Atmosphere of Mars

The atmosphere of Mars is the layer of gases surrounding the planet. It is primarily composed of carbon dioxide, which makes up 95% of the air. Molecular nitrogen accounts for 2.85%, and argon makes up 2%. Trace amounts of water vapor, oxygen, carbon monoxide, hydrogen, and noble gases are also present.

This atmosphere is much thinner and colder than Earth's. The maximum density is about 20 g/m3, which is only 2% of Earth's value. The average surface pressure is just 0.6% of what we experience on Earth. This thinness prevents liquid water from existing on the surface today.

Temperature on Mars is generally below zero, often reaching −60 °C. The planet is colder than Earth because it is further from the Sun. This distance means Mars receives less solar energy. The average surface emission temperature is comparable to inland Antarctica. The greenhouse effect is also much weaker on Mars than on Earth. On Earth, gases trap heat effectively. On Mars, the carbon dioxide density is too low to provide significant warming. Additionally, Mars has much less water vapor, which is a key contributor to the greenhouse effect.

The atmosphere changes significantly with the seasons. During winter, the density of the atmosphere is reduced by 25%. This occurs because carbon dioxide partly freezes at the polar caps. In the spring and fall, the density is higher. Dust activity also creates massive changes in the atmosphere. Dust devils and dust storms are very common. Global dust storms occur on average every 5.5 Earth years, or every 3 Martian years.

These storms can be so large that they change the color and brightness of the planet. In 2018, these changes were even visible to the naked eye from Earth.

Scientists believe the Martian atmosphere was much different in the past. Evidence suggests it was once thicker, warmer, and wetter. A thicker atmosphere would have allowed liquid water bodies to exist. Researchers use isotopic composition to study these changes. Isotopes are different versions of the same element. For example, Mars is depleted in lighter stable isotopes compared to Earth. This indicates that mass-selected processes have changed the atmosphere over time.

One major cause of change is atmospheric escape. Mars has been losing mass to space since its core slowed down. One process is Jeans escape, which affects hydrogen. Molecular hydrogen is produced from water in the lower atmosphere. It diffuses to the exosphere and decomposes into hydrogen atoms. If these atoms have enough thermal energy, they escape Mars's gravity.

Other gases escape through more complex methods. For instance, carbon escapes through photolysis. This happens when ultraviolet light breaks apart carbon monoxide molecules.

Other processes like sputtering also play a role. Sputtering occurs when particles strike the atmosphere and knock gases away. Observations from the MAVEN orbiter suggest sputtering is important for heavy gases. It may have contributed to a 65% loss of argon in Martian history. Nitrogen also escapes through photochemical reactions. The escape rate of nitrogen is very sensitive to solar activity. These continuous losses mean the Martian atmosphere is constantly evolving.

There is also ongoing debate regarding methane in the atmosphere. While the ExoMars Trace Gas Orbiter failed to find methane across the whole planet, other missions have detected unexpected levels. Some scientists suggest these levels could be a biosignature, which is a sign of life. However, this interpretation is highly controversial and lacks scientific consensus. Understanding these chemical traces helps scientists piece together the history of the planet.

Studying the Martian atmosphere connects to our understanding of planetary evolution. By comparing Mars to Earth and Venus, we learn how planets lose their air. The low ratio of carbon on Mars is only 10% of the ratio found on Earth or Venus. This suggests the early Martian atmosphere may have had ten times more carbon dioxide than it does now. These studies help us understand the long-term life cycles of rocky planets in our solar system.