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Atmosphere of Mars

Dateline: 06/07/99

By Alan Bruzel

The Viking 1 and Viking 2 probes, which landed on Mars on July 20, 1976 and September 3, 1976, relayed detailed accounts of the martian atmosphere to Earth. The later missions of the Mars Pathfinder (landed July 4, 1997) and the Mars Global Surveyor (achieved mapping orbit February 19, 1999) provided more information, and the planned touchdown of the Mars Polar Lander on December 3, 1999 will add further data concerning planetary conditions. (These missions perform multiple experiments; only their examination of the martian atmosphere is explored here.)

The collected reports reveal a planet with a thin, cold, dry atmosphere reddened by iron oxide dust particles of about a micron in size. The average "sea level" atmospheric pressure is approximately 9 millibars (0.13 pounds per square inch). By way of comparison, Earth's atmospheric pressure at sea level is 1014 millibars (14.7 pounds per square inch). Two conditions contribute to this low atmospheric pressure: Mars has less atmosphere than Earth, and the gravity of Mars (the force pulling on its atmosphere) is only 38% of Earth's gravity.

At 95.3%, carbon dioxide is the major component of the martian atmosphere. In winter, much of this gaseous carbon dioxide – some 20 to 30 percent – alternately condenses to solid carbon dioxide at the northern pole (during the northern winters) and then at the southern pole (during the southern winters). As a result, winter atmospheric pressure dips to about 5 millibars.

Composition of Atmosphere (Percent by Volume)

The distinctive isotopic abundances of elements present in the martian atmosphere serve to identify meteorites of presumed martian origin. For example, the martian deuterium/hydrogen ratio is five times that of Earth. Also, when compared to Earth values, the martian ¹⁵N/¹⁴N ratio shows a 60% enrichment, and the martian ³⁸Ar/³⁶Ar ratio a 30% enrichment.

Discussion of martian meteorites (and the current controversy regarding their indications of living organisms) leads to questions regarding life on Mars. Because liquid water is a necessity of life on Earth, was there once (or is there now) liquid water on Mars? Geological formations such as gullies and flood channels suggest a time when running water was a common feature on Mars, but surface water (except for the frozen water ice at the poles) is not to be found. Some posit that not all of the water on Mars evaporated into space; considerable underground water may be present. The Mars Polar Lander, which will release probes designed to puncture the martian surface to a depth of two meters, may provide clues.

Recommended Web resources for additional information:

Atmosphere of Mars
From the Web site: Was There Ever Life on Mars? From Los Alamos National Laboratory.

Atmosphere of Mars
From Mars Madness, a teaching project from Thinkquest.

Atmospheric and Meteorological Properties
Mars Pathfinder Science Results. From NASA’s Jet Propulsion Lab's Mars Pathfinder Web site.

Geoscience and Public Policy
Atmospheric facts presented as background to global warming. From Alan L. Kafka, Boston College.

Life on Mars
Article by Bruce Jakosky for New Scientist magazine concerning liquid water on Mars.

Martian Temperature and Pressure Profiles
First atmospheric data from NASA’s Mars Global Surveyor project.

Martian Volatile and Isotopic Signatures
Background to hypotheses concerning martian atmosphere. By Donald D. Bogard, Lunar and Planetary Institute.

Water and Humidity
Terms used in measurement of water vapor content. From University of Georgia.

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