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region called Xanthe Terra.

      When Mariner 9 set forth from earth in 1971, no one had seen Xanthe in close-up. No one had seen the crater that would one day be named for the principal investigator on the television team, or the striking channel that runs next to it and quite probably once filled it with water, Tiu Vallis. No one knew that Mars offered such sights. Mariner 4 had seen a moonlike surface covered in craters. It had measured the atmospheric pressure as being much lower than most measurements from earth had suggested – about 1 per cent of the pressure at sea level on earth. The long-held picture of Mars as a basically earthlike if very marginal environment – something like a cold high-altitude desert, except worse – was demolished. The surface had to be very old to have accumulated so many craters; the atmosphere must always have been very thin and free of moisture for the craters not to have eroded away. From the composition of the atmosphere – 95 per cent carbon dioxide – and measurements of its temperature and pressure – both low – Leighton and Murray had been able to predict that the polar caps, which earthbound observers had seen as water ice that might moisten their imagined earthlike desert, were in fact made of frozen carbon dioxide. Mariner 7 seemed to confirm this theory when it passed over the south pole carrying infrared instruments capable of measuring the surface’s temperature and composition, and found it to be as Murray and Leighton had predicted.

      Admittedly, Mars was not all craters. Mariner 6 had seen that Hellas, known as a large bright region to the earthbound astronomers, was much smoother than the cratered terrain next to it, though no one could say why. The same spacecraft also sent back pictures of an odd terrain quickly termed ‘chaotic’, a collapsed jumble of a landscape from which a few table-top mesas stood proud. It was as though the land had rotted from within. But though such features might prove interesting, the general impression was of a dull, geologically inactive place, more or less unchanged since the creation of the solar system, a place little more interesting than the earth’s moon and far harder to get to. Bruce Murray, who unlike many in the business had never had a boyhood romance with the stars, took a certain delight in debunking the delusions of people who still wanted to think of Mars as at least a little earthlike. Murray has a certain intellectual aggression, as do many Caltechers – the USGS geologists on Mariner 9 used to be amazed by the frequency and ferocity of the arguments that Murray’s students on the team, Larry Soderblom and Jim Cutts, would get into. Nostalgic notions of an earthlike Mars gave Murray’s belligerence its casus belli. Mars was simply not what people had thought it to be. Rather than a world to be experienced in the imagination, it was a planet to be measured, a planet in the new space-age meaning of the term, something woven from digital data streams and ruled by the hard science of physics and chemistry.

      On 12 November 1971, the night before Mariner 9 was to go into orbit, Caltech held a public symposium on ‘Mars and the Mind of Man’ featuring Murray, Carl Sagan and the science fiction authors Arthur C. Clarke and Ray Bradbury: it was the genteel ancestor of the bigger, brasher Planetfests which accompany today’s missions. Murray cast himself in wrestling terms as ‘the heavy – the guy with the black trunks’. He acknowledged people’s ‘deep-seated desire to find another place where we can make another start … that is not just a popular thing [but] affects science deeply’. He then set about using his experience of Mariners 4, 6 and 7 to pour cold water – in fact frozen carbon dioxide – on such fancies. Carl Sagan, a new member of the television team and already a passionate advocate of the search for life in planetary exploration, responded by saying that nothing seen so far had ruled out life on Mars – it had just made it harder to imagine if you were parochial enough to imagine all life must be like earth life. Clarke optimistically suggested that if there wasn’t life on Mars in 1971, there certainly would be by the end of the century.

