Mars, but not as we know it

This book begins by satisfying our curiosity about how the maps were made: how human ingenuity gets you from orbital photographs, all taken from different angles, to reasonable certainty about features down below that human eyes have never inspected directly. (It's the inverse of the Superman problem. Not "Is it a bird? Is it a plane?" but "Is it a hill? Is it a valley?")

Oliver Morton writes impeccably skilled science journalism, so his explanation is a model of memorable clarity. If it doesn't quite leave you equipped to build your own "control net" for other planets, it certainly leaves you knowing how one works, and how triumphantly Martian geography was won from a litter of equations, photomosaics and air-brush interpretations.

The pictures from Mariner 4 in 1964 each took eight hours to reach the Jet Propulsion Laboratory in Pasadena, pixel by pixel. They built the photographs up from paper strips hand-coloured in chalk.

So Mapping Mars is the book to consult for a luminous summary of extra-terrestrial mapping techniques; and for lucid expositions of the current state of thought about Martian rocks, Martian water, and Martian microbes. Morton is very good at letting you into the different ways that scientists in different disciplines think. He even manages to be tactful about the revelation that the ground on Mars isn't the " light-brownish pink" of the maps. Or the rich crimson-brown of the Viking photographs. That was a camera calibration error. Apparently, it's a rather manky yellowish-brown instead. And the sky isn't pink. It's brownish-yellow.

But Morton has something much more ambitious in mind. The same quality in him that makes him alert to the imaginative implications of the colour changes, gives him a vivid feel for the speculative worlds that tremble into existence, re-form, merge, and dissipate each time a new theory about Mars is entertained by the scientific community. From his appreciation of the various Marses that science projects, he has woven a beautifully intelligent meditation on place, and on the paradoxes of place that apply to a place like Mars. Think of an Annie Dillard or a Barry Lopez and you have some idea of the kind of the free-ranging essayistic attention Oliver Morton brings to Martian landscape; only he is writing about what can only be experienced second-hand, through a prism of ideas, so the spirit of Calvino is in there too. "Everything we know tells us that big pictures come from little pictures, that the truth about something being mapped comes from the bottom up ... But for Mars, the maps are all we have. They are the result of zooming in, not pulling out."

This is a book about Mars, but also about the way our idea of Mars is quietly influenced by our ideas of America, of how nature works, of death. "One can imagine a world in which one is dead: one simply cannot grace the imagining with full subjectivity. In the way we are forced to imagine them, death and Mars are much alike."

The conventions of modern non-fiction writing say you shouldn't dig deep in case you alienate your reader. Oliver Morton digs deep on the sly, in sentences that provoke continuing thought, like epigrams. As a store of current data, Mapping Mars will go out of date the next time a probe manages a soft landing on the yellowish-brown terrain; as a map of ways to think about the planet, it will be around for a long time to come.