Peak Shock Pressure Research Articles

Shock petrographic and numerical modeling constraints on the morphology and size of the Morokweng impact structure, South Africa

AbstractThe 369 m deep M4 drill hole, located ~18 km NNW of the center of the 146 Ma Morokweng impact structure (MIS), intersects shocked Archean granitoid gneisses and subsidiary dolerite intrusions that are cut by faults, cataclasites and mm‐ to m‐wide suevitic and pseudotachylitic breccia dikes. The shock features in quartz in the gneisses and breccia dikes include decorated planar deformation features (PDFs), planar fractures, feather features, and toasting. Other minerals show features that may be shock‐related, such as multiple sets of planar features and alternate twin ladder structures in feldspars, kink bands in biotite, and planar features in titanite, apatite, and zircon; however, these are variably annealed and/or overprinted by hydrothermal alteration effects, and confirmation of their origin awaits further study. Universal Stage measurements of PDF sets in quartz from 12 gneissic target rocks and from lithic and mineral clasts in three suevitic and three pseudotachylitic breccia dikes reveal four dominant sets: (0001), {}, {} and {}. Based on these observations, the average peak shock pressure in these rocks is estimated at ≤16 GPa, which supports the original proximity (within one transient cavity radius) of these rocks to the point of impact. No discernible depth‐dependent shock attenuation was noted in the core. These shock levels and the elevated structural position of the rocks in the M4 core relative to the impact melt sheet intersected in drill holes closer to the center of the MIS suggest that the M4 lithologies represent part of the parautochthonous peak ring volume that subsequently experienced 1.5–2 km of post‐impact erosion before it was buried beneath younger sediments. Numerical modeling using the iSALE‐2D code suggests that the original Morokweng crater had a rim‐to‐rim diameter of between 70 and 80 km, and that the rocks in the M4 core were originally located at a depth of 7–8 km and a radial distance of 8–9 km from the point of impact.

Determining Peak Shock Pressure and Cratering due to Hypervelocity Meteoroid Impact on Lunar Structures for Engineering Design

Determining Peak Shock Pressure and Cratering due to Hypervelocity Meteoroid Impact on Lunar Structures for Engineering Design

Shock‐induced pervasive remelting of Fe sulfides in the basaltic shergottite Northwest Africa 14672: A benchmark for shock stages S6/S7 on Mars

AbstractNorthwest Africa (NWA) 14672, the most highly shocked Martian meteorite so far, has experienced >50% melting, compatible with peak pressure >~65 Gpa, at a transition stage 6/7. Despite these extreme shock conditions, the meteorite still preserves a population of “large” Fe sulfide blebs from the pre‐shock igneous assemblage. These primary blebs preserve characteristics of basaltic shergottites in term of modal abundance, preferential occurrence in interstitial pores along with late‐crystallized phases (ilmenite, merrillite), and Ni‐free pyrrhotite compositions. Primary sulfides underwent widespread shock‐induced remelting, as indicated by perfect spherical morphologies when embedded in fine‐grained silicate melt zones and a wealth of mineral/glass/vesicle inclusions. Extensive melting of Fe‐sulfides is consistent with the decompression path experienced by NWA 14672 after the peak shock pressure at ~70 GPa. Primary sulfides acted as preferential sites for nucleation of vesicles of all sizes which helped sulfur degassing during decompression, leading to partial resorption of Fe‐sulfide blebs and reequilibration of pyrrhotite metal/sulfur ratios (0.96–0.98) toward the low oxygen fugacity conditions indicated by Fe‐Ti oxides hosted in fine‐grained materials. The extreme shock intensity also provided suitable conditions for widespread in situ redistribution of igneous sulfur as micrometric globules concentrated in glassy portions of fine‐grained lithologies. These globules exsolved early on quenching, allowing dendritic skeletal Fe‐Ti oxide overgrowths to nucleate on sulfides.

