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The results were reported and interpreted in 1966 in conjunction with C. Sagan and J. B. Pollack.

It was observed that high radar reflection seemed to be associated with the dark areas on Mars, although the positions of maximum radar reflection were always displaced somewhat from the locations of the dark areas. An examination of the factors which might cause such enhanced reflection, taken in conjunction with other considerations, led to the conclusion that the dark regions are highlands. The mean slopes and maximum elevations above adjacent bright areas of a few dark regions on Mars, as estimated from the radar reflections, are given in the table on page 123. It should be noted that the slopes and elevations quoted are uncertain by a factor of about 2. Nevertheless, the results imply that the major dark areas have mean slopes of a few degrees and their maximum elevations above the nearby bright areas may be as much as 10 kilometers (33 000 feet) or more.

The dark areas, such as Nodus Lacoontis, Trivium Charontis, and Nepenthes, which are known to exhibit prominent changes in appearance, are seen to have gentle slopes, between 1 and 2 degrees, and moderate heights of about 5 kilometers (16 500 feet). Moreover, these relatively small dark regions are completely (or largely) surrounded by bright areas. The three existing conditions—gentle slope, moderate elevation, and adjacent bright regions—are just those which may be expected to favor coverage by .and removal of small particles carried by the wind.

The Moeris Lacus, Niliacus Lacus, and Syrtis Major regions, which do not undergo significant secular changes, on the other hand, have steeper slopes, higher elevations, and are only partially bordered by bright areas. The conclusions from the radar studies are thus in harmony with the view that secular changes in dark (elevated) areas are the result of particles blown off (or onto) them from adjacent bright (lower) regions.

The largest maximum altitudes given above for Mars, about 15 or 16 kilometers (50 000 feet), may be taken as the difference in elevation between the lowest and highest surface areas. On Earth, this would be equivalent to the vertical distance between the ocean bottom and the top of a high mountain. It is of interest that this distance has much the same magnitude as the difference between the lowest and highest elevations on Mars.

From radar studies made in 1967, G. H. Pettingill, of the Massachusetts Institute of Technology, arrived at altitude differences and slopes on the Martian surface of the same order of magnitude as those given above. But the correlation between dark areas and high elevations was much less clear. The measurement technique used by Pettingill differed from that employed by Goldstein, however, and sources of possible discrepancy between the results given by the two methods are being investigated.

An observation which may have significance concerning the relationship between elevation and the bright and dark areas of Mars was reported in 1968 by P. D. Lowman of the Goddard Space Flight Center. He pointed out that in the photographs of Earth taken from the manned Gemini spacecraft, the lower and level desert areas appear brighter than the mountainous regions. Although a comparison of photographs of Earth taken at a distance of the order of 100 miles with the appearance of Mars in the telescope may not be completely justifiable, the results do suggest the possibility that the bright and dark areas on Mars may be at lower and higher elevations, respectively.

A few words may be said here about the possibility of determining elevations on Mars by methods other than that based on radar reflection. Differences of altitude have been measured on the Moon by observing shadows cast near the terminator at times, close to the quarter phases, when the Sun is low on the lunar horizon. As a result of applying the same principle to Mars, P. Lowell concluded, in 1906, from his failure to observe shadows at the terminator, that abrupt heights on the planet did not exceed some 0.8 kilometer (2500 feet).

A reexamination, reported in 1961, of Lowell's estimates was made at the New Mexico State University by C. W. Tombaugh, the discoverer of the planet Pluto. He concluded that the smallest vertical height on Mars that could be detected from the shadow at the terminator is about 8.4 kilometers (27 500 feet). "This means of mapping relief [on Mars]," wrote Tombaugh, "is beyond ground-based capabilities." Moreover, the calculation of elevation from shadow lengths assumes steep slopes. Elevated areas on Mars with gentle slopes, such as appear to be indicated by the radar measurements, would not be detected by the shadow technique even if considerably higher than 8.4 kilometers.

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