The interior of a planet holds important clues to its origin and thermal and dynamic evolution. Exploring a planet’s deeper layers can reveal how a planet accreted and differentiated into layers, whether its core sustains a geodynamo that creates a magnetic field, and what the origin is of any tectonic and volcanic activity. New clues are now uncovered for Mars. On pages 438, 434, and 443 of this issue, Knapmeyer-Endrun et al. (1), Khan et al. (2), and Stähler et al. (3), respectively, present the first findings of the interior structure of Mars based on data from the NASA lander InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport). These studies provide the first direct observations of the crust, mantle, and core structure on another rocky planet, for which the results and implications can be compared and contrasted with the characteristics of Earth.