On the basis of the sample provided by Mariner IV one can say that the number of large craters per unit area on the Martian surface and their size distribution resemble closely the size and distribution of craters on the high-lands of the moon [(see top illustration at right)]. The Martian craters have rims that rise about 100 meters above the surrounding surface and depths that extend several hundred meters below the rims. The crater wails slope at angles tip to about 10,000 degrees [If Mariner IV's sample of photographs is representative, there must be more than 10,000 craters on the surface of Mars.
Judging by the Mariner IV's sample Mars seems to have fewer craters of 10 kilometers in diameter and smaller than would be expected if their distribution in size were similar to that on the moon. Moreover,] there seems to be a tendency for the small craters to appear on the rims of large craters. [This suggests that there may be something special about the composition or texture of the crater rims that resists the forces that tend to erode1 small craters when they are formed elsewhere on the Martian surface.]
In some of the pictures taken deep in the Martian southern hemisphere one can see areas that seem to have a light covering of frost. [One can also see that many of the craters, instead of being circular, are flattened along a portion of their circumference. This phenomenon, also observed in lunar craters, is believed to result from structural faults below the surface. In at least one picture (No. 11) a pronounced line, quite straight, intersects a crater and continues across the rim. This too might be caused by a fault. So far we have not been able to complete the computer processing needed to draw any conclusions from the pained red arid green pictures, or to prepare them in a form suitable for combining their; overlapping areas into a color picture.] (7)
A mystery of considerable interest is presented by the high light levels recorded near the limb of the planet in the first picture. Where we had expected to find a black sky, the sky was more than half as bright as the planet! The other pictures also show evidence of "fogging", as if the Martian atmosphere were, enormously brighter and more extended than anyone had expected.
[Our first thought was that the fogging represented some kind of detect.iri the optical system. We wondered, for example, if the surface of the telescope mirror could have been pitted by the impact of meteoritic dust, but this seems to be ruled out by the fact that the meteorite detector, fulló exposed outside the spacecraft, received only a few hundred hits. We have also considered the possibility that volatile substances from the foam cushions used to protect the Vidicon tube might have whitened the black inside surface of the telescope tube and created internal reflections. We found, however, that we could not duplicate the fogging even by inserting white cardboard baffles in place of the black ones in the optical system.
Finally, we considered the possibility that the nickel compound that provides the top coat on the telescope mirror before it receives final polishing might have blistered after long exposure to the vacuum of space. We simulated blisters by putting drops of glue on a mirror but were still unable to duplicate the fogging seen in the Mariner IV pictures. We have tentatively decided that the cause of the fogging is really on Mars.] Recent models of the Martian atmosphere seem to suggest that tiny crystals of frozen carbon dioxide are present at all times even at great heights. Whatever the cause of the fogging [in July 1965] it must have extended to at least 100 kilometers above the surface of the planet and therefore it may be distinguishable from the earth with careful observation.
[LIFE ON MARS?]
There was never any expectation that these photographs, with their coarse one-kilometer resolution, would settle the question of whether or not life exists on Mars. We and others [(notably Carl Sagan of the Smithsonian Astrophysical Observatory)] have examined mànó pictures of the earth taken by the Tiros and Nimbus weather satellites, whose narrow-angle cameras provide somewhat better resolution than the Mariner IV camera, and can find only one or two examples of a picture that shows a human work of engineering [(see illustration)]. And this is even when one knows what to look for. Still more surprising, the Tiros arid Nimbus pictures fail to provide any evidence of vegetation, or seasonal changes in the earth's ground cover, except for snow and floods. It is certainly true that Mars looks inhospitable to life as we know it, but the question of whether there is life on the planet remains open.
[After an experiment such as Mariner IV's is concluded one always has second thoughts. For example, it might have been better to photograph a different area, or to use a camera system that provided a wider field of view, it would have been desirable, of course, to have sent Mariner Â with its two cameras. One would like to see the entire disk of Mars with, say, five-kilometer resolution. Still, there will beopportunities to make other, photographs in the future. We feel satisfied that the first close-up views of Mars made possible bythe ingenuity and hard work of hundreds of people, have shown the importance of an exploratory approach to the study of our planetary neighbors, and that they will be remembered as among the outstanding photographs of the early space age.] (8) (From Scientific American, April 1966, v. 214, No. 4, pp. 54—68.)