found as a gas in the air. Some evidence
suggests that in the past Mars had some
liquid water on its surface. For example,
Mars has hundreds of channels that look
like dried-up riverbeds. Some scientists
think Mars was once a warm planet with
large seas.
Orbit and Spin
Like all planets, Mars has two types of
motion: orbit and spin. Mars orbits, or
travels around, the sun. It completes one
orbit every 687 Earth days. In other
words, a year on Mars lasts 687 Earth
days. Mars spins about its center at
nearly the same rate as Earth does. Mars
takes about 24.6 hours to complete one
rotation. So a Mars day lasts about as
long as an Earth day.
Observation and Exploration
People have observed Mars since
ancient times. Since 1964 many
unmanned spacecraft have collected
information about the planet. The
United States and the Soviet Union
each sent several spacecraft to Mars
from the 1960s to the 1980s. Some of
them flew past Mars or orbited around
it. Others, including the U.S. Viking
crafts, landed on Mars.
Since the 1990s the United States and
Europe have sent several spacecraft to
orbit the planet. They include the U.S.
Mars Global Surveyor and Odyssey and
Europe’s Mars Express.
The U.S. spacecraft Pathfinder landed on
Mars in the 1990s. It released a robot
A camera on the spacecraft Pathfinder photographed the rover called Sojourner after the
rover was released onto the surface of Mars in 1997.
A volcano on
Mars named
Olympus
Mons is the
biggest known
volcano in the
solar system.
BRITANNICA STUDENT ENCYCLOPEDIA Mars 59
called a rover onto the surface. They both
sent back information about the surface,
interior, and atmosphere ofMars. The
United States sent two more rovers, called
Spirit and Opportunity, to the planet in
2003. The rovers moved around on the
surface. They performed experiments
and took many photographs.
#More to explore
Planets • Solar System • Space
Exploration
Mars, god
#see Ares.
Marsh
A marsh is a type of wetland with watery
mud that is rich in minerals. Marshes
are very similar to swamps. The main
difference between them is the type of
plant life they support. Marshes have
mostly grasses, while swamps have
mostly trees.
Marshes form in low-lying areas near
rivers and along seacoasts. They can have
either fresh or salty water. Marshes are
especially common in the areas called
deltas. These form at the mouths of rivers.
The flow of a river slows down near
the mouth. The rock and soil that the
river is carrying then settles to the bottom.
This material is called sediment. It
creates a rich mud in which marsh
plants grow.
Grasses, sedges, reeds, and rushes are
common marsh plants. Rice grown in
marshes provides a major portion of the
world’s grain. It is the most important
Marshland covers a delta region in Alaska. Marshes often form in deltas, which are areas
of land at the mouths of rivers.
60 Mars, god BRITANNICA STUDENT ENCYCLOPEDIA
kind of marsh plant. Crabs, snails,
turtles, and many kinds of fish and birds
are some of the animals that are found
in marshes.
#More to explore
Delta • Swamp •Wetland
Marshall,
Thurgood
Thurgood Marshall was the first African
American to serve as a justice (judge) on
the U.S. Supreme Court. Marshall
strongly supported equal rights for African
Americans.
Marshall was born in Baltimore, Maryland,
on July 2, 1908. After attending
high school in Baltimore, Marshall
graduated with honors from Lincoln
University in Pennsylvania. In 1933 he
graduated from Howard University Law
School inWashington, D.C. He was the
best student in his class.
In 1936 Marshall became a lawyer for
the National Association for the
Advancement of Colored People
(NAACP). At that time African Americans
did not have the same rights as
whites. The NAACP fought in the
courts to win equal rights for blacks. In
the 1940s and 1950s Marshall argued
32 cases before the Supreme Court. He
won 29 of those cases.
Marshall’s most important courtroom
victory was in the case of Brown vs.
Board of Education of Topeka in 1954.
Before this case, the law allowed states to
have separate schools for whites and
blacks. Marshall convinced the Supreme
Court that this practice went against the
U.S. Constitution. This ruling helped to
end all forms of legalized segregation, or
separation, by race.
In 1961 President John F. Kennedy
made Marshall a judge. In 1965 President
Lyndon B. Johnson made Marshall
the U.S. solicitor general. In that job he
represented the government in cases
before the Supreme Court. Then in
1967 Johnson put Marshall on the
Supreme Court.
As a Supreme Court justice, Marshall
continued to support equal rights for
blacks. He also opposed the death penalty.
Marshall retired in 1991. He died
in Bethesda, Maryland, on January 24,
1993.
#More to explore
African Americans • National
Association for the Advancement of
Colored People • United States Supreme
Court
Thurgood Marshall
BRITANNICA STUDENT ENCYCLOPEDIA Marshall, Thurgood 61
Marshall Islands
The Republic of the Marshall Islands
includes 29 atolls and five islands in the
central Pacific Ocean. Atolls are reefs, or
chains, of coral that surround a lagoon
(a shallow area of water). The country’s
