Australian Sky & Telescope - April 2018

12 AUSTRALIAN SKY & TELESCOPE April 2018

DISCOVERIES by David Ellyard

Riddle of the rings

Centuries of study have opened our eyes to Saturn’s wonders.

T

he rings that surround Saturn

are among the great wonders

of the Solar System, even more

so nowadays when spacecraft and

powerful telescopes have revealed their

astonishingly intricate beauty.

Galileo was the first to glimpse them

with his primitive telescope in 1610, but

he couldn’t understand what he saw. The

planet appeared to be not a simple disk

like Mars or Jupiter, but displayed some

sort of protuberances on either side. Did

Saturn have ‘ears,’ or was the planet

really three planets in close proximity,

with the big one in the middle? He was

even more puzzled two years later when

the phenomenon disappeared (as a result

of the Earth passing through the plane

of the ‘rings’ and seeing them edge-on).

In time it returned.

By the end of the 17th century, using

much better telescopes, observers such

as Christian Huygens and Robert Hooke

had solved the mystery. As Huygens

announced in 1655 in a statement

hidden in an anagram (to protect his

priority in the discovery, as Galileo had

done), “Saturn is surrounded by a flat

ring that nowhere touches the planet”.

Confirming the discovery, Hooke was

able to see that the rings cast a shadow

on the planet and the planet cast a

shadow on the rings.

As time went by and technology

improved, we learned more. Jean-

Dominique Cassini found there were

several rings, not just one, and his

name became synonymous with the

major division within them. Now we

know there are many rings, dozens,

hundreds depending on how they are

counted. When the Pioneer and Voyager

spacecraft visited the planet late in the

20th century, they showed the rings

looking like the grooves of an LP record.

But what were they made from,

and how had they formed? A century

after Cassini, the great mathematician

Pierre-Simon Laplace proved the ring

or rings could not be solid, since the

powerful gravity of the planet, pulling

differentially on various parts of the

ring(s), would tear it apart. Perhaps a

number of ringlets might survive, or a

ring made out of some sort of liquid.

Half a century on again, James Clark

Maxwell sorted it out. No pattern of

solid rings or ringlets or liquid rings

would survive. The rings must be made

of countless tiny fragments, all orbiting

the planet independently like so many

minute moons. If that were so, each

fragment would be controlled by the

same laws that guide the motions of

the planets... laws devised in the early

17th century by Johannes Kepler. This

would require the particles making up

the inner edge of a ‘ring’ to move more

quickly that those near the outer edge.

And so it proved. In 1895 — on April

9, which is why were are celebrating the

discovery this month — US astronomer

James Keeler of Allegheny Observatory

announced the results of a study

undertaken with a spectroscope (which

breaks light up into its many colours).

Comparing light reflected from the

inner part of a ring with that bouncing

off an outer region, he saw shifts in the

colour of the light (due to the Doppler

Effect) consistent with the predicted

differences in orbital speed. So Maxwell

was right. A few years later Russian

astronomer Aristarkh Belopolsky of

Pulkovo Observatory found the same

thing, using methods that also showed

that the clouds of Jupiter moved at

different speeds at different latitudes.

As to how the rings were formed,

again the gravity of the planet is the key

factor. The region of the rings lies with

the ‘Roche Limit’ for Saturn, where

gravitational forces from the planet

would tear apart any large object. The

rings are made up from matter left over

from the formation of the planet and

which might have formed into a moon,

had Saturn allowed it.

So the major mystery of the rings

of Saturn was solved, but recent

observations, such as by the successful

(and recently ended) Cassini mission,

have thrown up many more questions.

More rings, the intricate patterns and

movements within them, and the role

of tiny embedded moons, will keep

researchers going for many years yet.

■ DAVID ELLYARD presented SkyWatch

on ABC TV. His StarWatch StarWheel

sold over 100,000 copies. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE