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Results Project 2

Subsurface deformation in impacted Seeberger Sandstein

Introduction

Since most of the planetary surfaces are porous, a considerable number of impacts occur on porous target material such as sedimentary rocks on earth, or regolith on planetary bodies. Understanding the influence of pore space on impact mechanisms is hence essential for the investigation of natural impact phenomena in the solar system. The role of interstitial water or water ice in porous rocks, which is common in terrestrial and Martian target materials is not yet well understood. Several studies, including the investigation of natural impact craters, impact experiments, and numerical simulations have shown, that porosity and pore space saturation have a major influence on crater formation. Nevertheless, only sparse literature is concerned with impact cratering in solid and porous materials. Furthermore only parts of these experiments have been investigated with regard to sub-surface processes such as pore collapse, shock metamorphism, target damaging, shear localization, fracture propagation and dilatancy. To overcome this lack of information project 2 was founded in the framework of the MEMIN program.

Experiments and Results

A set of 18 hypervelocity impact experiments was performed at the acceleration facilities of Fraunhofer Ernst-Mach-Institut (EMI). For the study of the pore water influence on cratering mechanisms one unsaturated (“dry”) and one saturated (“wet”) sandstone block were investigated. The projectiles for these experiments were 2.5 mm steel spheres accelerated to 4.8 km/s and 5.3 km/s.

tl_files/fotos/Results/TP2/block_drill.jpg
Water saturated target block before the experiment.

tl_files/fotos/Results/TP2/Target_crater.png
Post impact crater cavity in the wet target experiment.

The targets were 20 cm cubes of Seeberger Sandstein. This quartz sandstone has a mean grain size of 100 ±25 µm and an overall porosity of 23 ± 1 vol. %. In the wet target experiment a maximum saturation of 91 % was achieved.  

tl_files/fotos/Results/TP2/Lithology.png
Seeberger Sandstein
a) BSE micrograph b) Mineral composition

Different levels of damage correspond to the appearance of different deformation modes in the rock. In a detailed BSE image analysis for the “dry” experiment, we distinguished and mapped four distinct zones, characterized by different deformation microstructures (c, d, e, f).

tl_files/fotos/Results/TP2/Profile_dry.png
a) Crosssection through the dry experiment. Deformation zones in color. b) Plane view of the crater c-f) Micrographs of the deformed zones.

In the microscopic analysis of the wet target experiment we have identified three different zones of characteristic deformation. The main structural difference between the dry and the wet target experiment is that the zone of pervasive grain crushing and compaction is missing in the wet block (c, d, e).

tl_files/fotos/Results/TP2/Profile_wet.png
a) Crosssection through the wet experiment. Deformation zones in color. b) Plane view of the crater c-e) Micrographs of the deformed zones.

The deformation zones were named and described as follows: 

I. Zone of Tensile Failure. (yellow). The microstructure is dominated by pervasive grain crushing. The Grains are disaggregated or crushed forming fine fragments. This fabric is cut by larger tensile fractures. The open tensile fractures are larger in the wet experiment than in the dry experiment.
II. Zone of Pervasive Grain Crushing and Compaction (orange). The structure is characterized by pervasive grain crushing and by a nearly complete loss of porosity. Note that this zone is missing in the wet experiment.
III. Zone of Localized Deformation (red). Characterized by areas of localized and intense deformation are enclosed by mostly intact parts of the sandstone. The overall porosity is slightly reduced due to the alternation between compacted and uncompacted domains.
IV. Zone of Incipient Spallation (magenta). Characterized by intragranular fractures and not ejected spall plates. The fragments are usually tabular. The spall fractures did not fully propagate to the free surface.

 

A determination of the pre-impact porosity and the post-impact porosity was carried out applying quantitative image analysis. The post impact porosity varies with distance to the crater floor. The area with the lowest porosity is the zone of pervasive grain crushing and compaction. Above this zone the porosity is enhanced by the tensile fractures. Below the zone of compaction the porosity gradually approaches the porosity of the starting material.

tl_files/fotos/Results/TP2/Pore_space_image.png
Porosity (red) determined by image analysis a) Zone of pervasive grain crushing and compaction b) Undeformed sandstone.

tl_files/fotos/Results/TP2/Porosity.png
Porosity with increasing distance from the impact point spource 


During microstructural mapping, conspicuous fractures have been recognized. To analyze these ca. 1800 fractures were mapped per experiment. To measure the difference between the true fracture orientation and the radial direction angular deviations were determined. The impact point source was used as reference system. The analyses reveal for both experiments a preferred orientation of cracks in a radial direction from the impact point source.

tl_files/fotos/Results/TP2/Histogram.png
a) Concussion fractures b) Angular deviation of mapped fractures

Discussion

The presence of pore water in impact experiments on sandstone has shown to have a major influence on the crater topography. Our experiments have shown a decrease of the depth-to-diameter ratios from the dry case to the wet case. Zones of increasing deformation could be defined with increasing proximity to the point of impact. The most severe target deformation in both experiments is found directly at the central crater floor.

