Possibly one of the oldest and most mysterious phenomena that telescopic observers have reported through the centuries is a faint glow emanating from the night side of Venus. This effect is known as the "Ashen Light" and to this very day it still remains a mystery in spite of several good theories. Unfortunately, none hold up as spacecraft missions to Venus have wrecked all the theories, but cannot give any reason for the mysterious glow.
Ashen Light gives Venus the appearance of a crescent Moon where the dark side is illuminated faintly by light reflected from Earth, known as the old Moon in the new Moon's arms, or Earthshine. Since Venus has no moon, this cannot explain the Ashen Light. Earth is too far away to illuminate the dark side of Venus. Ashen Light is best seen when Venus is a thin crescent, 30% illuminated or less, and against a dark sky. The color of the Ashen Light is primarily a warm, ruddy hue, but violet, blue, and green are also observed, although more rarely. Viewing the Ashen Light through color filters reveals it to be brightest in the red region of the spectrum. It is possible that the Ashen Light is an optical illusion as it has not been photographed and spacecraft have not seen it. The eye could play tricks by trying to fill in the rest of the globe with light. The telescope can also potentially create an optical illusion. Venus is extremely bright and the violet aberration of refractors can give the illusion of Ashen Light. Reflectors also are affected by the brilliance of Venus as light diffracted by the spider supports of the secondary mirror can produce an illusion of airglow on the night side of Venus.
Ashen Light is a rare phenomenon usually seen as a uniform, structureless glow, but sometimes it can appear patchy. One theory for the glow involves aurora but Venus does not have a magnetic field. Another theory is that the Ashen Light could be airglow due to the ionization of atoms by solar radiation. Orbiting spacecraft have detected airglow but it is far too dim for the eyes to detect through even the largest and best telescopes. Lightning has been proposed as another theory, but lightning is more prevalent on the day side on Venus instead of the night and is mainly concentrated close to the equator. In order to be visible from Earth as the Ashen Light, lightning would have to flash at a rate of 1000 flashes per second, which is ten times greater than what has been detected.
A more recent theory involves the fact that the clouds of Venus vary in thickness and opacity, which may allow the dull red thermal glow of the nearly 900 degrees F surface to be dimly perceptible at visible wavelengths. Absorption and light scattering by aerosols in Venus' atmosphere are weak at visible and near-infrared wavelengths allowing the dim glow of the surface to vary depending on the thickness of the clouds. It is not impossible for a fully dark adapted eye to see the ruddy, infernal glow from the surface of Venus under excellent observing conditions. The hotter lowlands would be brighter than the highlands and could explain the patchy appearance of the Ashen Light. The clouds of Venus are transparent at several near-infrared wavelengths making it possible to image the deep cloud decks and even the surface. The hotter lowlands shine through nice and bright at these near-infrared wavelengths and this is something that anyone can photograph using a modest telescope, a camera or webcam, and a 1-micron infrared filter. Such photographs give the appearance of the Ashen Light, but the near-infrared spectrum is invisible to the eyes.
The Ashen Light remains one of astronomy's greatest unsolved mysteries, but amateur astronomers now have many powerful imaging tools at their disposal. It may not be long before the mystery of the Ashen Light of the inferno world of Venus is finally solved.