Australian Sky & Telescope — January 01, 2018

34 AUSTRALIAN SKY & TELESCOPE January 2018

14.COLD GEYSERSDramatic plumes of

water vapour and microscopic particles jet

into space from the ‘tiger stripes’ near the

southpoleofEnceladus.Theseeruptions,seen

clearlyinthisbacklitviewtakenonNovember

27,2005,likelycomefromasaltyoceandeep

within Enceladus. The ocean’s geochemistry

mightresemblethatofhydrothermalventsin

Earth’s oceans.

15.IRREGULAR MOONPhoebe is the

biggestofSaturn’sirregular satellites,socalled

because of their noncircular and highly inclined

orbits.Theseviewscameduringthecraft’s

approachtoSaturnin2004.Scientiststhinkthe

irregular satellites of Jupiter, Saturn, Uranus

andNeptuneinitiallyorbitedtheSunbut

werecapturedbytheirhostplanets.Cassini

observations indicate that Phoebe’s density

ishigherthanusualfortheSaturnsystemand

akintothoseofKuiperBeltobjects,supporting

the capture idea.

planet rotates. Mission scientists once thought that the period

of this variation was synced with the spin rate of the planet’s

interior and should not change with time. But Cassini found

complicated variations in the radio emission, so the planet’s

true spin period remains unknown.

Perhaps the most amazing discovery of the Cassini mission

is that Enceladus, the second smallest of Saturn’s ‘classical’

satellites, sprays thousands of tons of salty water into space

each day from geysers near the moon’s south pole. Beneath

its icy shell, Enceladus harbours an ocean, the source of the

observed plumes. The plumes contain both solids — ice grains

and small silica particles — and gases, including methane,

nitrogen, hydrogen and carbon dioxide. On Earth, primitive

microorganisms known as archaea use the chemical reaction of

carbon dioxide and hydrogen into methane as an energy source.

Enceladus’s ocean might even contain such simple lifeforms,

though that will only be determined by a future mission.

These are but a few of the discoveries by the 12 science

instruments of the Cassini spacecraft. All have provided

troves of observations that will fuel scientific discovery for

decades. Here we have showcased some of the almost 400,000

images taken with Cassini’s narrow- and wide-angle cameras

and what they’ve taught us about this incredible destination.

„LUKE DONES, a member of Cassini’s imaging team,

explores planetary rings, the dynamics of comets and

asteroids, and the impact history of the Solar System at

Southwest Research Institute.

CASSINI’S SATURN