Iapetus’s Equatorial Ridge: Evidence for Ring Capture During the Nice Model Instability

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Saturn’s moon Iapetus hosts the tallest and most enigmatic mountain range in the solar system, aligned with the equator to within ±2° and exclusively on the moon’s dark leading side. We demonstrate that this ridge formed from the gradual collapse of a captured equatorial debris ring during the Nice model instability (~700 Myr after solar system formation).

Our model explains all key observations—including equatorial precision, hemispheric asymmetry, icy composition, and massive scale—through a single mechanism: moon-ring accretion driven by Iapetus’s large Hill sphere, rapid early rotation, and despinning lithosphere. This scenario makes testable predictions about internal ridge structure, subsurface density anomalies, and surviving ring remnants.

This is the first recognized case of a moon capturing and accreting its own ring to form permanent topography. It transforms Iapetus from a planetary oddity into a unique geological witness of the outer solar system’s most chaotic epoch.