Sky & Telescope - March 2025 - 53

POLAR CAP: EMIRATES MARS MISSION / EXI / J. MAJOR
winter when Mars is near perihelion, so
it seldom extends below 60° latitude at
its maximum extent.
The north polar cap's permanent
core persists into late summer and consists
primarily of water ice. This remnant
contains an estimated 1.6 million
cubic kilometers of water ice (equivalent
to about 30% of the Greenland ice
sheet) and extends from the pole to
about 80° latitude.
During winter, carbon dioxide, the
principal constituent of the Martian
atmosphere, freezes into a layer of
frost and snow less than a meter thick
atop the permanent cap of water ice.
This seasonal veneer of frozen carbon
dioxide contains more than 1
/8 of the
mass of the tenuous Martian atmosphere.
The " dry ice " snow component
also contains water-ice crystals and
particles of dust that act as condensation
nuclei.
As the carbon dioxide snow sublimates,
it entrains considerable amounts
of water vapor. The volatiles released by
the retreating polar cap cause clouds
and haze to form more frequently at
low latitudes. Extensive haze on the
morning and evening limbs of Mars
should have begun to appear in February,
as well as discrete white clouds over
topographic elevations like the Tharsis
shield volcanoes and Libya Montes.
The visibility of these features is greatly
The United Arab
Emirates Hope orbiter
captured this
image of Mars in
May of 2021 when
a dense canopy
of clouds and icefogs
covered the
North Polar Cap.
pThis 1918 pastel drawing by the British
observer T. E. R. Phillips depicts the shrunken
summer polar cap surrounded by Lowell's
Band. The dark, shark-fin-shaped albedo feature,
Syrtis Major, had just emerged from under
haze on the morning limb at right. Orographic
clouds above the elevated volcanic province
Elysium appear as the bright enclosed area just
to the left of center.
enhanced through blue (Wratten 38A)
and violet (Wratten 47) eyepiece filters.
As northern hemisphere spring
arrives, longer hours of sunlight
rapidly warm the arctic regions and
the dense winter canopy of clouds and
ice-fogs known as the " polar hood "
begins to dissipate. But this year the
orbital motion of Mars is also causing
the planet's distance from the
Sun to increase throughout early
spring. (Mars will reach aphelion on
April 21st, when Ls = 70°.) As a result,
for several weeks the polar hood may
reform and the polar cap's retreat may
slow or even reverse. Best remembered
for his discovery of Pluto, Clyde Tombaugh
was also an avid Mars observer.
He coined the term " aphelic chill " for
this recurring phenomenon.
The retreating polar cap is often
bordered by a narrow, dark collar
known as Lowell's Band. Best seen
through orange (Wratten 21) and red
(Wratten 23A or 25) filters, this feature
was first recorded by Wilhelm Beer
and Johann Heinrich Mädler during
the 1830s. In the closing years of the
19th century, Percival Lowell imagined
that it was a temporary sea of meltwater
from the thawing polar cap that fed
a global network of irrigation canals.
Spacecraft images have revealed this
transient collar as dusky fields of dunes
swept clean of frost and snow by winds
blowing off the pole.
The Mars we know today isn't as captivating
as Lowell's romantic vision of
the home of a struggling race of canal
builders, but changes on the Red Planet
remain enticing targets for backyard
telescopes.
¢Contributing Editor TOM DOBBINS
is co-author of Epic Moon: A History of
Lunar Exploration in the Age of the Telescope,
available at shopatsky.com.
sk yand tele scope .o r g * MARCH 2025 53
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Sky & Telescope - March 2025

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