Sky & Telescope - May 2023 - 53

80% of the reddish thermal glow in the
visible region of the spectrum is absorbed
or scattered by Venus's dense atmosphere
and low-lying clouds, but about 20%
penetrates the blanket of gas and mist.
A decade ago, veteran NASA planetary
scientist Dale Cruikshank calculated
the intensity of the visible-light
component of the dull-red thermal
glow of the nightside of Venus and
its apparent brightness as seen from
Earth. His luminance value for red light
(650 nanometers wavelength) exceeds
the brightness of the Milky Way's star
clouds by a factor of 20. So, if 80% of
this light is absorbed or scattered, is it
possible to see the remaining 20% in
close proximity to the dazzling glare
of Venus's sunlit crescent? Richard
McKim, who served as Director of the
British Astronomical Association's
Venus Section from 2004 to 2019,
believes it is. He suspects that the
changing visibility of the ashen light is
caused by variations in the thickness of
low-level clouds and perhaps even in the
level of Venusian volcanic activity.
Naval Research Laboratory astrophysicist
Brian Wood hesitates to attribute
the ashen light to thermal glow
because the sensitivity of the human
eye is highest in green light (around 555
nanometers) and falls off by a factor of
almost 10 in red light. Wood suspects
that a green emission feature recorded
in Parker Solar Probe images may be a
more plausible explanation.
During the day, intense solar ultraviolet
radiation breaks down molecules
of carbon dioxide in the atmosphere
of Venus high above the cloud deck,
liberating atoms of oxygen. Fierce, 320
km/h (200 mph) winds rapidly transport
these energized atoms to the night
side of the planet, where they slowly
combine to form diatomic oxygen molecules
(O2). The energy released by this
reaction emits green light.
First detected during the 1970s by
the Soviet Venera 9 and Venera 10
spacecraft, the brightness of this visible
oxygen airglow fluctuates dramatically
depending on the level of solar activity.
In the Parker images it's very prominent
right along the planet's limb, where
pThe ashen light is often described as a
uniform glow emanating from the nightside
of Venus. However, many observers have
reported an irregular, dappled appearance
similar to what is portrayed in this 1932
drawing by Robert Cheveau using a 7½inch
refractor.
pThis sketch by Dale Cruikshank depicts the
appearance of the nightside of Venus during
a daylight observation in 1962. This intriguing
observation by a seasoned observer and
eminent planetary scientist lends credence to
the notion that the thermal glow of the planet's
torrid surface may occasionally be visible in
telescopes.
the depth of atmosphere along the line
of sight is greatest. This feature may
correspond to a narrow, glowing band
outlining the planet's nightside limb
that was seen on several occasions during
the 1950s and '60s and dubbed " the
woolen thread " by the British observer
Valdemar Firsoff.
Wood points out that Venus's dense
atmosphere is also very effective at
refracting sunlight from the dayside and
wonders if a perceptible amount reaches
all the way to center of the nightside.
" It's clearly there at some level in the
[Parker] images and older Venus Express
orbiter data, " he says. " A lot more work
has to be done to quantify it, but this
light would certainly be at wavelengths
the eye would be very sensitive to. "
I've seen the ashen light only once -
almost four decades ago - but I remain
convinced that what I saw was no illusion.
The Parker images seem to lend
credibility to my eyepiece impressions of
a ghostly light marred by a diffuse dark
blotch that's still a vivid memory after
so many years.
The Parker Solar Probe team plans to
acquire more data during the spacecraft's
final flyby of Venus in November
2024. Meanwhile, they encourage
observers to gather more data as well:
" It would be a worthwhile project for
both amateur and professional astronomers
to assess whether the optical
surface emission seen by [Parker] might
be sufficiently bright to be observable
from the ground. "
For observers in the Northern
Hemisphere, evenings in the late spring
and early summer of 2023 present an
unusually favorable opportunity to
attempt to image in visible wavelengths.
From 40° north latitude, on June 16 the
planet's 43%-illuminated crescent will
stand 32° above the horizon at sunset.
By July 17 the crescent will be only 18%
illuminated, and the planet's altitude at
sunset decreases to 15°.
Take advantage of this four-week
observing window to carefully monitor
the nightside of Venus for the ashen
light. Comparing visual impressions
with images taken through red and
green filters combined with an infrared
blocking filter may finally solve one of
observational astronomy's oldest and
most enduring mysteries.
¢Although Contributing Editor TOM
DOBBINS has witnessed the ashen light
only once, it made a lasting impression.
sk yand tele scope .o r g * MAY 2023 53
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Sky & Telescope - May 2023

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