Mars Facts: From Little Green Men to Robotic Geologists

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• The average temperature on Mars is a frigid minus 81 degrees Fahrenheit (minus 63 degrees Celsius), but the planet does experience seasonal changes. From Earth the most noticeable seasonal events are enormous dust storms, which occur during the southern Martian spring and summer. The storms can encompass nearly the entire planet.

• Mars currently has no running water but does have clouds of water vapor and carbon dioxide.

• Thousands of Earthlings became alarmed by news of a Martian attack during a 1938 radio broadcast of War of the Worlds. The Orson Welles broadcast of a four-decade-old H.G. Wells novel was so convincing that horrified listeners peered skyward for glimpses of tentacled invaders arriving in their war machines. Many people even evacuated their homes.

• On July 25, 1976, the Viking Orbiter I captured an image of a Martian surface formation that resembled a human face. The picture became a Martian icon and, for some, a sure sign of intelligent life on the red planet. Subsequent images at higher resolutions appear to confirm that the "face" is simply the effect of lighting on a set of ordinary topographical features.

• Soil and dust give the red planet its color. Both are rich in iron oxide, commonly called rust.

• The distance between Earth and Mars varies from about 56 million kilometers (about 35 million miles) to 400 million kilometers (about 249 million miles) as the planets travel along their respective orbits. Because of this change, the apparent size of Mars in the sky varies by a factor of 5 and its brightness by a factor of 25. Times of close proximity are also a boon to spacecraft launches.

• Tuesday, known as mardi in French and martes in Spanish, has Latin roots as the day of Mars. The month of March is also named for Mars.

• At least two dozen Martian meteorites have been found on Earth. Perhaps the most famous of these is the Allan Hills meteorite, which was discovered in Antarctica in 1984 and may be 4.5 billion years old.

• Spacecraft have conducted photographic flybys of Mars, orbited it, and landed on its surface. Currently the NASA Mars rovers Spirit and Opportunity are combing the surface of the red planet. When and where they'll stop is anyone's guess. The pair completed their scheduled 90-day excursions on opposite sides of the planet in April 2004. But since then the solar-powered vehicles have kept on searching for watery clues to possible life on the frosty planet.

• The rover Opportunity found geological and chemical evidence of an extinct salty body of water that might have once held favorable conditions for life. Scientists now conclude that Mars once had abundant water.

• Mars's thin atmosphere is mostly carbon dioxide and some water vapor. There is no ozone layer, which on Earth functions as a critical shield from deadly doses of ultraviolet radiation. Scientists don't have comprehensive data about radiation levels on Mars, but readings from NASA's Mars Odyssey explorer report that radiation levels are two to three times higher than on Earth—a challenge to human exploration.

• Earth is one and three-quarters times larger than Mars and some seven and a half times as heavy. Yet the planets' landmasses cover nearly the same area, because so much of Earth is inundated with water.

• Mars has only 37.5 percent of the gravity found on our planet, so we could jump three times higher, or hit a golf ball three times farther, than we could on Earth. Better yet, we'd all weigh about two-thirds less on Mars.

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