Mars atmosphere composition
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Composition of the Martian Atmosphere
Major Components: Carbon Dioxide Dominance
The Martian atmosphere is predominantly composed of carbon dioxide (CO2), which constitutes about 96% of the atmospheric volume. This has been consistently observed through various missions, including the Viking 1 and the Curiosity rover 34. The high concentration of CO2 is a defining characteristic of Mars' atmosphere, influencing its overall chemical and physical properties.
Trace Gases: Nitrogen, Argon, and Oxygen
In addition to CO2, the Martian atmosphere contains trace amounts of nitrogen (N2), argon (Ar), and oxygen (O2). The volume mixing ratios relative to CO2 for these gases are approximately 0.0189 for N2, 0.0193 for Ar, and 0.00145 for O2 . These trace gases play crucial roles in the atmospheric chemistry and dynamics of Mars.
Isotopic Composition: Insights from Viking and Curiosity
The isotopic composition of the Martian atmosphere provides insights into its history and evolution. Measurements from the Viking 1 mission indicated that Mars is enriched in 15N relative to Earth by about 75%, suggesting significant atmospheric escape over time . The Curiosity rover's Sample Analysis at Mars (SAM) instrument has further refined these measurements, confirming the isotopic ratios of 40Ar/36Ar and the isotopic signatures of carbon and oxygen in CO2 45. These isotopic ratios are consistent with those found in Martian meteorites, supporting the hypothesis of substantial atmospheric loss in Mars' early history.
Historical Atmospheric Pressure: Evidence from Isotopes
The isotopic compositions of nitrogen and argon also provide clues about the historical atmospheric pressure on Mars. Studies suggest that the atmospheric pressure was higher than approximately 0.5 bar around 4 billion years ago, which would have been necessary to preserve the primitive isotopic compositions observed in Martian meteorites . This indicates that Mars had a denser atmosphere in its early history, which has since been significantly reduced.
Variability and Structure: MAVEN Mission Findings
The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has provided detailed measurements of the neutral gas and ion composition in the upper atmosphere. The MAVEN NGIMS instrument has detected nine atomic and molecular neutral species, revealing large amplitude density structures and variations with altitude, latitude, and solar zenith angle . These findings help map the scale height temperatures and provide a comprehensive view of the atmospheric structure.
Atmospheric Chemistry: Role of Water Vapor and Photochemistry
Water vapor (H2O) and its photolysis products play a significant role in the chemical composition of the Martian atmosphere. The GEM-Mars general circulation model highlights the complex cycle of water vapor, including its interactions with ice reservoirs and water ice clouds, which provide radiative feedbacks . The photochemical cycles involving CO2 and H2O are crucial for understanding the atmospheric chemistry and the presence of species like ozone (O3) and hydrogen peroxide (H2O2).
Conclusion
The Martian atmosphere is a complex system dominated by carbon dioxide, with significant contributions from nitrogen, argon, and oxygen. Isotopic measurements from various missions have provided valuable insights into the atmospheric history and evolution, indicating substantial atmospheric loss over billions of years. The MAVEN mission and atmospheric models like GEM-Mars continue to enhance our understanding of the atmospheric structure and chemistry, revealing the dynamic nature of Mars' atmosphere.
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