Using the second-degree coefficients of gravity field models and the new precession rate to constrain the size and density of Martian inner core

Recently, it is yet difficult to detect the existence of Martian solid inner core merely based on Mars seismic InSight data. To deal with this problem, our study intends to use the mean density and mean moment of inertia factor to constrain the size and density of Martian solid inner core. Using Mars high-degree gravity field models JGMRO120f and GMM3-120, and considering the recent precession rate, we obtained the mean density and mean moment of inertia factor, which are treated as observed values. Referring to the 4-layers internal structure model of Mars, and considering the 4 parameters including crustal density, mantle density, density of outer core, size and density of inner core, we calculated the modeled values of the Martian mean density and the mean moment of inertia factor. From the minimum residuals between observed and modeled values of mean density as well as that of mean moment of inertia factor, it is found that the two gravity fields models have the same result of distribution of free parameters. As to the optimized values of the free parameters, the two gravity field models even have the same results. Furthermore, the optimized crustal density, mantel density and density of outer core approach other studies, indicating the dependence of our results. Finally, our result demonstrates that Mars likely has a solid inner core with a size close to 840 km, and the density of inner core is near to 6950 kg·m-3. Our result implies that Mars has an inner core not fully composed of pure iron, which is consistent with the recent study that Mars requires a substantial complement of light elements in Martian core. However, it is further needed to constrain the size and composition of Martian inner core due to the non-uniqueness of inversion results. With the improvement of processing technology on the InSight data, it can be further constrained for the size and composition of Martian inner core.