The focus of this project is atmospheric erosion, in particular of the Earth's atmosphere and of the atmosphere or coma of a comet. In particular, the models describing a coma or the upper layers of the Earth's atmosphere are limited by the lack or inaccuracy of microscopic quantities such as reaction cross-sections and rate constants. Therefore, the aim of this project is to bring together different groups working on atomic and molecular ion structure and reactivity in order to provide the missing data.
On Earth, volcanic activity, plate tectonics, meteor impacts, glaciation and atmospheric escape processes all contribute to the evolution of the atmosphere over time. The present project focuses on atmospheric escape, and in particular on the role of chemistry in the upper atmosphere, which controls the properties of the escaping material. Earth, Mars, and Venus are relatively similar in size and location in the solar system and presumably formed from similar material, but they have very different atmospheres both in terms of density and composition. These differences may be due to different evolution pathways, in part because of different atmospheric escape processes. For instance, the Earth has an intrinsic magnetic field and thus a protective magnetosphere, while Mars and Venus have no comparable planetary magnetic field.
Another example of atmospheric evolution and erosion are comets: when a comet nucleus arrives in the vicinity of the Sun, its surface heats up, ices sublimate, and a temporary atmosphere is formed, the coma. The cometary nucleus is too small to gravitationally bind the coma gas which is thus lost into space. On their way out, the neutral volatile compounds (mostly water, CO, CO2) progressively ionize due to solar wind UV radiation and the interaction with the solar wind particles.
For terrestrial planets, the loss due to thermal escape processes (Jeans escape) is almost negligible for elements heavier than hydrogen. Non-thermal mechanisms are therefore important: photochemical escape, dissociative recombination, ion sputtering, charge transfer reactions, auroral electron bombardment, etc. In a cometary atmosphere, all volatile compounds escape. There, too, photochemistry is of utmost importance, as well as charge transfer reactions and reactions caused by solar wind energetic electron bombardment.
While chemistry appears to be a crucial ingredient in the study of atmospheric escape, there is actually a lot of uncertainty concerning the reaction cross-sections of many pertinent reactions. One reason may be the unimportance or nonexistence of certain reactive compounds in normal laboratory conditions. Another aspect is that some of these rate constants are known for room temperatures, but their temperature dependence might be unknown. The present project is therefore composed of the following elements: inventarization of ill-known reaction rates, sensitivity studies to find out which of these uncertainties really matter, e.g. in cometary chemistry, research to compute or to measure reaction rates for photoionization of atomic species, photodissociation of molecular compounds, electron and ion impact reactions, charge transfer reactions, recombination reactions (important in the ionosphere); and finally to examine isotope dependencies of the reaction rates (relevant for studies of the origin of comet material).
The results will then be incorporated into the reaction databases used in two different software packages: the TRANS4 model of the Earth ionosphere (a variant for Mars'atmosphere exists as well) and the MIM ROSINA/DFMS comet modeling software. The proposed project is very timely: for over two years Rosetta measures Churyumov-Gerasimenko's coma composition in full detail. An up-to-date reaction database will be instrumental in helping to understand the observations. And while the study of the Earth's ionosphere is a longer-term project, it will certainly be useful to exploit the updated cross-sections in modeling studies of atmospheric escape for Earth, Venus, and Mars.
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