Fuzzy blobs in sharp focus

Today's vast growth in human knowledge is nowhere more in evidence than in our awareness of the solar system. Kepler, Copernicus and Galileo helped to solve the mystery of the lights in the sky called planets, and the number of known planets was swelled in subsequent centuries by the discoveries of Uranus, Neptune, Pluto and a welter of satellites and asteroids. But they were still ill-glimpsed globes, too distant for genuine scientific study. By the time of the second world war, the United States employed just one scientist, Gerald Kuiper, to study the planets.

Now, in the space age, there are thousands; and the raw material they work on has expanded vastly. From the Moon to Neptune, spacecraft have made fuzzy blobs into familiar worlds. Telescopes on and orbiting the Earth have even given significant insights into Pluto, the only planet yet to receive a robot visitor.

Television series such as The Planets have brought these developments to a wide audience. These two books do something similar, but in much more depth. Both are spectacular, and each works well for its intended market.

Even the authors' CVs have something to tell us about how science has changed. Fred Taylor is Halley professor at Oxford, an expert on atmospheres, who started out in terrestrial meteorology and is now comfortable describing bodies of the solar system whether solid, liquid or gaseous. Ronald Greeley and Raymond Batson carry certificates in geology, a science that some decades ago began to incorporate fresh planets, satellites and asteroids, starting with the Moon and gradually extending its reach.

Their book is the pricier and heftier. Its atlas-quality maps, complete with latitude and longitude grids, of every significant object in the solar system and some bit-part actors as well, would have been impossible a few years ago. Many of these objects are represented by photo-mosaics and geological as well as topographical maps. The Moon has geological periods with names such as Imbrian and Nectarian to rival our Jurassic and Cambrian, while on Mars the prospector can look forward to mineral hunting among the Hesperian and Noachian strata. Buying this volume is like bragging about being a member of Homo sapiens . Readers will be rewarded by a feast of highly systematised knowledge that is, at the same time, visually astounding.

Mapping other worlds avoids some of the problems of terrestrial cartography - which name to use, Las Malvinas or The Falklands, for example - but it throws up others. Defining zero longitude, in effect finding Greenwich for dozens of unvisited worlds, is not easy. And then there are all those ravines, volcanoes, mountains and, most numerous of all, craters, to be named. On Venus, most things are called after women, fictional or real: Guinevere, Cleopatra, Boadicea, De Beauvoir and Helen. On Mercury, the international committees have favoured arty types, with the main regions called after Beethoven, Renoir and Shakespeare. On other worlds, caves, spas and characters from the life of Charlemagne have had their day.

To its visual treats, the book adds intelligent text. But there are some problems. The scale diagrams of the satellite and ring systems are unclear. And although the giant planets and their moons make up "systems" in a full sense, there is no such thing as the "Mars system", just a planet that happens to have captured a couple of asteroids.

Taylor's book, while less massive, is still substantial. But its production has involved compromises, especially the concentration of most of the pictures into special sections at the end of each chapter and the grotesque lack of an index. But, although his facts overlap with those of Greeley and Batson, Taylor's writing is sharper and there is more of it. His analysis of the surface and atmospheric development of the objects we encounter is more detailed, and he is not afraid to engage with tricky topics such as the possibility of life on Mars, or on Europa and Titan, satellites of Jupiter and Saturn respectively.

His synoptic view of the solar system is more ambitious than Greeley and Batson's. He is informative about its origin and about other once-mysterious topics such as the formation of the Moon. He pays closer attention to the small objects of the solar system, which Greeley and Batson place in a "sin bin" at the end of their book. Asteroids, comets and meteorites provide the basis for theories about how the more highly processed, bigger objects of the solar system came to be, and Taylor covers this territory well.

As he points out, erosion, volcanism, cratering and tectonic action all act to change a planetary surface, and on the Earth matters are made worse by the actions of water and living things. Fortunately, we have a lab nearby - the Moon - where these two effects are unknown. Here solar system evolution can be seen in detail and it is close enough for material to be available for examination along with the meteorites that arrive on Earth for free.

Both books show that our knowledge of the planets is profound. We have a detailed understanding of the sulphur volcanoes of Io, which a few decades ago would have seemed closer to science fiction, and we know that complex chemistry is afoot in the atmosphere of Titan. In 2004, the descent of the European Huygens probe through its clouds will reveal far more. This mission, alongside a clutch of new Mars craft, several probes to small objects in the solar system, possible new missions to the Moon and Mercury, and continuing interest in a trip to Pluto, will ensure fresh sights and insights for the grateful taxpayer back on Earth.

Martin Ince is deputy editor, The THES .