By Duncan Lunan

As Saturn approached opposition, at its nearest to Earth in mid-January 2005, the major topic was the Titan atmosphere entry and the landing on January 14^th by the Huygens probe, released from Cassini on December 25^th – on the anniversary of the loss of Beagle 2. 

The cover painting by Gavin Roberts for my book Man and the Planets showed a cluster of Huygens-type probes approaching Titan on the back of a Waverider vehicle, originally designed for the British space programme by the late Prof. Terence Nonweiler.  The painting dates from 1981 and remarkably enough, since we didn’t know then when Cassini-Huygens would arrive, the Cassini approach pictures show the ring shadow in exactly the phase of Gavin’s painting.

Cassini was launched in October 1997, gaining speed in two later flybys of Venus and one of the Earth and making a distant pass by Jupiter in December 2000.  In February that year a communications problem was discovered between Cassini and Huygens, requiring a change in the mission profile at Saturn. 

Cassini entered orbit around Saturn on July 1^st 2004, performing a rocket braking manoeuvre above the sunlit face of the rings, after approaching them on the shadowed side, as Pioneer 11 did in 1979. 

The Huygens lander was named after Christiaan Huygens, who first recognised the nature of the rings and discovered Titan;  the Cassini orbiter after Jean Dominique Cassini, the Italian astronomer who became the first Director of the Paris Observatory under Louis XIV, and discovered Iapetus and the largest gap in the ring system.  

Parking orbit in the Cassini Division, between the A and B rings, seemed like a good idea at one time, but the Voyager photographs show that the ring divisions are not empty.  In fact they have many very narrow rings, for reasons not understood, and at least one elliptical ring, which seems downright impossible but is a fact!  Plans for Cassini  (then called Mariner Saturn Orbiter)  to fly through the Cassini Division were quietly dropped, but it still had to make two dangerous crossings of the ring plane further out, beyond the edge of the visible rings.  All three of the previous probes survived that, but Pioneer 11 missed one of the previously unknown shepherd satellites of the F-ring by only 100 km, and Voyager 2’s camera platform seized up there temporarily due to overwork.   Even at what looked like a safe distance Cassini was hit by around 10,000 particles, none of them large enough to do significant damage, fortunately.  

Phoebe

Cassini’s Saturn approach passed Phoebe, the outermost moon, which is a very long way out, in a retrograde orbit, with high orbital eccentricity, but in the plane of the Ecliptic, whereas all the other moons are in the plane of Saturn’s equator.  All this indicates that Phoebe is a captured object, but remarkably, although it’s less than 100 km in diameter, it looked spherical, and dark, with a blotchy surface, in the far-encounter Voyager pictures.  All that led Drs. Victor Clube and Bill Napier to suggest that Phoebe may be a captured ‘super-comet’, like the one which they suggest broke up in the inner Solar System around 3000 BC  (The Cosmic Serpent, Faber, 1982).   The new photos were amazing, showing a most irregular surface which is far from spherical, with water, methane and carbon dioxide ice, huge craters, extraordinary changes of level and a smooth crust which seemed to show evidence of melting. 

Iapetus

The next moon in, Iapetus had a story of its own to tell, of which we literally don’t know the half.   The curious thing about Iapetus is that one side of it is more than ten times brighter than the other.  In the novel of 2001, a Space Odyssey, Arthur C. Clarke gives us a landing on Iapetus on top of a gigantic version of the black monolith, which suddenly turns hollow and “full of stars”.  A depressing number of people will still tell you that they didn’t understand the ending of the film until they read the book, although the two endings are totally different.  To begin with, in the film Stanley Kubrick dropped the idea of going on from Jupiter to Saturn because he thought the two planets looked too similar, apart from the rings.  The ring footage was afterwards used by Douglas Trumbull in the deeply disappointing Silent Running.

The Voyager 2 flyby showed us Iapetus but only added to the mystery.  “Most of the surface is bright and apparently consists of ordinary water ice, just like Rhea  [one of the other moons].  But the leading face  (except near the poles)  is black, as black as tar or asphalt… Presumably, the dark face represents a relatively thin coating on a basically icy satellite, although this is a conclusion based on the indirect evidence of the low overall density of Iapetus, not on direct observations.  In particular, there are no white spots or bright ray craters in the dark hemisphere, as would be expected if occasional meteoric impacts punched through a thin surface layer of black material.  [Some of the individual photo captions disagree with that – DL.]   The dark material is either thick or it is constantly renewed to cover up bright impact ejecta.

