Sky & Telescope - November 2022 - 52

NOVEMBER 2022 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
Seeing the True Colors of Mars
L
A lesson for astronomers from a chemist
ike the Moon, Mars displays a
distinctive pattern of light and dark
areas that early astronomers designated
as terrae (lands) and maria (seas). The
notion that the dark areas were actual
bodies of water was abandoned late in
the 19th century in favor of the belief
that they were tracts of vegetation.
Unlike their lunar counterparts, the
Martian maria exhibit seasonal changes
in intensity and seasonal changes in
outline suggestive of the growth and
decay of vegetation like we experience
here on Earth. But the major underpinning
of the vegetation theory stemmed
from the fact that many observers
perceive the color of the dark areas on
Mars as distinctly bluish-green.
The palette of reported colors
includes grass green, moss green, olive,
teal, and even robin's-egg blue. These
eyepiece impressions are so common
that a popular 1953 book about the
possibility of life on Mars, written by
physiologist and space-medicine pioneer
Hubertus Strughold, bore the title
The Green and Red Planet. Yet all these
vivid hues are literally in the eye of the
52 NOVEMBER 2022 * SKY & TELESCOPE
beholder - the products of an optical
illusion discovered early in the previous
century by a French chemist.
Michel-Eugène Chevreul (17861889)
was a towering figure of chemistry
in the 19th century. In 1823 he
embarked on an exhaustive study of
natural dyes and was soon appointed
Director of Dyeing at the Manufacture
des Gobelins, France's national tapestry
workshop, where he was tasked with
improving color fastness and intensity
in woolen goods.
As part of his rigorous approach to
the technology of dyeing, Chevreul
also investigated color perception. He
discovered that the perceived color of
an object depends on the color and
brightness of its surroundings, which
he called " the law of simultaneous
contrast of colors. " He wrote: " In the
case when the eye sees at the same time
two contiguous colors, they will appear
as dissimilar as possible, both in their
optical composition and in the height of
their tone. "
Simultaneous contrast causes colored
highlights to impart their complemenThis
color composite of Mars captured by
the European Space Agency's Rosetta spacecraft
attempts to accurately depict the planet's
true hues.
tary hue to any adjacent low-luminosity
features. For Mars observers, the
implications should have been obvious
- a dark neutral marking viewed
against a bright reddish background
will take on a bluish-green cast. In his
1833 work A Treatise on Astronomy, the
British astronomer (and accomplished
chemist) Sir John Herschel dismissed
the greenish color of the Martian maria
as a contrast illusion arising from " a
general law in optics. " Indeed, many
impressionist painters made great use of
simultaneous contrast, yet only a handful
of astronomers seemed to be aware
of it. By the dawn of the 20th century,
leading Mars observers like Eugène
Antoniadi and Percival Lowell seemed
oblivious to the illusion.
Gerard Kuiper made a very painstaking
attempt to determine the true colors
of Mars during the planet's 1954 apparition.
Widely regarded as the father of
modern planetary science, Kuiper used
ESA & MPS FOR OSIRIS TEAM MPS / UPD / LAM /
IAA / RSSD / INTA / UPM / DASP / IDA
Sky & Telescope - November 2022

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