Ejecta Distribution

Geological field data show that, apart from the overturned flap of crystalline ejecta, which occurs within roughly 2.5 km from the boundary of the inner crater, fragments of granite ejecta with sizes from 0.1 to 10 m lie at distances r of up to 10 to 15 km from the crater center. This seems to be the norm for land impacts, where ejecta are continuously distributed over a large area around the crater with a density roughly decreasing with distance as r (McGetchin et al. 1973). However, this is not obvious for marine target impacts. Numerical simulations (Ormo et al. 2002) and the new simulations presented here show that the flight of basement ejecta is restricted by the transient water cavity and water ejecta curtain (at least at great water depth, which results in a large water cavity).

A typical situation is shown in Fig. 1. The angle of ejection and the angle of the ejecta curtain depend on material strength (Melosh 1989). In strengthless water the ejecta curtain is almost vertical and initial ejecta trajectory angles exceed 60o. In solid targets the angle of the ejecta curtain is about 45o and initial ejecta trajectory angles are smaller than 60o. This means that ejecta curtains must intersect. At shallow water depth the mass of ejected water is small and cannot strongly influence the flight of the solid ejecta. However, a large water transient cavity (necessary for shallow excavation) evolves at high water depth where most of the excavated mass is water. Thus we can conclude that the basement ejecta are absorbed by water and its late motion is defined by water flow and gravity sedimentation. Moreover, it is known that fast ejecta (those that are ejected to great distances) form only from the top target layers. In our case the top layer is water. To consider the interaction between small ejecta fragments (smaller than cell size) and water flow we should abandon the treatment of ejecta as a continuous medium.

A simple rough estimate of solid ejecta transport by water can be done from considering water flow velocities and velocities of sedimentation for ejecta particles of different size. Typical velocity time dependences at different distances from the crater center are shown in Fig. 3. The velocity of sedimentation can be estimated from equilibrium between particle weight, buoyancy force, and drag force:

where r is the radius of the spherical particle, pg and pw are granite and water densities, g is gravity, and Vis the velocity of sedimentation. For r = 0.1, 1, and 10 m, we obtain V = 3, 10, and 30 m/s, respectively. The time x of particle transport by water can be estimated as x = H/V, where H is the sea depth. For H = 800 m we have x = 250, 80, and 25 s for r = 0.1, 1, and 10 m correspondingly. Combining these values with data from Fig. 3 we can conclude that basement ejecta fragments could be transported to a distance of 1 to 3 km outside the water transient cavity. Figure 3 shows flow velocity averaged through a water depth. In reality, the velocity changes with water depth and in some layers it can considerably exceed the averaged value. In particular, when the water transient cavity reaches its maximum radius, water near the seabed begins to move inwards, forming the resurge, while water in the top layers and water surge continue to move outwards. This can increase the area of ejecta fragment distribution.

Fig. 3. Water flow velocity at different distances R from crater center versus time. A positive velocity indicates outward flow.

Fig. 4. Volume of basement and sedimentary rocks ejected at distances larger than R versus R (Distance from crater center).

To study this effect more thoroughly we performed numerical simulations where solid ejected material (described as continuous medium before ejection) was replaced by a set of discrete 1-m-radius particles with a density of 2.6 g/cm3. These particles move with flow velocity (like passive tracers), but simultaneously they descend with velocity V defined by (1). Figure 4 shows the ejecta distribution obtained with this approach for both sediments and granite rocks. Our results show that granite particles could reach a distance of 11 km from the crater center. The boundary of sediment ejecta distribution is at least 15 km. We do not consider here the influence of the size of the particles and the trajectory angle on the ejecta distribution, which would be the subject of a further study.

Fig. 5. Cratering flow after the oblique (45°) impact of a 300-m-radius granitic asteroid into a 800 m deep sea. Dark gray is basement material, light gray is water, black is impactor material (with bulk density exceeding 0.01 g/cm3). Atmospheric gas and ejecta with a low bulk density are not shown.

Fig. 5. Cratering flow after the oblique (45°) impact of a 300-m-radius granitic asteroid into a 800 m deep sea. Dark gray is basement material, light gray is water, black is impactor material (with bulk density exceeding 0.01 g/cm3). Atmospheric gas and ejecta with a low bulk density are not shown.

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