Semi-major axis distribution under the interaction of Earth and Mars. The semi-major axis histogram compares the case of all planets present (also indicated by the grey area) with either Earth or Mars absent (top) or with Mars as the only present planet (bottom) for β = 0.01. Vertical dashed lines indicate the locations of several first order as well as the co-orbital MMRs (including a shifting factor to account for radiation pressure). Removing Earth has no significant effect on Mars MMRs, while the impact of Mars on Earth MMRs is more notable. Removing all other planets increases occupancy of external Mars MMRs slightly, although remaining low compared to MMRs of Earth.