Unraveling
the
Mysteries
of
Mars
By Dr. Steven Lee
Figure 1
Figure 2
For as long as humans have
looked up into the night
sky, the planet Mars has
sparked imaginations.
For thousands of years,
it was merely a bloodred star wandering
across the heavens. In
early cultures, it conjured
up images of war and
bloodshed. Egyptians named it
Har décher, “the Red One.” The
Babylonians called it Nergal, “the Star
of Death.” The Greeks considered it Ares,
“the Fiery One,” and to Romans it was
the god of war, called Mars.
In 1610, Galileo Galilei turned his newly
invented telescope on Mars, seeing only a
tiny reddish disk. Christiaan Huygens
sketched the first crude maps of the planet
in 1659 (Figure 1), noting that the visible
dark and bright markings drifted across the
disk in about 24 hours, indicating the length of
the Martian day was similar to Earth’s. By the 18th
century, astronomers were routinely mapping
clouds, polar caps, and surface markings that
seemed to change with the seasons. Dark markings
were assumed to be oceans and seas; similar
assumptions were applied to features on the Moon.
In 1877, Italian astronomer Giovanni Schiaparelli
drew maps showing a global network of
interconnected dark lines (Figure 2). His term
canali—Italian for channels—was mistranslated into
English as “canals,” leading to a “life on Mars”
hypothesis that held sway for many decades. An
annual “wave of darkening,” in which the dark
surface features became larger, darker, and better
defined in the spring and summer seasons, was
assumed to result from the springtime growth of
vegetation nourished by water collected from the
retreating polar ice caps. The supposed canals were
taken as evidence of an advanced Martian
civilization’s global-scale engineering project,
designed to carry the springtime melt from the polar
caps to irrigate forests and fields in the temperate
regions.
Since the mid-1960s, observations by spacecraft
have allowed Earthbound scientists to begin
unraveling the mysteries of the Red Planet and to
finally put many old myths to rest. In 1965, the
Mariner 4 spacecraft zipped within 6,200 miles
Figure 3
THE RED ARROW IN THE MARINER 4
IMAGE (ABOVE) POINTS TO THE
LOCATION OF THE MARS GLOBAL
SURVEYOR IMAGE (RIGHT). THE 1999
IMAGE SHOWS FEATURES ONLY 10 FEET
(3 METERS) ACROSS—AN IMPROVEMENT
IN RESOLUTION OF ABOUT 400 TIMES.
(10,000 km) of Mars and radioed back a scant 22
images collected over half an hour. This small
sampling revealed a lunarlike surface scarred with
impact craters (Figure 3) and sheathed in a thin
carbon dioxide atmosphere. Scientific interest in the
geological and possible biological history of Mars
waned—it was assumed Mars was as “dead” as the
Moon. The planned missions to Mars continued,
however, and in 1971 Mariner 9 became the first
spacecraft to orbit another planet. Over the next year,
7,329 images allowed the entire surface to be mapped. To the delight of scientists, a host of fantastic
landscapes was revealed. Mars became known as a
world of great extremes. Its features include the
largest known volcano in the solar system at about
360 miles across (600 km) and 15 miles high (25
km)—nearly three times taller than Mount Everest; a
canyon system 2,500 miles long (4,000 km), hundreds
of miles across, and up to 4 miles deep (7 km)—on
Earth, it would span North America; and surfaces
that range from those being slowly covered by dust
to those whipped by intense dust storms (Figure 4,
before and during a dust storm).
Figure 4
In 1976, the Viking missions (two orbiters as well
as landers in two locations) yielded several years of
both surface and global observations. The Viking
landers carried out the first “biology experiments”
on the surface. Although the somewhat ambiguous
results are still debated, the scientific consensus is
that the tests revealed exotic chemical processes
rather than biological activity. Recent years have seen
ongoing exploration of Mars. In 1997, the Pathfinder
mission delivered the first roving vehicle to the
Martian surface, while the Mars Global Surveyor
(arriving 1997) and Mars Odyssey (arriving 2001)
missions are continuing the orbital reconnaissance
with unprecedented spy-cameralike images of the
surface (Figure 3).
From the accumulated spacecraft observations, it
has become obvious that Mars is a very dynamic
world but one very different than previously
imagined. At 4,222 miles (6,794 km) Mars is about
half the diameter of Earth, and the surface gravity is
38 percent of Earth’s. The Martian day lasts 24 hours
and 37 minutes (Huygens was close in his
estimation!), but the year is 687 days long. The
rotation axis is tilted 25.1 degrees (similar to Earth’s
23.5 degrees), but each Martian season is about twice
as long as the terrestrial counterpart. The atmosphere
is very different—composed of 95 percent carbon
dioxide and with a surface pressure less than one
percent that found at sea level on Earth (equivalent to
flying in an aircraft at an altitude of about 80,000
feet). On the surface, the daytime high temperatures
rarely climb above the freezing point of water but
plunge to at least -220 degrees F (-140 degrees C)
every night. In the winter, the polar regions become
cold enough to freeze carbon dioxide out of the
atmosphere causing dry ice “snow” to fall! Under
current conditions, liquid water cannot exist for long
on the surface and will either freeze or quickly
evaporate. Only about a hair’s thickness of water
vapor exists in the Martian atmosphere, but Hubble
Space Telescope observations have revealed that is
enough to form widespread bands of water-ice
clouds at some times of the year (Figure 5).
The Mars Global Surveyor and Mars
Odyssey missions are busy revising
many of the “common wisdom”
aspects of Mars that were
largely based on the
Mariner and Viking
results. Near-surface
winds seem to be
more effective than
once thought,
forming
enormous sand
dunes in many
regions. Huge
tornadolike dust
devils sweep dust from the surface to high in the
atmosphere. Most intriguing is the evidence for
recent gullies, perhaps carved by running water; to a
geologist, “recent” could be 10,000 years ago or
yesterday. Several hundred examples of these gullies
have been discovered (Figure 6), supporting the idea
that subsurface aquifers containing liquid water or
brines may exist in many areas. The life-on-Mars
pendulum is swinging into
the maybe camp once again.
Many of the missions
planned for the coming
decade will attempt to
“follow the water” in search
of environments that may
have once supported—or
may still support—microbial
life. In early 2004, two Mars
Exploration Rovers will act
as robotic field geologists,
roaming for several months
and examining two separate
landing sites. Arriving in
2006, the Mars Reconnaissance Orbiter will expand on
the legacy of orbital
exploration. Later in the
decade, a “smart lander”
may place one or more
rovers within driving
distance of a fresh gully,
perhaps determining if near-surface deposits of water
are present. There are plans for a mission—perhaps
as early as 2013—that will return a few hundred
grams of Martian soil and rock samples for detailed
analyses in laboratories on Earth. At that point, the
Golden Age of Mars exploration will be
nearly complete and serious plans
can begin for human missions.
Stay tuned because the
Red Planet is slowly but
surely revealing its
secrets, and many
fantastic new
discoveries are
sure to come in
the months and
years ahead.
Figure 5
Dr. Steven Lee is
DMNS curator of
planetary science. He
also serves as a
coinvestigator on the
Mars Color Imager, a
camera system slated
to be launched on the
Mars Reconnaissance
Orbiter in 2005.
Figure 6