A new look at the Martian 'violet haze' problem. I - Syrtis Major-Arabia, 1969.

Photographic and photoelectric data obtained in 1969 have been combined to provide a relatively accurate and complete description of the so-called blue clearing phenomenon on Mars. The dark feature Syrtis Major and its surroundings are the subject of the present study. A series of comparison images of Mars in blue light was assembled to form a scale of blue clearing. The scale steps represent about the smallest readily recognizable difference in apparent contrast. The relationship between the scale number and the brightness of Syrtis Major as compared to its surroundings has been established microphotometrically. International Planetary Patrol Program photographs in blue light having Syrtis Major near the center of the disk were compared to the scale and assigned a value. The results indicate that the blue clearing of Syrtis Major was strongly dependent upon phase angle in 1969, reaching a maximum at opposition. Syrtis Major displayed a diurnal variation of blue clearing near opposition. This variation disappeared away from opposition.