Spring 2001
Mars
is the fourth
planet from the Sun in the solar system. Its red color
inspired the Greeks and Romans to name it after their god of
war Mars. The distance of Mars from Earth, and hence its
brightness, vary considerably. At times it is the
third-brightest object in the night sky, surpassed only by
the Moon and Venus. PHYSICAL
CHARACTERISTICS The orbit of Mars lies
about 1.5 times as far away from the Sun as the Earth's
does. The orbit is somewhat elliptical, so the planet's
distance from the Sun varies from a minimum, at perihelion,
of 206.7 million km (128.4 million mi) to a maximum, at
aphelion, of 249.2 million km (154.8 million mi). Because
Mars is farther from the Sun than Earth is, it takes longer
to complete a revolution. Its year is 687 Earth days long.
Mars speeds along at 24 km/sec (15 mi/sec) in the same
counterclockwise direction (when viewed from above the
planet's north pole) as all the other
planets. The Martian day is 24
hours, 37 minutes, and 23 seconds long. The present tilt of
the planet's axis is about 25 deg, producing seasonal
changes similar to those on Earth. Because of the elliptical
orbit of Mars, summer in its southern hemisphere occurs when
the planet is nearest the Sun, as does winter in its
northern hemisphere. The planet has an
average diameter of 6,780 km (4,217 mi), making it about
half the size of Earth but nearly twice the size of the
Moon. Because of its rotation it is slightly flattened,
having an actual diameter of 6,794 km (4,222 mi) at the
equator and 6,752 km (4,195 mi) at the poles. The bulk
density of the planet (3.9 g/cm3) is lower than that of
Earth (5.5 g/cm3). In addition, no measurable magnetic field
for Mars has been detected, which indicates that the core is
solid and explains why Mars has no radiation belt. The total
mass of the planet is only one-tenth that of Earth, and thus
Martian gravity is only 38% as strong. Mars is orbited by two
irregularly shaped satellites. The larger is named PHOBOS
("fear") and the smaller DEIMOS ("terror"), after attributes
personified in Greek mythology as sons of Ares. Each is only
a few kilometers wide. The moons are heavily cratered and
may be asteroids (see ASTEROID) captured by Mars, or the
accumulated remnants of the materials that formed
it. ATMOSPHERE AND
CLIMATE The major constituents
of the Martian atmosphere are carbon dioxide (95.3%),
nitrogen (2.7%), and argon (1.6%). Minor amounts of oxygen,
carbon monoxide, water vapor, and other trace constituents
make up the rest. The average surface pressure of the
atmosphere is less than 1/100th of the average surface
pressure of Earth's atmosphere, and it varies with season
and elevation. The Martian atmosphere undergoes dramatic
daily and seasonal temperature changes. It averages about
220 K (-64 deg F) and varies from 145 K (-199 deg F) during
the polar night to 300 K (80 deg F) at the equator during
midday at perihelion. Although thin and
frigid, the Martian atmosphere is very active and complex.
Mars and Earth have similar global atmospheric circulation
patterns because of their other similarities. In the Martian
atmosphere, as in Earth's, warm air rises at the equator,
moves poleward, deflects to the east, and then descends at
middle latitudes and returns to the equator. At middle to
high latitudes, winds blowing from the west contain narrow
bands of high winds called jet streams, which produce storm
systems near the surface. In addition, Mars has
seasonal climate changes driven by solar heating and by the
exchange of carbon dioxide between polar ice and frost
(discussed below) and the atmosphere. During each Martian
hemisphere's fall and winter, carbon dioxide freezes out of
the atmosphere to form a frost hood that stretches from the
pole to nearly halfway to the equator. As spring comes,
strong winds are produced by the temperature contrast, at
the edge of the retreating polar cap, between the ice and
the sun-heated soil. Augmenting this effect is the hotter
southern summer, when the planet is closer to the Sun. The
strong southern summer winds lift vast amounts of dust that
rise into great storms. These storms have been observed to
cover the entire planet.
http://www.nasm.edu/ceps/etp/mars/MARS.html
The Bilingual Mars Team at BTW