High-pressure minerals and new lunar mineral changesite-(Y) in Chang’e-5 regolith

Forty-five years after the Apollo and Luna missions, China’s Chang’e-5 (CE-5) mission collected ∼1.73 kg of new lunar materials from one of the youngest basalt units on the Moon. The CE-5 lunar samples provide opportunities to address some key scientific questions related to the Moon, including the discovery of high-pressure silica polymorphs (seifertite and stishovite) and a new lunar mineral, changesite-(Y). Seifertite was found to be coexist with stishovite in a silica fragment from CE-5 lunar regolith. This is the first confirmed seifertite in returned lunar samples. Seifertite has two space group symmetries (Pnc2 and Pbcn) and formed from an α-cristobalite-like phase during “cold” compression during a shock event. The aftershock heating process changes some seifertite to stishovite. Thus, this silica fragment records different stages of an impact process, and the peak shock pressure is estimated to be ∼11 to 40 GPa, which is much lower than the pressure condition for coexistence of seifertite and stishovite on the phase diagram. Changesite-(Y), with ideal formula (Ca8Y)□Fe2+(PO4)7 (where □ denotes a vacancy) is the first new lunar mineral to be discovered in CE-5 regolith samples. This newly identified phosphate mineral is in the form of columnar crystals and was found in CE-5 basalt fragments. It contains high concentrations of Y and rare earth elements (REE), reaching up to ∼14 wt. % (Y,REE)2O3. The occurrence of changesite-(Y) marks the late-stage fractional crystallization processes of CE-5 basalts combined with silicate liquid immiscibility. These new findings demonstrate the significance of studies on high-pressure minerals in lunar materials and the special nature of lunar magmatic evolution.

Variations in lunar regolith properties with depth as revealed by Chang'e-5 samples

The lunar regolith layer preserves abundant information on the internal properties of the Moon as well as the space-surface interactions. By gardening and overturning through time, the regolith layer is heterogenous and soils from different depths could record the complex evolution history of the Moon. Chang'e-5 (CE-5) successfully returned scooped samples and drilled samples from the northeastern Oceanus Procellarum of the Moon. In this study, one scooped sample (0–3 cm depth, S0) and two drilled samples (~10 cm depth, S10; and ~ 65 cm depth, S65) were employed to analyze their granulometry and mineralogy by micro Raman spectroscopy. The grain size increases with depth, and the median grain size of S0, S10, and S65 are Φ5.45 (30.2 μm), Φ4.47 (45.0 μm), and Φ3.37 (96.4 μm), respectively. Their mineral types and contents are pyroxene (33.4–40.5 vol%), plagioclase (30.4–33.8 vol%), olivine (9.0–13.1 vol%), glass (6.9–10.7 vol%), FeTi oxide (6.7–7.5 vol%), cristobalite (0.2–3.8 vol%), quartz (0.4–0.9 vol%), and phosphate (0.2–0.6 vol%). The size-dependent mineral modes reveal the relationship between grain size and mineralogy. The mineral modal abundances of 3 soil samples at different depths are roughly similar to that of CE-5 basalts. Raman peak position of quartz may record a peak shock pressure of 31.4 GPa. 5–9 vol% plagioclase amorphization reflects that at least 2–9% exogenous materials in CE-5 soil samples are ejected from craters with diameters larger than 3 km located beyond 50 km from the landing site.

Vibrational spectroscopic, crystallographic, and microscopic investigation on the Aioun El Atrouss diogenite

AbstractDiogenites are thought to have originated from asteroid 4 Vesta asteroid, and it has been previously shown that they mostly consist of orthopyroxene, chromite, and olivine minerals. In this study, the Aioun El Atrouss (AAT‐1) diogenite was analyzed by vibrational spectroscopic (FT‐IR and micro‐Raman), EDXRF, and XRD techniques. AAT‐001 is mainly composed of pyroxene and with lesser olivine. XRD investigation confirmed magnetite, kamacite, ilmenite, and albite. A troilite‐orthopyroxene intergrowth was revealed by XRF. The olivine composition was estimated to be approximately Fo66 by Raman olivine characteristic doublets. Pyroxenes in AAT‐01 were found to be En80Fs20. Finally, approximate peak shock pressure of more than 86 GPa for AAT‐1 was suggested based on previous hypervelocity impact experiments on olivine.

High-pressure shock compression of olivine: Dynamic pulverization and frictional melting

