Page 35 - BBC Sky at Night - September 2017 UK

CASSINI AT SATURN SEPTEMBER 35

The sunlit side of Saturn’s
iconic rings, with bright
spokes visible above
! Cassini imaged Titan through its murky atmosphere,
SATURN’S SPARKLING
revealing what we now know to be lakes of methane
another worry arose. The risk of a long-dead
and out-of-control Cassini someday hitting and
contaminating pristine Enceladus did not bear SPOKES
thinking about. A safe disposal was imperative.
Saturn’s hydrogen-rich atmosphere provided
that means of disposal, as well as offering the Cassini has taught scientists much about the
potential to gain new insights into the formation mysterious features in the planet’s rings
of the giant planets. Specifically, an enigmatic
Although Pioneer 11 and the two high above the plane of the rings by
region of liquid metallic hydrogen, deep inside
Voyager spacecraft observed electrostatic charging. Cassini showed
the gaseous planet, is thought to power Saturn’s Saturn’s magnificent assemblage these spokes appeared and dissipated
magnetic field. “Cassini will look for the of water-ice rings more than three in a matter of hours, and its data
boundary between the molecular hydrogen decades ago, it was not until the suggested that they might represent
layer and the metallic hydrogen layer in the arrival of Cassini that their nature a seasonal phenomenon throughout
was more precisely understood. Saturn’s 29-year orbit, vanishing in
magnetometer data,” says Spilker. “Gravity
Even as the spacecraft completes midwinter and midsummer, and
data may reveal whether or not Saturn has a
its Grand Finale of 22 ‘dives’ reappearing as the giant planet
rocky core, one or two Earth diameters in size, between the innermost edge of approaches the equinoxes.
and the depth of Saturn’s winds.” the rings and the gas giant itself, With a breadth of 282,000 km, the
The result was the Grand Finale: a multi- it is revealing astonishing new rings extend the equivalent of three-
month campaign that required the spacecraft details, including features likened quarters of the distance between Earth
to straw and propellers, on and the Moon, and their dazzling
to execute 22 death-defying ‘dives’ between the
scales the size of the Earth’s reflectivity owes itself to the fact that they
planet and its rings from April to September 2017.
tallest buildings. are composed of 99.9 per cent water-ice,
On five of those dives, Cassini would actually The Voyagers observed strange along with contaminants. Cassini’s radar
enter the upper atmosphere, approaching > radial ‘spokes’, up to 16,000km and visible observations – together with
long. The spacecrafts’ data on the stellar occultations – have shown varying
spokes’ formation and dissipation dimensions of ring particles, which
lifecycles implied that they were extend from sand-like grains to
indeed small particles, suspended mountain-sized boulders.

! The propeller ‘Earheart’, a disturbance ! Straw structure due to clumping,
! Cassini not only spotted jets spewing from Saturn’s in the ring caused by a central moonlet observed on the edge of Saturn’s B ring
moon Enceladus; it determined they were mainly water

skyatnightmagazine.com 2017