EMPLACEMENT OF IMPACT CRATER EJECTA - THE VIEW FROM THE MOON (Invited Presentation)

The emplacement of proximal and distal ejecta surrounding an impact crater is a complicated process of ballistic ejection and flight, collision with the surface, movement along the surface with associated erosion or deposition of material, and internal deformation of the ejecta. Ejecta can be composed of clastic material (individual particles moving individually or en masse) or liquid (in the case of a silicate planet - silicate impact melt). The presence of an atmosphere will influence the distribution and deposition of ejected material. Impacts into an ocean will also strongly influence the ejection and distribution of material. Ground-water or ground-ice has been suggested to influence the morphology of ejecta deposition on Mars and perhaps on the Earth (so called lobate ejecta craters). Erosion and burial of the surface and spatial resolution of planetary remote sensing data typically preclude detailed study of the ejecta morphology and characteristics to understand the details of its emplacement. The Moon, however, provides a unique environment (no atmosphere, no water, high-resolution remote sensing data) to understand the details of the ejecta character, composition and emplacement processes.

Lunar impact ejecta exhibits a typical pattern of the clastic ejecta being largely deposited before deposition of impact melt. Impact melt embays boulder ejecta near the rim and fills depressions on the proximal ejecta surface. Unusual crater forms on melt surfaces suggest that some clastic debris impacts the surface before the melt completely solidifies. Near the crater rim the ejecta is bouldery to hummocky. At greater distances the ejecta can exhibit lobes having well-defined lineated texture and lobate margins. Orientation of the lineation indicates shearing within the ejecta within a ground hugging flow whose flow direction is dictated by the momentum of the flow rather than local topography. Lobe orientation demonstrates that the flow direction varies as lobes are observed to overlap. The presence of so-called “self secondary” craters demonstrates that a small but significant component of ejecta is launched into near-vertical trajectories such it does not return to the surface until most of the clastic ejecta and impact melt have been deposited.