Studies of the surface of Mars (very early in the era of spacecraft reconnaissance)

There is probably no moment in the history of studies of Mars, including the year 1877, in which our knowledge of that planet is changing more rapidly. Prior to 1965 it was generally believed that the surface pressure on Mars was about 85 mb. As a result of ground-based spectroscopy, rocket ultraviolet spectroscopy, and the Mariner 4 flyby mission we now know that the actual pressure is about a factor of ten smaller. Prior to 1966 most students of the subject believed that the topographical relief on Mars was quite minimal. At first on rather inferential grounds and later in a much more direct manner, radar observations have shown that very major elevation differences exist on Mars. In ground-based studies generally, a major assault on Mars is now being performed. On the question of elevation differences, in 1969 two major radar observatories and at least four optical observatories working in the near infrared were attacking the problem with an investment of time hitherto reserved for the most distant objects from earth. At the same time two space vehicles, Mariners 6 and 7, were speeding toward a rendezvous with Mars equipped with a wide range of scientific instruments for remote reconnaissance. Except for the far encounter photographic sequences the spacecraft observed Mars only for a time on the order of an hour, as the space vehicles flew by Mars. However, in 1971 a Mariner mission is scheduled which, if successful, will place two orbiting spacecraft into orbits about Mars with nominal instrumental lifetimes of 90 days. Such a mission should permit complete mapping of the Martian surface at a variety of wavelengths, thorough searches for anomalous features, and systematic investigation of diurnal and seasonal changes on the planet. In 1975 the National Aeronautics and Space Administration plans, in addition to two more orbiters, two landers which will be deposited in selected areas of the planet again with 90 day nominal lifetimes. If all these spacecraft work as intended, it is clear that our knowledge of Mars will have increased by many orders of magnitude in a very few years. For these reasons one could hardly have picked a worse moment to review current knowledge of the surface of Mars. We proceed with this review in full expectation that much of it will be obsolete in the near future.