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SYOFTHE^ B.A.A.skyandtelescope.com • SEPTEMBER 2019 53February 18, 1897: Observing from
the island of Malta with a 10-inch New-
tonian refl ector at 250×, the Reverend T.
H. Foulkes reported “good air. Every-
thing in favour of perfect observations.
Had two most remarkable observations
of Venus.... I distinctly saw a clearly
defi ned ‘hump’... like a lunar moun-
tain on limb at full, very white.... a
little further [south] appeared an intense
white circular spot... exactly like one
of Jupiter’s satellites, coming on or off
the disc.”
April 17, 1940: Observing Venus in
daylight near superior conjunction with
a 5-inch refractor, Henry McEwen, the
Glasgow engineer who directed the
Mercury and Venus Section of the Brit-
ish Astronomical Association from 1895
until shortly before his death in 1955,
recorded: “Good images... through
thin cirro-stratus... Three shining
points of light visible... reminiscent of
[the lunar crater] Aristarchus on Mare
Procellarum — an impressive sight!”
On March 19, 1985 I was observing
Venus in evening twilight with a hum-
ble 4-inch refl ector when I was struck
by a well-defi ned spot on the limb of
the crescent. It was so brilliant that it
appeared to protrude into the surround-
ing sky, deforming the planet’s limb in
a fashion identical to Foulkes’ descrip-
tion of the feature witnessed 88 years
earlier. This seeming projection was an
illusion known as irradiation, a physi-
ological phenomenon caused by the
spreading of excitation from the area of
the retina that is actually stimulated by
light. Irradiation occurs at the bound-
ary between areas of markedly unequal
brightness, causing brighter areas to
appear to encroach upon fainter ones.
This extraordinary feature was
captured on fi lm 19 hours later by the
British amateur David Greenwood, who
was unaware of my observation in those
days before the rapid communication
made possible by the internet. Green-
wood pointed out that the interval of
time that had elapsed cast grave doubts
on whether the phenomenon was
atmospheric in nature or even the same
phenomenon at all. He noted that the
atmosphere of Venus rotates every 4days, so in the 19-hour interval between
our observations any atmospheric fea-
ture would have been displaced by about
70° of longitude.
So how could the location of such an
undoubtedly real feature have remained
virtually stationary? I can’t help but
think that it was somehow tethered
to the planet’s surface. The globe of
Venus spins backwards on its axis once
every 243 days, much slower than the
‘super-rotating’ overlying atmosphere.
I wondered if upwelling plumes pro-
duced by volcanic eruptions account for
the “star-spots” reported by telescopic
observers throughout the past century.
The surface of Venus is peppered
with hundreds of volcanic mountains,
basaltic lava fl ows, and other features
that make the planet’s history of vol-
canism unmistakable. While most of
the volcanoes appear to be long dead, in
recent decades the emergence of several
“smoking guns” suggests continuing
volcanic activity. Radar mapping by
NASA’s Magellan orbiter during the
early 1990s revealed a host of fresh-
looking lava fl ows. More recently, the
Infrared Thermal Imaging Spectrometer
aboard the European Space Agency’s
Venus Express probe detected several
transient hot spots.Perhaps the
most compelling
evidence consists
of the sudden,
dramatic surges
in the concentra-
tion of sulfur dioxide
in the upper atmo-
sphere of Venus detected by
Venus Express and NASA’s Pioneer
Venus Orbiter. One of the principal
gases released by volcanic eruptions
on Earth, sulfur dioxide is rapidly
destroyed by photochemical reactions
in sunlight. Any increase of the gas
in Venus’s upper atmosphere can only
signify a very recent origin.
A volcanic eruption would have to be
extraordinarily violent to blast sulfur
dioxide through the canopy of clouds
and haze located 50 to 70 kilome-
ters above the planet’s surface. Many
planetary geologists believe the planet’s
crushing atmospheric surface pressure,
which is 90 times greater than Earth’s,
muffl es volcanic eruptions on Venus.
They envision eruptions on Venus as
slow, oozing effusions of lava.
Another possibility is that a gust of
charged particles from the Sun could
create glows by energizing a patch of
the planet’s upper atmosphere. Alterna-
tively, atmospheric waves, which trigger
convective turbulence and are thought
to transport material up and down,
could briefl y concentrate bright material
to create the star-spots.
In this era of space probes and orbit-
ing telescopes, mysteries endure that
are accessible to the backyard astrono-
mer. That’s why I always have a sense
of anticipation when I peer into the
eyepiece of a telescope, even when it’s
pointed at a chronically unrewarding
target like Venus.¢Contributing Editor TOM DOBBINS
has witnessed several transient features
in the Venusian atmosphere during his
nearly fi ve decades of observing.pThe late observer and historian Richard
Baum is seen here with his venerable 4.5-inch
Cooke refractor.uThree “star-spots” in the
clouds of Venus as seen
by Henry McEwen on
April 17, 1940.