__1643: __Italian astronomer Giovanni Riccioli discovers a faint glow on the night side of the planet Venus. Other astronomers over the ensuing centuries will also observe the Ashen Light, but one of the longest-running mysteries of astronomy still defies conclusive explanations.

Riccioli was an astronomer of some repute. Working in the first generation after Galileo, he discovered that Mizar (the middle star in the handle of the Big Dipper) is actually a double star – the first one known. He also discovered satellite shadows on Jupiter and published a map of our moon's surface. The names he assigned (e.g., Sea of Tranquility, Sea of Storms) are still used today.

The faint luminescence Riccioli saw 366 years ago has been seen many times since, by professionals and amateurs alike. It's also not been seen by many who were looking for it. Its apparent intermittence and the lack of a satisfactory explanation has led some to chalk it up to observer error, distortion caused by Earth's atmosphere and/or artifacts induced by telescope optics.

But, still: 366 years of similar observations? Those who've seen the Ashen Light of Venus report it looks a lot like the reflected "Earthshine" that sometimes casts a dull glow on the moon, but not even that bright. It's most easily sighted when the dusk edge of the sunlight on Venus faces Earth.

The U.S. Pioneer mission and the Soviet Venera 11 and 12 landers looked for it without any luck. The Keck I telescope in Hawaii did spot a faint, green glow consistent with the 558-nanometer emission of oxygen atoms. It seemed possible that UV sunlight breaks abundant carbon dioxide molecules into carbon monoxide molecules and oxygen, with the single oxygen atoms emitting green light when they recombine into two-atom oxygen molecules. However, that emission would be too weak for all the amateur telescopes to have detected over the years.

Another possibility is multiple lightning flashes. During Venus flybys in 1998 and 1999, the Cassini spacecraft failed to detect the high-frequency radio noise that lightning would be expected to generate – like AM radio static during terrestrial thunderstorms. On the other hand, "observations of Venus' ionosphere ... reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz [that] have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds."

It's also possible the Ashen Light of Venus is caused by solar particles energizing the atmosphere like the terrestrial Aurorae Borealis and Australis – hence its evanescence.

Or it's some previously unknown combination of things we understand.

Or something we don't understand at all.

Source: Eastbay Astronomical Society