Elisabetta Pierazzo

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

Hydrocode simulation of the Chicxulub impact event and the production of climatically active gases

We constructed a numerical model of the Chicxulub impact event using the Chart-D Squared (CSQ) code coupled with the ANalytic Equation Of State (ANEOS) package. In the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact-related extinction hypothesis: carbonates (calcite) and evaporites (anhydrite). Simulations explored the effects of different projectile sizes (10 to 30 km in diameter) and porosity (0 to 50%). The effect of impact speed is addressed by doing simulations of asteroid impacts (vi = 20 km/s) and comet impacts (vi = 50 km/s). The masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event, ranging from 350 to 3500 Gt for CO2, from 40 to 560 Gt for S, and from 200 to 1400 Gt for water vapor. While our results are in good agreement with those of Ivanov et al. [1996], our estimated CO2 production is 1 to 2 orders of magnitude lower than the results of Takata and Ahrens [1994], indicating that the impact event enhanced the end-Cretaceous atmospheric CO2 inventory by, at most, 40%. Consequently, sulfur may have been the most important climatically active gas injected into the stratosphere. The amount of S released by the impact is several orders of magnitude higher than any known volcanic eruption and, with H2O, is high enough to produce a sudden and significant perturbation of Earths climate.

Hydrocode modeling of Chicxulub as an oblique impact event

Abstract Since the confirmation that the buried Chicxulub structure is the long-sought K/T boundary crater, numerous efforts have been devoted to modeling the impact event and estimating the amount of target material that underwent melting and vaporization. Previous hydrocode simulations modeled the Chicxulub event as a vertical impact. We carried out a series of three-dimensional (3D) hydrocode simulations of the Chicxulub impact event to study how the impact angle affects the results of impact events. The simulations model an asteroid, 10 km in diameter, impacting at 20 km/s on a target resembling the lithology of the Chicxulub site. The angles of impact modeled are 90° (vertical), 60°, 45°, 30°, and 15°. We find that the amount of sediments (surface layer) vaporized in the impact reaches a maximum for an impact angle of 30° from the surface, corresponding to less than two times the amount of vaporization for the vertical case. The degassing of the sedimentary layer, however, drops abruptly for a 15° impact angle. The amount of continental crust melted in the impact decreases monotonically (a consequence of the decrease in the maximum depth of melting) from the vertical impact case to the 15° impact. Melting and vaporization occur primarily in the downrange direction for oblique impacts, due to asymmetries in the strength of the shock wave with respect to the point of impact. The results can be used to scale the information from the available vertical simulations to correct for the angle of impact. A comparison of a 3D vertical impact simulation with a similar two-dimensional (2D) simulation shows good agreement between vertical 3D and 2D simulations.

Starting Conditions for Hydrothermal Systems Underneath Martian Craters: Hydrocode Modeling

Mars is the most Earth-like of the Solar System s planets, and the first place to look for any sign of present or past extraterrestrial life. Its surface shows many features indicative of the presence of surface and sub-surface water, while impact cratering and volcanism have provided temporary and local surface heat sources throughout Mars geologic history. Impact craters are widely used ubiquitous indicators for the presence of sub-surface water or ice on Mars. In particular, the presence of significant amounts of ground ice or water would cause impact-induced hydrothermal alteration at Martian impact sites. The realization that hydrothermal systems are possible sites for the origin and early evolution of life on Earth has given rise to the hypothesis that hydrothermal systems may have had the same role on Mars. Rough estimates of the heat generated in impact events have been based on scaling relations, or thermal data based on terrestrial impacts on crystalline basements. Preliminary studies also suggest that melt sheets and target uplift are equally important heat sources for the development of a hydrothermal system, while its lifetime depends on the volume and cooling rate of the heat source, as well as the permeability of the host rocks. We present initial results of two-dimensional (2D) and three-dimensional (3D) simulations of impacts on Mars aimed at constraining the initial conditions for modeling the onset and evolution of a hydrothermal system on the red planet. Simulations of the early stages of impact cratering provide an estimate of the amount of shock melting and the pressure-temperature distribution in the target caused by various impacts on the Martian surface. Modeling of the late stage of crater collapse is necessary to characterize the final thermal state of the target, including crater uplift, and distribution of the heated target material (including the melt pool) and hot ejecta around the crater.Introduction: Mars is the most Earth-like of the Solar System’s planets, and the first place to look for any sign of present or past extraterrestrial life. Its surface shows many features indicative of the presence of surface and sub-surface water [1], while impact cratering and volcanism have provided temporary and local surface heat sources throughout Mars geologic history. Impact craters are widely used ubiquitous indicators for the presence of sub-surface water or ice on Mars [2]. In particular, the presence of significant amounts of ground ice or water would cause impactinduced hydrothermal alteration at Martian impact sites [3]. The realization that hydrothermal systems are possible sites for the origin and early evolution of life on Earth [4,5,6] has given rise to the hypothesis that hydrothermal systems may have had the same role on Mars [7,8,9,10]. Rough estimates of the heat generated in impact events have been based on scaling relations [11,12], or thermal data based on terrestrial impacts on crystalline basements [13]. Preliminary studies [14,15] also suggest that melt sheets and target uplift are equally important heat sources for the development of a hydrothermal system, while its lifetime depends on the volume and cooling rate of the heat source, as well as the permeability of the host rocks. We present initial results of two-dimensional (2D) and three-dimensional (3D) simulations of impacts on Mars aimed at constraining the initial conditions for modeling the onset and evolution of a hydrothermal system on the red planet. Simulations of the early stages of impact cratering provide an estimate of the amount of shock melting and the pressure-temperature distribution in the target caused by various impacts on the Martian surface. Modeling of the late stage of crater collapse is necessary to characterize the final thermal state of the target, including crater uplift, and distribution of the heated target material (including the melt pool) and hot ejecta around the crater.

