Читать книгу Mapping Mars: Science, Imagination and the Birth of a World - Oliver Morton - Страница 13

‘I think it’s part of the nature of man to start with romance and build to a reality.’

Ray Bradbury, in Mars and the Mind of Man

Mars Polar Lander was JPL’s thirteenth mission to Mars and its fifth failure. Mariner 3 died with its solar panels pinned to its side by the wrapping in which it had been launched in 1964; Mariner 8 fell into the Atlantic in 1971; Mars Observer exploded as it was trying to go into orbit round Mars in 1993; Mars Climate Orbiter burned up in the atmosphere in 1999; Mars Polar Lander made its mistake just forty metres up a few months later. An optimist might point out that each got closer to the target than the previous failure. A pessimist might point out that the frequency of failure seems to be on the increase.

It’s hardly surprising that, with so few missions, everything that has not been a failure has been counted a terrific success. Mars exploration is still too new for there to have been any hey-ho, business-as-usual missions. But among all these successes one stands out: Mariner 9. Mariner 9 was the first American spacecraft to go into orbit round another planet. It was the first interplanetary probe to send back data in a flood, rather than a trickle. It was the first mission to Mars to provide images of the entire surface and record the full diversity of its landscapes. It was the first spacecraft to see a planet change dramatically beneath its eyes, to watch weather on another world. Mariner 9 revealed a Mars that was fascinating in its own right, rather that disappointing in the light of previous earthly expectations. And Mariner 9 allowed a small team of artists and artisans to make the first detailed, reliable maps of another planet.

There were two big differences between Mariner 9 and its earlier siblings (two of which, Mariner 2 and Mariner 5, went to Venus, not Mars). One was that Mariner 9 had a largish rocket system on board, its cluster of spherical fuel tanks hiding the distinctive octagonal magnesium body that all the Mariner family shared. This engine was needed to slow the spacecraft down when it got to Mars, thus allowing it to go into orbit round its target rather than flying past it at breakneck speed, as the previous probes had. The other, less visible, difference was that Mariner 9 would have the opportunity to send back serious amounts of data.

When Mariner 4 flew past Mars in 1965, it seemed extraordinary that the signal it sent back could be heard at all. Mariner 4’s radio transmitter had a power of ten watts; it had to send data back to a target – the earth – much less than an arc minute across (an arc minute is a sixtieth of a degree). Only a small fraction of the spacecraft’s ten-watt beam actually hit the earth, and only one ten-billionth of that fraction hit the actual receiver – a steerable radio telescope sixty-eight metres in diameter built specifically for the Mars missions at a site a couple of hours’ drive into the Mojave Desert from JPL. But the power of electronic engineers to decode such staggeringly faint signals has been one of the least celebrated wonders of the space age.^* It’s an ability at least as wonderful as that of actually launching things into space, and compared with rocketry it’s both grown in capability far faster and been a good sight more dependable. That sixty-eight-metre Goldstone dish in the Mojave, along with companions near Madrid and Canberra, now brings data back from the edges of the solar system, a hundred times further away than Mars, and handles data rates as high as 110,000 bits per second. Even in the early days of Mariner 4 the limiting constraint on the rate at which data could be sent back was not the radio link, but the speed at which the tape recorder which stored the data on board the spacecraft could play it back. And that was staggeringly slow: eight bits per second. It took weeks to send back data recorded in minutes.

Mariner 4’s pictures each contained less than a thousandth of the data in a nine-inch aerial photograph. The frames were just 200 pixels wide by 200 pixels deep; the brightness of each pixel was recorded as six bits of data, providing sixty-four gradations of tone between black and white. The total amount of data in every frame (thirty kilobytes) was just a little bit more than the amount of disk-space taken up by an utterly empty document in the version of Word with which I am writing this book. In principle I could download the equivalent of Mariner 4’s entire twenty-two-image data-set from the Internet in a matter of seconds using my utterly unexceptional modem. In 1964, though, it took eight hours to get each picture back to JPL. The process was so slow that the waiting scientists printed out the numerical value for each pixel on a long ribbon of ticker tape, cut the ribbon into 200-number-long strips and then coloured each pixel in with chalk according to its numerical value. Every two and a half minutes another strip could be added to the picture. The first space-age image of Mars, taken by the first entirely digital camera ever built and transmitted over 170 million kilometres of empty space, was put together like an infant school painting-by-numbers project.