“If the black material is just a coating of strange composition, we can ask whether it is of internal or external origin.  The fact that it is distributed so uniformly on one hemisphere and oriented so precisely toward the direction of motion cries out for an external origin, perhaps in the form of dust drifting inward from the outer satellite, Phoebe.  (Phoebe, however, has a slightly different colour from the dark side of Iapetus.)  Such an origin would also explain the ability of the dark material to renew itself.  But the best Voyager images also reveal convincing evidence that dark material on the trailing side is concentrated in crater floors, as if it had been generated from inside.  Thus, a number of Voyager scientists are convinced that the black material has an internal origin, and they suggest that it may have been produced by eruptions of methane from the interior.  According to this theory, Iapetus is unique in having the black coating because it is the only icy satellite that included methane in its original inventory of materials.

“Even after the Voyager 2 flyby, which yielded pictures of Iapetus with a resolution better than 20 kilometres, the basic question of the origin of its black coating remains open…”  (David Morrison, Voyages to Saturn, NASA SP-451, US Government Printing Office, 1982).   Cassini’s photos of Iapetus don’t explain it, but it must have to do with the ring in which both moons are enveloped.

But the new images of Iapetus are truly astonishing nonetheless.  The dark hemisphere is lower overall than the bright one:  at the division, the wall of a large crater has slumped in a huge landslide like those in Valles Marineris on Mars  (‘Sights of Saturn’, Astronomy Now, February 2005).  In the 1960s, similar features in the dark-floored crater Tsiolkovsky on the lunar Farside were thought possibly to indicate buried glaciers, because they resembled the burial of the Sherman glacier in Alaska by the ‘Good Friday’ earthquake of 1964.  The presence of ice wasn’t confirmed in detailed studies of Tsiolkovsky by the Apollo missions  (H. Masursky, G.W. Colton, F. El-Baz, Apollo over the Moon, a view from orbit, NASA SP 362, US Govt. Printing Office, 1978), but on Iapetus it’s almost certain.  Cassini has also discovered that Iapetus has a quite extraordinary ring of mountains, up to 20 km in height like Olympus Mons on Mars, running right around Iapetus ‘like a seam’ almost at right angles to the division between the light and dark hemispheres.  Now we know what to look for, it’s visible, though blurred, in the Voyager images of 1981.  Since then, images compiled by Cassini between 2005 and 2007 have revealed that the small moons Pan and Atlas are saucer-shaped, with huge equatorial ridges, which suggests that some previously unknown process has formed all three  (JPL Photojournal composite image ‘PIA08405: Saturn’s Saucer Moons’.)

Titan

In 1948 Kuiper discovered that Titan, Saturn’s largest moon, had an atmosphere of methane.   Chesley Bonestell produced a beautiful painting of a crescent Saturn in a dark blue Titan sky, reproduced in Willy Ley’s The Conquest of Space  (first published 1949, UK Sidgwick & Jackson 1952), which was also the cover of Life on Other Worlds by the Astronomer-Royal, Sir Harold Spenser-Jones  (Hodder & Stoughton paperback, 1959).   Many other artists followed suit.   In Nine Planets by Alan E. Nourse  (Harper, 1960)  Mel Hunter portrayed it in a bright blue sky, and Ludek Pesek gave it a green one, due to the methane, in The Moon and the Planets  (Paul Hamlyn, 1963).  

During the 1970s discussion project on the exploration and development of the Solar System, for Man and the Planets, illustrated by the late Ed Buckley and Gavin Roberts.  During it we learned from the book Mars and the Mind of Man  (Harper & Row, 1973)  that there were red clouds in the Titan atmosphere.  I urged Ed Buckley to be the first to paint Saturn in a red sky, but as we had no more information on the clouds, Ed decided to hedge his bets.  He painted Titan seen from space with small red clouds, and a view from the surface with a pink sky – but it was a clear sky, at sunrise.  Unfortunately the first publisher decided not to go ahead with the book, and Ed was beaten to the punch by other artists including David Hardy in his book Challenge of the Stars, with Patrick Moore.  Meantime much more information was coming in from probes exploring the Solar System and eventually the project grew into two books New Worlds for Old  (David & Charles, 1979)  and Man and the Planets  (Ashgrove Press, 1983).   Ed’s Titan paintings appeared in New Worlds for Old, only just in time before they went out of date.