In order to study seismo-mechanochemical processes, two kinds of shock compression experiments – the recovery and the in-situ VISAR (velocity interferometer system for any reflector) experiments – were performed, using a single crystal of forsteritic olivine and a single-stage powder propellant gun. The generated peak shock pressures were 31.1 GPa for both the recovery and the VISAR experiments. Many shear planes were generated in the olivine crystal. The VISAR experiment revealed detailed time profiles for the particle and shock wave velocities during the compression and yielded the Hugoniot elastic limit (HEL) of the olivine to be 7.44 GPa. We observed, in one recovered sample, a clear local melting occurring along a shear plane. The microstructural observations of the shear plane and its walls using a field-emission scanning electron microscopy (SEM) and an analytical transmission electron microscopy (ATEM) revealed that plastically-deformed olivine was pulverized into a few hundred-nanometer size particle assemblages within a few-micrometers narrow zone along the shear plane and, locally have partially melted. Moreover, injection veins filled with olivine melt were found in the wall of the same shear plane. It was shown from the estimate of the critical resolved shear stress (CRSS) along the shear planes and the inferred slip velocity using the Hugoniot elastic limit (HEL) data that the power generated by the friction was large enough for melting to occur in the olivine. The whole process occurred in a time scale <0.6 microsecond. Pulverization (comminution) before melting is considered to be a common phenomenon in the formation of pseudotachylytes and in dynamically expanding seismic faults upon earthquakes.

Formation and shock impact history of the Csatalja ordinary chondrite

AbstractThe analysis of the Csatalja H4 chondrite (which was found in August 2012) suggests shock‐related textures and spatial inhomogeneities, indicating a complex geological history. In the most heavily fractured and sheared units, small opaque grains and older fractures have locally enhanced the shock effect, producing melt. While the impact textures were evident in most units of the meteorite, mechanical shearing is apparent in only two units, suggesting that these units might have been present at somewhat different locations inside the parent body. Shearing also occurred at the border of the so‐called xenolith unit, confirming its mechanical mixing with the other units. Besides fragmentation and melting, chemical changes due to impact have also been identified, producing compositional homogenization of olivines in 30% of the investigated area of the sample's thin section (23 mm2), and moderate accumulation of Fe, Ca, and Na in the strongly shocked zones, initiating crystallization of feldspar in veins with a specific spatial distribution (feldspar glass with metal–sulfide globules). Analyzing the high P–T minerals, the peak shock pressure and temperature values differed substantially in the various units, ranging between 2 and 17 GPa, 100 and &gt;1200 °C. The xenolith unit crystallized more slowly after the impact event and does not show shock impact alterations, suggesting that it was formed in a deeper region of the parent body. This was later shifted to its current surroundings and was lithified (fixed) to the rest of the sample. This “randomly selected” Csatalja sample provides information on the range of the formation temperatures, pressures, and processes that contributed to the heterogeneity of meteorites at the mm spatial scale, in general. The identified heterogeneity is a result not purely of the shock effects but also of the different pre‐shock structural characteristics. The shock also mixed fragments mechanically that have been formed at different environments, with at least several dozens or even 100 m depth in the parent body.

Quantification of Impact‐Induced Melt Production in Numerical Modeling Revisited

AbstractMelting and vaporization of rocks in impact cratering is mostly attributed to be a consequence of shock compression. However, other mechanism such as plastic work and decompression by structural uplift also contribute to melt production. In this study we expand the commonly used method to determine shock‐induced melting in numerical models from the peak shock pressure by a new approach to account for additional heating due plastic work and internal friction. We compare our new approach with the straight‐forward method to simply quantify melting from the temperature relative to the solidus temperature at any arbitrary point in time in the course of crater formation. This much simpler method does account for plastic work but suffers from reduced accuracy due to numerical diffusion inherent to ongoing advection in impact crater formation models. We demonstrate that our new approach is more accurate than previous methods in particular for quantitative determination of impact melt distribution in final crater structures. In addition, we assess the contribution of plastic work to the overall melt volume and find, that melting is dominated by plastic work for impacts at velocities smaller than 7.5–12.5 km/s in rocks, depending on the material strength. At higher impact velocities shock compression is the dominating mechanism for melting. Here, the conventional peak shock pressure method provides similar results compared with our new model. Our method serves as a powerful tool to accurately determine impact‐induced heating in particular at relatively low‐velocity impacts.

New Occurrence of Seifertite and Stishovite in Chang’E‐5 Regolith

AbstractHigh‐pressure minerals in impact ejecta quantify shock conditions and provide clues for provenance analysis. The sampling site of Chang’E‐5 may contain distal ejecta, which is critical for constraining the impact events and interpreting the constitution of the target geological units. Here, we report a silica fragment consisting of seifertite, stishovite, α‐cristobalite‐like phase, and silica glass in Chang’E‐5 regolith. Both seifertite and stishovite formed via solid‐state transformation mechanism, in which seifertite formed by transition from α‐cristobalite upon pressure loading whereas stishovite may form from seifertite when postshock temperature was significant during pressure release. The coexistence of seifertite and stishovite suggests that their host rock has experienced a peak shock pressure of 11–40 GPa and records different stages of the impact process. Crater size calculation revealed that the host rock of stishovite and seifertite could have been transported from distant craters, confirming the retention of distal ejecta in Chang’E‐5 landing site.

