Impact Earth: A review of the terrestrial impact record

Over the past few decades, it has become increasingly clear that the impact of interplanetary bodies on other planetary bodies is one of the most ubiquitous and important geological processes in the Solar System. This impact process has played a fundamental role throughout the history of the Earth and other planetary bodies, resulting in both destructive and beneficial effects. The impact cratering record of Earth is critical to our understanding of the processes, products, and effects of impact events. In this contribution, we provide an up-to-date review and synthesis of the impact cratering record on Earth. Following a brief history of the Impact Earth Database (available online at http://www.impactearth.com), the definition of the main categories of impact features listed in the database, and an overview of the impact cratering process, we review and summarize the required evidence to confirm impact events. Based on these definitions and criteria, we list 188 hypervelocity impact craters and 13 impact craters (i.e., impact sites lacking evidence for shock metamorphism). For each crater, we provide details on key attributes, such as location, date confirmed, erosional level, age, target properties, diameter, and an overview of the shock metamorphic effects and impactites that have been described in the literature. We also list a large number of impact deposits, which we have classified into four main categories: tektites, spherule layers, occurrences of other types of glass, and breccias. We discuss the challenges of recognizing and confirming impact events and highlight weaknesses, contradictions, and inconsistencies in the literature. We then address the morphology and morphometry of hypervelocity impact craters. Based on the Impact Earth Database, it is apparent that the transition diameter from simple to complex craters for craters developed in sedimentary versus crystalline target rocks is less pronounced than previously reported, at approximately 3 km for both. Our analysis also yields an estimate for stratigraphic uplift of 0.0945D0.6862, which is lower than previous estimates. We ascribe this to more accurate diameter estimates plus the variable effects of erosion. It is also clear that central topographic peaks in terrestrial complex impact craters are, in general, more subdued than their lunar counterparts. Furthermore, a number of relatively well-preserved terrestrial complex impact structures lack central peaks entirely. The final section of this review provides an overview of impactites preserved in terrestrial hypervelocity impact craters. While approximately three quarters of hypervelocity impact craters on Earth preserve some portion of their crater-fill impactites, ejecta deposits are known from less than 10%. In summary, the Impact Earth Database provides an important new resource for researchers interested in impact craters and the impact cratering process and we welcome input from the community to ensure that the Impact Earth website (http://www.impactearth.com) is a living resource that is as accurate and as up-to-date, as possible

Surface Morphology and Subsurface Ice Content Relationships in Arcadia Planitia, Mars and the Canadian High Arctic

As NASA and SpaceX prepare for future human missions to Mars as part of an In-situ Resource Utilization (ISRU) Space Act Agreement (SAA), we need more detailed characterization of ice at proposed landing sites to constrain ice accessibility, landing safety, and scientific value. Obtaining near-surface in situ water-ice can be used for rocket fuel and life support needs which would significantly reduce the mass needed for transport to and from Mars. Arcadia Planitia is the lowest-lying region in the northern hemisphere of Mars where abundant evidence exists for an ice-rich subsurface. Shallow Radar observations indicate a decameters-thick layer of water-ice (i.e., buried ice sheet) extends across much of Arcadia. The goal of my Ph.D. research is to characterize the ice-related features at Arcadia Planitia, a proposed future human mission landing site, in detail to assist in the identification of a safe landing site where water-ice is present and accessible for ISRU. By utilizing multiple orbital datasets (i.e., morphology, albedo, thermal infrared reflectance, thermal inertia, and subsurface radar reflections) and identification criteria for Viscous Flow Features (VFFs) on Mars, I mapped six glacial-related features in Arcadia. These units consist of conventional VFFs, such as Lobate Debris Aprons, and non-conventional VFFs. Three sinuous features in the flat-lying plains of Arcadia show surface morphologies and spectral properties indicating these are non-conventional VFFs of channelized ice that once flowed. I propose these sinuous features to be analogous to terrestrial ice streams. Brain terrain is proposed to represent a lag deposit formed atop thick glacial ice as a result of ice sublimation. However, we observe brain terrain to occur only within a narrow latitudinal band within the study site with minimal examples of brain terrain found on the six glacial-related features mapped. We utilize the Canadian High Arctic to investigate analogous brain terrain, that we have termed Vermicular Ridge Features (VRFs), to identify surface-subsurface relationships with ground-penetrating radar, photogrammetry, grain size analysis, and LiDAR. We interpret VRFs to be produced from the passive ablation of stagnant glacial ice. We interpret the lack of brain terrain on the six glacial-related features we mapped at Arcadia Planitia to represent regions where thick units of ice persist, have experienced less degradation than the surrounding terrain, and, therefore, where massive ice is shallower from the surface making our mapped regions areas where ice is more accessible for ISRU

High Arctic channel incision modulated by climate change and the emergence of polygonal ground

Abstract Stream networks in Arctic and high-elevation regions underlain by frozen ground (i.e., permafrost) are expanding and developing in response to accelerating global warming, and intensifying summertime climate variability. The underlying processes governing landscape dissection in these environments are varied, complex and challenging to unravel due to air-temperature-regulated feedbacks and shifts to new erosional regimes as climate change progresses. Here we use multiple sources of environmental information and physical models to reconstruct and understand a 60-year history of landscape-scale channelization and evolution of the Muskox Valley, Axel Heiberg Island. A time series of air photographs indicates that freeze-thaw-related polygon fields can form rapidly, over decadal time scales. Supporting numerical simulations show that the presence of polygons can control how surface runoff is routed through the landscape, exerting a basic control on channelization, which is sensitive to the timing, duration and magnitude of hydrograph events, as well as seasonal air temperature trends. These results collectively highlight that the occurrence and dynamics of polygon fields modulate channel network establishment in permafrost-rich settings undergoing changes related to a warming climate

High Arctic channel incision modulated by climate change and the emergence of polygonal ground

Stream networks in Arctic and high-elevation regions underlain by frozen ground (i.e., permafrost) are expanding and developing in response to accelerating global warming, and intensifying summertime climate variability. The underlying processes governing landscape dissection in these environments are varied, complex and challenging to unravel due to air-temperature-regulated feedbacks and shifts to new erosional regimes as climate change progresses. Here we use multiple sources of environmental information and physical models to reconstruct and understand a 60-year history of landscape-scale channelization and evolution of the Muskox Valley, Axel Heiberg Island. A time series of air photographs indicates that freeze-thaw-related polygon fields can form rapidly, over decadal time scales. Supporting numerical simulations show that the presence of polygons can control how surface runoff is routed through the landscape, exerting a basic control on channelization, which is sensitive to the timing, duration and magnitude of hydrograph events, as well as seasonal air temperature trends. These results collectively highlight that the occurrence and dynamics of polygon fields modulate channel network establishment in permafrost-rich settings undergoing changes related to a warming climate

Early revascularization is beneficial across all ages and a wide spectrum of cardiogenic shock severity : a pooled analysis of trials

A pooled analysis in cardiogenic shock due to acute coronary syndromes is desirable to assess the effect of early revascularization (ERV) across all ages and a wide spectrum of disease severity