Relation Soleil ? Mars: description et analyse des échanges présents et passés entre magnétosphère et atmosphère – HELIOSARES

It is now well established that Mars' atmosphere evolved from an early Earth's like atmosphere 4 billions years ago to the present dry and thin atmosphere. However, we still ignore why and how most of the water, still present on Earth, disappeared from Mars' surface. Actually, we even do not know if the present Martian atmosphere is a trace of a past thick and wet atmosphere or simply the result of an equilibrium between present or recent degassing at the surface and atmospheric escape to space. NASA has recently decided to launch in 2013 a Martian mission dedicated to the study of the present atmospheric escape and its dependency on the solar conditions, a straightforward way to constrain the past evolution of Mars' atmosphere. On the European side, MarsNext, a mission to be launched in 2016, is under study and will carry also a package of instruments dedicated to the measurement of the impact of the solar wind and flux on the Martian thermosphere/ionosphere, on its exosphere, on its magnetosphere and on their relations to the present atmospheric escape. However, as shown by Mars Express, any measurement of the present atmospheric escape and more generally of the Martian environment needs, in order to be clearly understood, the use of complex model of Mars' interaction with the solar wind. We have developed such a kind of model by coupling, for the first time, a 3D model of the Martian exosphere (that is the part of the extended atmosphere in direct interaction with the solar wind) and a 3D model of the Martian magnetosphere formed by Mars' interaction with the solar wind. This work highlighted, in particular, the need to describe in a much detailed way the few hundred kilometres above the surface in order to understand the main channels of atmospheric escape and to extrapolate them to the past. In this proposal, we would like to develop, test and apply to Mars a detailed approach to describe Mars' interaction with the solar wind and its relations with Mars' upper atmosphere. The aims of this proposal are to improve three existing models of different regions of Mars' environment, in order to couple them consistently, that is : - An existing magnetospheric hybrid model in order to adapt his spatial resolution to the scale height of the upper atmosphere and ionosphere, - An existing exospheric models to couple it with the European Martian Global Circulation model (EMGCM) and in particular to its Martian thermosphere and ionosphere parts, - The EMGCM to couple it with the magnetospheric hybrid model, that is, to describe the ionosphere up to the exobase and therefore consistently include the ionospheric contributions calculated by the EMGCM in the magnetospheric model. The main expected results will be to obtain the first consistent description of Mars' environment from its surface up to the interplanetary medium. We will illustrate this achievement by modelling the Martian environment for various solar conditions. Such type of simulation will in particular provide us a much better view of the relations between magnetosphere, exosphere, thermosphere and ionosphere. Therefore, we will be able, for the first time, to track the key parameters leading to atmospheric escape. In this way, we will be able to extrapolate consistently the atmospheric escape to Mars' past conditions, an issue that cannot be reached properly with the present modelling tools. The second important results will be done thanks to the analysis of the existing data obtained by the two space missions Mars Express and Mars Global Surveyor, thanks to the developments of simulation tools of these measurements and thanks to the definition and study of test cases which will serve as validation tests of our theoretical developments but also which will help us to better track the mechanisms driving the solar wind and flux interaction with Mars' atmosphere. The work foreseen in this proposal will provide to these teams the opportunity to consistently couple their own description of a part of Mars' environment so that it will add considerable values to their present efforts to describe the present and past Mars' interaction with the solar wind. This work will be done in a generic way such that we should be able to describe the role of any incident plasma in shaping an atmosphere, Mars' case being a close analogue of Venus, Titan and Europa cases, which are planetary objects that have been, are or will be explored by European or/and US space missions. More generally, this proposal will provide to our teams a generic approach to describe the environments of any planet or satellite of the solar system or of any stellar system.