Atmospheric composition of Mars
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Major Atmospheric Components of Mars
Mars' atmosphere is dominated by carbon dioxide (CO₂), which makes up about 95% of its volume. Other significant components include nitrogen (N₂) at around 2.7%, argon (Ar) at about 1.6%, with smaller amounts of oxygen (O₂), carbon monoxide (CO), and water vapor (H₂O) present as trace gases 2379. These proportions have been confirmed by multiple missions and instruments, including the Viking landers, Mars Express, ExoMars Trace Gas Orbiter, and the Curiosity rover 25910.
Trace Gases and Seasonal Variations
Trace gases such as ozone (O₃), hydrogen chloride (HCl), and methane (CH₄) have been detected in very small quantities. Ozone is present at an average of 0.03 parts per million (ppm), HCl has been observed up to 4 parts per billion by volume (ppbv) below 30 km, and methane, if present, is below 20 parts per trillion by volume (pptv) 57. Water vapor is also a minor but seasonally variable component, typically ranging from 150 to 200 ppm 37. Seasonal cycles affect the abundance of CO₂, N₂, and Ar, with their concentrations lagging behind the pressure changes caused by CO₂ condensation and sublimation at the poles .
Noble Gases and Isotopic Composition
Mars' atmosphere contains noble gases such as neon (Ne), krypton (Kr), xenon (Xe), and isotopes of argon (Ar). The abundance pattern of these gases is similar to that found in Earth's atmosphere and in meteoritic gases, though xenon is underabundant compared to meteoritic ratios . Isotopic ratios, such as ^15N/^14N, ^40Ar/^36Ar, and ^129Xe/^132Xe, differ from those on Earth, indicating different evolutionary histories for the two planets' atmospheres 46. Mars is notably enriched in ^15N, suggesting significant atmospheric escape over time .
Vertical Structure and Temperature Profile
The Martian atmosphere is thin, with surface pressures less than 1% of Earth's. The surface temperature averages around 210 K, with a tropopause at 14 km altitude where the temperature drops to about 140 K. Above this, the temperature continues to decrease, reaching about 85 K at 100 km altitude, and becomes isothermal at higher altitudes . The upper atmosphere, between 140 and 200 km, has an average temperature of about 180 K .
Photochemistry and Atmospheric Stability
Photochemical reactions driven by solar ultraviolet light play a key role in shaping the composition of Mars' atmosphere. The breakdown of CO₂ and H₂O leads to the formation of CO, O₂, O₃, and other minor species. O₂ and CO remain well-mixed due to their long photochemical lifetimes, while ozone levels are inversely related to water vapor abundance . The stability of CO₂, O₂, and CO is influenced by both gas-phase and heterogeneous reactions involving dust and ice aerosols .
Local and Vertical Variations
Observations have revealed local and vertical variations in the abundance of minor constituents like CO and H₂O, as well as deuterated water (HDO). These variations provide insights into atmospheric dynamics, thermal profiles, and the vertical distribution of gases 110. Instruments on recent missions have provided the first detailed vertical profiles of isotopologues such as ^13C and ^18O in CO₂ and CO, and D/H ratios in water vapor .
Conclusion
Mars' atmosphere is primarily composed of carbon dioxide, with nitrogen, argon, and trace amounts of other gases. Seasonal and local variations, as well as unique isotopic signatures, reflect the planet's dynamic atmospheric processes and evolutionary history. Ongoing and recent missions continue to refine our understanding of the Martian atmosphere's composition, structure, and chemistry 12345678+2 MORE.
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