For the moon mappers the shadows with which they defined the landscape’s features were not an evocative extra to ease interpretation or please the eye. They were the essence of the map, the ultimate expression of the surface’s form. As such they needed to be rendered with minute fidelity, and the tool of choice was the airbrush, capable of capturing both the finest details – which is why people who retouched photographs relied on it in the days before Photoshop and similar software – and producing precisely graded washes, which was what commercial artists liked about it. There are other ways of producing maps of the planets: using Mariner 9 pictures and Mert Davies’s control net, a British astronomical artist called Charles Cross did a very pretty and accurate set of maps using pencil and charcoal. These were used to make the first ever comprehensive atlas of Mars, with text by Britain’s leading populariser of astronomy, Patrick Moore. Cross’s work was fine; but compare it with the far greater precision of Bridges’s moon work and you see immediately why, when the USGS started planning the production of official maps of Mars, the airbrush technique would have been the obvious one to use even if its leading proponents had not been located in the same town as the USGS astrogeology branch. Ray Batson, the USGS cartographer whom Masursky had chosen to run the map-making team, made recruiting Bridges, who had left the Air Force mappers in 1968 but still lived in Flagstaff, one of his first priorities.
Another recruit from the Lowell team was Jay Inge. Inge had been a keen stargazer from boyhood on, but bit off more than he could chew, mathematically, when he enrolled for physics and astronomy at the University of California, Los Angeles in the 1960s. After the first semester he was ‘casting around for things to do’ and ended up taking a degree in bio-medical illustration. Then he heard from a friend – one of his childhood telescope buddies – about what was going on at the Lowell Observatory. The moon mappers needed his illustrating skills and they offered a way back into stargazing. So Inge joined the team at Lowell.
Inge augmented the techniques Bridges had developed in various subtle ways. One particular gift he brought was a dexterous use of the powered eraser, not to get rid of errors – ‘an eraser is never used to rescue a poor drawing,’ he wrote sternly in a manual on shaded relief mapping – but as a technique for highlighting things. This was, in a way, an adaptation to the airbrush of the ‘dark plate’ map-making technique that was then sometimes used for charts of the ocean floor; dark plates double illustrators’ options by allowing them to both add and subtract from what was on the page to begin with. By taking ink away from the airbrushed original with a trusty K&E Motoraser, the illustrator could clarify and accentuate fine details, especially in the more deeply shaded parts of the maps.
By the time they made a start on the Mariner 9 images Bridges and Inge were highly accomplished, and the techniques they had developed for the moon were being taken up elsewhere. Inge had a fair amount of experience with Mars, too; while at Lowell he compiled telescope observations into a number of ‘albedo’ maps that showed the light and dark markings familiar for centuries (albedo is an astronomical term for the brightness with which an object reflects sunlight). But the spacecraft data offered new challenges. The television images from Mariner 9 were far better than any previous pictures of Mars, but they were very poor compared with the best images of the moon seen from the earth. (Even those observations were not as good as the pictures taken by the high-resolution camera designed for national security work that flew on board the Lunar Orbiter missions, which in the late 1960s overtook airbrush work as the state of the art for lunar mapping.) And with Mars there was no running up to the telescope in the middle of the night to get a better look. It wasn’t all the spacecraft’s fault: Mars was not a terribly good photographic subject. Its surface was pretty uniformly dark, and even after the great storm of 1971 had died down the atmosphere carried a residual obscuring burden of dust, not to mention occasional clouds.
The pictures were a lot less than ideal. Their saving grace, though, was that they were stored in a digital format. And even in the 1970s, there was a lot you could do with digital data to make it look better. The distortions in shape and brightness due to the design of the TV tubes could be dealt with. So could the after-image effect caused by the fact that vestiges of the previous picture would be mixed in with the current one. (If all this makes the cameras sound bad, well, they were: but they were also the best that could be sent to Mars.) Contrast could be increased spectacularly with new image-processing algorithms which massaged the data so that small variations in brightness were exaggerated into large ones. The computers could also ‘rectify’ images in which the camera had been pointed off at an angle, rather than straight down, putting them into a form suitable for mapping. Points from Merton Davies’s control net would be identified in a set of pictures and a graph would be created that showed how those points would be arranged in a given map projection. Then the image files would be stretched and squashed until the control points in the images matched the pattern prescribed in the idealised graph. An easy way to check that the system was working correctly was to look at the shapes of craters before and after. In pictures the spacecraft had taken at an angle, perspective made the craters on the surface look elliptical; in pictures the computers had given a correct projection, they were circular.
This time-consuming process produced ‘photomosaics’ with their proportions corrected and their features enhanced. But these mosaics still had their shortcomings. Some of the individual images that made them up would be darker than others, giving a sort of fish-scale effect to the assemblage. The images would also have been taken at different times of day and thus different pieces of the landscape would be lit from different directions – confusing to the inexpert eye and irritating to the expert one. Imperfections in the control net squashed and stretched some areas (in the case of the north polar region the small number of distinctive landmarks was particularly problematic, and would cause Inge no end of grief). And many useful images were simply excluded. Much of the Martian surface had been visited repeatedly by Mariner 9’s cameras, but only one image of any given feature could make it into any given photomosaic. The others had to be left out, even if they offered extra information. In short, even when rectified, the primary Mariner 9 mosaics were ugly, confusing and less detailed than they could have been.
This was where the airbrush mappers came in: Bridges, Inge and their junior colleagues Susan Davis, Barbara Hall and Anthony Sanchez. They overlaid the photomosaics with Cronaflex, a Mylar film covered in a translucent gel on to which they would apply their ink. For the most part – different mappers had different styles – they would first trace the obvious features, such as rims of craters and edges of valleys, then start to work in the detail. As well as looking through their working surface at the mosaic beneath, they would also look at any other pictures they had that showed the same features. They built up a mental image of the forms they were trying to portray, their imaginations reaching into the images for detail, their discipline pulling them back from self-delusion.* They made their Mars in their minds and their airbrushes whispered it on to the Cronaflex. The concentration required was phenomenal. Ralph Aeschliman, the only airbrush artist still working at Flagstaff in 2000, likened it to being a bathroom plunger stuck to a television screen: ‘If you got interrupted there was this schwooup noise as you tore yourself away.’
Making the maps was a way of working through the data, one that did so in images rather than words. Inge talks of it as an act of interpretation, a way of precisely describing the television team’s data. But these were not just descriptions; they were pictures. Indeed, to some they were art. Aeschliman was scraping a living as a landscape artist in the Pacific north-west – he had an intriguing style that drew on Chinese influences – when a reawakened interest in astronomy led him to buy some of