|Chaisson, Eric, & Steve McMillian. Astronomy Today. Engelwood Cliffs, NJ: Prentice Hall, 1993: 254, 258.||"In 1964 Mariner 4 confirmed these results, finding that the atmospheric pressure is only 1/150 the pressure of Earth's atmosphere at sea level and that carbon dioxide (CO2) makes up at least 95 percent of the total atmosphere."||0.675 kPa|
|Mars (planet). Encarta. Redmond, WA: Microsoft, 1993-2000.||"The pressure of Mars's atmosphere varies with the season, ranging from 6 to 10 millibars (1 millibar is approximately one one-thousandth of the air pressure at the surface of Earth)."||0.6–1 kPa|
|Kieffer, H.H, & B.M. Jakosky. Mars. Tucson, Arizona: University of Arizona Press, 1992. as cited in Basic Mars Facts. NASA.||"The average air pressure at the surface of Mars is 6 millibars (compared to 1013 millibars on Earth)."||0.6 kPa|
|Zubrin, Robert & Richard Wagner. The Case for Mars. New York: Touchstone, 1996: 148.||"The atmosphere pressure measured at the two Viking sites varied over a Martian year between 7 and 10 millibars (1 bar is Earth sea-level atmospheric pressure, or 14.7 psi; 10 millibar, or mbar, is 1 percent Earth sea-level atmospheric pressure), with a year-round average of about 8 mbar observed at the higher altitude Viking 1 landing site on Chryse Planitia."||0.7–1 kPa|
|Wilders, Shaun. The Planets of our Solar System Explored and Explained. 1998.||"The atmosphere is made up mainly of carbon dioxide, and its pressure is only about one-hundredth of that on Earth."||1 kPa|
Mars, the red planet, is the fourth planet from the Sun and orbits the Sun at a distance of 228 million km. Mars is named for the Roman god of war because it appears fiery red in Earth's night sky. The surface of Mars would be a harsh place for humans, but has more surface characteristics similarities with the Earth than any other planet.
Exploration of Mars began in 1960, when the USSR launched its first probe to Mars. The United States launched the Mariner and Viking missions in the 1960s and 1970s. The programs returned a vast amount of data about the chemical and physical characteristics of Mars and a large number of photographs of its surface. One of the key problems in the spacecraft mission to Mars in the early 1960s was whether the density of the Martian atmosphere could be accurately determined. In 1975, National Aeronautics and Space Administration (NASA) launched a series of probes to Mars, Viking 1 and 2. These spacecraft provided scientists with most of the current data about Mars.
The pressure at the surface of Mars is approximately 1 kPa and depends upon the Martian weather. Some sources don't give precise data, but only compare it to the earth's characteristics.
Lauren Mikulski -- 2000