There aren't many four-hundred-year-old astronomical mysteries. A lot of the questions that early astronomers asked, modern astronomers have answered. And yet, on our astronomical neighbor, there's a weird glow that after hundreds of years of research that we still can't understand.
In 1643, Giovanni Riccioli was observing Venus when he noticed a sort of glow emanating from the night side of the planet. Venus isn't always presenting its fully-lit-up side to us. In the summer months, just at dusk, and when Venus is illuminated (for us) only by a thin crescent of sunlight, it's possible to see a kind of glow. This is what Riccioli named "The Ashen Light of Venus."
Throughout the centuries, astronomers have consistently reported it. They say it looks like earthshine — the faint glow of sunlight off the Earth that illuminates the dark side of the moon — but the Earth is too far away for it to work on Venus. Where is the glow coming from? This is one of the mysteries that has emphatically not been solved by any space mission. Some space crafts looking for it haven't even been able to spot it — leading some astronomers to believe that it's just an enduring myth. When the Keck I telescope spotted bright spots on Venus, however, some of the Riccioli's credibility was regained.
What is the glow? There are many theories. Some believe that it's the carbon dioxide in the atmosphere of Venus being split by solar radiation. Some believe that it's the result of the lightning storms in the atmosphere of the planet. There's always a possibility it's the Venusian equivalent of the Northern Lights. No one knows for sure.
[Via Universe Today, UCLA, NASA]