Titan is larger than Mercury, the second-largest moon in the Solar System and only 50 miles less in diameter than Jupiter’s Ganymede, but its density is low and surface gravity is one-quarter of Earth’s, like Mars, and possibly even lower.  The Pioneer 11 flyby of 1979 was intended as a pathfinder for the Voyagers, and when Pioneer’s Titan data was lost due to a tracking station failure, detailed study of Titan was considered to be so important that it was decided to send both Voyagers to Titan rather than sending Voyager 1 to Pluto.  Instead, at Carl Sagan’s instigation, its cameras were activated one last time in early February 1990, to take a composite image of the entire Solar System in which Earth appeared as the classic Pale Blue Dot  (title of Carl Sagan’s book for Headline, 1995).  

In ASTRA’s project, Gavin Roberts had taken the opposite view from Ed’s and guessed that Titan would be completely cloud-covered.  He produced a painting which I intended for the Man and the Planets cover, and it proved accurate except that he had only a hint of red in the clouds.  The Voyager flybys of the Saturn system showed Titan’s atmosphere was bright orange-red, filled with a deep smog of organic compounds synthesised from the methane by the action of solar ultraviolet light – perhaps like the primal Earth, frozen half-way through the processes which led to the origin of life.

It seems unlikely that Saturn could be seen from Titan’s surface, but some surface features had been seen through the clouds in the infra-red even from Earth.  As Cassini began its mission in the Saturn system, first images showed an impact crater and complex features, possibly mountains and rivers, near the south pole.  At –200^o C, any liquid on Titan must be methane or more complex organic compounds.  The landing was on January 14^th, 2005, and as we didn’t know what it would find down there, Huygens had been built to survive anything including solid rock, ice, liquid methane or an ocean of tar.  In the event, it landed on the equivalent of wet sand – see Part 2, next week. 

At ground level atmospheric pressure on Titan is roughly one and a half times Earth’s, which means that aircraft will require comparatively small wings since the gravity is lower, possibly one-seventh of Earth’s.  Robert Zubrin, the architect of the proposed ‘Mars Direct’ manned missions, has designed the NIFTE  (Nuclear Indigenous Fuelled Titan Explorer)  which would have a mass of 8 tonnes and would require only 4 square metres of wing at 160 kilometres per hour, “(practically no wings at all!)”, or 100 square metres at 32 kph..  NIFTE could release a range of 10 kg sub-probes with rotors, lifting cells or other combinations  (Robert Zubrin, ‘The Case for Titan’, Ad Astra, June 1991).  Because the surface air density is 1.5 times that on Earth, “a human standing on Titan could fly by strapping wings on to his or her arms in the manner of Daedalus and Icarus”.  My colleague Gordon Ross came independently to the same conclusion, looking for a way to explore Titan without generating enough heat to disrupt the environment.  Interestingly, if the gearing problem could be solved for Titan, ornithopters could be made to work on Earth, at last, with modern compact power supplies and high-strength, lightweight materials.  Leonardo da Vinci wasn’t wrong, just a long way ahead of his time!  In October 2005 NASA’s outer Planets Assessment Group proposed a Titan Orbiter Aerorover Mission, using an airship 14 metres long and able to travel round Titan’s equator in two weeks  (Keith Cooper, ‘Return to Titan’, Astronomy Now, January 2007.)  Studies for aerial Titan explorers are ongoing, and have been given a boost by the success of the Ingenuity helicopter on Mars in 2021.

The complexity of conditions on Titan was revealed by the extreme ultra-violet spectrometer on Voyager 2, one of whose creators, Darrell Strobel of the US Naval Research Laboratory, engagingly described it as ‘a hobby’ on a visit to Scotland in October 1980.  Over billions of years a complex organic chemistry has built up including hydrogen cyanide, propane and acetylene.  At the surface there could be seas of liquid methane or a thick layer of hydrocarbon tar, Dr. Strobel predicted;  Arthur C. Clarke described even more elaborate possibilities in Imperial Earth  (1975), and the late James Blish envisaged ultracold life-forms in ‘How Beautiful with Banners’  (Best Science Fiction Stories of James Blish revised edition, Faber, 1973).  Could there be internal heating?  If so, at the surface water would flow like lava out of water volcanoes.  The vapour would freeze as the Sun went down:  ‘red lumps in the sunset…’  In February 2005 Cassini confirmed that there are water volcanoes on Titan which are also driven by ammonia.     

(To be continued) See also: Beginners Astronomy: Saturn and its Rings