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If you could visit any object in the solar system, which one would you choose? Would you decide to travel to the Earth's airless moon, to desolate Mercury, or to the baking-hot surface of Venus? Or, would you choose Mars, a planet with orange sandy deserts, great canyons, magnificent volcanoes, and gleaming ice caps?

Mars is the fourth planet from the sun, and the first planet beyond the Earth. Although Mars is smaller than the Earth, it has more land area because it has no oceans, lakes, or streams.

Mars looks distinctly orange or red in color, which has earned it the nickname the Red Planet. The color led the ancient Romans to name the planet Mars, after their god of war. Mars actually gets its color from minerals that contain oxides of iron. Ordinary rust is an oxide of iron, and it is dark orange in color. If you were to go for a walk on Mars, you would see dark orange or brown boulders and drifts of fine orange sand.

There is not much atmosphere surrounding Mars. In fact, there is 600 times more air around the Earth than there is around Mars. The air on Mars is usually crisp and clear. Even though almost every day is sunny, the temperature is "cool"—on a day in the middle of the Martian summer, the temperature may reach freezing, 32°F (0°C). You would need to wear a spacesuit to provide enough air to breathe and to keep you warm.

The Revolution of Mars

The revolution, or orbit, of Mars is more elliptical (oval shaped) than that of most of the other planets. Because of this, the distance from Mars to the sun varies throughout the Martian year. At its closest approach, called perihelion, Mars is 128.4 million miles (206.7 million kilometers) from the sun. When Mars is at its greatest distance from the sun, called aphelion, it is 154.8 million miles (249.2 million kilometers) away. Mars is farther from the sun than the Earth, so it travels more slowly in its orbit. One Martian year, the time Mars takes to orbit once around the sun, is 687 Earth days.

Because the Earth moves faster than Mars in its journey around the sun, the Earth catches up with and passes Mars about every two years. At these times the planets are said to be in opposition. When these planets are closest to one another, Mars is 34.6 million miles (55.7 million kilometers) from Earth. However, the distance between Earth and Mars can vary considerably from one opposition to the next because of the elliptical orbit of Mars.

Rotation and Size of Mars

The turning of a planet on its axis is called rotation. The Earth rotates on its axis every 23 hours 56 minutes. Mars has a similar period of rotation㬔 hours 37 minutes. Another similarity between the Earth and Mars is the tilt of the planet on its axis. The Earth is tilted 23.5 degrees; Mars is tilted 25 degrees. So, like the Earth, Mars has seasonal changes in temperature between its northern and southern hemispheres. Because of its longer period of revolution, however, seasons on Mars last longer than those on Earth.

Mars is only a little more than one ninth the mass of the Earth, and its density is less. The diameter of Mars is 4,200 miles (6,760 kilometers), a little more than one half that of the Earth. Because of its smaller mass, the pull of gravity on Mars' surface is weaker than the pull of gravity on the Earth's surface. A weak magnetic field was first detected by the Mars Global Surveyor spacecraft shortly after it entered orbit around Mars in 1997.

The Martian Surface The surface of the Earth's moon is covered with impact craters—the cup-shaped scars left when meteors and asteroids strike the surface—and so are the surfaces of Mercury and the moons of the outer planets. Astronomers expected that the surface of Mars would also be covered by craters. Instead, they discovered that only about two thirds of Mars is covered with them.

This tells astronomers that something may have removed the craters. On the Earth, wind and water erode craters, and sediments and lava from volcanic activity fill them. Much of the surface of the moon is still covered with craters because there is no wind or water and there is not as much volcanic activity on the moon as there is on the Earth.

On areas of Mars where there are no craters, astronomers do see signs that once there may have been strong wind and water action on the planet that could have eroded them. There are so many physical features that look like dry streambeds, gullies, and channels that it appears as if streams and rivers once flowed on the surface. But today Mars is so cold and so dry that liquid water would freeze or evaporate. However, the more astronomers learn about Mars, the more they think that the planet once had much more atmosphere and water than it has today.

Water on Mars. In order for running water to have existed on Mars, the atmosphere would have had to be far thicker than it is today and the temperatures above freezing at least some of the time. The big question is: If there once was water on Mars, what happened to it? One explanation for the present lack of surface water is that Martian air and water evaporated into space. Because the gravity on Mars is so weak, air and water can slowly escape. Over a few billion years, Mars may have lost most of its atmosphere and water.

Another possibility is that some of the water lies frozen beneath the surface. Still another explanation is that the channels and other features that appear to have been made by water were not actually formed that way.

