Culture

Beginners Astronomy: Saturn and its Rings

By Duncan Lunan

Saturn from Voyager 2

At an average distance from the Sun of 886 million miles, Saturn is so far out that Soviet astronomers proposed to place two telescopes in the Sun-Saturn Trojan points  (also called the Sun-Saturn Equilaterals).  The Lagrange points are special solutions to the 3-body problem, which has no general solution to how a small orbiting body moves within the pulls of two other bodies.  In three of the five such solutions, L1, L2 and L3, the bodies are all in a straight line;  these are dynamically unstable, and L3, on the far side of the Earth from the Moon, is especially so.  But L4 and L5, which form equilateral triangles with the Earth and the Moon, the Earth and the Sun, etc., are comparatively stable.  There are groups of asteroids called ‘Greeks’ and ‘Trojans’ circulating around the Sun-Jupiter L4 and L5 positions, and NASA’s Lucy mission is now on its way to explore them.  Both Earth and Mars have at least one Trojan asteroid, several of Saturn’s moons have Trojan companions, and there have been reports of dust clouds in the Earth- Moon Equilateral positions.   But with a separation of almost 1800 million miles, twice Saturn’s distance from the Sun, Saturn Trojan observatories would be able to map the Universe in three dimensions all the way to the red limit of observation.

Saturn takes 29½ years to orbit the Sun and is passed  by the Earth at intervals of little more than a year, so like Jupiter it stays in the night sky for months, less bright than Jupiter and moving more slowly relative to the stars.   A small telescope will not resolve the rings but will show that there’s something more than just a disc – Galileo thought there might be three planets!

Saturn ring orientation Hubble Space Telescope

There’s a famous view of Saturn’s rings from within the atmosphere in a painting by Chesley Bonestell in The Conquest of Space by Willy Ley, animated in the opening shots of the Pal/Bonestell War of the Worlds  (1953).  Back then the usual description of Saturn was of a rock and metal core overlaid with ice, as in the Wildt model of the giant planets which was then generally accepted  (see Jupiter notes). Jupiter and Jupiter’s Moons

Temperatures inside the gas giants are actually very high, although they’re very cold on the outside, and although Saturn does have a rock and metal core, the size of the Earth but with nine times its mass, the core is overlaid by liquid hydrogen and helium at very high temperatures, caused by the inward separation of the helium under the high gravity at those depths.  Saturn is the only one of the gas giants to have a density which is actually less than water, making gravity at the visible surface virtually equal to Earth’s, as it happens.  If Saturn was placed in a big enough bathtub, it would float – although, as Warren James of the Galileo project pointed out, when you took it out it would leave a ring behind.

Saturn inner system, rings and moons

Outwardly Saturn’s atmosphere looks less turbulent than Jupiter’s, but much of that is due to a haze of high-altitude ammonia crystals which blankets the planet.  Below it the storms are at least as violent as Jupiter’s and the winds even stronger – when ammonia thunderstorms appear as white spots in Saturn’s atmosphere, they spread swiftly in longitude far round the planet.  

It may be that each giant planet requires one huge ‘soliton’ storm to stabilise its atmosphere.   Jupiter’s Great Red Spot has persisted since the 17th century, and at the time of Voyager 2’s flyby of Neptune in 1989 there was one in the southern hemisphere, which disappeared, but the Hubble telescope imaged a similar one in the north.  Saturn has a huge hurricane with a central eye at the south pole, with another in the form of a hexagon at the north pole which has been there since the Voyager flybys, at least.  Only Uranus isn’t known to have one, but because of its bizarre ‘on its side’ rotation, the Sun was overhead at the south pole when Voyager 2 passed in 1986 and the north pole, remarkably bright when side-on in 2013-17, is only now turning more fully towards us.

The mystery of Saturn’s appearance which so baffled Galileo was solved by Huygens, who realised that there was a ring around it, nowhere touching the planet, and by Cassini who realised that the ring wasn’t rotating as a solid system but as a vast number of independently orbiting particles.  Cassini also discovered the biggest gap in the rings, named after him, which became the name of a military unit in Ken MacLeod’s novel The Cassini Division.  It lies between the A and B rings, with the darker C-ring  (‘the Crêpe Ring’)  on the inside and a much fainter D-ring inside that  (once called ‘the Loch Ness Monster of Saturn’ by professional astronomers who didn’t believe amateur sightings.)  But the neat system was spoiled in 1979 when the narrow F-ring, just outside the main system, was discovered by Pioneer 11 and later photographed by the Voyagers:  it proved to have a twisted structure, governed by the pulls of its two shepherding satellites, but initially provoking the baffled comment, “You have a mean sense of humour, Darth Vader”.  Amateurs had also reported a ring further out, now called the E-ring, which is generated by geysers of water at the south pole of the ice moon Enceladus  (see below).  Still more rings have been found still further out, the outermost and broadest apparently coming from Phoebe, the outermost known moon.  

Voyager 2 image of ring spokes
Voyager 1 F-ring braiding at 470,000 miles

Both Voyager spacecraft tried to photograph the rings edge-on as they flew through the Saturn system in 1980 and 1981, but neither succeeded.  At their speed, that meant the rings had to be less than half a mile thick.  As of January 2006 Cassini hadn’t succeeded in catching the rings edge-on either, but had reduced the maximum thickness to a hundred metres, and at Saturn’s equinox in 2009 when the rings were edge-on to the Sun, boulders 100 metres in diameters could be seen casting shadows across the rings, so the overall thickness was much less, although the diameter of the visible system is as large as the Moon’s orbit round the Earth.  For a time, it seemed that the brightness of the ice particles indicates that the ring system couldn’t be more than a few hundred million years old, but then we learned that they’re being replenished from eruptions on Enceladus, and by 2013 Cassini observations were indicating that the rings were very ancient, though resurfaced by ice from Enceladus.  The B-ring has also been resurfaced by reddish material, possibly iron oxide or organic compounds  (Nancy Atkinson, ‘Solar System Antiquities Abound in Saturn’s Rings’, Universe Today, March 28th 2013).   That might explain radar returns indicating metal in the B-ring, and perhaps explain the composition of the spokes.  

