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.
Other instruments found no sign of organic chemistry at either landing site, but they did provide a precise and definitive analysis of the composition of the Martian atmosphere and found previously undetected trace elements.
Mars Statistics Mass (kg) 6.421e+23 Mass (Earth = 1) 1.0745e-01 Equatorial radius (km) 3,397.2 Equatorial radius (Earth = 1) 5.3264e-01 Mean density (gm/cm^3) 3.94 Mean distance from the Sun (km) 227,940,000 Mean distance from the Sun (Earth = 1) 1.5237 Rotational period (hours) 24.6229 Rotational period (days) 1.025957 Orbital period (days) 686.98 Mean orbital velocity (km/sec) 24.13 Orbital eccentricity 0.0934 Tilt of axis (degrees) 25.19 Orbital inclination (degrees) 1.850 Equatorial surface gravity (m/sec^2) 3.72 Equatorial escape velocity (km/sec) 5.02 Visual geometric albedo 0.15 Magnitude (Vo) -2.01 Minimum surface temperature -140°C Mean surface temperature -63°C Maximum surface temperature 20°C Atmospheric pressure (bars) 0.007 Atmospheric composition