Sky & Telescope - October 2020 - 55

the desert regions of Mars . . . a little less
than the size of Texas." However, whatever was going on was short-lived.
Following the Great Dust Storm
of 1956, the new dark area vanished
as quickly as it had come. This storm
represented a turning point in the
history of Mars interpretation. It was
not the ﬁrst large dust storm observed
on the planet. But for the ﬁrst time
some astronomers - notably Dean
B. McLaughlin of the University of
Michigan - began to understand the
planet's albedo changes in terms of
the relentless movement of windblown
sand and dust, seeing their shifting
outlines as resembling the patterns of
an Etch-a-Sketch board. Among the ﬁrst
converts to McLaughlin's theory was
Yerkes Observatory astronomer Gerard
P. Kuiper. Earlier a strong advocate of
the vegetation theory, Kuiper completely
changed his mind after 1956.
We now know, of course, that there
is neither liquid water nor vegetation on
Mars. There is, however, lots of sand and
dust, as well as outcrops of weathered
basalt (see our photo essay on page 12).
There are also seasonal winds, capable of
raising dust devils, regional dust storms,
and planet-encircling storms.
A combination of warming and local
topographic differences still seems to

S Lighter dust fills Libya at top right, spilling
into the dark area of Syrtis Major.

Major Planum shield volcano. For a time,
the dust somewhat truncated that side of
the Syrtis's dark, triangular form. Later,
winds came and stripped most of that
dust away. It ﬁnally settled within the
ancient impact basin Isidis Planitia.
The Martian winds blow on, continuing to produce a phantasmagoria
of changes that fascinate telescopic
observers on Earth. What we are seeing
are the ever-shifting shapes caused by
wind and dust. But let's not entirely forget the changing shorelines of seas and
vast tracts of vegetation grown during
the last ebb of a dying race of Martians
imagined by our predecessors.
 Contributing Editor BILL SHEEHAN
is co-author, with Jim Bell, of The New
Planet Mars, coming soon from the University of Arizona Press.

SY RTIS M A JOR: N ASA / JPL / USGS; M A P:
WIKIMEDIA COMMONS / PUBLIC DOMAIN

T Giovanni Schiaparelli's 1878 map of Mars,
featuring the first "canali." South is up.

explain a subset of the storms. However,
thanks to orbiting spacecraft imagery,
we now know that many of the great
summer storms actually have a distant
origin, starting out along the tracks of
seasonally repeating winter dust storms
originating a hemisphere away. Plotting these tracks helps scientists better
understand the way that dust alters
the albedo features in places like Solis
Lacus, Pandorae Fretum, and Libya.
Like the polar vortices of Earth, large
cyclones develop in the polar regions of
Mars, especially around the North Polar
Cap during the northern hemisphere's
cold winter. Some of these cyclones
can become enormous. However, from
Earth they are hard to study, because
during much of autumn and winter
northern latitudes are hidden beneath
the clouds of the North Polar Hood.
During the 2001 planet-encircling
dust storm observed by NASA's Mars
Global Surveyor, bands of dust originating in circumpolar cyclone activity began
to move southward, picking up more
dust as they went through Mare Acidalium and Chryse, and then along the
equator into the Tharsis region. Bright
Tharsis dust, injected into Solis Planum,
caused the Solis Lacus albedo feature to
change shape. Here, the dust was caught
up in the strong winds of the subtropical jet and made its way across Noachis.
Much of it would end up far to the east,
piled up along the slopes of the Syrtis

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