Distribution of (a, e) of the orbits of the captured moons from the inner moon region of IG18 at different encounter distances. The encounter distances are given in the bottom right corner of each panel. Warmer colours mean higher numbers, and grey dots are the observed satellite (cf. Fig. 1). The red solid line indicates a tidal circularisation isochrone of 4 Gyr; the blue short-dashed line represents an equal pericentre distance q = 30 R_Nep; the black long-dashed line delineates an equal apocentre distance of Nereid’s pericentre Q = q_Ner; and the dash-dotted and dotted lines show tidal engulfment limit for prograde and retrograde orbits, respectively (cf. Fig. 2).