A steeply-inclined trajectory for the Chicxulub impact

Abstract

The environmental severity of large impacts on Earth is influenced by their impact trajectory. Impact direction and angle to the target plane affect the volume and depth of origin of vaporized target, as well as the trajectories of ejected material. The asteroid impact that formed the 66 Ma Chicxulub crater had a profound and catastrophic effect on Earth's environment, but the impact trajectory is debated. Here we show that impact angle and direction can be diagnosed by asymmetries in the subsurface structure of the Chicxulub crater. Comparison of 3D numerical simulations of Chicxulub-scale impacts with geophysical observations suggests that the Chicxulub crater was formed by a steeply-inclined (45-60° to horizontal) impact from the northeast; several lines of evidence rule out a low angle (<30°) impact. A steeply-inclined impact produces a nearly symmetric distribution of ejected rock and releases more climate-changing gases per impactor mass than either a very shallow or near-vertical impact.

Fig. 1. Asymmetries of the geophysical signature of the Chicxulub crater.

Background colourmap shows Bouguer gravity anomaly map in the vicinity of the crater (gravity data courtesy of Hildebrand and Pilkington). The red circle marks the nominal position of the crater centre; the green circle marks the centre of maximum mantle uplift; the blue circle marks the centre of the peak ring (as defined by the annular gravity low surrounding the central high); the white triangle marks the location of the Expedition 364 drill site through the peak ring (Hole M0077A). The coastline is displayed with a thin white line; cenotes and sinkholes with white dots, and the city of Mérida with a white square. The dotted lines offshore mark the approximate location of the inner crater rim and the extent of faulting as imaged by seismic data. Inset depicts the regional setting, with red rectangle outlining the region shown in the gravity map. Adapted from ref. .

Fig. 2. Development of the Chicxulub crater for a 60∘ impact.

The impact scenario depicted is for a 17-km diameter impactor with a density of 2630 kg m³ and a speed of 12 km/s. Evolution of the crater up to 5 min after impact is depicted. Shown are cross-sections through the numerical simulation along the plane of trajectory, with $x=0$ defined at the crater centre (measured at the pre-impact level; $z=0$); the direction of impact is from right to left. The upper 3 km of the pre-impact target, corresponding to the average thickness of sedimentary rocks at Chicxulub, is tracked by tracer particles (sandy brown). Deformation in the crust (mid-grey) and upper mantle (dark grey) is depicted by a grid of tracer particles (black). Tracer particles within the peak-ring material are highlighted based on the peak shock pressure recorded (white–blue colour scale); melted target material (>60 GPa) is highlighted in red.

Fig. 3. Development of the Chicxulub crater for a 30∘ impact.

The impact scenario depicted is for a 21-km diameter impactor with a density of 2630 kg m³ and a speed of 12 km/s. Evolution of the crater up to 5 min after impact is depicted. Shown are cross-sections through the numerical simulation along the plane of trajectory, with $x=0$ defined at the crater centre (measured at the pre-impact level); the direction of impact is from right to left. Colours and shading of material and tracer particles are the same as Fig. 2.

Fig. 4. Final simulated Chicxulub crater for a 30∘ and 60∘ impact angle.

Shown are cross-sections, along the plane of trajectory, through the final simulated craters formed by a $30^{\circ }$ (a) and $60^{\circ }$ (b) impact, with $x=0$ defined at the crater centre (measured at the pre-impact level); the direction of impact is from right to left. Sandy-brown tracers indicate the final position of the upper 3 km of the pre-impact target (sedimentary rock); red tracers indicate the position of melt; tracers with blue–white shading indicate shock pressures of simulated peak-ring materials. The geometric centre of the crater rim defines the coordinate origin ($x=0$); negative $x$-values are downrange.

Fig. 5. Offset of structural crater features relative to crater centre.

The offsets of the centre of mantle uplift (squares) and the centre of the simulated peak ring (circles), relative to the crater centre, are shown as a function of impact angle to the horizontal. Grey bands denote the approximate relative offsets of the peak-ring and mantle-uplift centres at Chicxulub, taking into account the uncertainty in crater diameter and locations of the different features (see Supplementary Fig. 1).