Flamsteed Astronomy Society

Saturn’s rings and the Cassini Mission

— November 2, 2004

Carl D. Murray — Professor of Mathematics & Astronomy, Queen Mary, University of London

Carl Murray talked to us about Saturn’s rings.  He has been a member of the Cassini Imaging Team since 1990 and with the mission well underway and producing breathtaking new pictures and data daily, Carl delighted us with his clarity and enthusiasm, and shared some of the excitement of the mission team.  He began with a short survey of the development of knowledge of Saturn’s rings.   Galileo was the first to observe them through his brand-new telescope in 1610.  The poor optics gave him a very shaky view and at first he thought Saturn was a triple planet.  The effect gradually disappeared though (in fact because Saturn’s orientation to Earth was changing and, unknown to Galileo, his view of the rings shifted to side-on where they are not easily visible at all).  Christiaan Huygens was the first to identify the ring system correctly for what it was.  His 1659 book Systema Saturnium (The Saturn System) remains still one of the clearest expositions, and Huygens also discovered Saturn’s moon Titan, now one of the prime targets of the Cassini Mission.  Working in Paris at the same time as Huygens, the Director of the Paris Observatory Giovanni Cassini discovered four more moons and in 1676 observed the gap in the rings later called the Cassini Division. 

One of Huygen’s drawings from The Saturn System 1659.  His diagram of the changing views of the rings remains one of the clearest ever.

Eventually in 1856 James Clerk Maxwell showed that the rings couldn’t be solid but must be composed of ‘an indefinite number of unconnected particles’.  Maxwell is also known (among one or two other small things like the theory of electromagnetic waves) for his ground-breaking work on dropping cats to see how fast they could turn to land on their feet.  Our knowledge of the rings exploded with the successful probe missions, Pioneer 11 in 1979 and the Voyagers in 1980/1.  It was the Voyager pictures that caught Carl’s imagination and started his investigation of the newly-discovered F-ring.

The Voyager pictures revealed the fascinating complexity of the ring system. Carl explained that our knowledge of the rings has now built prodigiously. We can explain all the features of the A-ring.  The culprits are Saturn’s moons which disturb the ring particles by their gravitational pull and ‘resonance’ effects—the time relationship between a moon’s orbit and the ring feature.  But the B-ring remains to be explained and many of the features continue to intrigue: why are some rings broad (A-D), some narrow (F,G), and some diffuse (E)?  What causes the radial features or ‘spokes’?  Why are there gaps in the rings, and rings in the gaps?  And what explains the lifetime of the ring system?  Why is it still there when the particles should be drifting in toward the planet, and the small moons drifting out?   There are high hopes that the Cassini-Huygens Mission will provide some answers.

Eyeing the culprit.  Mimas causes the ring gap in the foreground (NASA/JPL/SSI)

The Cassini/Huygens Spacecraft  (NASA/JPL/SSI)

The discovery of S/2004 S3 August 2004  (NASA/JPL/SSI)

Thousands of people in both Europe and the US have contributed to the Cassini-Huygens mission. Planning started in 1989 and UK scientists were very successful in winning key roles — 6 of 12 instruments on Cassini and 2 of 6 on Huygens are UK developments.  The probe was launched in October 1997 and arrived at Saturn in July 2004 after ‘gravity assist’ fly-bys of Venus (2), the Earth, and Jupiter.

Carl has had a high-profile role in the mission’s Imaging Team — at least in the UK media: challenged by John Humphries on Radio 4 one morning at 7:45am, Carl had to explain ‘what is the point of all this?’  He gently pointed-out that in the Saturn ring system we have a planet-forming solar system in miniature.  If we can’t understand this on our doorstep, how are we to explain systems we’re beginning to detect thousands of parsecs distant?   His finest hour (at least to date!) came in August 2004 when he spotted a new moon ‘S/2004 S3’ in the images (left) on the edge of the F-ring.   His excitement came partly from the fact that the last discovery in the UK of a solar system moon was of Jupiter JVIII in 1906 by Philip Melotte at Greenwich — just a hop, skip & jump from Carl’s desk in the Mile End Road!

We have another 4 years (maybe 6 years!) of the mission to look forward to.   The Huygens probe is planned to detach on Christmas Day and land on Titan in mid-January.  The Cassini Orbital Tour will provide 46 more fly-bys of Titan and a view of the rings from 70 degrees above the equatorial plane.

Watch this space!  (No pun intended, he lied)

Mike Dryland