Sky & Telescope - November 2022 - 53

COURTESY RICHARD MCKIM
the 82-inch Cassegrain reflector at the
University of Texas's McDonald Observatory,
employing a binocular viewer (a
rare accessory at the time) and a pair of
eyepieces that yielded a magnification of
900×. At this generous image scale, the
Martian disc appeared a whopping 6° in
diameter - 12 times the diameter of the
Moon seen with the unaided eye.
Kuiper placed a chart issued by the
Glidden Paint Company containing
180 rectangular swatches of various
color shades at arm's length just below
the eyepiece and illuminated it with a
lamp that was a close spectral match
for sunlight. The intensity of the lamp
was carefully adjusted to match the
apparent surface brightness of the
chart's color patches with the telescopic
image of Mars.
To Kuiper's surprise, the apparent
color of the planet's dark markings
seemed to depend greatly on the quality
of the seeing. He noted that Mare
Acidalium had a greenish hue in poor
or average seeing, but when the seeing
improved, it lost nearly all green color,
and whenever the seeing was truly
excellent, it had the same ochre color as
its surroundings. These visual impressions
were repeated on several nights.
Kuiper resumed his color observations
in 1956. Once again, he found that
with rare exceptions " the dark areas are,
to a first approximation, neutral, having
the same color as the rest of the planet,
only darker. "
The dark regions look somewhat
greenish at first, but this color
disappears when one looks straight at
the area in question. The initial color
impression seems to be due to contrast
with the ocher desert regions. If one
looks straight at the dark areas, there
is no apparent correlation between
seeing and color, as was reported in my
1954 observations. Apparently, this
correlation was due to the circumstance
that, with poor seeing, the eye tends to
wander over the planet while, with good
seeing, it fixes on the area being studied.
Kuiper's account points to a phenomenon
that perceptual psychologists
Eugène Antoniadi depicted the colors of Martian markings in shades of green in
this beautiful 1909 watercolor. Even the most gifted observers can be deceived by
the effects of simultaneous contrast. South is up.
call negative afterimages. We perceive
color using our retina's cone cells. There
are three types of cone cells, each containing
a different photopigment that
responds to light of long wavelengths
(orange-red), medium wavelengths (yellow-green),
or short wavelengths (blueviolet).
When these photoreceptors are
overstimulated, their photopigments are
depleted and they lose sensitivity. Normally,
images on the retina are moved
to fresh photoreceptors by tiny, random,
involuntary eye movements. But if you
stare at a brightly illuminated red object
for 20 to 30 seconds, then shift your
gaze to a white or grey surface, you will
briefly see a ghostly afterimage with a
cyan (blue-green) complementary color.
William K. Hartmann, who received
his doctorate in astronomy as a student
of Kuiper at the University of Arizona,
is a talented artist whose paintings of
planetary landscapes are among the
most realistic. In 1988, Hartmann
observed Mars with a 24-inch Cassegrain
reflector from the summit of
Mauna Kea. Although the dark markings
looked " bluish-grey " at the eyepiece,
he found that in " a textbook display of
simultaneous contrast " they could be
very realistically depicted in an acrylic
painting using only pigments on the
warm (reddish brown) side of neutral.
According to the renowned space
artist Don Davis, the appearance of the
dusty soil covering Mars is a drab, ruddy
brown that's a dead ringer for cumin
spice powder. The dark, feldspar-rich
basalt rocks that comprise the planet's
dusky features are generally neutral
grey, with thin, ephemeral coatings of
dust deposited and swept away according
to the whims of Martian winds.
When you observe Mars during this
apparition, bear in mind the phenomenon
of simultaneous contrast and how
it influences your eyepiece impressions.
Perhaps you'll see the planet in a whole
new light.
¢A chemist by profession, Contributing
Editor TOM DOBBINS has often
observed simultaneous contrast while
studying Mars at the eyepiece.
sk yand tele scope .o r g * NOVEMBER 2022 53
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