“False peak” creation in the Flynn Creek marine target impact crater

AbstractImpacts into marine targets are known to create abnormal crater morphologies. We investigate the formation of the ~4 km diameter Flynn Creek marine target impact crater using the iSALE hydrocode. We compare simulation results to topographic profiles, mineral pressure indicators, and breccia sequencing from drill cores to determine the most likely sea depth at this location at the time of impact (~360 Ma, Tennessee, USA): 700–800 m. Both the peak shock pressure produced by the impact and the mechanism(s) of central peak formation differ with sea depth. The large central mound of Flynn Creek could have been produced in three distinct ways, all requiring the presence of an ocean: (1) a relatively cohesive rim collapse deposit that reached the crater center as part of a ground flow and came to rest on top of the existing crater stratigraphy; (2) chaotic resurge of ejecta with the returning ocean that deposited at the crater center; (3) large uplift facilitated by the removal of overburden pressure from a deep ocean. The first two of these mechanisms create “false peaks” in which high‐shock uplifted material and original crater floor are buried beneath &gt; 200 m of relatively low shock material. Our simulations suggest that drilling of marine impact sites might require deeper than expected drill cores, so that any high‐pressure mineralogical indictors at depth can be accessed.

Shock response of pre-existing spall damage in copper

We perform molecular dynamics simulations to investigate second-shock-induced recompaction and the subsequent re-spallation process in Cu with pre-existing spall damage. Compared with the conventional spalling of pristine Cu free of damage, a clear disturbance is observed in the free-surface velocity under the second shock of a pre-spalled Cu sample. This is due to the complex interactions of shock waves during the collapse of existing voids. Full recompaction accompanied by local melting of residual damage is also observed in our second-shock simulations. Secondary spallation occurs after the recompaction process, and the effect of secondary loading conditions, including different values of the peak shock pressure and different flat-top widths of the shock wave, on second spallation is taken into account. The findings clearly show that the spall strength of pre-spalled Cu is lower than that of pristine Cu under the same loading conditions when the loading time is sufficiently long. This result is evidence of the locally disordered state of the recovered sample. Re-solidification accompanied by atomic rearrangement is observed in the disordered region after second-shock compression.

Shock‐metamorphic microstructures in quartz grains from Albian sandstones from the Tin Bider impact structure, Algeria

AbstractTin Bider is a 6‐km‐diameter complex impact structure, the largest one recognized in Algeria. The crater was excavated in Cretaceous sedimentary rocks composed of, from the base to the top, Albian sandstones, Cenomanian clays, Cenomanian‐Turonian limestones, undifferentiated Coniacian to Maastrichtian clays and limestones. The age of the impact event is poorly constrained to &lt;66 Ma by stratigraphy, the youngest geological unit affected by the event being the ~66 Myr old Maastrichtian limestones. Albian sandstones outcrop in the central sector of the structure and represent the only occurrence at outcrop of this geological unit in the structure. Here we report on a detailed petrographic analysis of eight Albian sandstone samples that were searched for shock‐metamorphic features. We confirm the presence of rare shocked quartz grains with planar deformation features (PDFs) and report on their crystallographic orientations as determined using the universal stage microscope. PDFs oriented parallel to the π{} and ω{} orientations are the most abundant ones. For the first time in impactites from Tin Bider, PDFs with basal (0001) orientation, corresponding to amorphized mechanical Brazil twins, are reported. Our results indicate that locally the peak shock pressure was of at least 20 GPa, but much lower in average for the investigated samples.

Stress and strain during shock metamorphism

Shock metamorphism is the process by which rocks, and the minerals within them, transform during the passage of a shock wave. Shock effects, such as shatter cones, are critical for the identification of impact structures and have commonly been used to infer mechanical and structural information on the cratering process. To make these inferences, the mechanics of shock wave behavior must be understood. Here, we use numerical simulations to demonstrate how stresses act during shock metamorphism across all regions of the target that experience solid-state shock metamorphism. Furthermore, our numerical simulations predict the strains that are produced as a consequence of those stresses. The results show that the magnitude and orientation of stress and strain during shock metamorphism are variable throughout the target, both spatially and temporally, even between rocks that experience the same peak shock pressure. The provenance of a sample relative to the point of impact and the timing/mechanism of the formation of a shock effect must both be considered when making structural interpretations. The stress and strain magnitudes and orientations as functions of time and location presented in this study provide the constraints that enable a greater understanding of the formation of a variety of shock deformation effects. We demonstrate this with a case study of shatter cones at the Gosses Bluff impact structure. Our results provide a useful guide to assist geologists and petrologists in the interpretation of shock metamorphic effects.

