Terrestrial consequences of hypervelocity impact – shock metamorphism, shock barometry, and newly discovered impact structures

Impact cratering was once considered a rare geological process of no, or little, importance to the evolution of the Solar System and planet Earth. After more than 50 years of space exploration and the discovery of numerous (~190 as of October 2016) impact structures on Earth, this view has changed, and it is now clear that impact craters are in fact one of the most common morphological features on solid bodies in the Solar System. The formation of a (hypervelocity) impact crater involves extreme conditions that cannot be compared with any other natural geological process, with extreme pressures and temperatures causing melting and/or vaporization of both projectile and portions of the target rocks. Upon impact, shock waves are generated at the projectile-target interface, which pass through the target rocks at supersonic velocity. The passage of the shock waves induce irreversible changes, so called shock metamorphic effects in the target rocks, including the formation of high pressure mineral polymorphs, diaplectic glasses, and microdeformation features in minerals. The most investigated of these microstructures are planar deformation features (PDFs) in quartz. These are straight, parallel, closely spaced (2-10 µm apart), sets of (when fresh) glass lamellae only naturally formed by impact cratering. PDFs are oriented parallel to specific crystallographic planes, with the most frequently reported orientations being parallel to low Miller-Bravais index planes (e.g., {10‾13}, {10‾12}). The orientation pattern of a PDF population differ depending on the pressure that the host quartz grain was subjected to, meaning that the orientations of PDFs can be used as a shock barometer, allowing e.g., production of shock barometry profiles that illustrate shock attenuation at impact structures. The research presented in this thesis focuses on impact craters, and the process by which they form, impact cratering, with special emphasis on shock metamorphic features in target rocks at the Siljan impact structure (Sweden). The results and discussion highlight the importance of the way datasets of PDF statistics are obtained and processed, using manual and/or automated methods of indexing. The interpretation of the dataset can influence the shock barometry models, and the need for a unified method is discussed. With regards to the Siljan impact structure, the pre-erosional rim-to-rim diameter of the crater was estimated to be on the order of 60 km, based on a combination of shock barometry and numerical simulation, produced by a collision between a ~5 km diameter projectile and Earth. Results of the numerical modeling are consistent with a sedimentary thickness overlying the crystalline basement at the time of impact of ~2.5 km, and post-impact erosion of the crater on the order of 3 to 3.5 km.The thesis also encompasses studies of two other, newly confirmed, Swedish impact structures, Målingen and Hummeln. The possible means of formation for both Målingen and Hummeln had been discussed for many years before the first bona fide evidence for the impact origin of the two structures was presented in papers included in this thesis.Furthermore, terrestrial impact structures with reliable ages (i.e., errors on age of less than 2 %) are discussed in the context of possible variations in the impactor flux to Earth over time. According to the results, there is presently no evidence for the existence of a periodic contribution to the terrestrial impact population

Geological, multispectral, and meteorological imaging results from the Mars 2020 Perseverance rover in Jezero crater

Perseverance’s Mastcam-Z instrument provides high-resolution stereo and multispectral images with a unique combination of spatial resolution, spatial coverage, and wavelength coverage along the rover’s traverse in Jezero crater, Mars. Images reveal rocks consistent with an igneous (including volcanic and/or volcaniclastic) and/or impactite origin and limited aqueous alteration, including polygonally fractured rocks with weathered coatings; massive boulder-forming bedrock consisting of mafic silicates, ferric oxides, and/or iron-bearing alteration minerals; and coarsely layered outcrops dominated by olivine. Pyroxene dominates the iron-bearing mineralogy in the fine-grained regolith, while olivine dominates the coarse-grained regolith. Solar and atmospheric imaging observations show significant intra- and intersol variations in dust optical depth and water ice clouds, as well as unique examples of boundary layer vortex action from both natural (dust devil) and Ingenuity helicopter–induced dust lifting. High-resolution stereo imaging also provides geologic context for rover operations, other instrument observations, and sample selection, characterization, and confirmation

Shocked quartz grains from the Malingen structure, Sweden-Evidence for a twin crater of the Lockne impact structure

The Malingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Malingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Malingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne-Malingen pair)

Shock deformation in zircon grains from the Mien impact structure, Sweden

Recognition of impact-induced deformation of minerals is crucial for the identification and confirmation of impact structures as well as for the understanding of shock wave behavior and crater formation. Shock deformed mineral grains from impact structures can also serve as important geochronometers, precisely dating the impact event. We investigated zircon grains from the Mien impact structure in southern Sweden with the aim of characterizing shock deformation. The grains were found in two samples of impact melt rock with varying clast content, and in one sample of suevitic breccia. We report the first documentation of so-called “FRIGN zircon” (former reidite in granular neoblastic zircon) from Mien (pre-erosion diameter 9 km), which confirms that this is an important impact signature also in relatively small impact structures. Furthermore, the majority of investigated zircon grains contain other shock-related microtextures, most notably granular and microporous textures, that occur more frequently in grains found in the impact melt than in the suevitic breccia. Our findings show that zircon grains that are prime candidates for establishing a new and improved age refinement of the Mien impact structure are present in the impact melt

