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Radon and its progeny as geophysical tracers of surficial and atmospheric processes on Mars



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Titre du congrès :3rd European Geosciences Union General Assembly 2006 Ville du congrès :Vienne Date du congrès :02/04/2006

Type de document > *Congrès/colloque

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Unité de recherche > IRSN/DSU/SERAC/LPMA

Auteurs > CHASSEFIERE Eric, MESLIN Pierre-Yves, SABROUX Jean-Christophe

Date de publication > 07/04/2006


Radon-222 and its short- and long-lived decay products in the Martian atmosphere have not been considered yet as potential tools for filling some gaps in our current knowledge of the planet. However, they can provide important information on surface exchange processes and on the composition of the regolith. There is indeed an exten-sive literature about the application of these isotopes to Earth's atmospheric studies, and results of the same kind could be obtained on Mars. Here we present a review of the different scientific objectives that could be achieved by the measurement of the natural atmospheric radioactivity at the surface of Mars. This includes the ura¬nium and water content of the first meters of the subsurface, below the penetration depth of cosmic rays and the sounding capability of the Mars Odyssey GRS, the at¬mospheric aerosol cycle, residence time and electrical charge, the atmospheric 4He abundance, and the present planet outgassing, as sought on the Moon since the 1960's. The soil water content at a given location can be determined by measuring the local radon-222 exhalation rate by alpha spectroscopy, as first suggested by Sabroux et al. (2003; 2004). In places where advective transport through fractured rocks may dom-inate diffusive transport, measurement of this tracer gas, inert and easily measurable, could provide the magnitude of the outgassing flux. Other objectives can be achieved by measuring its decay daughters, especially the short-lived ones (218Po, 214Po) and long-lived one (210Po) that are alpha emitters. For instance, measurement of the verti¬cal gradients of these isotopes nearthe surface can provide deposition rates of ultrafine particles and information about the boundary layer dynamics. Due to the long half-life of its parent lead-210, polonium-210 radioactivity can be used to infer the global av-erage radon flux, and thus to provide information on surface/atmosphere exchanges at a global scale. We present a simple box model relating radon and its progeny in the Martian atmosphere to illustrate these mechanisms, and underline the main differences with the terrestrial and lunar cases