The ‘ashen light’, or AL, is a faint, mysterious glow, or colouration, seen in the night-side hemisphere of the planet Venus. It’s often compared to the reflected ‘Earthshine’ that sometimes illuminates the dark side of the Moon.
First reported in 1643 by Italian astronomer Giovanni Riccioli, AL has been observed many times since, but its faint, transitory and elusive nature has prevented serious study.
More problematically, AL has only ever been detected by the human eye and no scientific instrument, either Earth-based or space-based, has ever recorded the phenomenon.
Some authorities have declared the phenomenon illusory, perhaps a contrast effect for the eye, or even ‘expectation bias’. Others suggest that instrumental defects may explain the phenomenon; light scattering, optical aberrations, the brightness of the background sky, weather and so on.
There are, however, enough credible reports of AL for some scientists to have offered explanations. These include reflected light from the Earth, aurorae, ‘airglow’ emission, lightning and infrared (heat) emission from the Venusian atmosphere.
Most of these explanations have been discounted for one reason or another. However, there’s good evidence that energetic solar wind particles, as well as ultraviolet light from the Sun, can excite oxygen atoms in the Venusian atmosphere.
This creates a faint green glow like that seen in aurorae on Earth. The process is somewhat different, though, since aurorae on our planet are due to the Earth's magnetic field interacting with solar particles, while Venus has no appreciable magnetic field.
Whether this explanation can account for all, or any, observations of AL is yet to be seen. So the long-standing mystery of AL may yet prove to be an illusion.
This article is an answer to the question (asked by Herman Townsend, Liverpool) 'What is Ashen Light?'
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