The Ashen Light of Venus: the oldest unsolved solar system mystery

The Ashen Light, a feeble glowing or colouration of the night-side of Venus, has been discussed ever since Riccioli reported it in 1643. It has been frequently, if intermittently, seen, under various guises, ever since; and though illusion (e.g. a contrast effect) has been suspected, its reality as a physical phenomenon of the planet has often been supported by observers. Theories to explain it are numerous and some have been rather far-fetched. In recent years, the most plausible have been a glimpse in the visible range of thermal emission from the hot lower atmosphere of the planet, or a glow produced by ultraviolet-, electron-, or ion-excited oxygen molecules in the upper atmosphere of the night-side. Two of the authors, Sheehan and Brasch, made a systematic study during the favourable Eastern elongation of spring 2012, consisting of visual observations and CCD images through a number of different telescopes and filters. With a special green filter, they recorded a glow. Artificial planet observations and imaging similar to those employed by previous investigators were also carried out, and showed similar features. We conclude that in all probability none of the known physical phenomena of Venus can explain the Ashen Light. Our evidence strongly supports the illusion theory. [continued…]

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