Venus’s Mysterious “Ashen Light”

January 9, 1643. A Jesuit priest and astronomer named Giovanni Battista Riccioli is observing Venus with his little refractor telescope. He notes the night side of Venus seems to be lighter than usual and names it “The Ashen Light of Venus”. Over 100 years later Sir William Herschel documents seeing this phenomenon.

It has been over 300 years since Riccioli reported seeing Ashen Light on Venus and the mystery remains. Many have looked for it and only a few have reported seeing it. Some place Ashen Light in the same category as Percival Lowell’s Martian “canals”, illusions.

If it’s real, what is Ashen Light?

Data from the Parker Solar Probe (PSP) might shed light on this mystery. You might ask, what does PSP have to do with Venus? Nothing in terms of study, but PSP is using Venus to gain momentum via seven fly-by encounters, using Venus’s gravity to slingshot it into its closest orbit of the Sun in 2025.

During its most recent two fly-bys, scientists took visible light images of Venus’s night side using PSP’s WISPR (Wide-field Imager for Solar PRobe camera). Why? To test a hypothesis that the cloud decks might be translucent to visible light. A way to test it is to look for a low-level glow coming through the clouds on Venus’s night side. Venus’s surface is around 900 degrees F even on its night side so it should glow a dull red. The handicap is these same clouds scatter and absorb infrared, allowing only about 20% makes it through. While this is plenty to detect at near-infrared wavelengths, there was concern it be too low for visible wavelength imaging.

The PSP team went ahead and took images of Venus’s night side and WOW!  They got a big surprise as the images showed blurry versions of the same surface features as previous radar mapping by NASA’s Magellan spacecraft. These are the first visible light images of Venus’s surface through its clouds! As a bonus, PSP’s camera picked up airglow, caused by ultraviolet radiation breaking down CO2 in the day side clouds and the released oxygen combining into O2 in the slightly cooler night side clouds. This produces a green glow.

The resultant images from nighttime Venus seem to corroborate multiple Earth-bound telescopic observation reports of a faint, mottled glow with dark splotches. The green glow that WISPR recorded might also be involved in this phenomenon. The human eye is very sensitive to green wavelengths, so the investigators believe it does have a role. In any event, more data will be collected during PSP’s remaining fly-bys and hopefully provide evidence to finally explain Venus’s Ashen Light.

What’s in the Sky?

Astronomy Night at Tye Preston Memorial Library, Canyon Lake. Tonight, Saturday, March 26th, 8:30 PM. New Braunfels Astronomy Club members will be there rain or shine, telescopes at the ready outside or a fun presentation inside.

March 28; 45 minutes before sunrise; southeast: A waning crescent Moon shines below Venus, Saturn, and Mars