Mapping Mars: Science, Imagination and the Birth of a World

Airy was, by all accounts, an uninspiring but meticulous man. He recorded his every thought and expenditure from the day he went up to Cambridge University to more or less the day he died, throwing no note away, delighting in doing his own double-entry bookkeeping. He applied a similar thoroughness to his stewardship over the Royal Greenwich Observatory, bringing to its workings little interest in theory or discovery but a profound concern for order, which meant that the production of tables for the Admiralty (the core of the observatory’s job) was accomplished with mechanical accuracy. He looked at the heavens and the earth with precision, not wonder, and though he had his fancies, they were fancies in a similar vein – ecstasies of exactitude such as calculating the date of the Roman invasion of Britain from Caesar’s account of the timing of the tides, or meticulously celebrating the geographical accuracy of Sir Walter Scott’s poem ‘The Lady of the Lake’. This was a man whose love of a world where everything was in its place would lead him to devote his own time to sticking labels saying ‘empty’ on empty boxes rather than disturb the smooth efficiency of the observatory by taking an underling from his allotted labours to do so for him. After more than forty-five years of such service Airy eventually retired 200 yards across the park to the White House on Crooms Hill, where he died a decade later.

It’s a little sad that the White House doesn’t carry a blue circular plaque to commemorate Airy’s part in the happiness brought to humanity by a single agreed-upon meridian, but surely there are monuments elsewhere. Maybe Ipswich has an Airy Street; he grew up there and remained fond of the place, arranging for his great transit circle to be made at an Ipswich workshop. There must be a bust of him in the Royal Astronomical Society. Or a portrait in some Cambridge common room. And even if there are none of these things, there is something far grander. Wherever else astronomers go when they die, those who have shown even the faintest interest in the place are welcomed on to the planet Mars, at least in name. By international agreement, craters on Mars are named after people who have studied the planet or evoked it in their creative work – which mostly makes Mars a mausoleum for astronomers, with a few science fiction writers thrown in for spice. In the decades since the craters of Mars were first discovered by space probes, hundreds of astronomers have been thus immortalised. But none of them has a crater more fitting than Airy’s.

A Point of Warlike Light (#ulink_fc42b324-8bd9-5580-b925-edf6203ddeea)

‘I’ve never been to Mars, but I imagine it to be quite lovely.’

Cosmo Kramer, in Seinfeld

(‘The Pilot (I)’, written by Larry David)

Mars had an internationally agreed prime meridian before the earth did. In 1830 the German astronomers Wilhelm Beer and Johann von Mädler, famous now mostly for their maps of the moon, turned their telescope in Berlin’s Tiergarten to Mars. The planet had been observed before. Its polar caps were known, and so was its changeability; the face of Mars varies from minute to minute, due to the earth’s distorting atmosphere, and from season to season, due to quite different atmospheric effects on Mars itself. There are, though, some features that can be counted on to stick around from minute to minute and season to season, the most notable being the dark region now called Syrtis Major, then known as the Hourglass Sea. To calculate the length of the Martian day, Mädler (Beer owned the telescope – Mädler did most of the work) chose another, smaller dark region, precisely timing its reappearance night after night. He got a figure of 24 hours 37 minutes and 9.9 seconds, 12.76 seconds less than the currently accepted figure. That this length of time is so similar to the length of an earthly day is complete coincidence, one of three coincidental similarities between the earth and Mars. The second coincidence is that the obliquity of Mars – the angle that its axis of rotation makes with a notional line perpendicular to the plane of its orbit – is, at 25.2°, very similar to the obliquity of the earth. The third is that though Mars is considerably smaller than the earth – a little more than half its radius, a little more than a tenth its mass – its surface area, at roughly a third of the earth’s, is quite similar to that of the earth’s continents.

