Mars and its Pathfinder may have been the star astronomical attraction of 1997, but most of the crucial evidence for the history of the solar system lies not on planets but on and in the moons of the outer planets—particularly the small moons.
In the face of the majestic bulk of Jupiter and Saturn, this truth may seem paradoxical, yet it is a reflection of the adage "the greater they are the harder they fall." The sheer mass of the planets means that their material is much changed from its original form. Gravitational collapse and radioactivity have melted the cores, allowing high-density materials such as iron and nickel to concentrate at the centre beneath a mantle of oxides and silicates, of ices and liquids.
The planets as we know them are the products of some five billion years of chemical reactions occurring under a wide variety of conditions, brewing compounds unknown in the original mix. Even after the initial condensation of the planets of the solar system, the circum-solar disc contained a wealth of surplus material which was to bombard the newly-formed planets and their satellites, adding significant quantities of material to them. Thus the record of the original materials and formation conditions is smudged and jumbled, if not erased.
In the case of the gas giants there is clearly no hope of ever detecting the remains of even recently colliding bodies. Where and in what form are the fragments of Comet Shoemaker-Levy now? Even bodies as substantial as the gas giants' own major satellites, with radii of 2000 km, could disappear without trace, sinking and melting into the molten core.
For the terrestrial planets the case is little better. Volcanic activity and the never-ending processes of erosion, to which in the case of Earth is added plate tectonics, relentlessly refashion the surface, redistributing any foreign matter so that the further into history we delve the more confusing the geological story becomes and the harder it is to state with any certainty just what conditions were a billion or few years ago.
So it is amongst the least of the heavenly bodies that astronomers hope to find samples of the original stuff of the solar system. They believe that comets preserve in their interiors the material from which the outer solar system condensed, for since the formation of the solar system these chunks of frozen volatiles have been orbiting the Sun out beyond Pluto (where they form the Kuiper Belt) and beyond that the Oort Cloud, extending out to about two light years away—halfway to alpha Centauri, our nearest star. Running a close second in the putative purity stakes are some of the asteroids and the small moons, and these have the great attraction that their exploration is within our reach with current technology and budgets.
The Voyager fly-bys in 1979 and the Galileo pictures still being transmitted to Earth show that the four largest moons of Jupiter have landscapes stranger and more various than any illustration in Astounding Science Fiction. There is Io, a world covered with sulphur, which is ejected up to nearly 200 km above the surface in vast symmetrical umbrella-shaped plumes from a number of very active volcanoes. Europa, smooth as a billiard ball, is marred by a random network of mysterious "tracks." Then there is the icy surface of Ganymede with its strange and yet-to-be-explained grooved terrain, a mesh of strands each composed of several parallel shallow troughs, possibly graben formed by sinkage between parallel faults. Callisto's rocky surface appears to be unique, for it is nothing but craters, and craters on craters—quite unlike our Moon, Mercury or Mars, which, although completely cratered, all have areas of their surfaces covered by later flows of magma. But Callisto apparently solidified early and so it became a totally inert target in spite of impacts great enough to shock a significant part of the surface.
In addition to the four large moons of Jupiter, there are 11 small satellites which have radii of only a few tens of kilometres, and Amalthea, with a radius of 120 km. All have dark surfaces and form two groups which share Jupiter's orbit but cluster ahead and astern of it. The four inner satellites and Amalthea orbit in the hostile environment of Jupiter's magnetosphere where their surfaces are bombarded by charged particles, gas and dust ejected from Io as well as meteorites on their final approach to the planet.
Beyond Jupiter, the only close-up images that we have are those made by the Voyagers in 1979/80. More will come from the Cassini-Huygens orbiter/lander, which was launched on its six-year trip to Saturn and its satellites in October 1997. Yet even the evidence which we do have is of unexpected variety, even drama.
Saturn's largest moon, Titan, intermediate in size between Mercury and Mars, is the only satellite known to have a substantial atmosphere, composed of molecular nitrogen, argon and methane with an upper-atmosphere photochemical smog formed from the ultraviolet photolysis of the methane. The surface was long believed to be a sea of methane and ethane. However, recent radar analysis and observations with the Hubble Space Telescope indicate that there are areas of solid surface, probably frozen water, methane and possibly ammonia—ices which, at a mean temperature of 95°K on the surface, are more like granite than the stuff in the refrigerator. The density, chemistry and orbital characteristics of Titan suggest that it was formed at the same time and from the same mix as Saturn.