Overview of experimental parameters and resulting volumes.

Parameter

Exp. A6-5126
Dry sandstone

Exp. A11-5181
Wet sandstone

Grain size [µm]

70-125

~70-125

Porosity [%]

23 ± 1

23 ± 1

Water saturation

Dry

Ca 90%

Projectile mass [g]

0.0671

0.0670

Velocity [km/s]

4.8

5.3

Kinetic energy [J]

773

941

Crater diameter [mm]

57.6

101.6

Crater depth [mm]

11.0

14.3

Crater volume [mm³]

7600

31100

Volume zone A1 [mm³]/[% crater vol.]

107/1.4

608/2

Volume zone B2 [mm³]/[% crater vol.]

659/9

-/-

Volume zone C3 [mm³]/[% crater vol.]

1209/16

1960/6

Max. thickness zone A1 [mm]/[dp]

0.9/0.36

1.9/0.76

Max. thickness zone B2 [mm]/[dp]

3.4/1.36

-/-

Max. thickness zone C3 [mm]/[dp]

2.4/0.96

6.0/2.4

1Zone of Tensile Failure, ²Zone of Pervasive Grain Crushing and Compaction, ³Zone of Localized Deformation

The development of the deformation microstructures with increasing strain is illustrated schematically in the figure:

tl_files/fotos/Results/TP2/Model.png
Development of deformation with increasing strain

The onset of deformation is characterized by grain boundary break-up along the phyllosilicate cementation. First fractures occur along force chains of quartz grains. With increasing strain, grains are crushed and the initial pore space collapses. New fractures occur along the contacts and bands of strong deformation form along the initial force chain. At even higher strain these bands gain width and finally coalesce to a complete zone of pervasive grain crushing and compaction.

Interestingly the degree of comminution and pore space collapse in the proximity to the crater floor is higher in the dry than in the wet experiment. The zone of pervasive grain crushing and comminution described for the dry experiment has not been recognized in the wet experiment. Supposably the pore water effectively prevented intensive pore collapse and grain comminution. Additionally the zone of localized deformation constitutes a significantly larger volume of deformed material in the wet target experiment. This also suggests that shock wave attenuation was more effective in the dry experiment. However, in the investigated ejecta a higher fraction of very fine-grained fragments was found for the wet experiments. This would imply that grain comminution was enhanced in comparison to the dry experiment, but all of the fine fragmented debris was excavated and thus cannot be seen in the sub-surface. A structural cause for a more effective ejection of comminuted material in the wet target experiment is possibly volumetric expansion of interstitial water upon pressure release.

The zone of tensile fracturing beneath the central crater can be attributed to the pressure release in a later deformation stage.

tl_files/fotos/Results/TP2/Tensile_cracks.png
Tensile fracture in comminuted sandstone

Likewise, the fractures at the crater rims are a consequence of spallation. Their geometries differ between the dry and the wet experiment. All spall fractures emerge from the crater walls, propagate outward and at least one branch reaches the target surface. In contrast to the dry experiment, the spallation fractures in the wet target show bifurcation. The secondary branches tend to propagate into the target block.

tl_files/fotos/Results/TP2/Crater_profile.png
Fractures in the wet experiment compared with fractures found by Polanskey & Ahrens 1990

Conclusions

The motivation of this study is a better understanding of deformation mechanisms in porous solid rock during hypervelocity impacts. Different modes of brittle deformation have been recognized and mapped for both cases. These differences are in good agreement with planar shock recovery experiments on dry and wet sandstone (Hiltl et al. 2000). The presence of water inhibits pore space collapse and comminution, at least in the craters sub-surface, and thereby causes decreased shock wave attenuation. This phenomenon results in the absence of a zone of complete compaction and a greater extent of the zone of localized deformation in the wet target. The development of concussion fractures is unaffected by the presence of water, as they are ubiquitous in both wet and dry target rocks. Their radial orientation is consistent with their supposed development as tensional Mode I fractures.

Elmar Buhl

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