NINETEEN close-up pictures of Mars, showing details down to a few kilometres across, were recorded by Mariner IV before the cameras crossed the terminator into the night hemisphere. The first seven pictures are not rich in detail because the Sun was less than 30° from zenith and shadows were absent. Later pictures, taken under a lower Sun, show progressively more relief: what may be afternoon shadows occur even on the ninth frame, composed at a solar altitude of more than 50°.
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Proceedings of the Geologists' Association (1970)
A survey of long-term observational behavior of various martian features that affect some recently proposed interpretations
International Geology Review (1968)