A Martian day lasts twenty-four hours and thirty-nine minutes and is called a sol. It takes Mars 687 days to travel around the Sun. The planet has different seasons throughout its orbit, because it is tilted, just like Earth. During a Martian winter, at the poles the temperature can drop to −200° Fahrenheit. At the equator during the summer, the temperature can reach 80° Fahrenheit during the day.
The force of gravity is much weaker on Mars than it is on Earth. An astronaut standing on Mars would feel only 38% as much gravity as on Earth.
Martian Geology and Atmosphere
Mars is called a terrestrial planet, because it is composed of rocky material, like Mercury, Venus, and Earth. Mars has some of the same geological features as Earth, including volcanoes, valleys, ridges, plains, and canyons.
Most Martian features have two-word names. One of the words is a geological term, and is usually from Latin or Greek, for example Mons for mountain, Planitia for plains, and Vallis for valley. The other word comes from the classical naming system begun by Schiaparelli during the 1800s or from later astronomers. Beginning in 1919, the International Astronomical Union (IAU) became the official designator of names for celestial objects and the features upon them. Only the IAU has that authority.
There are two particularly prominent features on Mars. One is a volcano called Olympus Mons that is about seventeen miles high. This is three times higher than Mount Everest on Earth. In English Olympus Mons means Mount Olympus. This was the home of the gods in ancient Greek mythology. The other notable geological feature on Mars is a canyon called Valles Marineris (Mariner Valleys in English). This enormous canyon is 2,500 miles long by sixty miles wide and up to six miles deep in places. It was named after the Mariner spacecraft that photographed it during the 1960s.
The surface of Mars is covered with a fine powdery dust with a pale reddish tint. This is due to the presence of oxidized iron minerals (like rust) on the planet's surface. The Martian atmosphere is thin and contains more than 95% carbon dioxide. There is a tiny amount of oxygen, but not enough for humans to breathe. It is windy on Mars. Strong winds sometimes engulf the planet in dust storms that turn the atmosphere a hazy yellowish-brown color. The wind also blows clouds around the sky.
The poles of Mars are covered by solid carbon dioxide (dry ice) layered with dust and water ice. These polar caps change in size as the seasons change. Sometimes during the summer the uppermost dry ice evaporates away, only to reform again when the weather turns cold.
Martian Moons
The two Martian moons Phobos and Deimos are not round spheres like Earth's Moon. They are shaped like lopsided potatoes. Phobos is approximately 5,800 miles from Mars, while Deimos is nearly 15,000 miles away.
The Martian moons are very small compared to other moons in the solar system. Many scientists believe that Phobos and Deimos are actually asteroids that wandered too close to Mars and were captured by its gravity. There is a large asteroid belt located between the orbits of Mars and Jupiter. This could be where Phobos and Deimos originated.
Mars in Orbit and Opposition
Because their orbital paths are different, the Earth and Mars each take a different amount of time to complete an orbit around the Sun. This means that Mars and Earth are constantly changing position in relation to each other. At their most distant point the two planets are 233 million miles apart. At their closest point they are less than thirty-five million miles apart. This explains why in some years Mars looks closer to Earth than in others. During August 2003 Mars was only 34.7 million miles from Earth. It will not be that close again until the year 2287.
About every twenty-six months the Sun, Earth, and Mars line up in a row with Earth lying directly in the middle. This configuration is called Mars in opposition. It means that Mars is closer to Earth than usual and easier to observe. Most of the historic discoveries about Mars occurred when the planet was in opposition. This was particularly true for the 1877 opposition associated with the findings of Schiaparelli and Hall. Scientists now know that Mars was only thirty-five million miles from Earth during that opposition.
The most recent Mars opposition occurred in November 2005. The next one will be in December 2007. Oppositions are the best times to send spacecraft to Mars.
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