Observing Project 7D The Tricks of Venus Atmosphere

The ashen light is one of the great mysteries of the inner solar system. Giovanni Riccioli first reported it in 1643. Looking through his telescope at the crescent

Venus he reported seeing a strange and unexplainable luminescence on Venus night side similar to Earthshine on a crescent Moon. Sir William Herschel also reportedly viewed the ashen light. The sharp-eyed E.E. Barnard never saw it but much more recent contemporary accounts made by prominent scientists reveal simultaneous viewing of the ashen light from two separate locations.

So is the ashen light for real and if so then what causes it? Astronomers using the 10-meter Keck I telescope on Mauna Kea have recently detected a greenish luminescence from the dark side of Venus. Spectrographic analysis of the glow is consistent with the 5,580-angstrom emission line of molecular oxygen (O2). Astronomers have hypothesized that ultraviolet radiation at the upper levels of the Venusian atmosphere might break down carbon dioxide into its constituent atoms, then high-level winds in the Venusian atmosphere would rapidly carry those atoms into darkness where the atoms would recombine into molecular oxygen, emitting green light as it does so. The problem with using this as the source of the ashen light is that molecular oxygen emissions are very weak and likely undetectable with amateur telescopes, especially in such close quarters with the -4 magnitude crescent Venus.

Could lighting flashes be the cause? If this is so and if they occur in sufficient quantities then it is possible that it may excite Venus' upper atmosphere to glow. But many scientists think this unlikely as well. In 1998 and 1999, the Cassini spacecraft made two close flybys of Venus to gain a gravitational boost from the planet to enable it to reach Saturn. Cassini buzzed just a few hundred miles above the cloud tops of Venus each time. If lighting strikes were common in the Venusian atmosphere, it would have created distortions in the spacecrafts low-frequency radio transmissions, much the same as lighting on Earth distorts AM radio. No such distortions were heard.

Many believe the ashen light is simply an optical illusion, a retinal after-effect created by the dazzling brilliance of Venus itself. But others believe it is real. On rare occasion an opportunity presents itself to search for the ashen light when the Moon intervenes and passes in front of a crescent Venus. When a waxing crescent Moon passes in front of a crescent Venus, then the Moon will first block the bright crescent. The Moon will then require as much as sixty to ninety seconds depending upon how big Venus itself appears to occult the entire planet. During this time, the ashen light may present itself. But such lunar occulations are rare and because they must occur close to sunset, only a very narrow area will get to see the event in a dark sky.

Through our telescopes,Venus does not ordinarily reveal much beyond its blinding white brilliance. But in the late 1970s, NASA's approaching Pioneer Venus orbiter imaged the planet's cloud tops in ultraviolet light and the planet revealed itself in surprising ways. The uniformly white cloud tops gave way to patterns and swirls as they raced around the planet. Though you cannot see ultraviolet light even if it could penetrate to Earth's surface, you might be able to pick out some detail in the Venusian surface with a violet colored filter, which screens out all visible light but that at the very end of the spectrum. Many amateurs have had success with this technique beginning to see hints of detail in the Venusian clouds. Seeing patterns and features in the clouds of Venus will require patience and a steady atmosphere for those precious few seconds you need to see, but you can see them.

Try to see these effects for yourself. For the ashen light, do not use any more magnification than is absolutely necessary or the high power may squelch the faint light you're trying to see. Do not use any filters either because the first light to be lost will be the ashen light. During four of its five evening apparitions, Venus is high enough in a dark sky while crescent that it sets at least two and a half hours after the Sun. The ashen light has escaped the notice of some of the world's most renowned astronomers. How about you? Many of the great astronomers of antiquity have never seen the movement of Venus' cloud tops either. But they also did not have the advantages of modern equipment. With the help of a quality telescope and a piece of purple colored glass, here's your chance to see what few others have and that the great astronomers of two and three centuries ago could only have dreamed of seeing.

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