The fourth planet from the Sun in the Earth's solar system is Mars, which is sometimes called the Red Planet. This is because the rocks, soil and the sky give off a red or pink hue. Mars is about 142 million miles from the Sun, about 45 million miles from the Earth, and is a little more than 50% the size of Earth. It also receives half as much as the sunlight of Earth.

Mars' atmosphere is very thin compared to Earth. If someone on Earth weighed 100 pounds they would only weigh about 40 pounds on Mars. It takes about 690 days for Mars to orbit around the Sun and a little more than 24 hours to rotate one time on its axis.

Before the exploration of Mars, many scientists had believed Mars could have been another planet which would have supported life. Astronomers thought they had seen lines crisscrossing its surface, which were thought to have been canals built by intelligent people. In addition, the different colors emitted by the planet led scientists to believe the seasons were changing on Mars, which meant there could have been plants growing to support life. In 1965, Mariner 4, a NASA spacecraft, sent back some pictures showing Mars as a planet containing many craters and no evidence of water, which meant life could not exist on the planet. The pictures were the first close-up photos of another planet.

The atmosphere of Mars is mostly carbon dioxide and small amounts of five other gases, including about 0.13% of oxygen. The air on Mars contains only about 1/1,000 as much water as Earth's air. The small amount though, can still condense and form clouds it its atmosphere. There is evidence that there was the possibility of water at one time during Mars early existence. Mars has features that look like shorelines, gorges, islands, and riverbeds, most likely having been formed by rivers a very long time ago. Mars also has the largest volcano in the solar system.

The temperature on Mars is much cooler than Earth. The average temperature of Mars is about 81°F below zero, but has reached as high as 68°F above zero and as low as negative 220°F.

There are no rings surrounding Mars, but it does have two moons named Phobos and Deimos. Both moons were discovered in 1877 by the same person, an American astronomer, Asaph Hall. Some scientists believe that both the moons of Mars at one time were asteroids but were caught in the gravitational pull of Mars, and then remained there. The moons have craters just like the Earth's moon which is evidence the moons had not been asteroids.

Much more information is known today about Mars than many years ago. There have been 45 missions since 1965 to discover as much as possible about the Red Planet, including there may have been huge floods on the planet over 3.5 billion years ago. In the future, NASA will one-day send astronauts on a mission to the Red Planet to discover much more about the fourth planet from the Sun.

In summary, much more is known about Mars than many years ago, but scientists hope to send astronauts to visit the unexplored planet. Mars is further from the Sun than Earth and is called the Red Planet. There is very little oxygen on the planet and currently there are no living organisms, but at one time in the past, astronauts thought they had seen evidence of life.

A: 45 million
B: 142 million
C: 93 million
D: 690 million

A: About twice as large.
B: About twice as small.
C: About half its size.
D: About three times as large.

A: Apollo 1
B: Challenger 4
C: Apollo 10
D: Mariner 4

A: Carbon dioxide
B: Oxygen
C: Helium
D: Nitrogen

A: -220°
B: -81°
C: 68°
D: 32°

A: The reason for its red color
B: A volcano was active on the planet
C: The planet's two moons, Phobos and Deimos
D: Craters on its moon

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