When Mädler came to compile his observations into a chart in 1840, mathematically transforming his sketches of the disc of Mars into a rectangular Mercator projection, he declined to name the features he recorded, but did single out the small dark region he had used to time the Martian day as the site of his prime meridian, centring his map on it. Future astronomers followed him in the matter of the meridian while eagerly making good his oversight in the matter of names. Father Angelo Secchi, a Jesuit at the Vatican observatory, turned the light and dark patches into continents and seas, respectively, as astronomers had done for the moon, and gave the resulting geographic features the names of famous explorers – save for the Hourglass Sea, which he renamed the ‘Atlantic Canale’, seeing it as a division between Mars’s old world and its new. In 1867 Richard Proctor, an Englishman who wrote popular astronomy books, produced a nomenclature based on astronomers, rather than explorers, and gave astronomers associated with Mars pride of place. His map has a Mädler Land and a Beer Sea, along with a Secchi Continent. Observations made by the Astronomer Royal in the 1840s – he was interested in making more precise measurements of the planet’s diameter – were commemorated by the Airy Sea. Pride of place went to the Rev. William Rutter Dawes, a Mars observer of ferociously keen eyesight, perceiving, for example, that the dark patch Mädler had used to mark the prime meridian had two prongs. (Dawes’s far-field acuity was allegedly compensated by a visual deficit closer to home; it is said he could pass his wife in the street without recognising her.) So great was Dawes’s influence on Proctor – or so small was the number of astronomers associated with Mars – that his name was given not just to the biggest ocean but also to a Continent, a Sea, a Strait, an Isle and, marking the meridian, his very own Forked Bay.

Proctor’s names had two drawbacks, one immediately obvious, one revealed a decade later. The obvious drawback was that an unhealthy number of the people commemorated on Mars were now British. When the French astronomer Camille Flammarion revised Proctor’s nomenclature for his own map of 1876, various continentals – Kepler, Tycho, Galileo – were given grander markings. One continental on whom Proctor had looked with favour, though, was thrown off: perhaps influenced by the Franco-Prussian war, Flammarion resisted having the most prominent dark patch on the planet called the Kaiser Sea, even if Proctor had named it such in honour of Frederik Kaiser of the Leyden Observatory. The Hourglass Sea became an hourglass again, though this time in French: Mer du Sablier.

Proctor’s other problem was more fundamental. The features he had marked on his map, whatever their names, did not match what other people saw through their telescopes. In 1877, Mars was in the best possible position for observation; it was at its nearest to the sun (a situation called perihelion) and at its nearest to the earth (a situation called opposition), just 56 million kilometres away. Impressive new telescopes all over the world were turned to Mars and revealed its features in more detail than ever before. The maps based on observations made that year were almost all better than Proctor’s; and the map made by Giovanni Schiaparelli, a Milanese astronomer, on the basis of these observations, provided a new nomenclature that overturned all others.

Schiaparelli was not interested in celebrating his peers and forebears; he wanted to give Mars the high cultural tone of the classics. In the words of Percival Lowell, an American astronomer who was to make Mars his life work, it was an ‘at once appropriate and beautiful scheme, in which Clio [muse of poetry and history] does ancillary duty to Urania [muse of astronomy]’. To the west were the lands beyond the pillars of Hercules, such as Tharsis, an Iberian source of silver mentioned by Herodotus, and Elysium, the home of the blessed at the far end of the earth. Beneath them, part of the complex dark girdle strung around Mars below its equator, were the sea of sirens, Mare Sirenum, and Mare Cimmerium, the sea that Homer put next to Hades, ‘wrapped in mist and cloud’. Then we come to the Mediterranean regions: the Tyrrhenian Sea and the Gulf of Sidra (Syrtis Major, the long-observed hourglass) dividing bright Hellas and Arabia. Along the far side of Arabia sits the Sinus Sabeus, a gulf on the fragrant coast of Araby, home to the Queen of Sheba. Beyond Arabia begins the Orient, with Margaritifer Sinus, the bay of pearls on the southern coast of India, and the striking bright lands of Argyre (Burma) and Chryse (Thailand). Finally, in the dark region others had called the eye of Mars, Schiaparelli placed Solis Lacus, the lake of the sun, from which all dawns begin.

Do not think for a moment that this means a good classical education will help you find your way around Mars. For a start, due to the way telescopes invert images, everything is flipped around: Greece is south of Libya, Burma west of Arabia. What’s more, Schiaparelli’s geography was often more allusive than topographical. His planet is 360° of free association. Thus Solis Lacus is surrounded by areas named for others associated with the sun; Phoenix, Daedalus and Icarus. The sea of the sirens borders on the sea of the muses, presumably because Schiaparelli wanted to provide opportunity for their earthly feud to continue. Elysium leads to utopia.

