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Results Project 1

Experimental impact cratering with the novel EMI two-stage light-gas gun: ground-breaking crater dimensions and improved material models

Cratering experiments in the laboratory are of particular importance for the investigation of the highly dynamic and complex processes associated with hypervelocity impacts on the earth’s and planetary surfaces. In this context project 1 plays a central role within the research group MEMIN by conducting the cratering experiments and providing the experimental data for the other subprojects. Of specific scientific interest is the investigation of physical effects like crater formation and ejecta dynamics. Many of the natural target materials are porous and contain pore fluids. For this reason the investigation of the influence of parameters such as pore water and target porosity on crater morphology, crater growth and ejecta behavior is significant. The acceleration of projectiles up to several kilometers per second requires the use of highly sophisticated two-stage light-gas accelerators (Figure 1).

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Figure 1. Two-stage light-gas accelerators of EMI used for the impact experiments. Top:
large-size facility; bottom: small-size facility.

The smaller gun allows a more detailed study of the impact and the ejection process because high-speed cameras and high-power flashes can be set up closer to the target yielding better illuminated high-speed images and a better resolution. The larger facility, on the other hand, allows acceleration of much larger projectiles and thus studying impacts at significantly higher impact energies. Hence, conducting impact experiments at different energies permits the investigation of scaling effects.

Due to the extremely short time-scales associated with the impact, the crater formation and the ejection process, special scientific methods and sophisticated technical equipment are required. This involves high-speed cameras with frame rates up to 100 kfps (kiloframes per second) and high-power flashes which are required due to the very short exposure times (Figure 2).

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Figure 2. Adjustment of the high-speed camera and the high-power flashes.

A typical evolution of the ejection process is shown in Figure 3 for an impact on dry sandstone. In the first frame, a cone with high-speed ejecta has formed. In the central part, an orange-colored ejecta plume is observed which is luminescent and radiates heat. The second and third frame show the fully developed ejecta cone. The lower part of the ejecta cone becomes increasingly steeply inclined and finally develops into a “tube”-like shape. This tube keeps its shape for a comparatively long time. At later time steps, the ejecta tube has increased in length and has developed a “neck”. A ring vortex caused by ejecta-atmosphere interaction is observed in the upper part of the ejecta cone. Furthermore, spallation fragments leaving the target surface at comparatively low velocities (denoted by red arrow) appear at later time steps.

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Figure 3. Typical evolution of the ejecta at different time steps. The first image shows a presumably hot central plume and the initial ejecta cone. The cone rapidly expands and steepens. At later time steps the cone transforms to a tube in which more and more large-sized spall plates become entrained.

The crater formation and the ejection process strongly depend on the water saturation level of the target material: Craters formed in wet sandstone targets have larger radii and volumes. In Figure 4 two craters from impacts on dry and water saturated sandstones are shown. The “dry” craters are characterized by (i) a white central depression consisting of highly crushed target material, (ii) an outer spallation zone and (iii) areas of incipient spallation where spall plates are partially detached from the target material but are still fixed. Craters formed in water-saturated sandstone targets (b)) are larger in diameter and volume and show a “terrace-shaped” spallation zone.

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Figure 4. Impact craters formed in dry (left) and wet (right) sandstone targets. Note the difference in scale bars.

Larger ejecta curtain angles (i.e. the angle between the ejecta cone and the target surface) are observed if the target is water-saturated. Figure 5 compares the ejecta evolution of two impact experiments: an impact on a dry sandstone target (upper half) and an impact on a sandstone target with a water-saturation level of about 50 % (lower half). The ejecta cone for the wet sandstone is straight and displays a larger angle to the target surface. Both the cone and the presumably hot ejecta plume move faster than in the dry target experiment. The luminous effect in the center of the ejecta cone is brighter for the dry sandstone compared to the wet sandstone. Ejecta-atmosphere interactions are different in both experiments. In the dry target experiment, turbulences affect the geometry of the curtain right from the beginning so that the ejecta could not develop the straight cone geometry. In the wet target experiments, vortices develop at a later stage but hardly change the straight cone shape.

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Figure 5. Comparison between ejecta clouds for impacts on dry (upper half) and wet (lower half) sandstone. The ejecta of the dry sandstone experiment appear brighter. The hot central ejecta plume of the wet target experiment is obscured by the darker and presumably cooler cone particles and is visible only in its uppermost part. The maximum ejection velocity of cone particles is slightly higher than that of the dry experiment. The central plume of the wet target experiment also has a higher ejection velocity than the plume of the dry experiment. Ambient pressure in the target chamber is 300 mbar.

Outlook:

A porosity study using highly porous tuff as well as low porous quartzite has recently been conducted and the data are analyzed currently. First results show similar (i.e. steep) ejecta cone angles of quartzite and wet sandstone. This leads to the assumption that porosity has a strong influence on ejection behavior: the reduction of pore space within the sandstone by means of interstitial water leads to ejecta cone angles comparable to those observed in target materials with low porosity.

 

Tobias Hoerth

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