Kai Wünnemann

How big was the Chesapeake Bay impact? Insight from numerical modeling

The Chesapeake Bay impact structure, Virginia, is the largest impact crater in the United States. The important question of how large the impactor was that formed the crater remains unanswered. This is primarily due to peculiarities of the crater structure, revealed by seismic exploration, that complicate comparisons with other terrestrial and extraterrestrial craters. One potential explanation for the unusual structure of the Chesapeake Bay crater is that the cratering process was affected by rheologic variations in the target at the time of impact. Using numerical modeling, we find that for a sufficient contrast in material strength between the sedimentary and crystalline units at the Chesapeake Bay impact site, we can produce a model of the final crater that is consistent with observational constraints, and hence we can infer the energy released during the impact, ;1.75 3 10 6 Mt. In the absence of target strength variations, the final crater diameter is likely to have been ;40 km, rather than the observed 80‐90 km.

Hybrid modeling of the mega-tsunami runup in Lituya Bay after half a century

The largest mega-tsunami dates back half a century to 10 July 1958, when almost unnoticed by the general public, an earthquake of M w 8.3 at the Fairweather Fault triggered a rockslide into Lituya Bay. The rockslide impact generated a giant tsunami at the head of Lituya Bay resulting in an unprecedented tsunami runup of 524 m on a spur ridge in direct prolongation of the slide axis. A forest trim line and erosion down to bedrock mark the largest runup in recorded history. While these observations have not been challenged directly, they have been largely ignored in hazard mitigation studies, because of the difficulties of even posing ― much less solving - a well-defined physical problem for investigation. We study the mega-tsunami runup with a hybrid modeling approach applying physical and numerical models of slide processes of deformable bodies into a U-shaped trench similar to the geometry found at Lituya Bay. C.

Numerical modeling of the ejecta distribution and formation of the Orientale basin on the Moon

The formation and structure of the Orientale basin on the Moon has been extensively studied in the past; however, estimates of its transient crater size, excavated volume and depth, and ejecta distribution remain uncertain. Here we present a new numerical model to reinvestigate the formation and structure of Orientale basin and better constrain impact parameters such as impactor size and velocity. Unlike previous models, the observed ejecta distribution and ejecta thickness were used as the primary constraints to estimate transient crater size—the best measure of impact energy. Models were also compared to basin morphology and morphometry, and subsurface structures derived from high-resolution remote sensing observations and gravity data, respectively. The best fit model suggests a 100 km diameter impactor with a velocity of ~12 km s−1 formed the Orientale basin on a relatively “cold” Moon. In this impact scenario the transient crater diameter is ~400 km or 460 km depending on whether the crater is defined using the diameter of the excavation zone or the diameter of the growing cavity at the time of maximum crater volume, respectively. The volume of ejecta material is ~4.70 × 106 km3, in agreement with recent estimates of the Orientale ejecta blanket thickness from remote sensing studies. The model also confirms the remote sensing spectroscopic observations that no mantle material was excavated and deposited at Orientales rim.

Earth-like habitats in planetary systems

Understanding the concept of habitability is clearly related to an evolutionary knowledge of the particular planet-in-question. However, additional indications so-called “systemic aspects” of the planetary system as a whole governs a particular planet׳s claim on habitability. In this paper we focus on such systemic aspects and discuss their relevance to the formation of an “Earth-like” habitable planet. This contribution summarizes our results obtained by lunar sample work and numerical models within the framework of the Research Alliance “Planetary Evolution and Life”. We consider various scenarios which simulate the dynamical evolution of the Solar System and discuss the consequences for the likelihood of forming an Earth-like world orbiting another star. Our model approach is constrained by observations of the modern Solar System and the knowledge of its history. Results suggest that on the one hand the long-term presence of terrestrial planets is jeopardized due to gravitational interactions if giant planets are present. On the other hand the habitability of inner rocky planets may be supported in those planetary systems hosting giant planets. Gravitational interactions within a complex multiple-body structure including giant planets may supply terrestrial planets with materials which formed in the colder region of the proto-planetary disk. During these processes, water, the prime requisite for habitability, is delivered to the inner system. This may occur either during the main accretion phase of terrestrial planets or via impacts during a post-accretion bombardment. Results for both processes are summarized and discussed with reference to the lunar crater record. Starting from a scenario involving migration of the giant planets this contribution discusses the delivery of water to Earth, the modification of atmospheres by impacts in a planetary system context and the likelihood of the existence of extrasolar Earth-like habitable worlds.

