Mars is the fourth planet from the sun and orbits the sun at a distance of about

141 million mi. Mars is named for the Roman god of war because it appears fiery
red in the earthís night sky. Mars is a small planet that has about half the
diameter of Earth and about one-tenth Earthís mass. The force of gravity on
the surface of Mars is about one-third of that on Earth. Mars has twice the
diameter and twice the surface gravity of Earthís moon. The surface area of

Mars is almost exactly the same as the surface area of the dry land on Earth.

The Martian day, or the time it takes Mars to rotate once on its axis, is about
a half an hour longer than an Earth day. Its year, or the time it takes to
revolve once around the sun, is about two Earth years long. Mars has two moons,

Phobos and Deimos. THE INTERIOR OF MARS Scientists believe that Marsís
interior consists of a crust, mantle, and core like Earthís interior, but they
do not know the relative sizes of these components. Because no spacecraft has
ever brought instruments that can study Marsís interior to the planet, the
only real data that scientists have about the planetís structure are its mass,
size, and the structure of the gravity field. Compared to Earth, Mars probably
has a relatively thick crust. Beneath the surface is an area of volcanic
activity in the northern hemisphere, it may be as thick as 80 mi. Beneath the
landing site of the United States spacecraft Viking 2, it may be as thin as 9
mi. The core is probably consists of mostly iron, with a small amount of nickel.

Other light elements, mainly sulfur, could exist in the core also. If so, the
core may be quite large. Mars does not have a significant magnetic field, so
scientists believe that Marsís core is probably solid. Mars does not, and
probably did not ever, have active plate tectonics. Because Mars is so much
smaller than Earth, it must cooled quickly after formation and the crust
thickened, forming one solid piece and eliminating any possibility of plate
tectonics as it was on and still is on Earth. Though the Martian crust is not
broken into separate plates, Marsís liquid mantle has sculpted the planetís
surface. The molten rock has broken through the crust to form volcanoes and its
motion has cracked the crust to form large rifts. THE SURFACE OF MARS The
surface of Mars would be a harsh place for humans, but it is more like the
surface of Earth than any other planet. The temperature on Mars does not get
much cooler than the temperature at Antarctica. At the surface it ranges from
about -140į C to 15į C (about -225į F to 60į F). During most of the year
wind speeds are normally low around 4.5 mph, but during dust storms they can
approach 40 to 50 mph. These winds often originate in large basins in the
southern hemisphere and carry large volumes of dust from the basins to other
regions, sometimes covering the entire planet in the storm. The dust is not
sandy, as in a sandstorm on the earth, but has the consistency of flour. The
northern and southern hemispheres of Mars have different characteristics. The
southern hemisphere has many impact craters and has a generally much higher
elevation than the northern hemisphere. The southern highlands are probably the
oldest ground on Mars. The northern hemisphere of Mars contains a much wider
variety of geologic features, including large volcanoes, a great rift valley,
and a variety of channels. The northern hemisphere also contains large expanses
of relatively featureless plains. Mars has the largest volcano in the solar
system, Olympus Mons. It is 16 mi high (almost twice as high as the earthís

Mount Everest) and covers an area comparable to the state of Arizona. Near it,
three other volcanoes almost as large-Arsia Mons, Pavonis Mons, and Ascraeus

Mons-form a line running from southwest to northeast. These four volcanoes are
the most noticeable features of a large bulge in the surface of Mars, called

Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge, but is
quite different in appearance. It is probably less than 4 mi high, but has a
diameter of 1000 mi. None of Marsís volcanoes appear to be active. The Tharsis
bulge has had a large effect on the appearance of the surface of Mars. The

Tharsis bulge includes many smaller volcanoes and stress fractures, in addition
to the large