Seasonal developments of the dark markings on Mars, 1907-1956
Slipher's caption to this image: Undoubtedly the outstanding revelation of the photographs is the undeniable record of seasonal darkening of vast blue-green regions in Martian summer. In 1907 we see Sabaeus Sinus standing alone in the Martian spring, but in the summer of 1909, it is seen darkly flanked by the great Pandorae Fretum in the form of a broad, dark band covering millions of square miles. Obviously if such a development occurred only once, it might be considered merely a strange coincidence. However, in 1924, with the planet again in its early spring, we note that the dark band has completely vanished into ochre desert, leaving the weak Sabaeus Sinus standing quite alone again. Also in the summer of 1926 we note that the great dark band has returned once more as a complete replica of what it was in 1909. In 1939, the dark Pandorae Fretum has disappeared again and Sabaeus Sinus stands alone, but in 1941 the summer darkening is shown returning over the Pandorae, although not to the full extent of the previous examples. In 1954, the customary winter-spring aspect is also evident. In 1956, during late Martian spring, save for a partial veiling due to haze and clouds from the widespread dust storm over most of Mars at the time, we see the customary darkening of the Pandorae Fretum region just as it has appeared in the other summer photographs.
Although not described in connection with the foregoing series of seasonal changes, the Hellespontus, Mare Ionium, Mary Tyrrhenum also were partly involved.
In a similar manner the series of dark regions in the southern hemisphere of the planet undergo a summer darkening. However, the Pandorae Fretum stands so completely alone on the planet that its appearance and disappearance with the seasons is most obvious and the fact that it completely vanishes into the ochre desert in winter makes it by far the easiest seasonal change to recognize.
Original image data dated on or about June 7, 1956