colleagues calculated a value of 0.3662 ± 0.0017 for the moment-of-inertia factor, based on an improved estimate of the martian spin pole precession rate determined from Doppler range measurements to the Mars Pathfinder and Viking landers.⁶

Any compositional model of Mars must account for its low mean density and high moment-of-inertia factor compared with those of Earth. Uncompressed density is a more relevant indicator of planetary composition, but this is necessarily uncertain because of a lack of knowledge of temperatures in the planet and material properties at high pressure. The ranges of estimated uncompressed densities are 3.8 to 3.9 g/cm³ for Mars and 4.4 to 4.5 g/cm³ for Earth.⁷ The moment-of-inertia factor measured for Earth is 0.3315.⁸ The difference in moment-of-inertia factors indicates that the concentration of mass toward the center of Mars is less than that of Earth. Earth’s core size is well determined from seismic studies; thus, the measured moment-of-inertia factor for Earth directly constrains the ratio of the core density to the mantle density. However, on Mars the core size is still a free parameter. As shown in Figure 2.1, the moment-of-inertia factor and bulk density constraints allow a wide range of possible core sizes, with an attendant range of core and mantle compositions.

Two factors are thought to be responsible for the lower mean density of Mars compared with that of Earth.⁹ Mars has an Fe/Si ratio smaller than that of Earth, and it is somewhat enriched in oxygen. The additional O is chiefly in the mantle, associated with Fe²⁺ that would otherwise occur as Fe⁰ in the core. A smaller or lighter core alone would not account completely for the higher moment-of-inertia factor of Mars: It is also necessary that the average density of Mars’s mantle be greater than that of Earth’s mantle. No unique density can be specified by the geophysical constraints, however, since this value depends on assumptions of crustal thickness and core size. The

FIGURE 2.1 Density profiles for a range of model core compositions (solid lines). For each core composition, the thickness of the low-density crust is adjusted to give the correct mean density and moment of inertia for Mars. Dashed lines indicate the depth, or pressure, of the core/mantle boundary for model core compositions. The crust-mantle-and-core profile shown (heavy line) assumes a 50-km, 3.0-g/cm³ crust. SOURCE: Reprinted with permission from C.M. Bertka and Y. Fei, “Implications of Mars Pathfinder Data for the Accretion History of the Terrestrial Planets,” Science 281:1838–1840, 1998. Copyright 1998 by the American Association for the Advancement of Science.