6D). Given the high-temperature origin of impact glasses, it would seem unlikely that any biomarkers would be preserved, but this is exactly what has been reported in two studies. The exact duration of impact-generated hydrothermal systems remains a topic of ongoing study and debate. The largest meteorite ever found in the United States is called the Willamette Meteorite. These features were caused by the collision of large meteorites or comets with the Earth. Bethesda, MD 20894, Web Policies (A) Unshocked gneiss with essentially no porosity and few fractures. The significance of substrate attachment underscores the importance of rock surface area and physicochemical characteristics highlighting the role that both impact-induced fracturing and shock metamorphism play in creating subsurface habitats. As noted in Section 2.2, the Haughton impact structure is an excellent example of the preservation potential of crater lakes, where the stratigraphic record contained with the Haughton Formation (the crater lake sediments) represents the only record of the Miocene period in the Canadian Arctic (Hickey et al., As outlined in Sections 2.2 and 4.1, the creation of hydrothermal systems is expected to occur for all craters above a few kilometers in diameter on Earth. As a library, NLM provides access to scientific literature. (B) The West (W) and East (E) Clearwater Lake impact structures in Quebec are 36 and 26km in diameter, respectively, and both contain crater lakes. Furthermore, the resulting pH gradient between the highly alkaline effluent and the acidic ocean could have driven the first primitive proton pumps across a hydrothermal mound. We conclude with the recommendation that impact craters should be considered prime sites in the search for evidence of past life on Mars. Carbonaceous chondrites are one of the three major classes of chondrites and, as their name implies, contain variable amounts of organic molecules. Feldspar has undergone melting to form vesiculated glass. Also known as Barringer Crater, this impact crater can be found in northern . An excellent example of this is Saturn's moon Titan, where the average surface temperatures are too low ( -180C) for liquid water to exist. the contents by NLM or the National Institutes of Health. Furthermore, while modern mid-ocean-ridge-hosted hydrothermal systems have vent temperatures too high (>400C; Von Damm, 1990) to support biological activity inside the vents, numerical modelling of the impact-generated hydrothermal systems at the aforementioned Sudbury and Chicxulub marine impact structures show some regions of the structure are immediately at temperatures in the range of 50C to 100C, with fluid flow climbing to >20,000 km3 after only 100,000 years and higher volumes still at 1 million years (Abramov and Kring, 2004, 2007). (2014), Enigmatic tubular features in impact glass, Sapers H.M., Banerjee N.R., and Osinski G.R. 2018)and as clasts in breccias, both in crater-fill deposits and in ejecta blankets (Figs. (2015), The RNA world as a model system to study the origin of life, Hydrothermal systems associated with martian impact craters, A new global database of Mars impact craters 1km: 1. (1968): (1) contact and compression, (2) excavation, and (3) modification. 1970; Bar-Nun and Shaviv, 1975). 1A) lined with a mix of melted, shock metamorphosed, and fractured material (collectively termed crater-fill deposits), surrounded by impact ejecta deposits. (2013). Indeed, it has been shown that even within rocks that were highly shocked and encased in impact melt rocks, bioessential elements (Pontefract et al., As emphasized by Kirsime and Osinski (2012), the longer cooling times in large structures can be explained by the depth partitioning of cooling mechanisms/regimes. The formation of impact pumice has previously been described in Section 2.1 and shown in Fig. United States Meteorite Impact Craters - Beaverhead crater, Montana and IDAHO Beaverhead crater, Montana and IDAHO The Beaverhead impact structure, which spans the border between Montana and Idaho, is the second largest known meteorite impact crater in the United States. In many subsurface environments, life is generally not limited by space but by energy and nutrients (e.g., Hoehler and Jrgensen, 2013). Willamette Meteorite - Wikipedia Their upper reaches would have rapidly frozen, but hydrothermal venting may have kept their lower depths liquid for a substantial period of timeessentially for the duration of the impact-generated hydrothermal system. The third important extraterrestrial source for organics is meteorites (i.e., objects that are large enough to survive passage through Earth's atmosphere but small enough to not form hypervelocity impact craters). In addition to comets and asteroids delivering intact molecules, Chyba and Sagan (1992) proposed that organics could also be derived from