Mars is the second most studied planet in the universe. Owing to great progress in observational and modeling techniques, fundamental atmospheric processes can be studied in great detail on Earth, which helps us study similar physical processes in other planetary atmospheres. On Earth, we take it for granted that there is plenty of water on the surface and atmosphere. On Mars, the question of what happened to (liquid) water is an exciting aspect of Martian climate science. It is thought that the early Mars used to have more habitable conditions with plenty of water on its surface and in its atmosphere. Studying habitability of a planet is to a large extent related to characterizing its atmospheric circulation patterns (winds) and thermal structure (temperature), which are important factors that control the presence and distribution of water. Probably one of the first things researchers seek to find on other planets in the Solar System and beyond is others forms of life. The common sense suggests that where there is a sufficient amount of liquid water, there could also be life. Why we look for life in the universe is, I guess, a philosophical question.
On Mars, global dust storms are reoccurring phenomena and atmospheric gravity waves, generated by a variety of meteorological phenomena in the lower atmosphere, continuously populate the whole atmosphere system. Gravity (or buoyancy) waves are essentially small-scale short-period variations in atmospheric parameters such as winds, temperature, density, and pressure. A recent study based on (MAVEN) Mars Atmosphere Volatile Evolution Mission observation showed that during global dust storms, thermospheric gravity wave activity nearly doubles. It is quiet fascinating that processes taking place on the surface of a planet can influence upper layers of the atmosphere 200 km above the surface. This vertical coupling is meanwhile a major field of research in atmospheric sciences. This research finding immediately raises the question of how the processes of dust storms, gravity waves and Martian atmospheric escape are interrelated.
This brings me to my main motivation to write this contribution in a recent science perspective article, in which I proposed that lower atmospheric gravity waves are a key player in shaping Martian water escape especially during global dust storms. Gravity waves are probably the missing puzzle piece in the context of Martian water cycle. Gravity waves shape the circulation and thermal structure of the Martian middle and upper atmosphere during all seasons. By influencing the mean meridional circulation (north-south winds) and upward winds on Mars, they can control the degree of water vapor transport from the mesosphere to the thermosphere, where water can be dissociated to hydrogen and oxygen. Hydrogen, the lighter species of the both, can easily escape to space. During global dust storms an increased amount of thermospheric gravity wave activity in form of increased temperature perturbations implies enhanced Jeans escape, since it is related to temperature variations. This can lead to an irreversible loss of hydrogen into space, depleting the atmosphere of water constituents. Over the course of many million years, global dust storms together with enhanced atmospheric wave activity could have diminished Martian atmospheric water reservoirs. Coordinated observational and modeling studies are needed to provide further insight into the complexity of water transport and loss on Mars.
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