New probes may help answer such questions. In 1997, Mars Pathfinder transmitted the first images taken from the Martian surface in almost 20 years. Later that year, Mars Global Surveyor entered orbit to map the planet's features in detail. In 1999, Mars Climate Orbiter was lost because of a miscalculation. Mars Polar Lander was due to land on the planet's south pole later that year, but contact with the craft was lost. In 2001 Mars Odyssey entered orbit around Mars and began a three-year mission to map the planet's surface. In 2004, the Mars Exploration Rover Spirit landed on the planet's surface to begin a three-month search for evidence of water in rocks and soil. Spirit's twin, the Mars Exploration Rover Opportunity, was scheduled to touch down on the other side of Mars a few weeks later.

Volcanoes. When the Mariner 9 spacecraft arrived at Mars in 1971, a big dust storm was in progress. The probe's cameras showed nothing but dust and four mysterious spots. As the storm subsided, scientists were surprised to learn that the spots were the tops of four giant volcanoes. The largest of these, Olympus Mons ("Mons" means "mountain"), is about 373 miles (600 kilometers) across and rises about 17 miles (27 kilometers) above the surrounding plain. Olympus Mons is a shield volcano—a volcano built up from thousands of gentle eruptions of lava. The island of Hawaii, on Earth, is also a shield volcano, but Olympus Mons is three times as large. Geologists think that Olympus Mons took at least a billion years to grow and that it stopped erupting about a billion years ago.

Olympus Mons is located near the equator of Mars in a volcanic region called Tharsis. Any impact craters there have all been buried under vast flows of lava. Three other large shield volcanoes are in this region: Ascraeus Mons, Pavonis Mons, and Arsia Mons.

The surface of the Tharsis region tells geologists that the crust of Mars does not move, as the Earth's crust does. (The movement of the Earth's crust is called continental drift.) To form Tharsis, molten rock flowed from inside Mars for billions of years, always coming out in the same region. For this reason the Tharsis volcanoes are larger than any volcanoes on Earth. However, there are more volcanoes on Earth than there are on Mars. On the whole, Earth is more geologically active than Mars.

Gigantic Canyon. East of Tharsis is a canyon so big—it stretches over 2,500 miles (4,000 kilometers)—that the Grand Canyon and the Colorado River would be lost inside of it. From the rim of this vast canyon, called Valles Marineris, there is a drop of 6 miles (10 kilometers) to the floor, and the canyon is 100 miles (160 kilometers) wide in places. Scientists think Valles Marineris is a rift valley, a huge crack that formed where the crust of Mars pulled apart.

Along the walls of Valles Marineris, especially at the eastern end, are channels that look as if they were cut by flowing water. Once again, scientists think these channels may be evidence that there once was water on Mars.

Polar Caps. The polar regions of Mars are quite cold. Astronomers think that much of the ice at the poles is not water but frozen carbon dioxide. Carbon dioxide freezes at a temperature of ¯195°F (¯126°C).

During the Martian summers, the polar ice caps shrink, only to expand again with the coming of winter. In the warm sunshine, some of the polar ice melts or evaporates. In the winter, water and carbon dioxide freeze and enlarge the polar caps. The ice near the edges of the polar caps is probably quite thin. Ice near the centers never melts, so the ice could be quite thick. The reason astronomers think there could be such thick ice is that pictures of the polar caps show dozens of layers of ice. If each layer were 10 to 15 feet (3 to 5 meters) thick, the total thickness would be hundreds of feet.

However, there is not enough water in the polar caps to cause much erosion if it became liquid. If all the carbon dioxide and water in the polar caps melted, Mars would still be drier than the driest desert on Earth.


Two small satellites, or moons, orbit Mars. They are named for the two horses, Phobos ("fear") and Deimos ("terror"), that pulled the chariot of Ares, the god of war in Greek mythology. Phobos is the closer of the two to Mars. Its orbit is only 2,462 miles (3,964 kilometers) above the surface of Mars. It takes Phobos only 71/2 hours to complete one orbit. Phobos resembles a potato in shape. In its longest dimension, Phobos measures 81/2 miles (13.7 kilometers), and it is 6 miles (9.7 kilometers) wide in the middle. The surface of Phobos is covered with craters, and there are cracks in it. A large meteorite may have caused these cracks. Deimos is smaller than Phobos and orbits above Mars at a distance of 14,700 miles (23,600 kilometers). It is 5 miles (8 kilometers) long and 31/2 miles (5.6 kilometers) wide. It appears that Deimos has a smoother surface than that of Phobos.

Phobos and Deimos are very dark. Astronomers believe that they are made of carbonaceous chondrite, which is a dark-colored mix of rock and tarlike compounds. Asteroids and meteorites sometimes contain carbonaceous material. Some astronomers suspect that Phobos and Deimos were once asteroids that strayed close to Mars and were captured by the planet's gravitational field.

If you were standing on the surface of Phobos, Mars would fill half of the sky above you. As you gazed toward the Red Planet, you would see the mighty volcano Olympus Mons and the vast canyon of Valles Marineris. Imagine what an adventure awaits the astronauts who will someday explore Mars.

Richard Berry Author, Discover the Stars