, that is,
   where the Sun is directly behind Cassini (the

illumination phase angle is 0°). The size of the

spot varies from ring to ring and depends upon

the microscopic structure of the ring particles.

This opposition effect provides a way to

investigate the structure of the rings on scales

far too small to be imaged directly. The fainter C

ring is at left in this view recorded June 26, 2005.

6. FLYING SAUCERS Each of these tiny moons
   orbits within or near one of Saturn’s rings.
   Over time they’ve accumulated so many ring
   particles around their equators that they look
   like enormous ‘flying saucers’ or ravioli. Ring
   particles seem to cover almost all of Atlas,
   which orbits just beyond the A ring’s outer edge.


7. TOTAL ECLIPSE Cassini recorded the
   images for this mosaic on September 15, 2006,
   as the spacecraft briefly passed through the
   planet’s shadow. The narrow G ring and broad
   E ring, both full of tiny particles that strongly
   scatter sunlight, lie outside of the main rings.
   Aegaeon, a tiny moon discovered by Cassini,
   is a source of the particles in the G ring, while
   Enceladus’s plumes provide the particles in the
   E ring. Earth is faintly visible at the 10 o’clock
   position just within the G ring.

1.

Since its arrival, the spacecraft witnessed a storm erupt

on Saturn; viewed the planet’s north and south poles clearly

for the first time; observed all of Saturn’s inner moons at

close range and dozens of ‘irregular’ moons from a distance;

partnered with the Hubble Space Telescope to investigate

Saturn’s aurorae; mapped the surface of haze-shrouded Titan;

measured the composition of its complex atmosphere; plunged

through the watery plumes of Enceladus; and much more.

Titan, a moon bigger than the planet Mercury and cloaked

in an atmosphere denser than Earth’s, proved to be a fully

stocked organic-chemistry lab, featuring giant lakes of

hydrocarbons, dynamic clouds and storms, and methane rain.

Saturn’s brilliant rings are ever changing, particularly

the narrow, multi-stranded F Ring, which inhabits the

gravitational ‘Roche zone’ where satellites and rings co-exist.

Cassini — the man — discovered Iapetus, Saturn’s distant,

third-largest moon, in 1671, and he soon realised that Iapetus

has a bright hemisphere and a dark hemisphere. Cassini

— the spacecraft — discovered that this object’s two-toned

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