Voyager & Cassini image contrast

Shadows aren’t visible in space and only become apparent if they fall on something, like the shadow of Saturn’s rings on the planet or the shadow of the planet on the rings.  When a band of the surface falls under the ring shadow, it doesn’t see the Sun again, except through the gaps, for 15 years.  (W.M. Higgins, Researchers in the Solar Realm, Hall, Virtue & Co., 1852.)   The artistic device of making the shadows of planets and moons or other objects visible is often used in diagrams illustrating the processes of eclipses:  it looks good, though it isn’t physically possible unless there’s some medium for the shadow to be projected through. 

satellite Pan shadow on A-ring from within the Encke Gap

Remarkably enough, above Saturn’s rings there is one, though probably not thick enough to produce this effect.  But in 1966, when the rings were edge-on to Earth, the Soviet spectroscopist Kozyrev found that the ammonia band was stronger and the methane one was weaker than elsewhere on the planet.  That meant that Saturn’s surface was warmer in the shadow of the rings, due to the greenhouse effect of an atmosphere of water vapour, coming off from the sunlit face of the rings and disassociating into hydrogen and oxygen under the action of sunlight  (M.S. Dobrov, The Rings of Saturn, Nauka Press, Moscow;  trans. NASA TT F-701, US Government Printing Office, June 1972).  It seems nobody read Soviet scientific literature, even in NASA’s Technical Translations, because when Pioneer 11 confirmed the ring atmosphere in 1979, NASA announced it as a new discovery, for which they were rightly rebuked by the Soviet Academy of Sciences.  In 2005 the Cassini Saturn orbiter ‘discovered’ the ring atmosphere all over again.

Since 1981 scientists have been puzzled by Saturn Kilometric Radiation  (SKR), radio bursts detected by the Voyager spacecraft, not synchronised with the planet’s rotation and not perfectly in synch with its magnetosphere either.  Richard C. Hoagland, better known for the controversy about ‘the Face on Mars’, published a two-part article in Analog entitled ‘The Blivit in the B-Ring’  (December 1982/ January 1983).  My US friends told me that it had to be a hoax, because in baseball a ‘blivit’ is a bag of unmentionable material hurled on to the field to disrupt a game.  When I met Hoagland at the ‘Case for Mars 2’ conference at the University of Colorado in Boulder in 1984, I challenged him on that and he conceded that the word did have that secondary meaning, but normally meant something strange or inexplicable.  He claimed that the Voyager data showed powerful electrical discharges in the middle of the B-ring, and a clear gap right at the same location.  I’ve seen a film which shows a plot of the light of the star Delta Scorpii, monitored as it passed behind the rings, and there’s no gap where Hoagland says that there should be – but the film version is a simulation, and although I’ve asked, I haven’t seen the actual photomultiplier trace.  

There certainly are powerful magnetic and electrical forces operating in the rings, and the Voyagers discovered extraordinary radial spokes silhouetted over them, revolving around Saturn with the planet’s magnetic field.  The material couldn’t be in orbit and retain that radial form, and photographs of the spokes backlit by the Sun confirmed that they’re composed of dust which is presumably diamagnetic and levitated out of the ring plane by the magnetic field.  But that would indicate that they’re metallic, where the rings seem to be composed overwhelmingly of water ice.  The Cassini orbiter didn’t see any spokes until after September 2005, because it hadn’t yet viewed the rings from the same perspective as the Voyager approach;  when they did appear, surprisingly they could be seen on the shadowed side of the rings  (Barry Shanko, ‘Saturn’s Spokes Have Spoken’, Astronomy Now, November 2005).

Saturn-Earth sizes, Mt. Palomar and Apollo 17 images

Jupiter’s system emits similar decametric radio bursts as its volcanic moon Io interacts with its magnetosphere, which is fed by eruptions from Io which form a torus of ions surrounding the planet.  Since the Pioneer 11 flyby of Saturn it’s been known that the rings absorb most of the particles which the planet’s magnetic field traps out of the Solar Wind, but Cassini has found a much more complex situation in the orbit of Enceladus, where the plumes of dust and water vapour are absorbing charged particles at the same time as generating new ones, forming complex structures within a similar torus  (Nancy Atkinson, ‘Enceladus is Blowing Bubbles’, Universe Today, April 15th, 2010).  Now it seems that interactions between the dust and the magnetosphere particles may also explain the kilometric radiation, without the need for Hoagland’s mysterious ‘blivit’ – but the detailed mechanism has still to be worked out.   (Tammy Plotner, ‘Dusty Plasma from Enceladus Might Affect Saturn’s Magnetosphere’, Universe Today, January 3rd, 2012.)

For a really detailed description of Saturn and its rings, with the seasonal changes of 2004-20, I recommend the Astronomy Now book Saturn, Exploring the Ringed Planet, written and edited by Keith Cooper  (Pole Star Publications, 2015).  As it says on the cover it has ‘hundreds of amazing pictures of Saturn, its rings and moons’.  It covers the entire Cassini mission except for the last few orbits and final plunge into the planet’s atmosphere in 2017.  On the website, https://astronomynow.com, it’s under magazines/special editions, and costs £9.99, but last time I looked it was on offer to subscribers at £5 – two investments well worth making.

Voyager Badges

(To be continued)

Interested in Space and Astronomy? There are many articles in The Orkney News About this subject. The Sky Above You – October 2021

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