atmospheric heating due to fast-expanding vapor plumes and via the post-impact recombination of simple organic components. The best-preserved impact crater in the world is the Barringer Meteorite Crater, near Winslow, Arizona. The role that impact cratering plays in fracturing planetary crusts and its effects on deep subsurface habitats for life are also discussed. Whereas the initial thermobaric phase lasts only a few minutes, modelling and dating studies suggest that the heat-driven effects of the impact event persist over timescales of significance to microbial colonization and evolution. (2019), Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration, The origin of lifea review of facts and speculations, Prebiotic adenine revisited: eutectics and photochemistry, Hydrothermal activity associated with the Ries impact event, Germany, Impact Earth: a new resource for outreach, teaching, and research, Intra-crater sedimentary deposits at the Haughton impact structure, Devon Island, Canadian High Arctic, Osinski G.R., Spray J.G., and Lee P. (2001), Impact-induced hydrothermal activity within the Haughton impact structure, Arctic Canada: generation of a transient, warm, wet oasis, Osinski G.R., Lee P., Parnell J., Spray J.G., and Baron M. (2005a), A case study of impact-induced hydrothermal activity: The Haughton impact structure, Devon Island, Canadian High Arctic, Osinski G.R., Lee P., Spray J.G., Parnell J., Lim D.S.S., Bunch T.E., Cockell C.S., and Glass B.J. 2). In Depth | Meteors & Meteorites - NASA Solar System Exploration The organic inventory of the Murchison meteorite, a CM2 carbonaceous chondrite that fell in Australia in 1969, represents the most thoroughly studied object of this type due to the available mass (100kg) and minor terrestrial contamination. Due to its remote location, it was discovered fairly recently 1948. Backscattered electron images showing the progressive physical changes in crystalline rocks from the Haughton impact structure with increasing shock pressure. This work is grounded in our own extensive field and laboratory studies of the impact cratering record on Earth, which now numbers 198 confirmed impact craters (Osinski and Grieve, 2019; see also www.impactearth.com for an up-to-date inventory), and synthesized with results from other studies. (2012), Osinski G.R., Tornabene L.L., Banerjee N.R., Cockell C.S., Flemming R.L., Izawa M.R.M., McCutcheon J., Parnell J., Preston L.J., Pickersgill A.E., Pontefract A., Sapers H.M., and Southam G. (2013), Impact-generated hydrothermal systems on Earth and Mars, Osinski G.R., Grieve R.A.F., Bleacher J.M., Neish C.D., Pilles E.A., and Tornabene L.L. 2005b). Once the impact-generated heat has fully dissipated from a crater and hydrothermal circulation has ceased, ambient surface temperatures return to pre-impact values: we consider this as the beginning of the next and longest-lived phase of post-impact ecological succession (Fig. The first meteorite impact craters recognized were simple, bowl shaped depressions with raised rims. Airbursts are a specific category of impacts where the projectile explodes at some height in the atmosphere but leaves no crater. Later studies of comets Hyakutake (1996) and Hale-Bopp (1997) revealed the presence of over two dozen biologically relevant organic molecules including ammonia, methane, acetylene, acetonitrile, hydrogen isocyanide, formic acid, isocyanic acid, cyanoacetylene, formamide, and thioformaldehyde (e.g., Biver et al., Fossil remains of non-avian dinosaurs (body fossils, egg . Meteorite Collision - an overview | ScienceDirect Topics 2011; Lollar et al., Meteorites Stardust Genesis Cosmic Dust Microparticle Impacts Hayabusa Research Groups Research Papers Laboratories Mars Science Laboratory Mars Exploration Rover OSIRIS-REx About Exploration Science Projects (ESP) Flight Experiments Science Operations & Environments ISRU & Simulants Astromaterials 3D HyperVelocity Impact Technology The recognition of alteration phases within such lithologies led to the initial concept of impact-generated hydrothermal systems (e.g., Newsom, 1980). Its believed that one such impact with a very large meteorite is what led to the extinction of the dinosaurs. 2014). These pipe structures have been interpreted as fossil hydrothermal vents, whose surficial expressions were likely hot springs and/or fumaroles. 2017) impact structures that demonstrated extreme sulfur isotopic fractionation in hydrothermal sulfides relative to original sulfate lithologies, consistent with microbial sulfate reduction by thermophiles. 7D), inconsistent with known mineralogical crystallization mechanisms. However, as noted in the previous section, during the thermobaric phase of crater formation, rocks from tens of kilometers' depth will be exhumed during the formation of 1001000km diameter craters that would have been common during the Hadean. Iron meteorites are comprised of >95% Fe and Ni. Driven by impact-generated hydrothermal system. 