By the time Mariner 6 and Mariner 7 flew past Mars in 1969, communications were far faster (though the on board tape recorders, which outweighed the cameras whose data they stored, were still a problem). Each of the 1969 Mariners returned a hundred times more data to earth than Mariner 4 had four years earlier. In 1971 Mariner 9 – with a data rate 2000 times that of Mariner 4 and a year in which to transmit, rather than a week – did 100 times better still. And this meant that the whole scale of the operation was different. The ‘television teams’ – so called because their instrument was basically a TV camera – on Mariners 4, 6 and 7 had been small: Leighton, who masterminded the camera design; a few other Caltech faculty members; some JPL people; and a few select outsiders, such as Mert Davies. But Mariner 9 was going to provide far more data than such a team could digest and the data were to be used not just for analytical science but for the practical business of mapping. Among other things, America was committed to landing robot probes on Mars to look for life in 1976. Those probes – the Vikings – needed landing sites, and choosing landing sites required maps.

NASA would have been happy to make the maps itself. But in the mid-1960s Congress noticed that almost every government agency had its own map makers and decided that the money-hungry, fast-growing space agency would be an exception to this rule. So the mapping of the planets was instead made the duty of the United States Geological Survey. This was not entirely arbitrary; the USGS already had an astrogeology branch, headquartered in Flagstaff, Arizona, which was deeply involved in the study of the moon and was helping to train the Apollo astronauts. The USGS gave primary responsibility for its study of Mars to a team of five geologists, three from Flagstaff, two from the survey’s California centre in Menlo Park, south of San Francisco. The senior member of the USGS team was a man called Hal Masursky: in part because Murray was at the same time working on a mission to Venus and Mercury, Masursky became one of the television team’s two principal investigators. The other PI was a young man called Brad Smith, a highly rated expert on Mars as observed through telescopes who had yet to complete his PhD.

Up to the point when he joined the astrogeology branch in the early 1960s, Hal Masursky’s career had not been stellar. He had never completed his Ph.D.; his terrestrial work had been uneventful. But Masursky became fascinated by the possibilities of geology on other worlds, and turned out to be a great success at it. The success lay not in his own scientific work – though he was a perceptive observer, his complete inability actually to write things up was something of a limitation – but in his ability to get things done within the sometimes bureaucratic world of space exploration and to explain these achievements to the world at large. Some of his colleagues considered him as vivid an off-the-cuff communicator as Carl Sagan.

Hal was at the same time a bright spark and a consummate committee man. He was charming but dogged, willing to get down into the details of sequencing spacecraft manoeuvres and download times whenever necessary, but also keeping a clear eye on the overall objectives. His astrogeological life became in large part devoted to the teamwork necessary for planning and running space missions, and he played a role in almost every major mission of the 1970s and 1980s, making sure they would send back pictures geologists could make use of. If Hal was on a committee, a planetary scientist who learned the political ropes back then once told me, it would get things done; if he wasn’t on a committee, then you didn’t want to be on it either. It was probably not an important one, and it might well not get anywhere.

Masursky was good at getting committees to work; in his personal life his gift for structure was less evident. Committee work meant he was endlessly travelling. (It’s said that at times he lived in Flagstaff without a car, preferring simply to rent one when he flew in just as he would anywhere else.) His ability to keep projects he was administering within budgets was famously poor. He was married at least four times, religious and passionate in argument. He was diabetic, but rather than accepting the discipline of managing the condition he let his team do so for him. Jurrie van der Woude, an image-processing specialist then at Caltech and later at JPL, remembers finding Masursky passed out on the floor of his office late one night during the Mariner 9 mission. Jurrie shouted for help and people came running – people already armed with candies and orange juice, because they knew what to expect. ‘From that point on I was part of the club. No matter where you went around the lab you’d carry orange juice with you. Nobody talked about it, but in press briefings there’d be four or five of us like secret servicemen, waiting and watching for the right time to bring him orange juice. He had this kind of a smile and every so often you’d realise that behind it he was just gone.’ Eventually diabetes took its toll; in the late 1980s Masursky sickened, dying in 1990. During his sad decline, he would occasionally elude his last, devoted wife and wander off to Flagstaff’s little airport, sure he should be going somewhere. Now he has a crater on Mars: 12.0°N, 32.5°W, a hole 110 kilometres across in the 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, built up by eruption after eruption of smooth-flowing basaltic lava. Yes, earth’s ‘shield volcanoes’ – like Ararat in Turkey, or Kilimanjaro in Tanzania, or Mauna Kea in Hawaii – were built in a similar way and have much the same profile. But the scale of the thing is incomparably grander. Mauna Kea, earth’s biggest volcano, would fit into the huge crater at the summit of Olympus Mons with room to spare. If you strung the arc of Japan’s home islands round its base the two ends wouldn’t meet; nor would the peak of Fuji clear the top of the great cliff that they were failing to encompass. An Everest on top of Everest would not come to the summit of Olympus Mons.

This single brutish Martian lump is larger than whole earthly mountain ranges. Its bulk – some 3½ million cubic kilometres of rock – is about four times the volume of all the Alps put together. If you wanted to build one on earth, you’d have to excavate all of Texas to a depth of five miles for the raw material – and you’d still be doomed to failure, because the planet’s very crust would buckle under the strain.

