Friedrich Hörz

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Thermal infrared spectroscopy of experimentally shocked anorthosite and pyroxenite: Implications for remote sensing of Mars

We performed shock recovery experiments at JSC (17-63 GPa) on samples of Stillwater pyroxenite and anorthosite and acquired their thermal infrared spectra (3-50 micron) to investigate the degradation of spectral features at high pressures. Additional information is contained in the original extended abstract.

Modification of amino acids at shock pressures of 3.5 to 32 GPa.

Amino acids were subjected to shock impact over a pressure range of 3.5 to 32 GPa both within and without meteoritic mineral matrices. The extent of amino acid destruction, racemization, and conversion to secondary amino acids was examined. Abundances of parent compounds decreased by a factor of 10(3) over this pressure range. Racemization also occurred, but some residual optical activity remained in the amino acids surviving shocks up to 32 GPa. Secondary amino acids formed in the high peak pressure range; those identified were beta-alanine, glycine, alanine, gamma-aminobutyric acid, and beta-aminoisobutyric acid. At 30 GPa, the abundances of these daughter compounds exceeded those of the remaining initial amino acids. As the concomitant effects of high mechanical stress and temperature accompanying shocks cannot be separated in this work, their relative contribution to the observed transformations cannot be estimated. The survival of amino acids in shock experiments suggests that, after formation or emplacement of amino acids in carbonaceous chondrite parent bodies, these objects never experienced impact velocities greater than 5 km/s, which suffices to generate 30 GPa for typical silicate/silicate impacts. These results also provide guidelines for choosing appropriate capture media for interplanetary dust particles on Earth-orbiting platforms.

Thermal infrared spectroscopy and modeling of experimentally shocked plagioclase feldspars

Abstract Thermal infrared emission and reflectance spectra (250-1400 cm-1; ~7-40 mm) of experimentally shocked albite- and anorthite-rich rocks (17-56 GPa) demonstrate that plagioclase feldspars exhibit characteristic degradations in spectral features with increasing pressure. New measurements of albite (Ab98) presented here display major spectral absorptions between 1000-1250 cm-1 (8-10 mm) (due to Si-O antisymmetric stretch motions of the silica tetrahedra) and weaker absorptions between 350-700 cm-1 (14-29 mm) (due to Si-O-Si octahedral bending vibrations). Many of these features persist to higher pressures compared to similar features in measurements of shocked anorthite, consistent with previous thermal infrared absorption studies of shocked feldspars. A transparency feature at 855 cm-1 (11.7 μm) observed in powdered albite spectra also degrades with increasing pressure, similar to the 830 cm-1 (12.0 μm) transparency feature in spectra of powders of shocked anorthite. Linear deconvolution models demonstrate that combinations of common mineral and glass spectra can replicate the spectra of shocked anorthite relatively well until shock pressures of 20-25 GPa, above which model errors increase substantially, coincident with the onset of diaplectic glass formation. Albite deconvolutions exhibit higher errors overall but do not change significantly with pressure, likely because certain clay minerals selected by the model exhibit absorption features similar to those in highly shocked albite. The implication for deconvolution of thermal infrared spectra of planetary surfaces (or laboratory spectra of samples) is that the use of highly shocked anorthite spectra in end-member libraries could be helpful in identifying highly shocked calcic plagioclase feldspars.

Dimensionality scaled penetration experiments: Aluminum targets and glass projectiles 50 μm to 3.2 MM in diameter

Summary Spherical soda-lime glass projectiles 50, 150, 1000 and 3175 μm in diameter (Dp) in aluminum targets (series 1100; “annealed”) of variable thickness T, were used to determine how the penetration-hole diameter (Dh) varied as a function of Dp/T at a constant impact velocity of 6 km/s. The target thickness ranged from infinite half-space geometries to 0.8 μm thick foils. Virtually identical morphologies characterize the penetration holes, no matter what projectile size, at equivalent Dp/T conditions. The relative hole diameter (Dh/Dp) decreases systematically with increasing Dp/T from Dh ≅ 4Dp for massive targets, to Dh = Dp for very thin foils. A modest dependence on the absolute projectile size is observed; comparatively small cracters, yet relatively large penetration holes are produced by the smallest (50 μm) impactors. Nevertheless, linear dimensional scaling seems suitable for first-order estimates of Dp from the measurement of Dh and T on space-exposed surfaces. The projectile fragments and the debris dislodged from the target were intercepted by witness plates that were located behind the target. The dispersion angle of this debris cloud depends on the thickness of the target. In addition, millimeter-sized impactors are collisionally fragmented with greater ease than small impactors. Furthermore, we observe that systematic changes in the specific energy of dislodged projectile and target material occur as a function of Dp/T. While linear scaling of target and projectile dimensions is a useful framework to explain many observations and associated shock processes, we suggest that consideration of the absolute and relative shock-pulse duration in the projectile (tp) and target (tt) may ultimately be the more useful approach. It implicitly accounts for all dimensions and, additionally, for specific impact velocities and pertinent material properties, via equations-of-state, for the impacting pair.

MORPHOLOGY OF METEOROID AND DEBRIS IMPACT CRATERS FORMED IN SOFT METAL TARGETS ON THE LDEF SATELLITE

Abstract We have measured the depths, average diameters and circularity indices of over 600 micrometeoroid and space debris impact craters formed in surfaces exposed to space aboard the Long Duration Exposure Facility satellite. The target surfaces had a variety of orientations and physical properties. The average depth-diameter ratio of craters formed in aluminum targets by nearly normal impacts is between 0.56 and 0.60, higher than the canonical and widely accepted value of 0.50 which corresponds to a hemispherical shape. The depth-diameter ratio does not change significantly with target Brinell hardness values between 40 and 90, or with average impact velocity above 5 km s−1. The depth-diameter ratio is found to vary as roughly the one-tenth power of target density. Less than 10% of the craters examined had major-to-minor axis ratios higher than 1.5, consistent with the production of shallow, elongated craters exclusively by grazing impacts. The variation in depth-diameter ratio for circular craters most likely results from variation in projectile shapes.

