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THE PHOTOGRAPHS ARE REPRODUCED

Meanwhile] (6) the tape-recorded version of Picture No. 1 was fed into a television-like picture tube and photographed, to produce a picture in a [more] familiar form. [Because the sun would be striking the Martian surface almost vertically in the first few pictures, we knew there would be no strong shadows to bring out surface details. Nonetheless, we were all, I think, somewhat shocked by the almost total absence of surface features in the first few pictures when they were viewed just as they arrived, without enhancement of any kind. In fact, we were ïît sure that the few surface-features visible were real until we saw, on close inspection, that certain markings in the first picture coincided with similar markings in the overlapping, second picture.]

The first unmistakable craters turned up in Picture No. 7 and continued to appear prominently through No. 14. Beginning with No. 15 the light level dropped faster than the automatic gain control could adjust, [in part because a light level that was acceptable for a green-filter picture was not adequate for the subsequent red-filter picture. There may also be significant atmospheric obscuration in pictures No16 through No. 18.] The camera crossed the terminator to the dark side of Mars in Picture No. 19, and from then on no surface details can be seen.

The television scan path started on the limb of Mars at about 47 degrees North latitude, swept southward across the equator to about 53 degrees South latitude, then curved northward again and moved off the planet at about 30 degrees South latitude [(see illustration)]. As it turned out, the path crossed a region in which maps of Mars show only a few canal-like markings, and we have not yet been able to discern any such markings with certainty. [It appears that the camera just failed to catch the edge of an interesting feature called Trivium Charontis, which has the shape of a long, thin triangle. This region is significant because it has been observed to change considerably during the past few years, and because it is close to the “desert” area Elysium. Earlier in 1965 persistent white clouds had been seen in the Elysium area, and it is also known to be a strong reflector of radar signals.]

After the 22 pictures had been recorded once, a process that took a little more than eight days, Mariner IV was ordered to transmit the entire set a second time. [We were anxious to see how closely a replay would duplicate the initial values for the 40,000 picture elements in each picture. Any discrepancies between the two playbacks would indicate the number of errors that had occurred in transmission and would also tell us where they had occurred in each picture.] We were gratified to find that the second transmission differed from the first in only about 20 elements of the 40,000 in each pictures, making an average of 10 errors per picture in each transmission. [This was far fewer than we had dared hope to achieve, and represents a truly remarkable level of performance for such a complex system.



To those of us involved in the project] the major surprise in the pictures was the large number of craters; more than 70 of all sizes are clearly distinguishable. [We realize now that we should not have been so surprised. Both Ernst J. Öpik of the Armagh Observatory in Northern Ireland and Clyde W. Tombaugh of New Mexico State University, and probably others as well, had predicted that close-up pictures of Mars would reveal a cratered surface. In the Astronomical Journal for October, 1950, Tombaugh proposed that the “round ‘oases’ are sites of impact craters caused by the collisions of small asteroids”, and he also predicted that “the lack of water erosion on Mars would permit the surface to retain a visible record of major events that happened during the planet’s entire separate existence, similar to the moon.”]

After examining the Mariner IV pictures [(and without knowing of Öpik's and Tombaugh's predictions) my col­league] Murray pointed out that they apparently depicted an extremely ancient surface. We guessed that the surface might be as much as two to five billion years old. [We meant by this that features of that age would still be visible.] In contrast, surface features, on the earth are eroded and effaced in a few lens of millions of years. [Our estimate of the age of the Martian surface has since been challenged by other investigators, who believe the pictures would show even more craters if some had not been removed by erosion. In our opinion the matter cannot be settled until more of the Martian surface has been photographed and until more is known about the relative rates of impact of asteroid-sized bodies on the moon and Mars.


Date: 2015-04-20; view: 1142


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