Mars shows various features as it rotates on its axis,
thus presenting a different face toward Earth over a period of time.
Click on image for Enlarged View
Viewing the Features of Mars
Of all the worlds in the solar system, Mars is most Earthlike. It
has ever-changing weather, seasonal thawing of polar ice caps, clouds,
vast dust storms, and four seasons. The changes are what you want to
But to recognize changing features on the Martian disk, you
will have to learn the dark markings and features that stay the same
or change only slowly. These features will serve as your reference
marks. Use the maps of Mars that accompany this article to learn these
features as Mars approaches Earth this spring. During the three or
four months closest to opposition you will be able to spot changing
features—clouds, dust storms, the recession of the polar cap—with a
skilled and practiced eye.
Astronomers have mapped Mars with a
latitude and longitude grid just like the one on Earth, except that
longitude runs from 0 degrees to 360 degrees continuously for Mars.
The Central Meridian (CM) is the Martian longitude crossing the center
of the disk of Mars.
To learn the Martian features, you need only
look up the Central Meridian of Mars that’s facing Earth on a given
day and hour and compare it to maps and drawings of Mars on this page
to know which “face” of Mars you’re looking at.
Mars rotates once on
its axis in 24 hours, 37 minutes, and 23 seconds. The Martian day,
called a “sol” by space scientists, is thus a bit longer than a day on
Earth. Mars turns only 351 degrees of longitude in twenty-four hours.
Consequently, if you observe Mars with your telescope at the same time
each night, the Martian surface features will have turned 10 degrees
less far onto the visible side of the planet. (Thirty-seven minutes
later, of course, Mars will be oriented exactly as it was the night
The net effect is that each Martian feature can be seen
best every thirty-six days by an observer at a given longitude on
Earth, but someone living on the other side of Earth will see the
other side of Mars.
Martian Polar Regions
The Martian polar ice caps are bright, easy to see, and undergo
fascinating seasonal changes. They thaw in the spring in each Martian
hemisphere and re-form each autumn in an annual cycle of the seasons.
Mars’ widely varying distance from the Sun strongly affects its
seasons. It receives 45% more heat from the Sun when nearest than when
The southern polar region of Mars will be tilted toward
Earth during most of the 2003 apparition. The Martian northern
hemisphere winter solstice occurs on September 29, just one month
after closest approach to Earth. This means that the south polar cap
will be the only one visible during this apparition.
in the southern hemisphere on May 5, at which time the polar ice cap
should be easily visible. It will slowly melt off during June and
July. As the tilt of Mars toward Earth increases, the south polar ice
cap will rapidly melt off during the Martian spring. By the time of
the August opposition, the ice cap will be only the barest tiny
remnant as the Martian summer approaches. Then, as the Martian summer
ends as the planet recedes from Earth during our autumn months, the
carbon dioxide in the Martian atmosphere in the frigid polar zone will
begin to freeze again to the ground as the temperature drops. The
south polar cap should remain visible into 2004 as it enlarges at the
same time as the separation between Earth and Mars increases during
November and December and into next year.
Clouds and Hazes
Mars has an amazingly dynamic atmosphere considering it is less
than one hundredth as dense as Earth’s atmosphere. White water clouds,
bluish limb hazes, and bright surface ice-fogs and frosts are
regularly seen and tracked by Mars observers. Over many years
astronomers have learned a lot about when and where these atmospheric
features will occur; much of the knowledge is based on observations
made with small telescopes.
Orographic clouds form when moist air
rises over a high spot. These whitish water clouds form in spring and
summer on the upper slopes of the large Martian volcanoes—Olympus Mons,
Ascraeus Mons, Pavonis Mons, Arsia Mons, and Elysium Mons—and between
Tharsis Tholus and Valles Marineris. Look for orographic clouds using
blue and violet filters.
Limb haze appears as a bright, misty arc of
light on the sunrise or sunset limb of Mars. It appears at the limb
because there the observer looks through a long path in the upper
Martian atmosphere, which may contain carbon dioxide crystals, fine
dust, cirrus-type water clouds, or a mixture of these. Consequently
the presence of limb haze is a very sensitive way to detect unusual
weather activity or polar phenomena. When you see haze, note its
location, color, and density, as well as the filters used to see it.
Fogs and frosts, often called bright patches, form in the chill of the
Martian night, rotate with the planet, bake off in the morning
sunlight, and usually disappear by local noon. You can tell them from
elevated clouds by examining them with blue, blue-green, and yellow
filters. High clouds look brightest with a blue filter, while
low-lying fogs look brighter with a blue-green filter than they do in
a blue or a yellow filter. Surface frost looks brightest in green and
yellow filters and is hard to see with a blue filter. The behavior and
location of bright patches also help distinguish these patches from
clouds and limb haze. Fogs normally form in valleys, linear
depressions, basins, and on upper slopes. Frosts are usually seen on
deserts, plateaus, mountains, and floors of large craters. Pinpointing
the location and seasonal occurrence is important to the study of
Martian weather patterns.
Mars normally shows only a bright,
featureless disk in violet light because the Martian atmosphere
scatters short wavelengths. When a violet clearing occurs, the large,
dark surface features can be seen through the atmosphere with a
Wratten 47 filter. Although the debate over the cause of this
phenomenon has raged for decades, no explanation of violet clearing is
Observations of Mars indicate that yellow dust storms occur around
the time of southern summer solstice, soon after Mars reaches
perihelion. During the opposition in June 2001, dust storms broke out
in the Hellas basin just after the planet’s closest approach to Earth.
This dust storm quickly spread to the whole surface of Mars. This was
the biggest dust storm seen on Mars in three decades. It is unlikely
that two major dust storms will occur on successive apparitions.
Still, because little is known about the cause of Martian dust storms
and how they develop, it is impossible to predict what will happen.