The Paradoxical Ashen Light
By Mark Gingrich and Ellis Myers
Sir William Herschel saw it. Sharp-eyed E. E. Barnard
never did. Much more recently, though, Patrick Moore has seen it-and made sketches
of it-on several occasions. In addition, two contemporary planetary scientists,
Dale P. Cruikshank and William K. Hartmann, both saw it from well-separated
observing sites, simultaneously. Yet some claim it is of doubtful existence,
akin to Schiaparelli's observations of Martian "canali."
The "it" mentioned above is one of the
unsolved mysteries of the solar system, the Ashen Light of Venus. First reported
by Giovanni Riccioli on January 9, 1643, it is said to be a faint luminescence
on the night side of the planet, similar in appearance to "earthshine"
on the Moon, although not so bright. Ashen Light has been most often sighted
when Venus was in the evening sky, when the evening terminator of the planet
is toward the Earth. Studies have been attempted by several space missions,
including Pioneer and the Russian Venera 11 and 12 landers. Still, the phenomenon
remains sporadic and elusive. It is currently again in the news.
Working with the 10-meter Keck I telescope in Hawaii,
researchers recently identified a faint, green luminescence on the night side
of Venus, consistent with the 558-nanometer emission from oxygen atoms. The
scientists suggest that carbon dioxide, in high concentrations in the atmosphere,
are split by the ultraviolet of the daytime sunlight into carbon monoxide and
atomic oxygen. These oxygen atoms are carried by high-speed winds to the night
side, where they emit the green light as they combine to form molecular oxygen.
This emission is quite weak, however, and it is probably not such that it could
be detected with amateur telescopes. Thus it may not be the true source of Ashen
Light, after all.
Lightning flashes are another postulated mechanism
for Ashen Light. If numerous enough, their integrated effect may give an overall
glow in the dusk skies of Venus. But in an article appearing in a January issue
of Nature, a team of astronomers from the University of Iowa published a criticism
of this idea. Analyzing data from the Cassini spacecraft, which flew by Venus
at close range in 1998 and 1999, the team determined that high-frequency radio
noise was not detected. Such static-like disturbances to AM radio heard during
a thunderstorm on Earth would have been expected, unless lightning on Venus
is extremely rare.
But these latest findings don't completely rule out
the existence of Ashen Light-not yet, at least. Certainly Ashen Light could
be an intermittent occurrence. Some have even speculated that solar particles
bombarding Venus' s upper atmosphere might create a hemisphere-wide auroral
glow, one that is as capricious as gusts of the solar wind.
Experienced amateur astronomers attempting to view
the Ashen Light very make the attempt by using an occulting bar (that is, an
opaque mask in the eyepiece's focal plane) to block the dazzling sunlit portion
of Venus. This greatly cuts down extraneous light scattered in the eye, improving
the chance of seeing the feeble Ashen glow. Nevertheless, Venus light scattered
by the Earth's atmosphere and in the telescope's optics (in addition to diffracted
light from a reflector's secondary-mirror support vanes) conspire against the
Yet there indeed is a way to see Venus's night hemisphere
without having to suffer that veiling scatter. On July 17, 2001, the waning
crescent Moon occults the 68-percent-illuminated disk of Venus. Most significantly,
the reappearance event has the planet's night side emerging first from behind
the dark (albeit faintly aglow in earthshine) lunar limb, an occulting bar par
excellence! Here, then, is a chance-admittedly brief, of no more than a dozen
seconds or so-to look for Ashen Light without interference by Venus's sunlit
Clearly, if Venus's disk is discernible before its sunlit side emerges, that would constitute a compelling, positive data point bolstering the case for Ashen Light. Furthermore, a quick-and-dirty assessment could be made of the Ashen Light's surface brightness relative to the level of earthshine for that particular lunar phase (13% illuminated).
On the other hand, suppose the Ashen Light is not
seen during the brief interval when only Venus's night side is in the clear,
and suppose also the glow is then perceived a minute afterward, say, when the
planet's day side becomes visible. That scenario would support either the illusion
hypothesis or the instrument-artifact hypothesis.
Alas, this remarkable opportunity is only for a select
few. To witness it under ideal circumstances, one must be situated in a relatively
small swath of the Earth's surface, far enough east within the occultation zone
so that Venus is perched sufficiently high above the eastern horizon, and far
enough west to avoid dawn twilight. And if you wish also to watch from terra
firma (i.e. "solid ground"), your choices are restricted to the Wake
Atoll and the easternmost Marshall Islands in the Pacific Ocean tropics. Avoid
the rush and make your travel plans now!