2013). ), The origin and emergence of life under impact bombardment, The evolutionary and ecological benefits of asteroid and comet impacts, Impact-induced impoverishment and transformation of a sandstone habitat for lithophytic microorganisms, Cockell C.S., Lee P., Schuerger A.C., Hidalgo L., Jones J.A., and Stokes M.D. (1992), On the possibility of chemosynthetic ecosystems in subsurface habitats on Mars, Bowden S.A., Parnell J., and Burchell M.J. (2009), Survival of organic compounds in ejecta from hypervelocity impacts on ice, Life on Mars: chemical arguments and clues from martian meteorites, Brack A., Horneck G., Cockell C.S., Brces A., Belisheva N.K., Eiroa C., Henning T., Herbst T., Kaltenegger L., and Lger A. 2009) but also are the most fertile substrate for tens of kilometers around the Haughton structure (Cockell et al., Connect with D. Login (Image credit: Solarseven | Shutterstock.com ) Everyone dies of something, but. (2013) distinguished six distinct locations in a typical complex impact crater where impact-generated hydrothermal deposits and habitats can form (Fig. A detailed study of the basal Ries crater lake sediments is ongoing and confirms the hydrothermal origin of smectite clays but also highlights that there is a complex story of other secondary mineral formation through recent weathering (Svensson et al., (2017), Evidence for early life in Earth's oldest hydrothermal vent precipitates, Dressler B.O., Weiser T., and Brockmeyer P. (1996), Recrystallized impact glasses of the Onaping Formation and the Sudbury Igneous Complex, Sudbury Structure, Ontario, Canada, Ehlmann B.L., Mustard J.F., Murchie S.L., Bibring J.-P., Meunier A., Fraeman A.A., and Langevin Y. 2005a) (Figs. Higher impact rates prior to 3.8 Ga have also been used to suggest a thermal bottlenecking event that would result in the preferential selection of hyperthermophiles and lead to the observed hyperthermophilic bias in modern evolutionary 16S rRNA phylogenetic tree reconstructions (e.g., Nisbet and Sleep, 2001; Kring and Cohen, 2002). 5, ,6A,6A, ,6D).6D). The outer margin of central uplifts is also typically highly fractured and faulted and, correspondingly, provides sites of preferential fluid flow and hydrothermal mineralization (Hode et al., Importantly, these transient hydrothermal systems can provide the substrates (see Section 3) and habitats (see Section 4) necessary to support the origins and early evolution of life. The continuance and conservation of these chemical reactions is largely dependent on whether a crater lake forms within the impact basin. It is worth noting that impact glasses are a ubiquitous product of impact events on Earth, being documented in craters from a few tens of meters in diameter (e.g., the 45m diameter Kamil Crater, Egypt; Folco et al., 8C8E). The formation of subaerial hydrothermal systems driven by endogenic activity seems entirely plausible (e.g., Schulze-Makuch et al., (2010), Origin and evolution of life on terrestrial planets, Brakenridge G.R., Newsom H.E., and Baker V.R. 2001; Cockell, 2006; Sleep, 2018; Schmieder and Kring, 2020) and provide a modern comprehensive treatise of the role of meteorite impacts in the origin and early evolution of life. If life ever did exist on Mars, the ideal place to seek potential biosignatures of these last remnants, therefore, would be lithic habitats (cf. There, more than 50,000 years ago, a meteoriteimpact blasted a . The ability of microorganisms to grow within rocks has long been recognized as an advantageous trait in harsh environments such as hot and cold deserts, particularly in surface environments where photosynthesis is possible (e.g., Friedmann, 1982; Bell, 1993). (2011), Parnell J., Osinski G.R., Lee P., Green P.F., and Baron M.J. (2005), Thermal alteration of organic matter in an impact crater and the duration of post-impact heating, Parnell J., Bowden S., Lindgren P., Burchell M., Milner D., Price M., Baldwin E.C., and Crawford I.A. (E) Shock Level 6 (5560 GPa). Today, we know of about 190 . Work by Miller (1953) and Zahnle (1986) used lightning and ultraviolet radiation to drive the chemical generation of HCN; however, these products are quickly destroyed and were not able to be effectively concentrated. There is some consensus regarding the requisite conditions for the transition from prebiotic chemistry to living systems: a maintained excess of Gibbs free energy (-G), a solvent (water), a mode for the encapsulation and concentration of prebiotic molecules, a mechanism of information storage, and the presence of catalytic molecules such as enzymes (e.g., Pace, 2001; Monnard and Deamer, 2002). 