      While Clarke and his colleagues spoke in Caltech’s auditorium, events up at JPL were turning out quite dramatic enough without any added fiction. One of the reasons that 1971 was a good time to launch the first Mars orbiters was that Mars, which has a markedly eccentric orbit, would be at its closest to the sun at the time when it was most easily reached from the earth. Unfortunately, perihelion warms the Martian atmosphere up quite a lot and the resultant winds can kick up dust storms. This possibility had been discussed earlier in the year by the Mariner mission operations team. Brad Smith, Masursky’s partner at the helm of the television team, said it would not be a problem. But Smith was wrong. The great storm started on 22 September. Within a few days almost half the southern hemisphere was obscured by the brilliant cloud and a week later a second storm started further to the north. Soon the storms merged. Telescopes on earth saw a Mars utterly without features – and so did Mariner 9. Its first pictures, sent back on 8 November, revealed no detail whatsoever – wags joked that they had arrived at cloud-covered Venus by mistake. On 10 November, when the pre-orbital images should have been as good as those from Mariners 6 and 7, all that could be seen was the faint outline of the south polar cap and a faint dark spot. It turned out to correspond to the location which Schiaparelli had called ‘Nix Olympica’ – the Snows of Olympus. Two days later three more dark spots were seen a few thousand kilometres from Nix Olympica, forming a line from south-west to north-east across the region called Tharsis. The rest of the planet was still completely blank.

      Two days later, after the spacecraft had gone into orbit, new pictures revealed that each of these spots had a crater at its centre. Carl Sagan took a Polaroid of the computer screen and rushed to the geologists’ room. Masursky and his colleagues immediately realised what they were seeing. These were not impact craters like those seen by the previous Mariners, but volcanic calderas. Nix Olympica and the other features – dubbed North Spot, Middle Spot and South Spot – were volcanoes, volcanoes vast enough to stick out of the lower atmosphere into air too thin to carry the fine Martian dust. Within hours, Masursky was telling the waiting press corps all about it. Murray, who as well as sporting the black trunks of the killjoy was taking on a role as the television team’s prudent conscience, was aghast. Mars had previously shown no signs of volcanism; it was surely rash to jump to such a dramatic conclusion. But within days more detailed photos showed without doubt that Masursky was right.

      It’s easy now to scoff at Murray’s reluctance to see the truth. Mars’s volcanoes have become, along with its vast canyon system, the things for which the planet is best known. Inasmuch as there is a popular picture of Mars today, these features – four big lumps with a long set of deep gashes to one side, rendered in a reasonably garish red – are what make it up. In some ways, though, Murray’s reluctance to credit such things seems almost fitting, a greater tribute to their stature than straightforward acceptance. It may sound like a lack of imagination – but if you wanted to, you could read it as the opposite. Maybe Murray had the imagination to look beyond the simple images of calderas and see quite how dauntingly huge the volcanoes would have to be in order to show up on Mariner 9’s pictures of a planet wrapped in dust from pole to pole.

      Think of the commute that some of the USGS astrogeologists were making on a weekly basis between San Francisco and Los Angeles; like a few thousand people every day, I made it myself while researching this book. You come off the tarmac at San Francisco airport and wheel round over the South Bay; northern California drops away beneath you, views open up. By the time the plane is at its cruising altitude of 33,000 feet, the view has spread out across the state. The Coast Range beneath you is a set of soft creases in the earth’s crust, the Sierra Nevada a white rim on the horizon. After about half an hour’s flight at a fair fraction of the speed of sound, you start to drop down and pull out over the Pacific, then come back around into LAX. And if your plane could fly through solid basalt, that entire flight profile would fit easily inside the bulk of the volcano then known as Nix Olympica and now called Olympus Mons.

      Olympus Mons is a softly sloping cone sitting on a cylindrical pedestal, a flattened lampshade on a 70mm film canister. The face of the pedestal is a cliff that circles the whole mountain and rises on average four or five kilometres above the surrounding plain. Stick that pedestal on to California and it would cover the centre of the state from Marin County in the north to Orange County in the south. The mountain’s peak, more than fifteen kilometres above the top of its surrounding cliff, would be high in the stratosphere, far above the reach of any passenger jet. You would be able to see it halfway to Flagstaff, a gently humped impossibility peering over the western horizon.

      Yes, Olympus Mons is a mountain,

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