No one knows why dark side of Venus has a faint glow

Key Points:

• The ashen light on Venus, first noted by Giovanni Battista Riccioli in 1643, remains an unresolved astronomical mystery with sightings reported over nearly four centuries.
• The term "ashen light" was coined in the late 1800s, inspired by the analogy to earthshine, the faint light reflected from Earth onto the Moon's dark side.
• Various theories have been proposed, including ionization by solar ultraviolet light, aurorae, and lightning, but recent observations by Japan's Akatsuki orbiter found no evidence of lightning flashes on Venus's night side.
• Current leading explanations focus on nightglow caused by atmospheric oxygen emissions triggered by solar events, supported by data from NASA's Parker Solar Probe detecting visible light emissions at 557.7 nm on Venus.
• The ashen light may also be an optical illusion influenced by human visual sensitivity or the contrast effects of viewing Venus's bright crescent against its dark side.