On the Crustal Architecture of the Terrestrial Planets

Understanding the structure and composition of planetary crusts is fundamental for unraveling the diverse geologic pathways of rocky bodies in the solar system. In recent years, geophysical missions have shed light on the crustal architecture of the Moon and Mars. New missions are currently en route to Mercury and in preparation for Venus. Here, we provide an overview of our current knowledge of the crustal structure of the Moon, Mars, Mercury and Venus, and present nominal models for these planets. Planetary crusts are thought to have average thicknesses of ∼20 km (Venus), ∼30 km (Mercury, Moon) and higher (30–70 km, Mars), and generally represent a few percent of the silicate mass fraction of their planet. In comparison, crustal thickness on Earth is bimodal, with values of 40 and 7 km for the continental and oceanic crusts, respectively, for a global average of ∼19 km. We highlight that gravity inversions must account for the often-uneven resolution of gravity fields and show that the classical Bouguer anomaly filtering step can be avoided by simultaneously inverting for crustal density and thickness. Rather than discarding data, this method ascribes short-wavelength gravity anomalies to crustal density variations. For Mercury, Venus, and the Moon, we discuss the effect of having a laterally variable mantle density on crustal thickness inversions, and for Mars, we present an approach to consider a high-density basaltic crust. While crustal thickness inversions remain non-unique, we discuss that the distribution of tectonic and volcanic landforms can help constrain the range of plausible models.