Onset of chaos in a characteristic dynamical simulation. Jupiter and Saturn started in the JSREG configuration, locked in 2:1 MMR with low eccentricities (starting eccentricities of 0.025 for both planets) and exhibiting regular motion. The evolution of their orbital eccentricities/inclinations are shown in the top/bottom panel. Gravitational scattering of a Mars-mass embryo initially at 12 au triggered chaos in the giant planets’ orbits. The planets remained in resonance, as confirmed by libration of the 2:1 MMR critical angle ﹩2{\lambda }_{\mathrm{Sat}}-{\lambda }_{\mathrm{Jup}}-{\varpi }_{\mathrm{Jup}}﹩, on chaotic but long-term stable orbits. ﹩{\lambda }_{\mathrm{Sat}}﹩, ﹩{\lambda }_{\mathrm{Jup}}﹩, and ﹩{\varpi }_{\mathrm{Jup}}﹩ are Saturn’s mean longitude, Jupiter’s mean longitude and longitude of pericenter, respectively. The initial conditions of this simulation are provided in the Appendix.