Experimental Constraints on the Thermal Structure of the Martian Interior and Martian Magmatism

Introduction: Past space missions to Mars have provided important constraints on the planet's internal density distribution [1,2]. However, the thermal structure of the Martian interior remains highly uncertain. Although theoretical calculations have provided some basic knowledge of the thermal evolution of Mars [e.g., 3-9], the results from these computer simulations are strongly dependent on the model parameters. Estimating Martian core temperature is difficult because of the uncertainty in the chemical composition and physical state of the core. Knowledge of the physical state of the core is critical for placing bounds on core temperatures. Several lines of observation may shine some light on the physical state of the core. These observations include the present day weak Martian magnetic field and the existence of a strong magnetic field in the early history of Mars as discovered by Mars Global Surveyor. However, ultimate confirmation of the physical state of the core has to rely on future missions that will provide seismic data. If the physical state of the Martian core is known, core temperatures can be estimated from melting temperature of the core materials. Cosmochemical constraints indicate that Mars has a sulfur-bearing iron-nickel core. Our experimental study of melting relations in the system Fe-Ni-S at high pressures provides essential data to estimate the core temperatures. Recent data from Mars Global Surveyor revealed the distribution of Martian crust and provided new insight into Mars's thermal history [e.g., 9]. Understanding mantle convention on Mars is critical for explaining the crustal dichotomy and magma production on Mars [7, 10]. Our experimentally determined melting relations in an iron-rich mantle composition up to core-mantle boundary pressures provide important constraints on thermal history models. Experimental Procedure: We used a multi-anvil high-pressure apparatus to simulate the pressure-temperature conditions of the Martian interior. Experiments were conducted using an 8/3 high-pressure cell assembly [11] that is capable of generating pressures up to 28 GPa, covering the entire range of Martian mantle pressures and the upper portion of the core pressures. For the melting relations of the Martian core, we used a model core composition in the Fe-Ni-S system [12]. The starting materials were Fe-Ni-S mixtures , created by mixing pure metallic Fe, Fe-Ni alloy, and FeS. The starting materials were loaded into either