Occurrence of tuite and ahrensite in Zagami and their significance for shock-histories recorded in martian meteorites

AbstractWe report on the discovery of two high-pressure minerals, tuite and ahrensite, located in two small shock-induced melt pockets (SIMP 1 and 2) in the Zagami martian meteorite, coexisting with granular and acicular stishovite and seifertite. Tuite identified in this study has two formation pathways: decomposition of apatite and transformation of merrillite under high-P-T conditions. Chlorine-bearing products, presumably derived from the decomposition of apatite, are concentrated along the grain boundaries of tuite grains. Nanocrystalline ahrensite in the pyroxene clast in SIMP 2 is likely to be a decomposition product of pigeonite under high-P-T conditions by a solid-state transformation mechanism. The pressure and temperature conditions estimated from the high-pressure minerals in the shock-induced melt pockets are ~12–22 GPa and ~1100–1500 °C, respectively, although previous estimates of peak shock pressure are higher. This discrepancy probably represents the shift of kinetic relative to thermodynamic phase boundaries, in particular the comparatively small region that we examine here, rather than a principal disagreement between the peak shock conditions.

Structure Analysis of Natural Wangdaodeite—LiNbO3-Type FeTiO3

This paper reports the first structure refinement of natural wangdaodeite, LiNbO3-type FeTiO3 from the Ries impact structure. Wangdaodeite occurs together with recrystallized ilmenite clasts in shock melt veins which have experienced peak shock pressures of between 17 and 22 GPa. Comparison of natural and synthetic wangdaodeite points toward a correlation between the distortion of ferrate- and titanate-polyhedra and the c/a ratio of the unit cell. The Raman spectrum of wangdaodeite is calculated based on the refined structure. Comparison to the reported spectrum of the type-material shows that the Raman peak at 738–740 cm−1 is indicative for this phase, whereas other features in type-wangdaodeite are tentatively assigned to disordered ilmenite.

Characterization of shocked quartz grains from Chicxulub peak ring granites and shock pressure estimates

AbstractPlanar deformation features (PDFs) in quartz are a commonly used and well‐documented indicator of shock metamorphism in terrestrial rocks. The measurement of PDF orientations provides constraints on the shock pressure experienced by a rock sample. A total of 963 PDF sets were measured in 352 quartz grains in 11 granite samples from the basement of the Chicxulub impact structure’s peak ring (IODP‐ICDP Expedition 364 drill core), with the aim to quantify the shock pressure distribution and a possible decay of the recorded shock pressure with depth, in the attempt to better constrain shock wave propagation and attenuation within a peak ring. The investigated quartz grains are highly shocked (99.8% are shocked), with an average of 2.8 PDF sets per grain; this is significantly higher than in all previously investigated drill cores recovered from Chicxulub and also for most K‐Pg boundary samples (for which shocked quartz data are available). PDF orientations are roughly homogenous from a sample to another sample and mainly parallel to {103} and {104} orientations (these two orientations representing on average 68.6% of the total), then to {102} orientation, known to form at higher shock pressure. Our shock pressure estimates are within a narrow range, between ~16 and 18 GPa, with a slight shock attenuation with increasing depth in the drill core. The relatively high shock pressure estimates, coupled with the rare occurrence of basal PDFs, i.e., parallel to the (0001) orientation, suggest that the granite basement in the peak ring could be one of the sources of the shocked quartz grains found in the most distal K‐Pg boundary sites.

