Orbital parameters’ evolution of Jupiter and Saturn in the nominal simulation. The top panel shows the semi-major axes of Jupiter in red and Saturn in blue. The shaded areas mark the positions of the peri-and apo-centre, q = a(1 − e) and Q = a(1 + e), of the planets, respectively. The black dashed line marks the position of the 2:1 resonance with Jupiter. The bottom panel shows the eccentricities of the planets. At the start of these curves, Jupiter is fully grown, while Saturn reaches its final mass around 72 700 yr. The coloured markers indicate the times at which the surface density is represented in the four panels of Fig. 2.