Sky & Telescope - March 2022 - 53

DIAGRAM OF ATMOSPHERIC CONVECTION: GREGG DINDERMAN / S&T, VENUS: JAXA / ISAS / DARTS
of how these features arise. Convection
largely governs atmospheric circulation
on Venus. Near the equator, the intense
sunlight causes hot air to rise and flow
toward the poles. These strong laminar
winds extend to latitudes between 60°
and 70°, where air starts to descend and
flow back towards the equator beneath
the visible cloud canopy. This region
is the site of the dusky " cold collars, "
which appear narrow to earthbound
observers due to foreshortening. At even
higher latitudes, atmospheric circulation
is dominated by the whirlpool pattern
of powerful polar vortices, which
are covered with bright cloud hoods.
These features are the cusp caps.
In 1842, Gruithuisen wrote, " I find
that spots were seen sometimes at both
poles, at other times at one pole alone. "
This observation has also proved accurate.
Recent ground- and space-based
observations show that the speed and
strength of zonal atmospheric currents
on Venus can vary considerably and
often show hemispheric inequalities.
This discovery may account for the
results of a 1958 statistical review by
James Bartlett of 830 Venus observations
made by members of the Association
of Lunar and Planetary Observers
from 1944 to 1956. Bartlett found that
one or both cusp caps were visible in
54% of the observations: The south cap
alone was seen 11% of the time, the
north cap 7%, and both caps simultaneously
35%.
Cloud features on Venus stand
out boldly in images recorded using a
monochrome planetary camera and
ultraviolet filter. They are usually subtle
in visible light and always difficult to
discern visually, unless steps are taken
to reduce glare using a neutral-density
filter or, better yet, a variable polarizing
filter (S&T: Nov. 2020, p. 52). Color
filters can also dramatically increase
their visibility by boosting contrast.
While most observers prefer the Wratten
38A violet filter, prolific British
Venus observer Valdemar Axel Firsoff
claimed that the combination of a green
filter and a polarizing filter gives the
best views. Imagers equipped with highspeed
video cameras and an ultraviolet
filter rountinely record the cusp caps.
The romantic visions of polar snows
on Venus are banished forever. But
following the ever-changing appearance
of the cusp caps and cusp collars
remains a rewarding opportunity to
contribute to our understanding of
the dynamic atmosphere of our closest
planetary neighbor.
¢Contributing Editor TOM DOBBINS
has occasionally glimpsed details in the
clouds of Venus with the aid of filters but
never without them.
Polar vortex
Convection
Solar
heating
Convection
Polar vortex
The pattern of atmospheric circulation on Venus is evident in this false-color ultraviolet image
from the Japanese Akatsuki spacecraft. Convection from intense solar heating drives winds
toward high latitudes, where a zonal flow parallel to the equator eventually predominates near
the polar vortices.
In 1878, French astronomer and artist
Étienne Trouvelot depicted brilliant spots along
the margin of the southern cusp cap that he
imagined were snow-capped peaks.
British observer David Graham made this
rendering of gibbous Venus in 1988 showing
the cusp caps while observing through his
6-inch refractor at 286×.
sk yand tele scope .o r g * MARCH 2022 53

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