A Brief Introduction to Hydrocode Modeling of Impact Cratering

Numerical modeling is a fundamental tool for understanding the dynamics of impact cratering, especially at planetary scales. In particular, processes like melting/vaporization and crater collapse, typical of planetary-scale impacts, are not reproduced in the laboratory, and can only be investigated by numerical modeling. The continuum dynamics of impact cratering events is fairly well understood and implemented in numerical codes; however, the response of materials to shocks is governed by specific material properties. Accurate material models are thus crucial for realistic simulation of impact cratering, and still represent one of the major problems associated with numerical modeling of impacts.

Parallel 3D Hybrid Continuum/DSMC Method for Unsteady Expansions Into a Vacuum

We present the application of a unidirectional unsteady coupling between a continuum solver and a three dimensional parallel Direct Simulation Monte Carlo (DSMC) code. Two different problems have been considered: the spherically symmetric expansion of a water vapor cloud into a vacuum and the late stages of a comet impact on the Moon. In both cases, unsteady data pre-computed from a continuum solution are used as input to the DSMC code at a fixed interface. The DSMC results were then compared to the continuum results downstream of the interface in a region of mutual validity in order to validate our approach. The DSMC results for the expansion flow showed good agreement with the analytic solution for the density, velocity and temperature downstream of the interface. Similarly, for the comet impact simulations, the DSMC density agrees well with the solution from the continuum solver, the SOVA hydrocode. A slightly hotter solution is however obtained downstream of the interface using the DSMC code compared to the hydrocode solution.

Improving knowledge of impact cratering: Bringing together “Modelers” and “Observationalists”

Formation of a planetary-scale impact crater hundreds of meters to hundreds of kilometers in diameter involves the interplay of multiphase processes occurring over size- and time-scales that range over many orders of magnitude. To further complicate matters, a hypervelocity impact into geologic materials has never been observed at greater than laboratory scales. Understanding the formation process clearly is a very complicated problem that requires a highly interdisciplinary approach. Significant work has been done recently in several key areas of impact studies, but in many respects, there is a “disconnect” among groups employing different approaches. This pertains, in particular, to modeling versus observations.

Impact cratering in the solar system

Given that impact cratering is the most common geological process in the solar system, study of this topic provides an excellent means of interplanetary comparison. One can gain insight into the crustal properties of various solar system bodies, the population of asteroids and comets that potentially could cause impacts, and the varied ways in which impact cratering has influenced the geologic histories of the planets. The First International Conference on Impact Cratering in the Solar System recently was held at the 40th ESLAB (European Space Laboratory) Symposium at the European Space Agencys European Space Research and Technology Centre, in Noordwijk, the Netherlands. About 150 participants from around the world came to discuss the latest understanding of and issues related to impact cratering in the solar system. The uniqueness of this conference lay in its broad scope, covering everything from the late heavy bombardment controversy to impact-induced mass extinctions. Thus, the conference brought together many scientists of varying expertise who may otherwise have never interacted.

Impacts in Precambrian Shields

One of the most fundamental geologic processes that have shaped the surfaces of the solar systems planetary bodies is the impact of solid bodies. It affects the surface of a planetary body not only by creating a visible crater, but also by modifying, locally and temporally, the crater subsurface and a wide area around the excavated crater. In particular, terrestrial impact structures are in the unique position of providing the only ground truth available for investigating the impact cratering process. The establishment of shock metamorphic effects on rocks as reliable criteria for assigning impact origins to terrestrial structures has led to the recognition of over 160 impact structures on Earth over the past 40 years. Investigation of these structures is fundamental for validating theoretical and experimental impact studies.

A systematic comparison of two different models of cosmogenic nuclide production in meteorites

SummaryA systematic comparison of a variable cosmogenic-production rate model (proposed in a previous paper) with the conventional constant-production rate model is carried out. Attention is focussed on the time-integrated concentrations in meteorites. A graphical method for the estimate of the exposure and terrestrial ages is applied with reference to the ten pairs of the five long-lived cosmogenic nuclides36Cl,26Al,10Be,53Mn and129I of interest in the Accelerator Mass Spectrometry.RiassuntoViene eseguito un confronto sistematico tra un modello a produzione cosmogenica variabile, proposto in un precedente lavoro, e il modello convenzionale a produzione costante. Particolare attenzione viene dedicata alle concentrazioni cosmogeniche, integrate sul tempo, nelle meteoriti. Un metodo grafico, che usa coppie di concentrazioni di radionuclidi cosmogenici a vita media lunga in meteoriti, permette di stimarne le età di esposizione cosmica e le età terrestri. Si esegue un confronto incrociato, per ognuno dei due modelli, usando le dieci coppie ottenibili con i cinque radionuclidi consmogenici a vita media lunga,36Cl,26Al,10Be,53Mn and129I, di attuale interesse nella Spettrometria di Massa con Acceleratore.РезюмеПроводится систематическое сравнение модели переменной интенсивности космогенного образования (предложенной в предыдущей статье) с общепринятой моделью постоянной интенсивности космогенного образования. Основное внимание уделяется проинтегрированным по времени концентрациям в метеоритах. Графический метод для оценки экспозиции и возраста Эемли применяется к десяти парам пяти долгоживущих космогенных нуклидов36Cl,26Al,10Be,53Mn и129I, которые представляют интерес в ускорительной массспектрометрии.