Sky & Telescope - January 2025 - 52

JANUARY 2025 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
The Shifting
Sands of
Mars
Syrtis Major is an iconic
Martian feature that has
changed dramatically over
the years.
I
n 1877, Italian astronomer Giovanni
Schiaparelli introduced a naming system
for surface features on Mars based
on geographical names from classical
antiquity. This nomenclature is still in
use today. He named the Red Planet's
most conspicuous dark marking Syrtis
Major after the Roman name for the
body of water on the coast of Libya
that's known today as the Gulf of Sidra.
Centered near 290° longitude and
10° north latitude, Syrtis Major is a
dusky, wedge-shaped albedo feature (a
region that differs in reflectivity from
adjacent areas) that extends some 1,500
kilometers (930 miles) northward from
the Martian equator and spans 1,000
km from west to east. The first permanent
feature to be noted on
the surface of another
planet, it was recorded
in a series of 1659
sketches of Mars
by the Dutch
astronomer
Christiaan Huygens.
He was
able to glean
that the length
of a day on Mars
is slightly longer
than a day on
Earth by monitoring
the feature's passage
52 JANUARY 2025 * SKY & TELESCOPE
Syrtis Major is the centerpiece of this 1999 Hubble Space Telescope image.
across the face of Mars from night to
night through a primitive 2.7-inch
refractor that magnified only 87×.
A native of water-logged Holland,
Huygens compared the prominent
smudge to a bog. By the mid-19th century,
the numerous dark features that
cover about three-eighths of the Martian
surface came to be widely regarded
as bodies of water. Before the adoption
of Schiaparelli's nomenclature, cartographers
dubbed Syrtis Major the
" Atlantic Channel " or the
" Hourglass Sea. "
By the dawn of
the 20th century, it
became clear that
these markings
As this 1864
sketch by English
observer William
Rutter Dawes depicts,
during the 19th
century Syrtis Major
appeared considerably
narrower and more pointed
than it does today.
couldn't be bodies of water because
they never produced visible specular
reflections of sunlight. In addition,
steady seeing revealed that they weren't
uniformly dark, but rather complex
patchworks of spots and streaks. A new
consensus emerged that the markings
were the dried-up beds of ancient seas
surrounded by bleak ochre deserts - a
notion that persisted until NASA's three
Mariner spacecraft surveyed the planet
at close range in the 1960s.
In 1873 British astronomer Richard
Anthony Proctor wrote: " The principal
charm of astronomy, as indeed of all
observational science, lies in the study
of change - of progress, development,
and decay, and especially of systematic
variations taking place in regularlyrecurring
cycles. "
Changes on Mars have captivated
generations of observers. While the overall
pattern of its markings has remained
recognizable over the centuries, the
planet's albedo features vary in outline,
intensity, and coloration both seasonally
and from year to year. On a timescale of
MARS: NASA / ESA, S. LEE (U. COLORADO) J. BELL (CORNELL U.)
AND M. WOLFF (SSI); SKETCH: WILLIAM R. DAWES
Sky & Telescope - January 2025

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