Lost Impact Craters in Low-Impedance Materials

The research discussed in the Poster Abstract, below, by Schultz and Stickle, explains how shallow angle of incidence (oblique) impacts generate "impact" structures that are significantly different from the classic, better understood, crater planforms. Our proposed Saginaw impactor, being delivered on a near-tangential trajectory, would have generated an impact at angles of less than 5 degrees. The craters generated would be manifested as a quite shallow, elongated oval. The experiments using the NASA Ames Vertical Gun Range, were limited in scope, as the gun's lower limit of rotation is 15 degrees.

Lost Impacts

P. H. Schultz; A. M. Stickle
1. Department of Geological Sciences, Brown University, Providence, RI, USA.

The absence of a clearly identified crater (or craters) for the proposed YDB impact has raised questions concerning the reality of such an event. Geologic studies have identified impact deposits well before recognizing a causative crater (e.g., Chicxulub and Chesapeake Bay); some have yet to be discovered (e.g., Australasian tektite strewnfields). The absence of a crater, therefore, cannot be used as an argument against the reality of the YDB impact (and its possible consequences). The study here addresses how a large on-land impact during the late Pleistocene or early Holocene could avoid easy detection today. It does not argue the case for a YDB impact, since such evidence must come from the rock record.

During the late Pleistocene, the receding Laurentide ice sheet still covered a significant portion of Canada. While a large (1km) body impacting vertically (90°) would penetrate such a lowimpedance ice layer and excavate the substrate, an oblique impact couples more of its energy into the surface layer, thereby partially shielding the substrate. Three approaches address the effectiveness of this flak-jacket effect. First, hypervelocity impact experiments at the NASA Ames Vertical Gun Range investigated the effectiveness of low-impedance layers of different thicknesses for mitigating substrate damage. Second, selected experiments were compared with hydrocode models (see Stickle and Schultz, this volume) and extended to large scales. Third, comparisons were made with relict craters found in eroding sediment and ice covers on Mars.

Oblique impacts (30 degrees) into soft particulates (no. 24 sand) covering a solid substrate (aluminum) have no effect on the final crater diameter for layer thicknesses exceeding a projectile diameter and result in only plastic deformation in the substrate. In contrast, a vertical impact requires a surface layer at least 3 times the projectile diameter to achieve the same diameter (with significant substrate damage). Oblique impacts into ice and plasticene layers over clear acrylic blocks allow assessing internal damage. These experiments reveal that low-impedance surface layers approaching 1 to 2 projectile diameters effectively shield the substrate from shock damage for impact angles less than 30 degrees.

Missing craters (and relict crater roots) within ice-rich deposits on Mars illustrate the rapid erasure the impact record. Numerous small pedestal craters (crater diameter < 5km) occur at high latitudes and reflect the cyclic expansion and disappearance of polar ice/dust deposits up to 0.5 km thick. Much larger examples (> 50km), however, occur at low latitudes but are localized in certain regions where even thicker deposits (locally >2km) have been removed, uncovering a preserved Noachian landscape. Crater statistics further document this missing cratering record.

Thick Pleistocene ice sheets on Earth would have played a similar role for the removal of terrestrial cratering record. We calculate that a crater as large as 15km in diameter formed by an oblique impact could have been effectively erased, except for dispersed ejecta containing shocked impactor relicts and a disturbed substrate. While plausible, evidence for specific missing events (e.g., the proposed YB impact) must be found in still-preserved ice layers and sediments.