Histograms showing the merger probability vs. mass ratio (a)–(c) and initial outer moon semimajor axis (d)–(f) , for prograde–prograde simulations. Solid bars indicate the merger probability, and stacked shaded bars indicate system survival probability (merger or outer moonlet survival). Simulations per bin are indicated above each bar. Results for runs 1, 2, and 3, are plotted in the top, middle, and bottom rows, respectively.