Saturn's lesser moons Mimas, Enceladus, Tethys, Dione and Rhea, lying between Saturn and Titan—are all ice satellites with densities ranging from 1.4 to 1.2 g/cm3, yet their surfaces differ significantly. Mimas and Rhea preserve unchanged the record of cratering, whereas the surface of Enceladus bears ample evidence of reworking with flows and fractures.
Tethys and Dione are of very similar size, yet differ significantly in mass and composition, although both present surfaces on which some of the oldest craters have been obliterated by later events.
Iaptus, lying out beyond Titan, was first seen by Cassini in 1671, who observed that it has the odd property of being visible only when it is on one side of Saturn, for as it swings around in its orbit, so it dims and disappears—an effect, which Cassini quite correctly guessed, is due to there being a dark and a light hemisphere.
Like our Moon, Iaptus always presents the same face to Saturn, and indeed, the leading hemisphere is covered with a material which is six to ten times darker than that of the trailing face. Thus as it moves around in 3 its orbit we only see it retreating. Unfortunately the low (20 km) resolution photographs from Voyager 2 do nothing to explain this phenomenon and we must wait for Cassini-Huygens.
Orbiting between Titan and Iaptus is Hyperion, similar in size to Mimas but apparently just an irregular chunk of rather dark ices, perhaps a fragment of some larger body. In addition, there are 12 much smaller irregularly-shaped satellites, two recent discoveries yet to be confirmed and probably more to be found.
Finally and most famously, there are the rings which, in spite of all the wonders of deep-space astrophotography, still remain the most beautiful of astronomical objects. This stable yet mobile collection of small to microscopic fragments which appears as a solid plane to us is in fact transparent when viewed from other angles, and so thin that it casts just a fine linear shadow on the associated shepherd satellites.
Uranus, like Saturn, is a gas giant but has only half Saturn's mass and is twice as far from the Sun. It has nine narrow but distinct rings, and its satellites fall into two clear groups: four large (600800 km radii), and orbiting inside them eleven small (radii of a mere 40-200 km). The outermost of the small group, Miranda, with a radius of 242 kin, is a giant in comparison with the others and has been called the jewel of the Voyager survey in recognition of the variety and dramatic scale of its topography. Indeed, it is a virtual compendium of satellite landscapes, looking like a cut and paste job from images captured elsewhere. There are the cratered highlands cribbed from Mimas, fractures and canyons in imitation of Enceladus and parallel troughs like those on Ganymede. A famous cliff 5 km high lends support to the idea that this jumble is actually a reassemblage of the fragments of a shattered satellite.
Ariel, Umbriel, Titania and Oberon are all, like Miranda, composed of silicate-contaminated ices formed from water, methane, ammonia and carbon dioxide so cold (less than 50°K) that they have the physical characteristics of rocks. The inner twins, Ariel and Umbriel, are of the same size and mass, but their surfaces are quite different. Umbriel is not only much darker than any of the other satellites, but also presents an apparently completely cratered surface, whereas Ariel shows only a few small craters and a surface bearing witness to major geological activity with fault scarps, depressions, valleys and recently formed plains of ice.
The outermost pair, Oberon and Titania are also virtual twins in respect of size, mass and colour, yet their surface features bear testimony to very different histories. Titania, lying closer to Uranus than Oberon, seems to have been both more heavily and more recently cratered, perhaps producing sub-crustal heating, the consequent expansion producing a network of fractures in the crust. The only photographs we have of Oberon from Voyager are of low resolution but show significantly fewer but larger craters with extensive fields of ejecta.
Although a "standard" gas giant, Neptune is a puzzle, for it has no known "natural" satellites, as both Nereid and Triton which orbit the planet seem to be captured bodies. Apart from its highly eccentric orbit, we know virtually nothing of little Nereid, which is probably an ex-asteroid. In spite of its size, akin to that of Europa, Triton remains an enigma, as its retrograde orbit allows tidal forces to draw it down towards Neptune where, in less than 100 million years, it will be torn apart by gravity.
The most remote of the satellites is Charon, Pluto's moon. In relation to its primary, Charon is by far the largest satellite in the solar system, and there is now a tendency to regard the Pluto/Charon pair as gravitationally coupled asteroids rather than as a planet and satellite.
With nine planets and more than 50 moons, asteroids by the thousands and an unfailing supply of comets the possibilities for astonishment make counting on death and taxes look like a bit of a punt. And that is only for the publicity pits; the scientific data will be quite something again as we tease out the details of the history of the solar system.