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The superficial layer of rocky planets : a link between interior and atmosphere

The study of planetary subsurfaces at Pôle Système Solaire concerns various aspects of the subsurface-atmosphere interactions at Mars : search for and characterization of water in the subsurface, study of gaseous exchange fluxes between the subsurface and the atmosphere, study of past and present outgassing, history of chemical weathering at the surface of the planet… It involves also several Earth science laboratories, on Jussieu Campus : MAGIE, SISYPHE and in the South of Paris : IDES , IRSN . Most of these collaborations, which are recent and of interdisciplinary nature, mainly consist of the adaptation to space constraints of experimental techniques used in the laboratory or for field campaigns in the field of geosciences. The work of scientific modelling, necessary to prepare the interpretation of results, is in most of cases still in its starting phase.

Large amounts of water are trapped in the Martian subsurface, under the form of ice (permafrost) or liquid water at some depth. To characterize these reservoirs in terms of physical state, depth, volume is important to trace back the history of the atmosphere, assess the fraction of water which has escaped to space and the one which has been trapped in the subsurface, understand why Mars has been potentially able to host life, despite its further evolution to desertification. LATMOS, in cooperation with IDES, is implementing a radar which will be boarded on the Exomars mission of ESA (2013).

Beyond, several projects are under study for a possible geophysical network mission of ESA (MSR-NET, 2015). With SISYPHE and MAGIE, LATMOS is interested by the electrical sounding of the physical properties of the subsurface (porosity, conductivity), completed by magnetic susceptibilimetry, as promoted by CEREGE , allowing to characterize the oxidation state of iron. With LISA, LATMOS also plans to search in the subsurface for adsorbed gaseous oxidants (H2O2, O3) by using electrochemical sensors fixed on a « mole » (provided by the german DLR) penetrating the regolith down to a depth of five meters. Characterizing the depth of the oxidation layer is indeed crucial to prepare sample return, by providing the depth below which to drill to reach unaltered organic material.

Recent measurements suggest, without demonstrating it, the presence of a residual volcanism, or of an outgassing due to hydrothermalism. The possible detection of methane from Mars-Express and terrestrial observations is still controversial. The recent detection of polonium in the spectra of the APXS of the MER rovers, and the ongoing interpretation of gamma spectra obtained by the GRS instrument of Mars-Odyssey , suggest the presence of radon in the atmosphere. The SPHIR instrument from LATMOS and BIRA-IASB , an infrared spectrometer of high spectral resolution dedicated to the measurement of methane, is proposed for the TGE mission of NASA (one of the wo missions in competition for the Scout 2013). In the frame of a cooperation between LATMOS and IRSN, a specific instrument dedicated to the in-situ observation of radon by alpha spectrometry is under study. LMD, in cooperation with LERMA , plans to develop a microwave spectrometer partially devoted to the search for sulfur compounds in the atmosphere. All these instruments could be part of the payloads of the MSR-NET mission.

Author : Eric Chassefière
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