Atmosphere of Mars

The atmosphere of Mars is the layer of gas surrounding Mars. It is primarily composed of carbon dioxide (94.9%), molecular nitrogen (2.6%) and argon (1.9%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen and other noble gases. The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 Pascal (0.088 psi; 6.1 mbar), which is less than 1% of the Earth's value. The currently thin Martian atmosphere prohibits the existence of liquid water at the surface of Mars, but many studies suggest that the Martian atmosphere had been much thicker in the past. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today.Mars's thin atmosphere, visible on the horizon.Mars Pathfinder – Martian sky with water ice clouds.A storm front moves in The atmosphere of Mars is the layer of gas surrounding Mars. It is primarily composed of carbon dioxide (94.9%), molecular nitrogen (2.6%) and argon (1.9%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen and other noble gases. The atmosphere of Mars is much thinner than Earth's. The surface pressure is only about 610 Pascal (0.088 psi; 6.1 mbar), which is less than 1% of the Earth's value. The currently thin Martian atmosphere prohibits the existence of liquid water at the surface of Mars, but many studies suggest that the Martian atmosphere had been much thicker in the past. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today. The atmosphere of Mars is colder than Earth's. Owing to the larger distance from Sun, Mars receives less solar energy and has a lower effective temperature (about 210 K). The average surface emission temperature of Mars is just 215 K, which is comparable to inland Antarctica. The weaker greenhouse effect in the Martian atmosphere (5 °C, versus 33 °C on Earth) can be explained by the low abundance of other greenhouse gases. The daily range of temperature in the lower atmosphere is huge (can exceed 100 °C near the surface in some region) due to the low thermal inertia. The temperature of the upper part of the Martian atmosphere is also significantly lower than Earth's because of the absence of stratospheric ozone and the radiative cooling effect of carbon dioxide at higher altitudes. Dust devils and dust storms are prevalent on Mars, which are sometimes observable by telescopes from Earth. Planet-encircling dust storms (global dust storms) occur on average every 5.5 earth years on Mars and can threaten the operation of Mars rovers. However, the mechanism responsible for the development of large dust storms is still not well understood. The Martian atmosphere is an oxidizing atmosphere. The photochemical reactions in the atmosphere tend to oxidize the organic species and turn them into inorganic carbon dioxide or carbon monoxide. Although the most sensitive methane probe on the recently launched ExoMars Trace Gas Orbiter failed to find methane in the atmosphere over the whole Mars, several previous missions and ground-based telescope detected unexpected levels of methane in the Martian atmosphere, which may even be a biosignature for life on Mars. However, the interpretation of the measurements is still highly controversial and lacks a scientific consensus. In 1784, German-born British astronomer William Herschel published an article about his observations of the Martian atmosphere in Philosophical Transactions and noted the occasional movement of a brighter region on Mars, which he attributed to clouds and vapors. In 1809, French astronomer Honoré Flaugergues wrote about his observation of 'yellow clouds' on Mars, which are likely to be dust storm events. In 1864, William Rutter Dawes observed that 'the ruddy tint of the planet does not arise from any peculiarity of its atmosphere seems to be fully proved by the fact that the redness is always deepest near the centre, where the atmosphere is thinnest.' Spectroscopic observations in the 1860s and 1870s led many to think the atmosphere of Mars is similar to Earth's. In 1894, though, spectral analysis and other qualitative observations by William Wallace Campbell suggested Mars resembles the Moon, which has no appreciable atmosphere, in many respects. In 1926, photographic observations by William Hammond Wright at the Lick Observatory allowed Donald Howard Menzel to discover quantitative evidence of Mars's atmosphere. With an enhanced understanding of optical properties of atmospheric gases and advancement in spectrometer technology, scientists started to measure the composition of the Martian atmosphere in the mid-20th Century. Lewis David Kaplan and his team detected the signals of water vapor and carbon dioxide in the spectrogram of Mars in 1964, as well as carbon monoxide in 1969. In 1965, the measurements made during Mariner 4's flyby confirmed that the Martian atmosphere is constituted mostly of carbon dioxide, and the surface pressure is about 400 to 700 Pa. In the 1970s, two landers of the Viking program provided the first ever in-situ measurements of the composition of the Martian atmosphere. Since then, many orbiters and landers have been sent to Mars to measure different properties of the Martian atmosphere, such as concentration of trace gases and isotopic ratios. In addition, telescopic observations and analysis of Martian meteorites provide independent sources of information to verify the findings. The imageries and measurements made by these spacecrafts greatly improve our understanding of the atmospheric processes outside Earth. Curiosity and InSight are still operating at the surface of Mars to carry out experiments and report the local daily weather. Mars 2020 and Rosalind Franklin are scheduled to launch in year 2020. CO2 is the main component of the Martian atmosphere. It has a mean volume ratio of 94.9%. In winter polar regions, the surface temperature can be lower than the frost point of CO2. CO2 gas in the atmosphere can condense on the surface to form 1–2 m thick solid dry ice. In summer, the polar dry ice cap can melt and release the CO2 back to the atmosphere. As a result, significant annual variability in atmospheric pressure (~25%) and atmospheric composition can be observed on Mars. The condensation process can be approximated by the Clausius–Clapeyron relation for CO2. Despite of the high concentration of CO2 in the Martian atmosphere, the greenhouse effect is relatively weak on Mars (about 5 °C) because of the low concentration of water vapor and low atmospheric pressure. While water vapor in Earth's atmosphere has the largest contribution to greenhouse effect on modern Earth, it is present in only very low concentration in the Martian atmosphere. Moreover, under low atmospheric pressure, greenhouse gases cannot absorb infrared radiation effectively because the pressure-broadening effect is weak.