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The Mystery of Venus’ Ashen Light

Venus imaged by Magellan Image Credit: NASA/JPL

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, Venus 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!

For further reading, a paper on Ashen Light by C. T. Russell and J. L. Phillips

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Science writer at Urban Times URBNFUTR & 21st Floor. Broadcaster on Under British Skies for Astronomy.FM Radio. Amateur Astronomer. Skeptic. Rational Thinker. Science Geek. Music Lover. Harpist. Singer. Book Worm. BSL User. WooBasher. Dangly Earring Wearer. OU Junky

Comments on this entry are closed.

  • canderson April 27, 2012, 3:48 PM

    I’m confused. This article states that “nobody has managed to capture an image of the Ashen Light, yet,” but then later states that “The Keck 1 telescope on Hawaii reported seeing a subtle green glow.” Given that there is no way to look through the Keck with an eyepiece, I’m assuming that when the author says that the Keck “saw” a glow, she means that a spectrum of Venus’s dark side showed emission?

  • hofstaedter georg April 27, 2012, 4:28 PM

    Wer sucht in österreich einen Arbeitsplatz als Kellnerin,……? /die astojenny ! oder hab’ ich eine fata morgana beim gleichen artikel von bad astronomer gesehen !!!????

  • hofstaedter georg April 27, 2012, 4:43 PM

    ein ahha erlebniss,…!ad sense von google, werbung für travelgirl !!! Jedzt hab’ ich kappiert !

  • Abraham Samma April 27, 2012, 7:44 PM

    According to Russell and Phillips in the paper above:

    ” We would expect that the process causing ashen light should act equally well at all locations of Venus relative to the Earth. The variation in occurrence rate must be due mainly to observability from Earth.”

    (Observability in terms of how much light from the sunlit side is drowning the ashen light; shows how faint it is.) I initially thought that seasonality would figure also if not more. I tend to find aurora activity much more feasible than lightning as the latter should be occurring at an incredible rate as to have assured it’s detection by Cassini.

  • Abraham Samma April 27, 2012, 7:44 PM

    According to Russell and Phillips in the paper above:

    ” We would expect that the process causing ashen light should act equally well at all locations of Venus relative to the Earth. The variation in occurrence rate must be due mainly to observability from Earth.”

    (Observability in terms of how much light from the sunlit side is drowning the ashen light; shows how faint it is.) I initially thought that seasonality would figure also if not more. I tend to find aurora activity much more feasible than lightning as the latter should be occurring at an incredible rate as to have assured it’s detection by Cassini.

  • torque xtr April 27, 2012, 10:10 PM

    I wonder, why there are no photos and so little other information on that from Venus Express, which has been there for some years and should have made many observations of the nightside…

    • jjbreen April 28, 2012, 3:13 PM

      Let’s start a conspiracy about the Venus Express and it’s supposed lack of “News Releases”, they could equate to “News Black Outs” …. Imagine the fun we could have!!

      Go on the talk circuit and make a few $$$ …. I could easily spin a yarn that would rival Roswell! ;)

  • Prism2Spectrum April 29, 2012, 11:21 AM

    Mysterious “Ashen Light” glow from the dark side of Earth’s hellish neighbor (its terrible twin). Seems an appropriate description (whether phenomenon is real or not) – in gray-light of that world, enveloped in a smothering, acid-laced cloud deck (illuminated with bolts of fury?), storming around the planet with demon-like ferocity (under an eerie auroral sky-glow?). A burnt-world that have may have, at least once, undergone a volcanic, global resurfacing event (as of impacting catastrophe?). And consequently, one that might have been completely molten (before its deathly descent into hell?) – after it revolved as a finished, rock-surfaced, and ocean-adorned(?) planet (with an encircling moon?).

    That world today, which so brilliantly coruscates – like a diamond of beauty – above our Life-World’s twilight horizons, appears in the harsh Sun-light, a lifeless-gray, grim ash-enclosing tomb. An entire planet that may have once shone with life-promise. Densely buried, it now casts the most dramatic light on the wonder and glory of its next-door neighbor – our living Homeworld!

    (Hark! Do I hear the trumpets blare. And, lo, in my eyepiece, did I glimpse glittering displays, as of royal celebration? Or were they late-night fires of rejoicing and feasting? Alas, maybe it was just a volcanic eruption from the infernal regions
    below.)

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