Earth	Facts	Mars
23 Hrs, 56 Mins	Day Length	24 Hrs, 37 Mins
365.25 Earth Days	Year Length	687 Earth Days
1	Moons	2
4.5 Billion Years	Age	4.5 Billion Years
7,926 MileDiamter	Size	4,217 MileDiamter
29,028 Feet	Tallest Mountain	78,760 Feet
2 Miles	Deepest Canyon	4 Miles
57° F	Temperature	-81° F
Thicker, Wetter	Climate	Thin, Dry
About 2 2/3 that of mars	Gravity	About 1/3 that of Earth
Mostly Nitrogen	Atmosphere	Mostly Carbon Dioxide
4	Seasons	4

A Martian day is about 37 minutes longer than an Earth day. The twin Mars Exploration Rovers are built to travel up to 110 yards in one Mars day (called a sol), about as far as the 1997 Mars Pathfinder rover traveled in its entire 90-day mission. The sun is expected to power the solar-paneled ROVERS several hours a day.

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Because Mars is farther from the Sun than Earth, Mars takes about 320 more days to revolve around the sun than Earth does. That makes a Martian year almost twice as long as an Earth year. The Earth is 93 million miles from the sun, while Mars is 142 million miles on average.

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With diameters of 14 miles and 8 miles, Mars' two potato-shaped moons are tiny next to Earth's -- which is 2,159 miles across. The smaller of the two Martian moons, Deimos, revolves around Mars every 30 hours and 18 minutes, while the larger Phobos makes a revolution every seven hours, 39 minutes. In 1998 two Russian probes were launched to explore Phobos, but mission controllers lost contact with both probes.

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It is believed that Mars, like the sun, the Earth and all other planets, their moons, the asteroids and comets, was formed about 4.5 billion ago. Some scientists believe that Earth and its moon were a single entity before a planet-sized body slammed into it, breaking it apart. Some researchers say that Mars underwent an ice age 100,000 years ago, and that Mars still contains water ice that if melted would cover the planet with water several inches deep.

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Mars is about half the size of Earth and twice the size of the moon, yet its dramatic landscape features much deeper canyons and taller mountains than any on Earth.

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About 2 1/2 times taller than Earth's Mount Everest, the Martian volcano Olympus Mons is the tallest mountain in the solar system. Because plate tectonics are not present on Mars, numerous volcanic eruptions have piled up to become miles-high mountains over many years. The Earth's plates, in contrast, slide over its hot mantle, limiting the life of a volcano.

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The Grand Canyon of Mars, Valles Marineris, is 2,485 miles long, carving about one-fifth of the Martian circumference. The canyon is up to 370 miles wide and more than four miles deep. Compared to the Grand Canyon, the Mars canyon is almost 10 times longer, about 20 times wider and about four times deeper. But the Grand Canyon is not the Earth's deepest. The Mars canyon is two miles deeper than what is typically identified as the deepest canyon on Earth, Peru's Colca Canyon.

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The temperature on Mars is much colder than on Earth and is believed to swing by tens of degrees quickly, depending on polar ice masses, dust storms, the time of day and year. Because the Martian atmosphere is thin and because no bodies of water exist to hold the sun's heat, Martian temperatures cool quickly at night, or when dust or clouds block the sun's rays.

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With no oceans, lakes or rivers on the surface, little water exists in the Martian atmosphere. But Mars has not always been so dry. Researchers say the shapes of the planet's valleys, canyons and craters indicate that wet, warm periods probably existed. Some scientists suspect that water may be trapped under the planet's surface, or might even be flowing at or near the surface now. Mars' climate is in part determined by seasonal changes of carbon dioxide in the planet's polar ice caps, which releases the gas as it retreats. The thin Mars atmosphere and dust storms also play a role.

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The gravity on Mars is 38 percent that of Earth's. A person weighing 100 pounds on Earth would weigh 38 pounds on Mars, and could jump three times as high. Any future human Mars expedition would be tough on a person's body because the astronauts would have to adjust from Earth's gravity to weightlessness in space then to gravity again on Mars. People lose bone mass while in zero gravity.

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The atmosphere on Mars is made up primarily of carbon dioxide, which is 95 percent of the planet's makeup. At less than 1 percent, very little oxygen exists in the Martian atmosphere. The two main Earth gases are nitrogen and oxygen, which make up 78 percent and 21 percent of the air, respectively. The oxygen in Earth's atmosphere is produced and maintained by animals, including humans, and by plants. On Mars, the southern polar cap releases large volumes of carbon dioxide during spring and summer, when the ice turns to vapor.

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Because the orbit of Mars about the sun is more elliptical than Earth's, the seasons on Mars vary in duration, unlike Earth where each season lasts about one-quarter of the year. On Mars, summer is the longest season, equaling 199 Earth days. Its shortest season is winter, lasting 146 Earth days. A full Martian year is 687 Earth days long.

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