For the most part he did not explain his nominal reasoning very exactly, but there are exceptions, most notably right in the middle of the map, at the point where dark Sinus Sabeus gives way to Sinus Margaritifer, somewhere between Arabia and the Indies, a place he called Fastigium Aryn. ‘As Mädler,’ Schiaparelli wrote, ‘I have taken the zero-point of the areographic longitudes there, and following this idea I have given it the name of Aryn-peak or Aryn-dome, an imaginary point in the Arabian sea – which was long assumed by the Arabic geographers and astronomers as the origin of the terrestrian longitudes.’

By the time he was through with Mars, Schiaparelli had given 304 names to features on its surface and though there was a Proctorite resistance – ‘Dawes’ Forked Bay it will ever be to me, and I trust to all who respect his memory,’ wrote Nathaniel Green, who painted a lovely map of Mars after observing the planet from Madeira during the opposition of 1877 – it foundered. Schiaparelli’s proper names were triumphant and have in large part lasted until today. It was his common nouns that caused the problems. Schiaparelli saw a large number of linear features on the face of the planet and called them ‘canali’ – channels. Schiaparelli claimed to be agnostic as to the nature of these channels – they might have been natural, or they might have been artificial. Percival Lowell, his most famous disciple, plumped firmly for the artificial interpretation.

Lowell’s reasoning went like this. Mars is habitable, but its aridity makes the habitability marginal; if there were intelligences on Mars, they would do something about this; the obvious thing to do would be to build a network of long straight canals. And since this is what we see when we look at Mars, this is what must have happened.

With this leap of the imagination, Lowell created one of the most enduring tropes of science fiction: Mars as a dying planet. It would live on in the works of H. G. Wells, Edgar Rice Burroughs, Leigh Brackett and many, many others. And if his interpretation of what he saw did not win as much favour among his astronomical colleagues as it did in the popular imagination, it was not because the idea of life on Mars seemed too far-fetched. Observations of the way the planet’s brightness and colour seemed to change with the seasons made plant life there seem almost certain; if plants, why not animals and why not intelligence? The most weighty argument against Lowell’s Martians was simply that over time other, better observers consistently failed to see the canals as continuous and linear, if they saw them at all. The lack of evidence of engineering, not the implausibility of life on Mars, was what counted against Lowell – a belief in life on Mars was quite commonplace.

Today this easy acceptance seems rather remarkable. At the beginning of the twenty-first century, when the possibility of life elsewhere has become the central preoccupation of space exploration, its discovery is routinely held up as the most important discovery that could ever be made. What accounts for this change?

A large part of the answer lies in the nature of astronomy. Copernicus’s proposal that the earth was not the centre of the solar system changed the way that astronomers looked at the sky. If the earth was no longer the fixed centre, then it was a wandering star like the five which shuffled back and forth across the zodiac: a planet. Previously unique, now it was one member of a class and must have similarities to its classmates. The world had become a planet and so the planets must become worlds, a process accelerated by the Galilean discovery that, like the earth, the planets were round and had features. In this context it was quite normal to believe that one of the things that the planets had in common was life, especially since, after Copernicus, many astronomers tended to go out of their way to deny the earth any special attributes. As Lowell put it in Mars (1896), ‘That we are the only part of the cosmos possessing what we are pleased to call mind is so earth-centred a supposition, that it recalls the other earth-centred view once so devoutly held, that our little globe was the point about which the whole company of heaven was good enough to turn. Indeed, there was much more reason to think that then, than to think this now, for there was at least the appearance of turning, whereas there is no indication that we are sole denizens of all we survey, and every inference we are not.’ A Copernican stance could easily lead astronomers to the assumption of life, not lifelessness, as the status quo.

Another part of the answer is that in Lowell’s day a belief in life on Mars was largely without consequences. As Alfred Lord Tennyson noted as early as 1886, our astronomical observations of planets and our dreams of what might transpire on them were separated by a vast gulf:

Hesper – Venus – were we native

to that splendour or in Mars,

We should see the Globe we groan in,

fairest of their evening stars.

Could we dream of wars and carnage,

craft and madness, lust and spite,

Roaring London, raving Paris,

In that point of peaceful light?

Life on Mars might be likely, it might be inevitable, it might even be intelligent, but the possibility of people ever actually visiting Mars – or Martians visiting earth – was more or less pure fancy. This made Martians fascinating but not important, rather in the way of dinosaurs – another turn-of-the-century craze. Whatever evidence scientists might find of dinosaurs, or speculations they might produce about them, without a time machine encounters with dinosaurs were impossible. Similarly, without a space machine, encounters with Martians were impossible.