2010; Sapers et al., The Haughton example is one where the crater lake sediments preserved as the Haughton Formation date to the post-impact succession phase, as they were deposited following a period of a few million years of erosion (Osinski and Lee, 2005). Over the subsequent days to years, groundwater, if present, will flow into the newly formed hydraulic void represented by the crater structure. 2013). Impact Craters in America - Topozone 2) creating important new lithophytic (rock-based) microbial habitats (see Section 4.2). Not only did a significant proportion of both glycolaldehyde and montmorillonite survive (cf. Examination of glasses from the Ries impact structure in Germany has revealed tubular textures with remarkably similar morphologies (Fig. Latest Earthquakes | Live WebChatShare Social Media. (B) The 93km diameter Pettit Crater on Mars, a well-preserved complex crater with a central peak (cp). Barringer Crater (often called Meteor Crater), is located near the city of Winslow on Route 66 in Arizona, US, and was the first crater confirmed to have been caused by an extraterrestrial impact . Accessibility 1981). 2012) and even extant bacteria (Hazael et al., The history of meteor observation in the United States contains some surprises for today's reader. Main Attributes of Craters Mentioned in the Text, All attributes of terrestrial craters are from www.impactearth.com. Another crater lake in the United States can be found in southern . Life as we know it is based on a set of bioessential elements fundamental to known organic compounds, including hydrogen, oxygen, nitrogen, phosphorus, and sulfur, based on a carbon backbone (e.g., Ehrenfreund et al., As reviewed recently by Osinski et al. Others have argued that this concentration of impact ages at 3.9 Ga is an artifact that reflects sampling biases (e.g., Hartmann, 2003; Zellner, 2017) or age resetting (Boehnke and Harrison, 2016), which resulted in a monotonic decay rate since 4.5 Ga. 2011) (Fig. This is important as it has been proposed that CO was an important trace gas on prebiotic Earth because of its high free energy and its ability to catalyze important reactions involved in prebiotic synthesis (Kasting, 2014). At the end of thermobaric phases, the biosphere is significantly disrupted in the immediate area surrounding impact with local areas of sterilization. (2013) showed that after a small impact (<1km in diameter) the density of HCN created is approximately 10mol/m2 over a 100 km2 surface area, equivalent to between 500 and 10,000 years of HCN generation via lighting. This massive meteorite is displayed at the American Museum of Natural History. This event occurred about 15 million years ago, and a splatter of hot glass formed a strewn field across central Europe. It is one of the best-preserved "wet-target" impact craters in the world. 6A, ,6B).6B). Other meteoroids come from the asteroid belt, a zone between Mars and Jupiter filled with thousands of dwarf worlds that failed to coalesce into planets. (2017), Burton A.S., Stern J.C., Elsila J.E., Glavin D.P., and Dworkin J.P. (2012), Understanding prebiotic chemistry through the analysis of extraterrestrial amino acids and nucleobases in meteorites, Distribution, classification, and ages of martian impact crater lakes. However, as discussed by Neish et al. Impact events thus potentially provide a mechanism to generate habitable planets, satellites, and even asteroids throughout and beyond the Solar System with implications for investigating the habitability of exoplanets. Impact events can also deliver and/or generate many of the necessary chemical ingredients for life and catalytic substrates such as clays as well. 2014). Bottom line: Only 1878 meteorites have been found and verified in the U.S. in the past 212 years. 5 of the Most Significant Impact Craters in North America Meteorites have been found all over the world, and of the 1,671 verified in the United States as of April 2013, 158 came from Kansas (see Meteorites in the United States ). (2008), Deep drilling into the Chesapeake Bay impact structure, Gonzlez-Toril E., Martnez-Fras J., Gmez Gmez J.M., Rull F., and Amils R. (2005), Iron meteorites can support the growth of acidophilic chemolithoautotrophic microorganisms, Goudge T.A., Aureli K.L., Head J.W., Fassett C.I., and Mustard J.F. 2015). Schultz et al. Thus, impact-induced fracturing is suggested to improve fluid flow and thus access to nutrients and energy as is observed at Chesapeake (Cockell et al., Importantly, with increasing levels of shock (i.e., increasing pressure), there is a notable increase in porosity and permeability in target rocks (Singleton et al., (1997), Spectroscopic observations of comet C/1996 B2 (Hyakutake) with the Caltech Submillimeter Observatory, Lollar B.S., Onstott T.C., Lacrampe-Couloume G., and Ballentine C.J. 2004]) or as sedimentary basins providing unique climate records (e.g., Gale Crater currently being explored by the Mars Science Laboratory Curiosity rover [Grotzinger et al., 2009; Parnell et al., A portion of the organic molecules delivered from meteorites and cometary material thermally decomposes in the atmosphere and, reacting with the atmospheric N2, can then form CN radicals. acknowledges funding from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Space Agency, and MDA Maxar, for his Industrial Research Chair in Earth and Space Exploration. Although this process may result in sterilization of the deep subsurface near the point of impact, the subsequent generation of the hydrothermal system, and the eventual cooling and cessation of that system, would leave behind fracture networks that could be exploited by microorganisms (see Section 4.4). (2014), Reactivity and survivability of glycolaldehyde in simulated meteorite impact experiments, Planetary and Interstellar Processes Relevant to the Origins of Life, Lacustrine organic geochemistryan overview of indicators of organic matter sources and diagenesis in lake sediments, A production of amino acids under possible primitive Earth conditions. Ponds and flows of hot impact melt also occur in patches on top of the ballistic impact ejecta deposits in the crater exterior. (1993), Carbon abundance and silicate mineralogy of anhydrous interplanetary dust particles, Tornabene L.L., Osinski G.R., McEwen A.S., Wray J.J., Craig M.A., Sapers H.M., and Christensen P.R. 2019). United States Meteorite Impact Craters - Crooked Creek crater, missouri Only a few impact craters in the US can be seen from the ground. 2003). On Earth, plate tectonics is the main process responsible for crustal recycling and for transporting deep crustal and mantle materials to the surface or near subsurface (Condie, 2016). Several theories suggest a hot, aqueous environment for the origin of life (e.g., Pace, 1991; Stetter, 2005), and submarine hydrothermal vents have been widely proposed as candidate environments for prebiotic chemistry (e.g., Orgel, 1998; Nisbet and Sleep, 2001; Copley et al., Located in Ontario, this impact crater Scientists once believed that tthe impact crater was200 km across and about 10 km deep. Even if life did not originate in an impact crater or with building blocks provided by impact events, we propose that impact craters would have provided protected niches where life may have survived and eventually thrived during the Hadean and into the early Archean. Would love your thoughts, please comment. Such springs have been shown to have suitable fluid chemistries and sources of minerals and organics, in addition to generally lower temperature vent environments than black smoker submarine vents (see Cawood and Pirajno, 2008, and references therein). Moldavite - A green gem material formed by an asteroid impact. Lacustrine sediments in general offer not only a quiescent habitat allowing for the establishment of stable microbial communities but also excellent preservation potential of organics and other biomarkers (e.g., Meyers and Ishiwatari, 1993). When and where life originated on Earth and whether there existed, or exists, life elsewhere in our solar system represent some of the biggest unanswered scientific questions of our time. A wide range of hydrothermal minerals have been documented in impact structures around the world, with target lithology strongly influencing the mineral assemblages formed in impact-generated hydrothermal systems. This was a small event, with total energy estimated at 1 tonne TNT equivalent. These meteorite or impact craters appear as scars. 2004b; Zrcher and Kring, 2004)which both formed in a shallow marine setting. The crater has well-preserved ejecta deposits, and impact melt occurs both inside and outside the peak-ring. 2013) (see Section 2.2), an origin that has also been proposed to account for at least some of the clay detections on Mars (e.g., Newsom, 1980; Schwenzer and Kring, 2009; Tornabene et al., In the previous section we showed that fracturing, shock metamorphism, and melting during the thermobaric phase can create new surficial endolithic habitats. 2005). These 5 spectacular impact craters on Earth highlight our planet's wild They look very much like what one might intuitively expect from an object hitting the ground at high speed. (2018). In addition to the generation of necessary prebiotic chemical precursors, there is growing recognition that minerals may have played a role in both the formation of simple organic molecules such as formaldehyde and, possibly, in the formation of molecules as complex as RNA, through reactions mediated by minerals, in particular clays (Brack, 2006). There are three main sources of heat for impact-generated hydrothermal systems (Kirsime and Osinski, 2012; Osinski et al., 2014; Onstott et al., The crater is revealed on the Cook Station and Cherryville USGS 7.5 minute quadrangle maps. Cratering rates over the subsequent few hundred million years