North Spot, Middle Spot and South Spot, stretched out along the ridge of Tharsis, are smaller than Olympus Mons. But not by much.

The great storm, rather than obscuring Mars completely, had in fact served to highlight its most dramatic features. It also set Sagan – always alert for lessons from other planets with relevance to this one – to wondering whether similar phenomena might have any relevance to the earth. Mariner 9’s infrared spectrometers showed that the dust did not just obscure the Martian surface from earthly eyes; it also chilled it by shielding it from the sun. In 1976 Sagan, his student James Pollack and other colleagues produced papers showing how the dust thrown into the earth’s stratosphere by large volcanic eruptions could cool the home planet in a similar way. Such cooling was to be put forward in the early 1980s as the mechanism by which a large impact by an asteroid or comet – an event guaranteed to kick up a lot of dust – might have killed off the dinosaurs. This new mechanism for mass extinction led to Pollack and his colleagues being asked to model the sun-obscuring effects of nuclear war, and thus to the idea of ‘nuclear winter’. Having gone to Mars to look for signs of life, Sagan found intimations of planetary mortality.

As the cooling planet-wide pall of dust started to ebb down the volcanoes’ flanks in late 1971, the television team began to pick out the outlines of other features: depressions, in which there was more airborne dust to reflect sunlight back into space, started to stand out as bright blotches. By the middle of December a vast bright streak had become visible to the east of the three Tharsis volcanoes. When the dust had settled out further the streak was revealed to be a set of linked canyons thousands of kilometres long and five kilometres deep. It would come to be called Valles Marineris after the spacecraft through which it was discovered. By the time the dust subsided in 1972, large parts of the planet’s northern hemisphere had been revealed as plains much more sparsely cratered than those over which the first three Mariners had passed. At the same time, other features known from earthly observation, like bright Argyre and Hellas, turned out to be the remnants of absolutely vast impacts.

Most striking of all, particularly to Masursky, were the erosion features. In some places long, narrow valleys ran for hundreds of kilometres across the plains with few if any tributaries. In other regions there were branching networks of smaller valleys, suggestively similar to those that drain earthly landscapes. And elsewhere mere were vast, sweeping channels that seemed to have torn across the crust with unbelievable force, scouring clean areas the size of whole countries. Had water done this? Masursky seemed sure of it and waxed lyrical on the planet’s lost rains to journalists; Murray looked on, grinding his teeth. After all, this was an alien world of new possibilities. Streams of lava might have been responsible – or torrents of liquid carbon dioxide, or gushing hydrocarbons, or slow-grinding ice. Even the thin winds were suggested as possible scouring agents – and though that was a spectacular stretch, it was increasingly clear that wind did indeed play a large role in the way the planet looked. Everywhere there were streaks where dust had revealed or hidden the surface beneath; in some places there were full-blown dune fields. The seasonal changes observed from the earth and held by some to mark the spread of primitive vegetation – changes that would have been Mariner 9’s primary focus, had its sister ship, Mariner 8, not fallen into the Atlantic just after launch and thus bequeathed the main mapping mission to its sibling – were now explained by the wind, at least in principle.

And there was yet more for Masursky and Murray and their colleagues to wonder at and argue over. Strange parallel ridges and lineations running in step for hundreds of kilometres. The collapsed chaos features seen by Mariner 6, which now appeared to be sources for some of the great channels. Rippling bright clouds of solid carbon dioxide (such clouds, streaming off the heights of Olympus Mons, provided the intermittent bright white expanses that made Schiaparelli think of snow and call the area Nix Olympica). Most strikingly, there were regions at the poles where the interaction of wind-borne dust and expanding and contracting polar caps had built up a weird, laminated terrain. Each layer must correspond to a different set of conditions – different wind patterns, different climates. Millions, maybe billions of years of history were there in those layers, just waiting to be read if only you could get to them and figure out what made them. Murray, in particular, found these polar layered terrains fascinating. Thirty years on he still does. He was to be part of the science team on the ill-fated Scott and Amundsen microprobes that accompanied Mars Polar Lander.

The twenty-four people working shifts on the television team had more than enough data to keep them happy. Every twelve hours a new swathe of pictures would come back, covering the planet in seventeen days. There were always new things to see, new things to think about, new things to ask for close-ups of at the next opportunity. And in the end Mars’s rocky surface was stored in their computers and tacked up on their walls, almost seven gigabytes of data, 7329 images. Mars was now much more than one of Tennyson’s points of peaceful light – it was taking on, in Auden’s words, ‘the certainty that constitutes a thing’. It could be measured in detail, and properly mapped.

^* The excellent Australian film The Dish goes some way to redressing this oversight.