So while there might be intelligent Martians, there could be no links of history or interest between them and us. This gave the Martians an interesting rhetorical niche that they quickly made their own: ‘The man from Mars’ became the quintessential intelligent outsider, unswayed by any relevant prior worldliness, unattached to custom. He retains that position to this very day; his natural habitat is the newspaper op-ed page and other didactic or satirical environments, but he turns up elsewhere, too. Temple Grandin, the highly articulate woman with autism in Oliver Sacks’s An Anthropologist on Mars, applies the titular image to herself as a way of stressing her disassociation from the ways of the world around her; the wonderfully innocent yet artfully contrived metaphors of the poems in Craig Raine’s A Martian Sends a Postcard Home led to a whole school of poetry (if a small one) being dubbed ‘Martianism’. One of the most influential science fiction novels of the twentieth century, Robert Heinlein’s Stranger in a Strange Land, achieves its impact by showing us the earth through the eyes of a true ‘man from Mars’ – a human brought up on Mars by Martians.

Rhetorical devices aside, believing in Martians made little difference to the earthly lives of Lowell’s readers and this, I suspect, is one of the things that made them easy to believe in. Another spur to belief was the difference that the existence of Martian minds made to the way earthly imaginations saw Mars. One of the Copernican ways in which Martians made the planet Mars a world like the earth was that they made it a place experienced from the inside, a site for subjectivity. Without minds, Lowell argued, Mars and the other planets were ‘mere masses of matter’ – places without purpose, frightening voids. With minds, they were worlds.

To Lowell, there was no really useful or involving way to think about a planet except as a world inhabited and experienced by mind. The space age, though, has brought us new ways of seeing beyond the earth and changed our way of thinking about what we see. Our spacecraft, tools of observation but hardly observers in themselves, have shown us things we know cannot be witnessed directly or experienced subjectively, but which can still fascinate. The post-Copernican elision between worlds (structures of shared experience and history) and planets (vast lumps of rock and metal and gas that orbit a fire yet vaster) has been rewritten. Yes, the earth that is our world is also a planet. But not all planets are worlds. We no longer need the point of view of a mythical Martian to imagine Mars, or to convince us that Mars might be worth imagining. Now that our spacecraft have been there we can know it intimately from the outside, know it as an objective body rather than a subjective experience. We can measure and map its elemental composition and its wind patterns and its topography and its atmospheric chemistry and its surface mineralogy. The planet Mars can fascinate us just for what it is.

If the space age has opened new ways of seeing mere matter, though, it has also fostered a strange return to something reminiscent of the pre-Copernican universe. The life that Lowell and his like expected elsewhere has not appeared, and so the earth has become unique again. The now-iconic image of a blue-white planet floating in space, or hanging over the deadly deserts of the moon, reinforces the earth’s isolation and specialness. And it is this exceptionalism that drives the current scientific thirst for finding life elsewhere, for finding a cosmic mainstream of animation, even civilisation, in which the earth can take its place. It is both wonderful and unsettling to live on a planet that is unique.

Yet if the earth is a single isolated planet, the human world is less constrained. The breakdown of the equation between planets and worlds works both ways. If there can now be planets which are not worlds, then there can be worlds that spread beyond planets – and ours is doing so. Our spacecraft and our imaginations are expanding our world. This projection of our world beyond the earth is for the most part a very tenuous sort of affair. It is mostly a matter of imagery and fantasy. Mars, though, might make it real – which is why Mars matters.

Mars is not an independent world, held together by the memories and meanings of its own inhabitants. But nor is it no world at all. More than any other planet we have seen, Mars is like the earth. It’s not very like the earth. Its gravity is weak, its atmosphere thin, its surface sealess, its soil poisonous, its sunlight deadly in its levels of ultraviolet, its climate beyond frigid. It would kill you in an instant. But it is earthlike enough that it is possible to imagine some of us going there and experiencing this new part of our human world in the way we’ve always experienced the old part – from the inside. The fact that humans could feasibly become Martians is the strongest of the links between Mars and the earth.

