Effects of planet curvature and crust on the shock pressure field around impact basins

Sarah T Stewart,Karin L Louzada

doi:10.1029/2009gl037869

Abstract

We investigate the effects of planetary curvature and the crust‐mantle boundary on the shock pressure field around impact basins on Mars using acoustic ray path calculations and hydrocode simulations. Planet curvature and, to a lesser extent, increasing sound speed with depth shallow the zone of wave interference, where shock pressures decay rapidly to the surface. The depth to the interference zone boundary diverges from the flat surface solution for projectile‐to‐Mars radius ratios greater than ∼1% (transient craters greater than ∼300 km); the difference increases with distance from the impact point and projectile size. In hydrocode simulations (but not the simple ray path model), the presence of the crust‐mantle boundary produces nearly vertical pressure contours in the crust. Around Hellas basin, demagnetization occurs at shock pressures between 1.1 (±0.2) and 3.4 (±0.7) GPa, where the range is due to the uncertainty in the transient crater diameter.

Highlights

We investigate the effects of planet curvature and the crust-mantle boundary on the geometry of the interference zone and the shock pressure distribution in the crust around large impact events
Melosh [1984] calculated the depth of the interference zone boundary (DIZB) as a function of distance along the surface, s: ( ( ) ) ( ) DIZB (s) = cLτ !# 4 d 2 + s2 − cL2τ 2
The blue or grey dots in Figure 3A show DIZB for an impact on a homogeneous Mars by a 230-km radius projectile at 9 km/s. (Note that DIZB is independent of the pressure decay profile.) The results indicate that the Interference Zone Boundary (IZB) is shallower in a spherical planet and that the depth to the boundary decreases with increasing distance and impactor size (Figure 3B)

Summary

Introduction

Recent interest in the shock pressure field around impact basins has been fueled by observations of unmagnetized crust around the younger basins on Mars [e.g., Hellas basin, Acuña, et al, 1999] coupled with experimental evidence of pressure demagnetization of magnetic minerals at a few GPa [e.g., Hargraves and Perkins, 1969; Nagata, 1974; Cisowski and Fuller, 1978; Rochette, et al, 2003; Kletetschka, et al, 2004; Gattacceca, et al, 2007; Louzada, et al, 2007]. The shock pressure distribution in the crust is sensitive to the geometry of this interference zone. Previous estimates of the shock pressure distribution in the crust surrounding Martian impact basins have not included an interference zone [Hood, et al, 2003] or used an adaptation of the stress wave propagation and reflection model for flat homogeneous surfaces by Melosh [1984] [e.g., Kletetschka, et al, 2004; Mohit and Arkani-Hamed, 2004]. We investigate the effects of planet curvature and the crust-mantle boundary on the geometry of the interference zone and the shock pressure distribution in the crust around large impact events. We discuss the implications for shock demagnetization on Mars

Methods

Results

Discussion

Conclusion