are the topic of ongoing debate. (2013) showed that HCN formation in meteorite impacts during the LHB could have provided an excellent mechanism for the production and concentration of cyanides, a hypothesis recently supported by the numerical modeling of Devon et al. 8600 Rockville Pike (2017), Evidence for a spatially extensive hydrothermal system at the Ries impact structure, Germany, The Lappajrvi impact structure (Finland): age, duration of hydrothermal crater cooling, and implications for life, Earth's impact events through geologic time: a list of recommended ages for terrestrial impact structures and deposits, The diatom flora in the vicinity of the Pretoria Salt Pan, Transvaal, Republic of South Africa. (2015) further demonstrated that impact-generated fracturing is likely responsible for this high porosity. Most workers interpret this association to be the result of the impact excavation of pre-impact clay minerals. Meteorite Impact Craters Meteorites crash through the Earth's atmosphere with tremendous force. Support from the Polar Continental Shelf Program and Northern Scientific Training program was critical in supporting fieldwork at the Haughton and other impact structures that forms a major component of this research. Of course, our Earth has been battered by meteorites numerous times since its creation and there are thousands of impact craters dotting the Earth. Subsequently, primary succession of the landscape will occur (Cockell and Lee, 2002). 2014, 2016) illustrates the longevity of this phase. 2004; Matrajt et al., Whether they were formed by volcanic eruptions or meteorite impacts, these 15 crater lakes harbor a special kind of beauty. The final setting where impact-generated hydrothermal alteration has been proposed is within crater lake sediments (Figs. The evolution of lacustrine environments on Mars: is Mars only hydrologically dormant? An excellent example is the 27km diameter Tooting Crater, which may have formed as recently as 3 Ma (Mouginis-Mark and Boyce, 2012). In short, a large number of experiments conducted over a range of shock pressures up to 40 GPa have shown that a large fraction of amino acids and organic molecule biomarkers can survive hypervelocity impact (e.g., Blank et al., Highly vesicular volcanic glass formed silica-rich (felsic) magmas during explosive volcanism. 2019). We also get your email address to automatically create an account for you in our website. Coupled with precipitation and the melting of surface and subsurface ice deposits, where applicable, the influx of water into the crater depression can lead to the generation of a crater lake, potentially within days or weeks (Figs. List of impact craters on Earth - Wikipedia 1986; Osinski, 2005; Arp et al., Of course, our Earth has been battered by meteorites numerous times since its creation and there are thousands of impact craters dotting the Earth. Such habitats are able to provide protection against stressors such as large temperature shifts, low water availability, and high levels of UV radiation (e.g., Cockell et al., 2013). These impact-processed rocks contain a diverse set of nutrients essential to microorganisms, many of which are mobilized within glasses and thus can be accessed through the use of organic acids (Pontefract et al., How to Survive a Killer Asteroid | WIRED Some planets are more pockmarked with craters than others: the Moon is covered with thousands but the Earth has only 200 confirmed meteorite . 0 0 votes. Cooling in these small to midsized structures is relatively rapid due to mainly convective heat transport (Kirsime and Osinski, 2012). Furthermore, unlike other geological processes such as volcanism or plate tectonics, impact cratering is ubiquitous on rocky and icy bodies in the Solar System (Fig. 2009) and provides a present-day fertile substrate (Cockell et al., In relatively small impacts producing simple craters up to 2km in diameter, it is also well known that fragments of the projectile survive largely intact as meteorites and are distributed within the host crater and around it in ejecta deposits (e.g., Barringer Crater in Arizona; Nininger, 1956). (D) Altered impact melt-bearing breccia from the Ries impact structure, Germany. The NSERC Discovery Grant and Northern Supplement programs are also gratefully acknowledged. Many meteoroids are associted with comets . 2011) evidence for the presence of water on the surface of Mars early in its history. 8C, ,8D)8D) and surface outcrops (Fig. We return to this in Section 6. We argue that impact events are not just isolated catastrophic geological events but a fundamental process in planetary evolution that plays an important role in the origin of life and in controlling planetary habitability. 2013). There have been 156 observed and recovered meteorite falls in the U.S.
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