No one knows why dark side of Venus has a faint glow
by Ellsworth Toohey · Boing BoingOn January 9, 1643, the Italian astronomer Giovanni Battista Riccioli pointed his telescope at Venus and noticed a faint glow on the planet's unlit side. Riccioli figured it was an artifact of his glass — the same trick that makes a prism throw rainbows. Almost four centuries later, astronomers are still arguing about what he saw.
Wikipedia's entry on the persistent astronomical puzzle of ashen light traces sightings from Riccioli through Sir William Herschel, Sir Patrick Moore, and a string of 20th-century observers. The German priest Athanasius Kircher may still have noticed it five years earlier, during a 1638 visit to Sicily. The phenomenon got its name in the late 1800s, when astronomers compared it to earthshine — the dim ash-colored light reflected from Earth onto the dark side of the Moon.
Theories have come and gone. Harold Urey suggested in 1957 that ions whipped up by solar ultraviolet light were doing it. A 1969 paper proposed aurorae. Throughout the 1980s, lightning was the favorite, and the Soviet Venera probes, along with ESA's Venus Express, picked up electromagnetic hints of storms. But Japan's Akatsuki orbiter, which has been watching the night side with a dedicated Lightning and Airglow Camera, logged 16.8 hours of darkness without catching a single flash.
The current frontrunner is nightglow. Data from NASA's Parker Solar Probe in 2022 showed that the WISPR camera could see the hot Venusian surface right through the clouds in visible light. After a coronal mass ejection slams into Venus, the upper atmosphere lights up at 557.7 nm, an emission produced by oxygen and the same color that gives some of Earth's auroras their green tint. Human eyes are poor at detecting the deep red glow from the surface, so anyone who has ever claimed to see ashen light may have unusually red-sensitive vision, an optical illusion from staring at a bright crescent, or no glow at all.
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