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Results Project 6

Crater structure and excavation: The effects of projectile velocity, projectile mass and target porosity

The project

Since it is rarely possible to observe fresh natural impact craters on Earth, the examination of experimental craters and their ejected material provide useful insights into the cratering process. Most craters have differences and show distinct characteristics that can be correlated for instance to the properties of the target. Some rocks are porous, show a series of different layers or may contain groundwater. These properties define the magnitude and attenuation of the shock wave and therefor influence the cratering process. In this project we produce experimental impacts under changing parameters to determine their influence on cratering and on the ejection process.

As a substitute for meteorites we use spheres of steel and of iron meteorites (Campo del Cielo meteorite). Accelerated to high velocity they impact into a block of sandstone. The parameters changed are projectile mass, projectile velocity and water-saturation of the target:

  • The projectile mass is increased from 0,067 over 4,1 to 7,3 g,

  • the projectile velocity is increased from 2,5 over 3,5 to 4,6 km s-1,

  • and we change the water-saturation of the sandstone to 50 and 90%.

An important tool for our examinations is the ejecta catcher who preserves the ejected material in situ. The following image shows the installation of a catcher in the targets chamber.


After the shot the catcher is carefully removed for further investigations.


Experiments and natural craters

Analyzing the results from different-sized experiments indicates that we can draw conclusions for natural impacts. Comparing small-scale experiments, explosions in 10 to hundred meters scale, and natural craters is possible if the parameters for mass, energy, pressure and velocity are correlated with mathematical formulas. The figure below, for example, describes the ratio between the total ejecta mass of an impact to the mass of the largest ejected block of material on different scales.


The ejection process

The combined application of high-speed cameras and ejecta-catcher systems made it possible to analyze the ejection process and to classify four stages. In the first stage a fireball surrounded by a plume of dusty material can be observed immediately followed by a cone of fast moving fine grained particles as a second stage (a). In the third this cone is accompanied by turbulences or ring-winds forming a kink (b). In fourth stage the cone is replaced by a tube of coarse particles and spall pieces (c).


In every experiment this sequence is modified through the changing parameters for projectile mass, projectile velocity and water saturation. The figure below displays the different results for the lowest imprint angles in experiments.


The parameter studies indicate a correlation between projectile velocity, target properties and ejecta angle. With increasing velocity the ejecta angle decreases whereas with increasing water saturation the ejecta angle increases. The comparisons with data from related studies approve these results. We can observe this behavior also on planetary scale: on dry planetary bodies like Earth and Mars we find low ejecta angles and on icy bodies like Ganymede and Europa we find steep angles for the crater ejecta.


With the help of tracer particles it is possible to get more into detail. The following image shows the tracer paint on the expected impact point before the shot.


With increasing distance to the impact point we observe an increasing ejecta angle for dry targets, while on water-saturated targets we get a fan-like ejecta pattern of widening particle bands (see figure below).


Microscopic analysis of samples from the catcher reveals more details about the ejecta mechanics. In the figure below we can observe that particles found near the center of the catcher ("inner zone") display a stronger comminution than particles found in the "outer zones". The fine particles are crushed quartz grains. Position and the degree of comminution of the fine grained particles are typical for material from the plume stage, whereas the later material from the cone stage consists of mostly undamaged grains. Comparing the images of dry target experiments and water-saturated targets it can be seen that material from dry targets is quite uniform in size whereas the material of the wet target is more mixed.


The reason for this change in behavior can be found in the altered properties of the water-saturated sandstone:

  • The yield strength of the material is reduced.

  • The impedance contrast between the quartz grains and the pore space is reduced.

  • Under the conditions of an impact water can vaporize producing an explosive effect. Vapor has 1600x the volume of water.

Under the scanning electron microscope melting and high pressure deformations can be observed giving us information about the pressure regime during the impact. While shear fractures indicate pressures of around 3.5 GPa we need 50 GPa to achieve molten quartz.


If we combine our results with data of related MEMIN projects we can draw a first detailed picture of the ejection process. In the figure below the ejection stages for an experiment on dry sandstone are displayed combined with an approximated curve of the pressure in relation to the distance from the impact point. The reconstruction of the energy flow during the excavation is part of future research.


Frank Sommer

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