Sky & Telescope - March 2024 - 53

BARRY COWLEY
ing 170 by 200 kilometers (106 by 124
miles) located between Mare Imbrium
and Oceanus Procellarum. Wood suggested
that a deposit of sulfur might be
responsible for this anomalous feature,
which became known for much of the
20th century as Wood's Spot.
By far the most conspicuous area of
localized color on the Moon, Wood's
Spot is known today as the Aristarchus
Plateau. Towering 2 km above the
surrounding maria, these highlands
are more than 3.6 billion years old. As
early as 1647 the lunar cartographer
Johannes Hevelius commented on
their ruddy hue. Many visual observers
perceive their color as a dirty yellow
reminiscent of Dijon mustard, while
others see it as dusky tan or even a
brownish hue.
Readily visible through a 3-inch
telescope, this tint is due to a veneer
of iron-rich volcanic glass that erupted
from the feature known as the Cobra
Head, the source of the broad lava
channel Vallis Schröteri, or Schröter's
Valley. Just to the east of the Cobra
Head is the brilliant, 40-kilometerwide
crater Aristarchus, one of the
youngest large craters on the Moon.
The impact that formed Aristarchus
about 450 million years ago exposed
underlying bedrock composed of the
bluish mineral ilmenite. The cool cast
of the interior of Aristarchus contrasts
starkly with the warm hue of the
nearby pyroclastic materials.
Olivine is probably the mineral
responsible for the olive-green and
bottle-green hues that transiently
appear under low Sun angles in Mare
Crisium, Mare Humorum, Mare
Frigoris, and on the floors of the craters
Grimaldi and Ptolemaeus. Apollo
15 astronauts collected samples of
vivid-green, translucent volcanic glass
near Rima Hadley in the foothills of
Montes Apenninus, another location
with a history of telescopic observers
noting a greenish tint.
The persistent color differences
of large expanses of the lunar maria
correspond to basaltic lavas that differ
in age and composition. They are
best discerned under high-Sun angles
The bluish tones of Mare Tranquillitatis (center right) contrast markedly with the warm hue
that dominates Mare Serenitatis to its northwest. Mare Imbrium at upper left displays the
greatest variety of color variations once your eye becomes accustomed to the view.
at gibbous and full phases. The cool,
bluish hue of Mare Tranquillitatis is
due to titanium rich basalts, while the
warmer khaki hue of Mare Serenitatis
corresponds to older, titanium-poor
lavas. The sharply delineated boundary
of the adjacent mare makes their color
difference rather easy to discern. Using
basic image-processing software to
moderately increase the color saturation
of lunar images captured with a
one-shot color camera like a DSLR,
webcam, or even a cellphone camera
can be very effective at revealing these
elusive hues.
In his 1621 magnum opus The
Anatomy of Melancholy, Oxford scholar
Robert Burton described the Moon as
" . . . a ruined world, a globe burnt out, a
corpse upon the road of night. "
Our closest celestial neighbor may
be a dead orb, but it is by no means
uniformly ashen grey. Glimpsing its
tinted nuances is a challenging and
rewarding activity.
¢Contributing Editor TOM DOBBINS
is from time to time known to spend an
entire evening focused on observing
Earth's satellite.
sk yand tele scope .o r g * MARCH 2024 53

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