The mystery of Venus’ ashen light

April 30, 2012

May is the best time to try and spot one of the most enduring unsolved mysteries in our Solar System. Ashen Light is a faint glow allegedly seen on the unlit portion of Venus, during its crescent phase, similar to the earthshine often observed on the Moon, though not as bright. It is more commonly observed while Venus occupies the evening sky, as now, than when it is in the morning sky. But no one really knows for sure what causes it.

So what’s the history of our knowledge about this enigmatic glow?

The phenomenon was first noted in 1643, by Italian astronomer Giovanni Battista Riccioli. Though many notable astronomers have reported sightings in the 369 years since, including Sir William Herschel and more recently, Sir Patrick Moore, many others have failed to see the effect, leading to claims that it is due to nothing more than observer error, an illusion, atmospheric effect or equipment malfunction. Things are not helped by the fact that nobody has managed to capture an image of Ashen Light, yet.

As the month progresses, nears the Sun, ready for its transit on June 5th to 6th and the planet’s crescent phase will increase in diameter during the month, from 37 arcseconds to 56 arcseconds. The best option for amateur astronomers hoping to catch a fleeting glimpse is to use an occulting bar to block the bright crescent, making any glow present on the unlit portion of Venus, more visible.

There is much controversy and many theories as to the cause of Ashen Light. The Keck 1 telescope on Hawaii reported seeing a subtle green glow and suggested it could be produced as ultraviolet light from the Sun splits molecules of carbon dioxide, known to be common in Venus’ atmosphere, into carbon monoxide and oxygen, but the green light emitted as oxygen recombines to form O2 is thought too faint to explain the effect. Another more likely theory is that multiple lightning strikes are illuminating Venus’ skies. Though the Cassini spacecraft flew by Venus twice on it’s voyage to Saturn and failed to detect the high frequency radio noise we associate with thunderstorms on Earth, in 2007 Venus Express did detect low frequency ‘whistler waves’ that can also result from lightning. It could also be the Venusian equivalent of aurorae.

By far the most bizarre theory, and my personal favourite, was proposed in the early 19th century by the Bavarian astronomer Franz von Gruithuisen, who suggested that Ashen Light was the result of fires lit to clear land for farming on Venus, or to celebrate the coronation of a new Venusian Emperor!

Explore further: Pretty Sky Alert

More information: … ussell/papers/ashen/

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Apr 30, 2012
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not rated yet Apr 30, 2012
Astronomer Franz von Gruithuisen believed that ashen light was from the fires from celebration of a new Venusian emperor, and later believed that it was the inhabitants burning vegetation to make room for farmland. I do believe instead, it's auroral phenomenon.
not rated yet Apr 30, 2012
Please, someone, take some good pictures of it in May
Things are not helped by the fact that nobody has managed to capture an image of Ashen Light, yet. With a surface temperature of at least 900 degrees Fahrenheit and places with active lava flows far hotter than that, it is reasonable to expect the thermal glow of the surface should be seen at least occasionally from Earth.
1 / 5 (1) Apr 30, 2012
One very reasonable inference which you won't see mentioned by conventional thinkers is that the planet is possibly not in thermal equilibrium. It may actually be emitting a slight amount of light for the very reason that it is cooling off from some recent former event. Although it's certainly not something which is popular amongst people on physorg, it's very important to mention that numerous cultures of the world claim that Venus is actually a young planet. Whether or not they are right is up to us to establish -- not assume.

There is an interesting piece of evidence which used to be discussed, but which has long since fallen out of fashion in conversations about Venus: It appears to have an anomalous albedo. And I believe that the anomaly was given a boost when probes were sent down, as they repeatedly reported back that Venus' temperature spiked at the planet's surface.

I also recall that there was some debate over this, insofar as thermal equilibrium is being *assumed*.
5 / 5 (1) Apr 30, 2012
that numerous cultures of the world claim that Venus is actually a young planet.
Which other evidence they have? The Venus can contain a higher amount of heavy radioactive elements, which would keep it hotter than the Earth and Mars. For example it's speculated, the surface of Mercury is full of lead and tellurium, because of strange radar reflections in some craters. The strong green-house effect will serve as a heat insulator as well.
not rated yet May 11, 2012
In 1953, Nikolai Kozyrev attempted to analyse the phenomenon of Ashen light, a nocturnal air glow on Venus whose existence remains controversial to this day. He also made the earliest photometric measurements of the visible and ultraviolet spectrum of Venus. His calculation of the thermal balance of Venus disputed the popular theory that the clouds of Venus consisted of dust. Kozyrev argued that energy absorbed in the upper atmosphere created high altitude storms, but the surface of Venus would be still and dimly lit. This work had an impact on the theory of Venus and Nobel Laureate Harold Urey devoted a paper to the analysis and implications of it.

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