Large waves caused by oceanic impacts of meteorites

Impact craters can be observed on all terrestrial planets and their larger satellites. Basically every body in the solar system with a solid crust, no matter how small it is, exhibits evidence of impacts in the past. For example, the Moon provides an excellent data base of impact craters. However, the major fraction of impact events occurred between 4.6 and 3.9 Billion years ago. The impact frequency at that time was ∼ 100 times larger than it has been ever since. Figure 1 shows the craters Ptolomaeus, Alphonsus, Arzahchel and Albetegnius. The image depicts that impact craters vary form large basins of several 100 kilometres in diameter (the largest impact basin is Valhalla with 4000 km in diameter on the Jovian satellite Callisto) to structures that are only several 10’s of meters in diameter.

The effect of target properties on transient crater scaling for simple craters

The effects of the coefficient of friction and porosity on impact cratering are not sufficiently considered in scaling laws that predict the crater size from a known impactor size, velocity, and mass. We carried out a systematic numerical study employing more than 1000 two-dimensional models of simple crater formation under lunar conditions in targets with varying properties. A simple numerical setup is used where targets are approximated as granular or brecciated materials, and any compression of porous materials results in permanent compaction. The results are found to be consistent with impact laboratory experiments for water, low strength and low porosity materials (e.g., wet sand), and sands. Using this assumption, we found that both the friction coefficient and porosity are important for estimating transient crater diameters as is the strength term in crater scaling laws, i.e. the effective strength. The effects of porosity and friction coefficient on impact cratering were parameterized and incorporated into π-group scaling laws and predict transient crater diameters within an accuracy of ±5 % for targets with friction coefficients f ≥ 0.4 and porosities Φ = 0 – 30 %. Moreover, 90 crater scaling relationships are made available and can be used to estimate transient crater diameters on various terrains and geological units with different coefficient of friction, porosity and cohesion. The derived relationships are most robust for targets with Φ > 10 – 15 %, applicable for a lunar environment, and could therefore yield significant insights into the influence of target properties on cratering statistics.

Shock-darkening in ordinary chondrites: Determination of the pressure-temperature conditions by shock physics mesoscale modeling

We determined the shock-darkening pressure range in ordinary chondrites using the iSALE shock physics code. We simulated planar shock waves on a mesoscale in a sample layer at different nominal pressures. Iron and troilite grains were resolved in a porous olivine matrix in the sample layer. We used equations of state (Tillotson EoS and ANEOS) and basic strength and thermal properties to describe the material phases. We used Lagrangian tracers to record peak shock pressures in each material unit. The post-shock temperatures (and the fractions of tracers experiencing temperatures above the melting point) for each material were estimated after the passage of the shock wave and after reflections of the shock at grain boundaries in the heterogeneous materials. The results showed that shock-darkening, associated with troilite melt and the onset of olivine melt, happened between 40 and 50 GPa - with 52 GPa being the pressure at which all tracers in the troilite material reach the melting point. We demonstrate the difficulties of shock heating in iron and also the importance of porosity. Material impedances, grain shapes and the porosity models available in the iSALE code are discussed. We also discussed possible not-shock-related triggers for iron melt.

Effects of Moon's Thermal State on the Impact Basin Ejecta Distribution

We investigate how different temperature gradients of the Moon affect the ejection of lithic and molten materials for impact basin several hundred kilometers in diameter to quantify the thickness and melt content of ejecta blanket as a function of radial distance. We find, by means of numerical modeling, that the ejecta thickness and melt content, similar to the basin formation, is sensitive to the thermal properties of the target. For two similar impact scenarios, the ejecta thickness with radial distance is proportional to a power law, but for a “warm” target, it declines faster than for a “cold” target. In addition, the impact on the warm target produces more molten ejecta than in the case of the cold target. The thermal effects on the ejecta thickness distribution can be testified by the topographic variations around Imbrium and Orientale basins, which were thought to be formed on a warm and cold Moon, respectively. Our study demonstrates that the thermal effect needs to be taken into account to estimate the ejecta thickness distribution for large-scale impact basins on airless planetary surfaces.

Correlating laser-generated melts with impact-generated melts: An integrated thermodynamic-petrologic approach: Laser Melting of Planetary Materials

Planetary collisions in the solar system typically induce melting and vaporization of the impactor and a certain volume of the target. To study the dynamics of quasi-instantaneous melting and subsequent quenching under postshock P-T conditions of impact melting, we used continuous-wave laser irradiation to melt and vaporize sandstone, iron meteorite, and basalt. Using high-speed imaging, temperature measurements, and petrologic investigations of the irradiation targets, we show that laser-generated melts exhibit typical characteristics of impact melts (particularly ballistic ejecta). We then calculate the entropy gains of the laser-generated melts and compare them with the entropy gains associated with the thermodynamic states produced in hypervelocity impacts at various velocities. In conclusion, our experiments extend currently attainable postshock temperatures in impact experiments to ranges commensurate with impacts in the velocity range of 4–20 km s–1 and allow to study timescales and magnitudes of petrogenetic processes in impact melts.