Mars: interior structure and excitation of free oscillations

Abstract

Based on available chemical models of the planet [Philos. Trans. R. Soc. London 349 (1994) 285; Supply and loss of volatile constituents during the accretion of terrestrial planets, in: S.K. Attreya, J.B. Pollack, M.S. Matthews (Eds.), Origin and Evolution of Planetary and Satellite Atmospheres, Univ. Arizona Press, pp. 268–288; Icarus 126 (1997) 373; Phys. Earth Planet. Inter. 112 (1999) 43; Space Sci. Rev. 92 (2000) 34], a new set of global models of the Martian interior has been constructed. A model comprises four submodels—a model of the outer porous layer, a model of the crust, a model of the mantle and a model of the core. The first 10–11 km layer is considered as an averaged transition from regolith to consolidated rock. The mineral composition of the crustal basaltic rock varies with depth because of the gabbro-eclogite phase transition. As a starting point for mantle modeling the experimental data obtained by Bertka and Fei [J. Geophys. Res. 102 (1997) 525; Earth Planet. Sci. Lett. 157 (1998) 79] along the areotherm have been used, iron content of the mantle being varied. The measured or estimated up to now elastic properties for a set of mantle minerals are used. Seismic velocities determined from new high P– T data on elastic properties are 2–3% lower than the velocities calculated earlier. New high P-T measurements of the density of Fe (γ-Fe), FeS and FeH enable us to refine the core model. Taking into account available chemical models and the fact that noticeable amount of hydrogen could enter the Martian core during its formation [Solar Syst. Res., 30 (1996) 456], such parameters as a ferric number of the mantle (Fe#), sulfur and hydrogen content in the core are varied. The following tendency is seen: the presence of hydrogen leads to the increase of the Fe/Si ratio and decreases Fe# in the mantle due to the increase of the core radius. The higher sulfur and hydrogen content in the core and the smaller mantle Fe#, the less likely a perovskite layer exists. The modeling shows that to obtain the Fe/Si ratio up to the chondrite ratio of 1.71, more than 50 mol% of hydrogen should be incorporated into the core. In the second part of the paper, based on the available estimates of the Martian seismic activity and the sensitivity of current instruments, the amplitudes for different types of free oscillations have been estimated. It is found down to what depth the normal modes can sound the planetary interiors. A marsquake with a seismic moment of 10 25 dyn cm is required for spheroidal oscillations (with ℓ≥17) to be detected. These spheroidal modes are capable sounding the outer layers of Mars down to a depth of 700–800 km.