At the beginning of the space age – at the moment when it became clear to all that Mars might indeed one day be experienced subjectively – the International Astronomical Union stepped in to clean up the planet’s increasingly baroque nomenclature. Thanks to the efforts of Schiaparelli, Lowell and Eugène Michael Antoniadi, whose beautifully drawn charts had become the standard, the planet had come to boast 558 names for an uncertain number of features. In 1958 the IAU experts settled on 128 named regions and features, with 105 of the names coming from Schiaparelli. Then the first spacecraft images came back and the stalwarts of the IAU needed not only more names but also new rules by which names could be assigned. It was at this point that the convention of naming craters for people with an interest in the planet was laid down. Proctor’s astronomical pantheon was reconvened – Dawes, Secchi, Mädler, Beer and the rest of them all got craters, as did Proctor himself.

And in 1972 the International Astronomical Union established for all time the precise location of the Martian meridian. Lacking a transit circle made of good Ipswich steel – or, for that matter, any ancient monuments – the IAU’s working group had to use a natural landmark for their zero. They chose the geometrical centre of a small, nicely rounded crater in the middle of a larger crater fifty-six kilometres across. They called that larger crater Airy.

Mert Davies’s Net (#ulink_8208ae65-cc16-5b62-a51a-90a75aa3e9f2)

There is a passage in the oeuvre of William F. Buckley Jr, in which he remarks that no writer in the history of the world has ever successfully made clear to the layman the principles of celestial navigation. Then Buckley announces that celestial navigation is dead simple, and that he will pause in the development of his narrative to redress forever the failure of the literary class to elucidate this abecederian technology. There and then – and with awesome, intrepid courage – he begins his explication: and before he is through, the oceans are in orbit, their barren shoals are bright with shipwrecked stars.

John McPhee, In Suspect Terrain

It was Merton Davies who put Airy in his prime position. Mert is a kindly man, tall and thin, dignified but rather jolly. Everyone who knows him speaks fondly of him. You might imagine him embodying decent reliability in a Frank Capra film even before learning that he has worked for the same outfit over more than fifty years. But it’s hardly been a small-town life. Mert Davies was one of the pioneers of spy satellites, one of the small cadre of technical experts who changed the facts of geopolitical life by letting cold warriors see the world over which they were at war from a totally new perspective. After that, he became one of only two people to have played an active role in missions to every planet save Pluto.

(#ulink_df90643e-9f4a-5cd1-9f8c-e0f3eb7fc774) He has reshaped – quite literally – the way that earthlings see their neighbours in space. Davies is the man chiefly responsible for the ‘control nets’ of most of the solar system’s planets and moons – complex mathematical corsets that hold the scientific representations of those planetary surfaces together.

The first control net that he created served as the basis for the first maps of Mars made using data from spacecraft, rather than observations from earth. Compiled from fifty-seven pictures sent back when Mariner 6 and Mariner 7 flew past the planet in 1969, that first net tied together 115 points. When I met Davies in his office in Santa Monica thirty years later, his latest Martian control net held 36,397 points from 6320 images. Well into his eighties, Davies was still hard at work augmenting it further.

Davies had been interested in astronomy since boyhood, an interest he had shared with those close to him. In 1942, when he was working for the Douglas Aircraft corporation in El Segundo, California, he started courting a girl named Louise Darling. His interests made their dates a little unusual. Davies had started making a twelve-inch telescope, a demanding project. ‘I had a hard time finishing it,’ he recalls. ‘The amount of grinding it took and the difficulty of polishing that big a surface was a little bit over my head. I would take her with me to polish.’ And so she entered the world of grinding powder and the Foucault test, a simple but wonderfully precise way of gauging a mirror’s shape, which allows an amateur with simple equipment to detect imperfections as small as 50 billionths of a metre. Unorthodox courtship, but it worked. When I met Mert in 1999 he and Louise had been married for more than fifty years.

Just after the war, Davies heard that a think tank within Douglas was working on a paper for the Air Force about the possible uses of an artificial satellite. He applied to join the team more or less on the spot. The think tank soon became independent from Douglas and, as the RAND Corporation, it went on to play a major role in defining America’s national-security technologies and strategies throughout the Cold War. In the early 1950s Davies and his colleagues looked at ways to use television cameras in space in order to send back images of the Soviet Union. Then they developed the idea of using film instead of television – experience with spy cameras on balloons showed that the picture quality could be phenomenal – and returning the exposed frames to earth in little canisters. The idea grew into the Corona project, which after a seemingly endless run of technical glitches and launch failures at the end of the 1950s became a spectacularly successful spy-satellite programme.