Shock barometry of the Siljan impact structure, Sweden

The Siljan impact structure in Sweden is the largest confirmed impact structure in Western Europe. Despite this, the structure has been poorly studied in the past, and detailed studies of shock metamorphic features in the target lithologies are missing. Here, we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at Siljan. At 21 localities from an area approximately 20 km in diameter located centrally in the structure, the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured. Observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter. The most strongly shocked samples, recording pressures of up to 20 GPa, occur at the very central part of the structure, and locally in these samples, higher pressures causing melting conditions in the affected rocks were reached. Pressures recorded in the studied samples decrease outwards from the center of the structure, forming roughly circular envelopes around the proposed shock center. Based on the distribution pattern of shocked quartz at Siljan, the original transient cavity can be estimated at approximately 3238 km in diameter. After correcting for erosion, we conclude that the original rim to rim diameter of the Siljan crater was somewhere in the size range 5090 km

Shocked Mesosiderites: Hidden in Plain Sight?

Summary: Four mesosiderites were classified, and petrographic analyses were performed to look for indicators of shock. Of the four samples, Lamont and QUE 86900 were found to contain shock-related features, such as planar microstructures and quenched melts, respectively

Resolving the age of the Puchezh-Katunki impact structure (Russia) against alteration and inherited 40Ar* – No link with extinctions

The possibility of a “death from above” cause for biotic crises and extinction events is intriguing, to say the least, but such claims must be supported by reliable and reproducible data, not only impact diagnostic criteria, but also accurate and precise radioisotopic ages of the impact structures/events. To date, only one example of such an impact-related global extinction event is confirmed, at the end of the Cretaceous period. Here we present and discuss results of newly obtained 40Ar/39Ar data from step heating analysis of impact melt rock samples from the 40 km-in-diameter Puchezh-Katunki impact structure, Russia, which allow us to precisely and accurately date its formation at 195.9 ± 1.0 Ma (2σ; P = 0.10). Based on these new data, we challenge the proposed temporal correlation with as many as five different extinction events (including the end-Triassic mass extinction) that were based on previous age estimations ranging from ∼164 to 203 Ma. Our new age for the formation of the Puchezh-Katunki impact structure allows us to exclude a relationship between this impact event and a known extinction event. We also show that careful sample preparation and methodology can overcome problems with inherited and trapped 40Ar, issues that are common when dating impact melt rocks. This is supported by 40Ar* diffusion and mixing numerical models showing that the most prominent negative effects in the case of the Puchezh-Katunki impact melt rock samples are caused by hydrothermal alteration and undegassed melt rock domains present in an otherwise homogenized melt rock. Numerical modeling also shows that the 40Ar* from high-Ca inherited crystals or clasts is decoupled from the melt rock during step heating experiments allowing to safely recover a plateau age. Finally, our results highlight the importance of improving the database of ages of impact structures and show that caution should be practiced when suggesting connections between specific impact events and extinction events, especially in the case of poorly dated impact structures

Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body

Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar system's asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years

A tale of clusters : No resolvable periodicity in the terrestrial impact cratering record

Rampino & Caldeira carry out a circular spectral analysis (CSA) of the terrestrial impact cratering record over the past 260 million years (Ma), and suggest a ∼26 Ma periodicity of impact events. For some of the impacts in that analysis, new accurate and high-precision ('robust'; 2SE < 2 per cent) 40Ar-39Ar ages have recently been published, resulting in significant age shifts. In a CSA of the updated impact age list, the periodicity is strongly reduced. In a CSA of a list containing only impacts with robust ages, we find no significant periodicity for the last 500 Ma. We show that if we relax the assumption of a fully periodic impact record, assuming it to be a mix of a periodic and a random component instead, we should have found a periodic component if it contributes more than ∼80 per cent of the impacts in the last 260 Ma. The difference between our CSA and the one by Rampino & Caldeira originates in a subset of 'clustered' impacts (i.e. with overlapping ages). The ∼26 Ma periodicity seemingly carried by these clusters alone is strongly significant if tested against a random distribution of ages, but this significance disappears if it is tested against a distribution containing (randomly spaced) clusters. The presence of a few impact age clusters (e.g. from asteroid break-up events) in an otherwise random impact record can thus give rise to false periodicity peaks in a CSA. There is currently no evidence for periodicity in the impact record