Before space exploration， Mars was considered the best candidate for harboring extraterrestrial life. Astronomers thought they saw straight lines crisscrossing its surface. This led to the popular belief that irrigation canals on the planet had been constructed by intelligent beings. In 1938， when Orson Welles broadcasted a radio drama based on the science fiction classic War of the Worlds by H.G. Wells， enough people believed in the tale of invading Martians to cause a near panic.

　　Another reason for scientists to expect life on Mars had to do with the apparent seasonal color changes on the planet’s surface. This phenomenon led to speculation that conditions might support a bloom of Martian vegetation during the warmer months and cause plant life to become dormant during colder periods.

　　In July of 1965， Mariner 4， transmitted 22 close-up pictures of Mars. All that was revealed was a surface containing many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Finally， in July and September 1976， Viking Landers 1 and 2 touched down on the surface of Mars. The three biology experiments aboard the landers discovered unexpected and enigmatic chemical activity in the Martian soil， but provided no clear evidence for the presence of living microorganisms in the soil near the landing sites. According to mission biologists， Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface， the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open.

　　Mars is the fourth planet from the Sun and is commonly referred to as the Red Planet. The rocks， soil and sky have a red or pink hue. The distinct red color was observed by stargazers throughout history. It was given its name by the Romans in honor of their god of war. Other civilizations have had similar names. The ancient Egyptians named the planet Her Descher meaning the red one.

　　Before space exploration， Mars was considered the best candidate for harboring extraterrestrial life. Astronomers thought they saw straight lines crisscrossing its surface. This led to the popular belief that irrigation canals on the planet had been constructed by intelligent beings. In 1938， when Orson Welles broadcasted a radio drama based on the science fiction classic War of the Worlds by H.G. Wells， enough people believed in the tale of invading Martians to cause a near panic.

　　Another reason for scientists to expect life on Mars had to do with the apparent seasonal color changes on the planet’s surface. This phenomenon led to speculation that conditions might support a bloom of Martian vegetation during the warmer months and cause plant life to become dormant during colder periods.

　　In July of 1965， Mariner 4， transmitted 22 close-up pictures of Mars. All that was revealed was a surface containing many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Finally， in July and September 1976， Viking Landers 1 and 2 touched down on the surface of Mars. The three biology experiments aboard the landers discovered unexpected and enigmatic chemical activity in the Martian soil， but provided no clear evidence for the presence of living microorganisms in the soil near the landing sites. According to mission biologists， Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface， the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open.

　　Mars has two moons， Phobos and Deimos， which are small and irregularly shaped. These may be captured asteroids， similar to 5261 Eureka， a Martian Trojan asteroid. Mars can be seen from Earth with the naked eye. Its apparent magnitude reaches ?2.91，a brightness surpassed only by Venus， the Moon， and the Sun， although most of the time Jupiter will appear brighter to the naked eye than Mars. Mars has an average opposition distance of 78 million km but can come as close as 55.7 million km during a close approach， such as occurred in 2003.

　　Mars is currently host to three functional orbiting spacecraft： Mars Odyssey， Mars Express， and the Mars Reconnaissance Orbiter. On the surface are the two Mars Exploration Rovers (Spirit and Opportunity) and several inert landers and rovers， both successful and unsuccessful. The Phoenix lander completed its mission on the surface in 2008. Observations by NASA’s Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.