DUGi

Self-consistent Modelling of Mercury’s Surface Composition and Exosphere by Solar Wind Sputtering

http://dugi.udg.edu/item/http:@@@@hdl.handle.net@@2072@@273650

A Monte-Carlo model of exospheres was extended by treating the solar wind ion induced sputteringprocess, quantitatively in a self-consistent way starting with the actual release of particles from themineral surface of Mercury. Mercury is a body without a significant atmosphere, thus, the surface iseffected by different processes that are mainly related to the radiation and plasma environment ofthe Sun and to micrometeorites, which are delivered to Mercury’s surface. In such a case it can beassumed that the composition of Mercury’s thin collisionless atmosphere, the exosphere, is relatedto the composition of the planetary crustal materials. If so, then inferences regarding the bulkchemistry of the planet can be made from a study of atoms and molecules in the exosphere afterthey are released from the mineral surface by a variety of release processes. One difficult challengeis the identification of the main source of some elements like H, He, Na or K. Generally it isbelieved that H and He come primarily from the solar wind, while Na and K originate fromvolatilized materials partitioned between Mercury’s crust and impacts from meteorites. Besides thebefore mentioned elements corresponding to spectroscopic observations and experiments with soilanalogues, other elements such as O, Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Fe, Ni, Zn, OH should alsobe related with Mercury’s surface soils (Wurz et al., 2010, and references therein). Based onavailable observational data and literature data we established a global model for the surfacemineralogy of Mercury and from that derived the average elemental composition of the surface.Compositional data analysis has been employed for Mercury’s surface minerals recently by(Sprague et al., 2009). In these cases the applied method was based on simple correlation methods,which do not exploit the full potential of the available data. In addition, the closed nature ofcompositional data, i.e., the assumption that component concentrations have to sum up to 100% inan analysis, bears important implications for the statistical analysis of compositional data, which donot seem to have been sufficiently appreciated until now. To investigate the default of the classicaladditive analysis method our research group applied recently a more realistic multiplicative method(Aitchison, 1986) based on the Euclidean space geometry of the simplex (see the chapter Elementsof simplicial linear algebra and geometry). Our recent results presented in detail in Wurz et al.,(2010) for Mercury will be discussed. This model serves as a tool to estimate densities of species inthe exosphere depending on the release mechanism and the associated physical parametersquantitatively describing the particle release from the surface

Universitat de Girona. Departament d’Informàtica i Matemàtica Aplicada