While Corona was in its infancy, Davies was seconded to Air Force intelligence at the Pentagon, where he used the new American space technology to try to figure out what Russian space technology might be capable of. When he returned to Santa Monica in 1962, he was ready for a change. Spy satellites were no longer exciting future possibilities for think-tank dreamers, but practical programmes controlled by staff officers and their industry contractors. And there was another problem. ‘A lot of the work at RAND was going into Vietnam – my colleagues were working on reconnaissance issues there – and I wanted no part of that.’

Happily, an alternative offered itself in the form of Bruce Murray, an energetic young professor from the California Institute of Technology in Pasadena, on the other side of Los Angeles. Murray was an earth scientist, not an astronomer. His first glimpse of Mars through a telescope wasn’t a childhood epiphany in the backyard. It was a piece of professional work from the Mount Wilson Observatory. Late as it was, though, that first sight provided emotional confirmation for Murray’s earlier intellectual decision that the other planets were something worth devoting a lifetime’s study to. When Murray looked at Mars through the world-famous sixty-inch telescope, he was not just seeing an evocative light in the sky; he was seeing a world’s worth of new geology, a planet-sized puzzle that he and his Caltech colleagues were determined to crack. Their tool was to be the Jet Propulsion Laboratory, a facility that Caltech managed on behalf of the federal government. JPL, in the foothills of the San Gabriel mountains, had been a centre for military aerospace research since the war. In 1958 the Army ceded it to the newly founded National Aeronautics and Space Administration, as part of which it would become America’s main centre for planetary exploration. By 1961, JPL was planning NASA’s first Mars mission, Mariner 4. The man in charge of building a camera for it was Robert Leighton, a Caltech physics professor. He asked a geologist he knew on the faculty, Bob Sharp, to help him figure out what the camera might be looking at. Sharp asked his eager young colleague Murray to join the team.

Murray and Davies met in 1963; with three young children to support, Murray was keen for some extra income and so found consulting for RAND congenial. He and Davies quite quickly became close friends and Mert started to think he might want to get involved in Murray’s end of the space programme. After all, he had the right credentials: he had been in the space business since the days of the V2 and he had some experience in interpreting images of both the earth and moon as seen from orbit. (At the Pentagon he had analysed Russian pictures of the far side of the moon to see whether they might be fakes.) When Mariner 4’s television camera sent back its image-data – a string of twenty-one grainy pictures covering just 1 per cent of the planet’s surface – Davies was as surprised as almost everybody else to see that it looked not like an earthly desert but like the pock-marked face of the moon, or the aftermath of a terrible war. The space programme was important (Murray and his colleagues would brief the president) but it was also open (they briefed him in front of the cameras). Out among the planets there was no risk of finding yourself in a conflict you wanted no part of, or of having to keep work secret from all but your closest colleagues.

By the time Mariner 6 and Mariner 7 were sent to Mars four years later, in 1969, Davies was a key part of the team dealing with the images they sent back. His particular contribution was to work on the mathematical techniques needed to turn the disparate images into the most reliable possible representation of the planet.

Since the seventeenth century, when Willebrord Snell of Leiden first refined the procedure into something like its modern form, earthbound map makers have turned what can be seen into what can be precisely represented through surveying. Decide on a set of landmarks – Snell and his countrymen liked churches – and then, from each of these landmarks, take the bearings of the other landmarks nearby. From this survey data you can build up a network of fixed points all across the landscape. Plot every point on your map according to measurements made with respect to things in this well-defined network and it will be highly accurate. If, unlike Holland, your country is large, mountainous and only sparsely supplied with steeples, setting up a reliable network in the first place can be hard work – the United States wasn’t properly covered by a single mapping network until the 1930s, when abundant Works Progress Administration labour was available to help with the surveying. But the principle of measuring the angles between lines joining landmarks has been used in basically the same way all over the earth.

Two problems make the mapping of other planets different, one conceptual, one practical. On earth, experience allows you to know what the features you are mapping are: hills, valleys, forests and so on are easily recognised for what they are. While the pictures a spacecraft’s cameras send back may be very good, this level of understanding is just not immediately available. When the first images of Mars were sent back by Mariner 4 they were initially unintelligible to Murray and the rest of the imaging team. Before the researchers even started on a physical map, they needed a conceptual one, a way of categorising what was before their eyes. How to do this – how to see what had never been seen before – was the besetting problem of early planetary exploration.