The Planets. Professor Cox Brian

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      President Lyndon B. Johnson sharing in man’s ‘quest to extend the horizons of human knowledge’ as Dr William H. Pickering (left), director of the Jet Propulsion Laboratory, shows him the first Mariner 4 photos.

      Lyndon B. Johnson – extracts from Remarks Upon Viewing New Mariner 4 Pictures From Mars, 29 July 1965

      ‘Dr Webb, Dr Pickering, Dr Leighton, Members of Congress, distinguished guests:

      Unaccustomed as I am to welcoming men from Mars, I am very happy to see you gentlemen here this morning. As a member of the generation that Orson Welles scared out of its wits, I must confess that I am a little bit relieved that your photographs didn’t show more signs of life out there …

      The flight of Mariner 4 will long stand as one of the really great advances in man’s unending quest to extend the horizons of human knowledge. In the history books of tomorrow, unlike the headlines of today, the project’s name may be lost but the names of the men of vision, men of imagination and faith who made this enterprise such a historic success are going to be honored in the world for many generations to come.

      This advance for mankind is awe-inspiring. It is all the more so when we realize that such capabilities have come into being within a short span of a very few years …

      It may be – it may just be that life as we know it with its humanity is more unique than many have thought, and we must remember this.’

      In response, Carl Sagan co-authored a paper suggesting, somewhat playfully, that all was not lost. Mariner 4 took only 22 photographs with a resolution of over a kilometre in a strip crossing the region in which the astronomer Percival Lowell had sketched canals from his observatory in Flagstaff, Arizona, at the turn of the twentieth century. From the warmth of the Arizona Desert, Lowell wrote that Mars was chilly, but no more so than the South of England, which certainly supported a civilisation of sorts. Using several thousand photographs of a similar resolution taken by meteorological satellites in Earth’s orbit, Sagan and his co-authors found only a single feature that unambiguously indicated the presence of a civilisation – Interstate Highway 40 in Tennessee. They concluded that Mariner 4 would not have detected human civilisation had it flown by Earth. ‘We do not expect intelligent life on Mars, but if there were intelligent life on Mars, comparable to that on Earth, a photographic system considerably more sophisticated than Mariner 4 would be required to detect it.’

      The non-existence of an extant Martian civilisation was confirmed by the Mariner 9 mission in November 1971 – the first spacecraft to orbit another planet. Mariner 9 achieved a photographic resolution of 100m per pixel, and no sign of intelligent life, past or present, was detected. The twin Viking landers in 1976 failed to detect even microbial life, although the combined results of the suite of microbiology experiments carried by the spacecraft are not considered to be unequivocal because Martian soil chemistry is, to coin a phrase from the official NASA report, enigmatic, and could conceivably have masked any biological activity.

      In hindsight, the fact that Mars is not teeming with life today is not so surprising. Mars orbits 50 million miles further from the Sun than Earth and receives less than half the solar energy. It is a small world with a tenuous atmosphere that provides little insulation or greenhouse warming. NASA’s Curiosity rover in the Gale crater has measured midday temperatures above 20 degrees Celsius, but in the early hours of the morning it has experienced minus 120. As Alfred Russel Wallace wrote in 1907, any attempt to transport water across the Martian surface today would be ‘the work of mad men rather than of intelligent beings’. There are no canals, no cities and no envious eyes. The planet is a frozen hyper-arid desert too far from the Sun to support complex life.

      Yet it hasn’t always been this way. Observations from our fleet of orbiting spacecraft and landers have revealed a complex and varied past. Once upon a time the red planet was glistening blue. Streams ran down hillsides and rivers wound their way through valleys, carved by a water cycle from land to sky and down again from mountains and highlands to the sea. This presents a great challenge for planetary scientists. Put simply, nobody would have been surprised if Mars had always been an inert rock because it is a small planet far from its star. But the geological evidence is unequivocal; the surface tells a different story.

      Mars, then, remains an enigma. As a wandering red star, it stirred the imagination of the ancients. As a telescopic image, too small and shifting for visual or intellectual clarity, it became our twin. When spacecraft flew by, it shocked us into considering our cosmic isolation. The red planet was relegated in our collective consciousness to the status of just one more rock glistening in the night. Then we landed, and discovered a world that was once habitable, and could be again.