Assessing the survivability of biomarkers within terrestrial material impacting the lunar surface

The history of organic and biological markers (biomarkers) on the Earth is effectively non-existent in the geological record >3.8 Ga ago. Here, we investigate the potential for terrestrial material (i.e., terrestrial meteorites) to be transferred to the Moon by a large impact on Earth and subsequently survive impact with the lunar surface, using the iSALE shock physics code. Three-dimensional impact simulations show that a typical basin-forming impact on Earth can eject solid fragments equivalent to ~10−3 of an impactor mass at speeds sufficient to transfer from Earth to the Moon. Previous modelling of meteorite survivability has relied heavily upon the assumption that peak-shock pressures can be used as a proxy for gauging survival of projectiles and their possible biomarker constituents. Here, we show the importance of considering both pressure and temperature within the projectile, and the inclusion of both shock and shear heating, in assessing biomarker survival. Assuming that they survive launch from Earth, we show that some biomarker molecules within terrestrial meteorites are likely to survive impact with the Moon, especially at the lower end of the range of typical impact velocities for terrestrial meteorites (2.5 km s−1). The survival of larger biomarkers (e.g., microfossils) is also assessed, and we find limited, but significant, survival for low impact velocity and high target porosity scenarios. Thermal degradation of biomarkers shortly after impact depends heavily upon where the projectile material lands, whether it is buried or remains on the surface, and the related cooling timescales. Comparing sandstone and limestone projectiles shows similar temperature and pressure profiles for the same impact velocities, with limestone providing slightly more favourable conditions for biomarker survival.

Effect of impact shock on extremophilic Halomonas gomseoemensis EP-3 isolated from hypersaline sulphated lake Laguna de Peña Hueca, Spain

The geologic histories of planetary surfaces reveal that Earth and Mars have been pummeled by cataclysmic impact events. The surface of Mars has been perused to have an impact origin for its hemispheric dichotomy. The spallation during impact events causes the interplanetary transfer of material from Mars to Earth or Mars to Phobos/Deimos. Assessing the survival of micro-organisms in impact conditions is critical for the development of planetary protection strategies for future missions. Shock waves are generated during such major impact events. The objective of the present investigation was to explore the microbial diversity of the hypersaline sulphated Laguna de Peña Hueca, Spain and to study the effect of shock waves on extremophilic bacteria isolated from the lake. Peña Hueca is a hypersaline sulphated lagoon rich in Mg–Na–SO4–Cl, epsomite and hexahydrate and it potentially serves as Planetary field analogue site for Martian chloride deposits and salt-rich subsurface brines of Ocean worlds like Enceladus and Europa. The microbial community structure of the lagoon was studied by 16S rRNA metagenomic sequencing. The phylogenetic studies indicated the presence of phyla Euryarchaeota, Proteobacteria, and Bacteroides in the hypersaline brines of the lagoon. The anoxic sediments of Peña Hueca showed the presence of Haloanaerobiaeta and Hadesarchaeota including the anoxic genus of Haloanaerobium, Desulfosalsimonas and Desulfovermiculum. The effect of impact shock on the halophilic bacterium Halomonas gomseomensis EP-3 isolated from Laguna de Peña Hueca was studied in a Reddy shock tube. The halophilic bacterium was exposed to shock waves at a peak shock pressure of 300 ​kPa and a temperature of 400 ​K. The results of shock recovery experiments of halophilic bacteria reveal 97% killing at 300 ​kPa and Mach number of 1.47 in comparison with Bacillus sp. This study indicates that gram-positive spore-forming Bacillus sp. are better adapted to survival in impact shock waves in comparison to non-sporulating halophiles. In the current study, we present the first report on response of halophiles in impact shock. Furthermore, we demonstrate a novel application of the simple handheld Reddy shock tube in astrobiology. The survival studies of halophiles isolated from terrestrial analogue sites in impact shock can provide valuable insights in astrobiology and microbial physiology in impact shock stress.

The Magnitude and Waveform of Shock Waves Induced by X-ray Lasers in Water

The high energy densities deposited in materials by focused X-ray laser pulses generate shock waves which travel away from the irradiated region, and can generate complex wave patterns or induce phase changes. We determined the time-pressure histories of shocks induced by X-ray laser pulses in liquid water microdrops, by measuring the surface velocity of the microdrops from images recorded during the reflection of the shock at the surface. Measurements were made with ~30 µm diameter droplets using 10 keV X-rays, for X-ray pulse energies that deposited linear energy densities from 3.5 to 120 mJ/m; measurements were also made with ~60 µm diameter drops for a narrower energy range. At a distance of 15 µm from the X-ray beam, the peak shock pressures ranged from 44 to 472 MPa, and the corresponding time-pressure histories of the shocks had a fast quasi-exponential decay with positive pressure durations estimated to range from 2 to 5 ns. Knowledge of the amplitude and waveform of the shock waves enables accurate modeling of shock propagation and experiment designs that either maximize or minimize the effect of shocks.