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Mir animation The Red Planet

Mars is the fourth planet from our sun, making it our next-door neighbor in space. Thanks to early visits to Mars by Mariner and Viking missions, we know more about Mars than any other planetary body in our solar system, except for our moon. After a two-decade absence from the Red Planet, we sent Pathfinder to Mars in 1997. Pathfinder landed on July 4.

Mars is smaller than Earth -- its total surface area is about the same as Earth's land surface area (not including the area covered by water). Mars is a rocky planet with a cold, desert-like surface. Its atmosphere is made up mostly of carbon dioxide. Although the atmosphere is very thin compared to Earth's, it is thick enough to allow very strong winds and dust storms that can engulf the entire planet for months at a time.

Two moons, Deimos and Phobos, orbit Mars. The moons are very small -- only about 15 and 27 km (nine and 17 miles) at their longest. They are also carbon-rich, leading some scientists to believe that they were pulled into Mars's orbit from a nearby asteroid belt.

Because Mars is farther from the sun than Earth, it is cooler. The average temperature on Mars is about -55°C. A day on Mars is just slightly longer than a day on Earth (24.62 hours, versus 23.93 hours on Earth). But it takes Mars almost twice as long to make its way around the sun -- Earth's year is 365.26 days, but a Mars year is 686.98.

Mars has seasons that are similar to Earth's, because the tilt of its rotational axis is about the same as Earth's. However, Mars makes an elliptical orbit of the sun. Any body in a gravitationally bound orbit will travel faster in its orbit when it is close to the sun ("perihelion") than when it is distant ("aphelion"). One result is that Mars's seasons are not equal in length. On Earth, spring lasts 93 days, summer lasts 94 days and fall and winter each last 89 days. But on Mars, spring is equivalent to 171 Earth days, summer lasts 199 Earth days, fall is 171 Earth days and winter lasts only 146 Earth days. Another effect of Mars's elliptical orbit is a wide variation in surface temperatures, which can range from as low as -133°C (-207°F) to as high as 27°C (80°F) when Mars is closer to the sun.

The geography of Mars is often considered to be more interesting than on any other planet besides Earth. Gigantic volcanoes dot Mars's surface, although the planet shows no evidence of current volcanic activity. Some parts of Mars still bear the scars of an ancient bombardment by asteroids and comets that took place during the final stages of the planet's formation. (One ancient impact crater, Hellas Planitia, located in Mars's southern hemisphere, is more than three miles deep and 1,240 miles wide.) Other areas have seen enough geologic activity to erase those early markings. Some other important geologic features on Mars include:

  • Olympus Mons is the largest mountain in the solar system, rising 78,000 feet (24 km) high. Its base is more than 310 miles (500 km) in diameter and is rimmed by a cliff that is 20,000 feet (6 km) high.

  • Tharsis is a huge bulge on the surface of Mars, measuring about 2,480 miles (4,000 km) across and 6.2 miles (10 km) high.

  • Valles Marineris is a system of canyons large enough that it would span the entire continental U.S. It is 2,480 miles (4,000 km) long and ranges from one to five miles deep.

The surface of Mars also shows huge channel systems that were cut by water erosion. Evidence suggests that liquid water released from underground could have caused massive floods on the Martian surface. There may have been river systems and large lakes or even oceans. But today there is no evidence of liquid water anywhere on Mars's surface -- in fact, the erosion channels are thought to be as much as 4 billion years old.
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