SOUTH POLE-AITKEN BASIN FORMING IMPACT EXCAVATES MOON’S UPPER MANTLE

The South Pole-Aitken (SP-A) basin is the largest observable impact structure on the Moon. The 2500 km diameter SP-A basin was likely formed by a 200 to 300 km diameter asteroid striking at an oblique angle, giving the basin its elliptical shape. The current orientation of the basin suggests the impactor traveled from south to north at an angle from the horizon of 30° to 45°. Since impacts excavate material from below the surface and SP-A is the largest observable impact on the Moon, SP-A probed deeper into the lunar mantle than any other basin and its ejecta likely contain the deepest-seated material on the surface of the Moon.

We simulated the SP-A basin forming impact using the iSALE-3D hydrocode. We modeled a spherical Moon with high-resolution cells. We completed simulations using 200 to 420 km diameter impactors with a 15 km/s impact velocity and 30°, 45°, and 60° oblique angles. We used Lagrangian tracer particles to represent the ejected material and to track provenance and the trajectory of ejecta. We found SP-A excavated the upper mantle of the Moon to depths greater than 50 km. We calculated maximum excavation depths of material ejected beyond the SP-A basin transient crater rim of 105 km and 145 km for impactors of 200 and 300 km diameter respectively at 45°.

We determined the thickness and original depth of the debris deposited for each point on the Moon’s farside. The ejecta extends mainly downrange from the basin rim: very little is deposited uprange due to the obliquity of the impact. The range of the ejecta is inversely related to its initial depth. Crustal material (<50 km depth) travels farthest and blankets the largest area of the farside. Upper mantle material (>50 km depth) occupies a distorted annulus downrange of the transient crater rim. The ejecta volume and area decrease with increasing excavation depth. We find that the debris underlying the lunar farside highlands mainly consists of material excavated by the SP-A basin-forming impact. The Moon’s upper mantle material is most likely to be exposed in close proximity to the SP-A basin’s north rim.