Hydrocode modeling of Chicxulub as an oblique impact event
Introduction
One of the largest impact structures on Earth, the Chicxulub structure [1]in the Yucatán Peninsula, Mexico, is associated with one of the major mass extinctions in the Phanerozoic [2], the one that has become famous for the demise of the dinosaurs, 65 Myr ago. Several studies have focused on modeling the Chicxulub impact event with increasing accuracy 3, 4. In particular, special attention was devoted to accurately representing the target stratigraphy, which is considered to be crucial for an assessment of the environmental effects of the impact [4]. All previous hydrocode simulations of the Chicxulub impact event have modeled the impact as vertical (i.e., the impact velocity is perpendicular to the target's surface). However, Shoemaker [5]showed that, regardless of the target planet's gravitational field, the probability of a meteorite impacting a given surface area at an angle between θ and θ+dθ, where θ is measured from the vertical, is:
This means that the probability of vertical (θ=0°) as well as grazing (θ=90°) impacts is negligible, while impacts are most likely for θ=45°. Unfortunately, limitations in the available hydrocodes, computer power and disk space, have, so far, limited modeling efforts to 2-D impact simulations, which can only model vertical impacts. So called `2,5-D' numerical computations [6]have also been attempted, but because they prohibit lateral spreading of ejected material their validity is questionable.
The rim of the final craters is circular for all but the most oblique impacts (θ<10° 7, 8), although interior crater features might provide some information (not yet well understood, see discussion below) on the angle of impact [9]. On the other hand, the angle of impact has a profound influence on the compression and expansion stage of an impact event, including the evolution of the expansion plume, the distribution of ejecta around the crater, and, of particular importance for the environmental effects of the impact event, the amount and the shape of the regions of melting and vaporization in the target.
Attempts to estimate the angle of impact of the Chicxulub event have been made by looking at the geophysical evidence 10, 11and at the paleontologic record [10]. Although far from giving a complete picture, the available Bouguer gravity data on the Chicxulub structure indicate an elongated central gravity high trending northwest, encircled by a horseshoe-shaped gravity low. This asymmetry seems also to appear in the nature and distribution of ejecta from Chicxulub, although the data set is spotty and of low resolution. Assuming that the impact event was, at least partially, responsible for extinctions at the end of the Cretaceous, the paleontologic record seems to suggest that North America was affected most severely by the impact event. The paucity of data outside the North American continent, however, may introduce a bias in the conclusions. Even with the realization of the incompleteness of the data used, Schultz and D'Hondt [10]concluded that the trajectory for the Chicxulub bolide was from the southeast to the northwest at a 20–30° angle from the horizontal. Looking at the same geophysical evidence, however, Hildebrand et al. [11]reached a very different conclusion. They suggested that the offset of the crater's central structure with respect to the center, combined with the asymmetry in the collapsed cavity wall and peak ring and thrusting in the zone of slumping (these factors were also considered by Schultz and d'Hondt), suggests an angle of impact of about 60° and a direction from the southwest to the northeast. Clearly these very different conclusions indicate how little we yet understand about oblique impacts, especially when the data set available is limited and spotty, as in the case of the Chicxulub structure. However, they also raise the question of the importance of the angle of impact in assessing regional and global environmental effects of the Chicxulub impact event.
Section snippets
3D hydrocode simulations
To investigate the effect of angle of impact on the production of melt and vapor, we carried out a series of 3D simulations [12]using CTH, a hydrocode developed at the Sandia's National Laboratories (Albuquerque, NM) to model multidimensional, multi-material, large deformation, strong shock wave physics [13]. The code is based on the same principles as CSQ [14], i.e., it employs a two-step Eulerian scheme for solving the equations of hydrodynamic flow in finite difference form. It can be used
Model input for CTH
The target lithology used is similar to the 2D simulations in Pierazzo et al. [4]: a 100 m deep shallow sea overlain a 2.9 km thick sedimentary layer, a 30 km thick continental crust, and a dunite mantle. The equivalent of the terrestrial atmosphere is also included in the model. The projectile is treated as a spherical dunite asteroid, 10 km in diameter, impacting the surface at 20 km/s.
Available tabular EoS have been used for the materials. Although the sedimentary layer is a sequence of
Results of 3D simulations
The simulations cover the first 5 s of the impact event. While this length of time is enough to follow the propagation and decay of the shock wave, it cannot give a complete description of the evolution of the expansion plume. Much longer runs are necessary to better constrain the thermal evolution of the plume, as well as its expansion, and the fate of the projectile. Each simulation starts with the projectile already at the target surface, i.e., the atmospheric entry of the projectile is not
Melting/degassing in oblique impacts
The main reason for these 3D simulations is to study the effect of impact angle on the degassing of the sedimentary layer, and the subsequent production of climatically important gases. The resulting melting and vaporization regions are shown in Fig. 7b–f, that clearly show the marked asymmetry associated with oblique impacts: the bulk of the melting and vaporization occurs downrange of the impact point. Furthermore, the region of melting becomes shallower for smaller impact angles,
Conclusion
We carried out the first high resolution 3D simulations of the Chicxulub impact event for various angles of impact. The analysis of the simulations at different impact angles, suggests:
(1) The region of melting of continental crust become shallower as the angle of impact decreases, as is illustrated in Fig. 7b–f, and is summarized in Fig. 8a. In particular the depth of melting decreases significantly with angle of impact and the region of melting is displaced downrange of the impact point.
(2)
Acknowledgements
The research was supported by NASA grants NAGW-5159 and NAGW-428. Three dimensional simulations using the CTH hydrocode were performed by David Crawford at Sandia National Laboratories using material and geometry descriptions provided by the authors. This research was performed in part using parallel computing resources located at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of
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2022, Geochimica et Cosmochimica ActaCitation Excerpt :Below 60°, parts of the projectile may survive as solid fragments (from the trailing half of the projectile, subject to lesser shock pressures), with the vaporization becoming nearly non-existent for very shallow impact angles (∼15°). For the assumed 45–60° impact angle of the Chicxulub impact event, a substantial proportion of the impactor material was likely vaporized (∼10–30%, see Fig. 10) and melted (Pierazzo and Melosh, 1999, 2000), and deposited outside the impact crater, as indicated by the marine Os isotopic composition record and Ir concentrations (Paquay et al., 2008). The preservation within the K–Pg boundary ejecta of a significant meteoritic component (e.g., Quitté et al., 2007; Goderis et al., 2013), and of a small, 2.5 mm, (fossil) meteorite fragment (Kyte, 1998), is consistent with the scenario of an oblique impact event, probably below 60°, where parts of the impactor survived as solid phase and/or melted material, according to hydrocode modeling (Pierazzo and Melosh, 1999, 2000).
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