Craters in aluminum 1100 by soda-lime glass spheres at 1 to 7 km/s

To assist in the interpretation of crater populations on space-exposed surfaces from the Solar Maximum Satellite and the Long Duration Exposure Facility (LDEF), we conducted laboratory simulations of cosmic-dust impacts into aluminum 1100 targets with ∼3.2 mm diameter soda-lime projectiles at velocities (V) between 0.7 to 7 km/s. The resulting crater diameters (D c ) conform to current cratering equations, while crater depths (P c ) are somewhat deeper, yielding typical P c /D c ratios of ∼0.58 at V > 6 km/s. Similar values (P c /D c > 0.55) are found for craters in aluminum surfaces retrieved from the Long Duration Exposure Facility, which were produced at impact velocities as high as 20 km/s. The value of P c /D c = 0.5 that is being used by many should be abandoned ; a P c /D c = 0.58 is recommended. Similarly, we demonstrate that mass loss of the target, as determined by pre- and post-shot weight measurements, can be almost an order of magnitude less than mass losses that would be inferred from crater-volume measurements. This difference is due to substantial internal deformation of the target which permits large volumes of material to be deformed, and displaced without physical ejection and dislodgment. In addition, this study traces the behavior of the glass impactor. At V 2.5 km/s. The radially expanding melts develop, by geometric dispersion, into thin films that tear, leaving a discontinuous melt deposit upon cooling. At V > 5.5 km/s the entire impactor is essentially molten ; however, small quantities of unmelted residues still can be found at 7 km/s. Increasingly larger fractions of melt escape the crater between 5 and 7 km/s. Only modestly higher velocities would be needed to have the entire projectile escape the crater. This appears to be the case for ∼50% of all LDEF craters which lack detectable residues at sensitivity levels of Scanning Electron Microscopy - Energy Dispersive X-Ray analysis.

Transport and emplacement of crater and basin deposits

Material is ejected from impact craters in ballastic trajectories; it impacts first near the crater rim and then at progressively greater ranges. Ejecta from craters smaller than approximately 1 km is laid predominantly on top of the surrounding surface. With increasing crater size, however, more and more surrounding surface will be penetrated by secondary cratering action and these preexisting materials will be mixed with primary crater ejecta. Ejecta from large craters and especially basin forming events not only excavate preexisting, local materials, but also are capable of moving large amounts of material away from the crater. Thus mixing and lateral transport give rise to continuous deposits that contain materials from within and outside the primary crater. As a consequence ejecta of basins and large highland craters have eroded and mixed highland materials throughout geologic time and deposited them in depressions inside and between older crater structures.Because lunar mare surfaces contain few large craters, the mare regolith is built up by successive layers of predominantly primary ejecta. In contrast, the lunar highlands are dominated by the effects of large scale craters formed early in lunar history. These effects lead to thick fragmental deposits which are a mixture of primary crater material and local components. These deposits may also properly be named ‘regolith’ though the term has been traditionally applied only to the relatively thin fine grained surficial deposit on mare and highland terranes generated during the past few billion year. We believe that the surficial highland regolith - generated over long periods of time - rests on massive fragmental units that have been produced during the early lunar history.

Loss of radiogenic argon from shocked granitic clasts in suevite deposits from the Ries Crater

Five granitic clasts from the Otting and Aumuhle quarries in the suevite ejecta deposits beyond the rim of the Ries, Germany, impact crater have been characterized as to modal composition, degree of shock, and loss of radiogenic argon. The shock pressures experienced by this selected suite of samples range from 60 GPa, and the samples have been modestly shocked to heavily shocked, or even melted. Ages for these samples were determined by the 39Ar40Ar technique. The least shocked specimen (10–15 GPa) shows very little loss of radiogenic Ar relative to the approximately 320 My age of the Variscan basement at the Ries. All other samples (~28 GPa, ~42 GPa, ~52 GPa, and an impact melt) suggest complete to nearly complete loss of radiogenic Ar at the time of the 15 My old impact event. Most of the radiogenic Ar in these granitic samples is contained in feldspar, with a lesser amount in biotite. Diffusive loss of Ar occurs relatively easily from these highly disordered feldspars. The essentially complete loss of radiogenic Ar from 4 of the 5 samples is, in large part, ascribed to thermal activation during the cooling history of the suevite rather than during initial passage of the shock wave. These data are consistent with the conclusion from previous paleomagnetic studies that the suevite was deposited at temperatures above 500°C. The fact that Staudacheret al. (1982) found almost no loss of radiogenic Ar in a wide variety of Ries samples is ascribed to two causes: a) most of their samples were shocked to < 15 GPa and were collected in drill cores from the modestly shocked crater bottom, which was not as hot as the suevite; and b) their observations were primarily made on hornblende and biotite separates that are more resistant to shock effects and to diffusive loss of Ar, compared to shocked feldspars. Loss of argon from our suevite samples likely occurred in a “hot”, post-impact ejecta layer. From data presented here we estimate that < 50% of the material composing the “hot” Ries ejecta should show at least partially reset K-Ar ages; such materials compose no more than 5% of the total displaced crater volume.