The gravitational field of a planet depends on its internal density distribution, and can thus be used to constrain models of its interior structure. Gravity field models are often expressed in global spherical harmonics because these functions are a solution to Laplace's equation that describes the gravitational potential outside a sphere that encompasses the entire planet (e.g., Heiskanen & Moritz, 1984;Kaula, 1966). In general, models of a planet's gravity field are determined with some combination of radiometric tracking, for

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