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Knowledge of the average density of the crust of a planet is important in determining its interior structure. The combination of high-resolution gravity and topography data has yielded a low density for the Moon's crust, yet for other terrestrial planets the resolution of the gravity field models has hampered reasonable estimates. By using well-chosen constraints derived from topography during gravity field model determination using satellite tracking data, we show that we can robustly and independently determine the average bulk crustal density directly from the tracking data, using the admittance between topography and imperfect gravity. We find a low average bulk crustal density for Mars, 2582 +/- 209 kgm(-3). This bulk crustal density is lower than that assumed until now. Densities for volcanic complexes are higher, consistent with earlier estimates, implying large lateral variations in crustal density. In addition, we find indications that the crustal density increases with depth.

Plain Language Summary Knowledge of the structure of the crust of a planet is important because it informs us about the formation and evolution of the planet. The GRAIL mission to the Moon has showed us that the density of the Moon's crust is lower than assumed. This indicates the influence of impact craters on the Moon's history. For Mars, the models that we have of its gravity field (which gives information about the interior of the planet) are thought to be of poor quality to do similar studies as at the Moon. Here we show how we can obtain a value for the bulk density of Mars' crust. Our value is lower than the density that has been assumed until now. We also find strong variations of density on Mars, highlighting the differences between the volcanoes and southern highlands. We show new models of the thickness of Mars' crust. Our novel approach can be used for other planets for which we know both gravity and topography.