46th Lunar and Planetary Science Conference (2015)
and T. Kenkmann1, 1Institute of Earth and Environmental Sciences – Geology (Albert-Ludwigs-Universität (ALU)
Freiburg, Alberstraße 23B, 79104 Freiburg, Germany, [email protected]).
Introduction: Shatter cones are the only known
macroscopic feature considered as evidence for shock
metamorphism. Conveniently identifiable in the field,
they play an important role for the discovery and verification of impact structures. Still the occurrence of
shatter cones is heterogeneous throughout the crater
record and evidently bound to material parameters and
physical boundary conditions.
Most authors align the formation of shatter cones
with the passage of the shock front or the release from
shock loading [1-3]. However, none of the existing
models provide a satisfying concept that explains all
relevant aspects of shatter cones, namely, their (i) conical to hyperbolic shape, (ii) the presence of diverging
striations and grooves, and (iii) their hierarchical bifurcation that leads to the horsetailing effect.
In our study we aim at a better qualitative and
quantitative understanding of the geometrical parameters of shatter cones. Here we present the findings of
shatter cone like features found in MEMIN cratering
experiments. In this MEMIN project we are attempting
to constrain the physical boundary conditions necessary for the formation of shatter cones.
Figure 1: Fragment recovered from the ejecta of a 20 cm
sized sandstone cube impacted by an aluminum projectile.
Figure 2: WLI scan with the Bruker AXS Contour GT-K0 of
the fragment shown in figure 1, note the curved fracture surface and fine diverging striae.
Methods: By hand picking and soft stimulated
fracturation of the crater’s subsurface we thouroughly
examine the ejecta of our experiments and the crater
itself. A morphometric analysis of each specimen is
carried out with a Bruker AXS Contour GT-K0 white
light interferometer (WLI). We use this optical noncontact, non-intrusive technique with nm-µm spatial
resolution for characterizing surface topographie, in
particular to measure (i) apical angles of master cones
and their sub-cone apices, (ii) the groove-ridge wavelength and amplitude of striated cone surfaces, (iii) the
bifurcation distance of fractures and (iv) the curvature
of cones.
Results: So far millimeter to half centimeter sized
cones had been recovered from the ejecta of sandstone
and limestone blocks impacted by aluminum and steel
projectiles. The 20 cm sized cubes of sandstone were
impacted by aluminum projectiles with the diameters of
5 and 2.5 mm, with impact velocities ranging from
6.97 to 7.75 km/s [4]. In addition an impact experiment
with a 20 cm sized limestone cube that was impacted
by a 2.5 mm steel projectile at 5.32 km/s was investigated.
We found fracture surfaces of distinctly conical geometry and slightly curved surfaces, marked by fine
striations (Figs.1, 2). SEM analysis of the recovered
fragments showed vesicular melt films alternating with
smooth polished surfaces [5]. The vesicular melt films
predominantly form at strain releasing steps and suggest that shatter cones are probably shear fractures or
mixed mode fractures. The fragments are found in the
material with intense grain crushing and porosity reduction.
46th Lunar and Planetary Science Conference (2015)
ues e.g. for the spikiness of a scale limited surface (curtosis). In the measured fracture surfaces we also observe a qualitative self-affinity comparing the surface
roughness from fractals of the primary surface.
Continuing Work: We will extending our search
for shatter cones in the MEMIN experiments into the
crater subsurface to better constrain the physical
boundary conditions for their formation. Whereas a
thorough morphometrical analysis of shatter cone
fragments found in the ejecta is carried out to quantitatively describe the hierarchical structure of shatter cone
patterns, the surface roughness and their structure on
different scales.
References: [1] Gash P.J.S. (1971) Nature Phys.
Sci., 230:32-35. [2] Baratoux D. and Melosh H.J.
(2003) Earth & Planet. Sci. Letters, 216:43-54. [3]
Sagy A. et al. (2004) JGR, 111: 1-20. [4] Poelchau
M.H. et al. (2013) Meteoritics & Planet. Sci., 48:8-22.
[5] Kenkmann T. et al. (2012) MetSoc 75, Abstract
Figure 3: WLI surface meassurement of a shatter cone in
micritic limestone from Steinheim crater and surface approximation of two shatter cone profiles taken perpendicular to
the cones axis with the software Origin 9.1.
Prelimenary results of the extracted shatter cone
surfaces show that the subparallel arrangement of longitudinal ridges and grooves of shatter cones can be
described by multi-harmonic or multi-frequency wave
analysis (Fig.3). The sinusoidal topology perpendicular
to the shatter cones symmetry axis can conveniently be
descibed by standardized parameters, enabling a more
sophisticated and quantitative description of the alternating groove striae wave patterns and may offer a critical test for further model development.
In comparison by scale-invariant parameters the recovered fragments show surface characteristics comparable to shatter cones e.g. from the Steinheim impact
crater. With the WLI method we can easily extract val-