Ashen light

Ashen light is a hypothesised subtle glow that has been claimed to be seen on the night side of the planet Venus. The phenomenon has not been scientifically confirmed. Its potential causes remain disputed, but it may be associated with lightning on Venus.

The ashen light was first reported by the astronomer Giovanni Battista Riccioli on 9 January 1643, and he named it "The Ashen Light of Venus." Subsequent claims have been made by various observers including Sir William Herschel, Sir Patrick Moore, Dale P. Cruikshank, and William K. Hartmann.

The ashen light has often been sighted when Venus is in the evening sky, when the evening terminator of the planet is toward the Earth. Observation attempts were made on 17 July 2001, when a 67% Venus reappeared from behind a 13% moon. None of the observers of this occurrence (including some using 61 cm 'Super RADOTS' telescopes) reported seeing the ashen light. Video from the event was captured, but the camera was too insensitive to detect even the earthshine.

A particularly favourable viewing opportunity occurred on 8 October 2015, with a 40% illuminated Venus reappearing from behind the unlit limb of a 15% sunlit Moon. The event was visible in dark skies throughout Central Australia and was recorded by David and Joan Dunham (of the International Occultation Timing Association) using a 10" f/4 Newtonian with a Watec 120N+ video camera from a location just north of Alice Springs. They also observed the event visually with an 8" SCT telescope. Neither the real-time visual observation nor close visual inspection of the video recording showed any sign of the dark side of Venus. While not conclusive, these observations suggest the ashen light is more likely attributable to telescope optics and eye physiology rather than atmospheric phenomena on Venus.

The Keck telescope on Hawaii reported seeing a subtle green glow and suggested it could be produced as ultraviolet light from the Sun splits molecules of carbon dioxide (CO
2), known to be common in Venus' atmosphere, into carbon monoxide (CO) and oxygen (O
2), but the green light emitted as oxygen recombines to form O
2 is thought too faint to explain the effect, and it is too faint to have been observed with amateur telescopes.

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