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Wednesday, July 8, 2015


Arizona's Barringer Meteor Crater
Editor’s note: We’ve all scene meteor craters on the Moon and Mars.  The craters are everywhere.  Scientists on earth, however, say they’ve only been able to discover 160 craters—to date.  Why does the moon have so many craters and the earth so few?
If you don’t know the answer to this trick question then read on.
            If you go to the following address you will come across a 40-slide presentation of the crater activity on Earth:

GUEST BLOG--by Christian Koeberl and Virgil L. Sharpton writing for the Lunar and Planetary Institute--Impact cratering research has gained attention throughout the world following the suggestion that a large impact event caused the extinction of about 50% of all living species, including the dinosaurs, approximately 65 million years ago.

The evidence that a large asteroid or comet struck the Earth at that time came from detailed studies of the thin clay layer that globally marks the stratigraphic boundary between the Cretaceous and Tertiary (K-T) geological periods. This layer is enriched in the siderophile elements (such as iridium), indicating that the clay represents a mixture of normal crustal rocks, which typically have low siderophile-element abundances, and a small percentage of extraterrestrial material.

The worldwide integrated volume of the extraterrestrial material in the K-T boundary layer is equivalent to an asteroid approximately 10 kilometers in diameter — large enough to have produced a 200-kilometer-diameter crater. In the early 1990s, the subsurface Chicxulub structure in Mexico (slide #37) was confirmed as the long-sought Cretaceous-Tertiary boundary impact crater. An environmental crisis, triggered by the gigantic collision, contributed to the extinctions.

Based on apparent correspondences between periodicities observed in the marine extinction record and in the terrestrial impact record, some scientists have suggested that large meteorite impacts might be the metronome that sets the cadence of biological evolution on Earth — an unproved but intriguing hypothesis. Nevertheless, the study of the K-T extinction and its association with one of the largest impact structures known on Earth led to renewed and widespread interest in impacts.
Tomorrow in
Impact craters are formed when a large meteoroid (asteroid or comet) crashes into a larger planetary body that has a solid surface. All the bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The landscapes of the Moon, Mars, and Mercury have conspicuously preserved this bombardment record because the surfaces of these relatively small planetary bodies have remained unchanged over hundreds of millions of years.

Compared with the Moon, the Earth has been even more heavily bombarded over the course of its history due to its stronger gravitational attraction. However, impact craters are not immediately obvious on the surface of Earth because our planet is geologically active; the surface is in a constant state of change from erosion, infilling, volcanism, and tectonic activity.

These processes have led to the rapid removal or burial of Earth's impact structures. Thus, only about 160 terrestrial impact craters have been recognized to date. The majority of them are located within the geologically stable cratons of North America, Europe, southern Africa, and Australia; this is also where most of the crater searches have taken place. Spacecraft orbital imagery and geophysical surveys for resource exploration have helped to identify structures in more remote locations.

Daniel Moreau Barringer, early U.S.
explorer of Meteor Crater
Meteor Crater (also known as Barringer Crater), Arizona, with a diameter of approximately 1.2 kilometers, was the first terrestrial impact crater (slides #10 and #11) to be recognized as such. Its impact origin was first suspected late in the nineteenth century, when abundant iron meteorite fragments were discovered in the immediate vicinity of the crater.

This finding led the mining engineer Daniel Moreau Barringer to embark, between about 1905 and 1928, on a drilling project to find a suspected large iron meteorite body underneath the crater floor.

At this time, however, researchers did not yet have a clear understanding of the immense energy that is liberated when an extraterrestrial body hits the surface of the Earth with cosmic velocity. It was only in the 1920s that the first quantitative studies revealed the explosive nature of meteorite impact. Under impact conditions, tremendous amounts of energy are released instantaneously, completely destroying the cosmic projectile and generating a crater that is many times larger than the original meteoroid.

In the case of Meteor Crater, an iron meteorite body only about 30–50 meters in diameter was sufficient to create a crater 1.2 kilometers in diameter.

The key to understanding the explosive nature of an impact event is the high velocity with which a meteoroid hits the Earth. These velocities range between 11.2 kilometers per second (the escape velocity of the Earth-Moon system) and 72 kilometers per second (the orbital velocity of the Earth plus the escape velocity of the solar system at the distance of the Earth from the Sun). Because the kinetic energy liberated on impact of an object is proportional to the square of its velocity, these high-speed meteoroids can be, gram for gram, more than 100 times as explosive as TNT!

After the first studies on Meteor Crater, several other relatively small craters were also found to contain impactor fragments, and for many years these remnants were the only accepted evidence for impact origin. But because the projectile does not survive intact in large impact events, scientists have developed more sophisticated means of detecting the signatures of meteorite impact.

In some cases, nonterrestrial relative abundance of siderophile elements can be detected in the impact melt rocks within large craters (or in impact ejecta, as at the K-T boundary sediments mentioned above); this provides a chemical signature of the meteorite impactor.

The most commonly used chemical elements for such studies are the platinum group elements (e.g., iridium, osmium, and platinum). This is based on the fact that almost all meteorites have abundances of these elements that are higher by factors of 20,000 to 100,000 than those of average terrestrial crustal rocks. The addition of even a small meteoritic component (less than 1%) results in distinctly elevated platinum group element contents in the impact breccias or melt rocks.

Since the 1960s, numerous studies have documented another physical marker of meteorite impact: shock metamorphism. This refers to metastable or irreversible effects produced in various target rocks and minerals as the strong shock wave passes through them. As hypervelocity impact is the only naturally occurring process capable of generating strong shocks in crustal rocks, certain shock-metamorphic effects are unambiguous signatures of meteorite impact.

Diagnostic shock effects include shatter cones, multiple sets of microscopic planar deformation features (PDFs) in quartz, feldspar, and most other rock-forming and accessory minerals, diaplectic glass, and high-pressure mineral phases, such as stishovite (a high-pressure form of quartz). Even diamonds are formed by high-pressure conversion of graphite in target rocks.

Researchers have recognized that the presence of shock-metamorphic effects is a much better indicator of the impact origin of a geologic structure than the presence of meteorite fragments (which are rapidly destroyed by erosion anyway). Experimental studies over the past three to four decades have provided a good database that shows which types of shock features form at which pressures. It was also recognized that the effects resulting from shock (nonequilibrium processes) are different from those resulting from static high pressures (an equilibrium process).

Today, terrestrial impact structures are confirmed based on the presence of some or all of these shock effects.

Impact craters are divided into two main groups, based on their morphology: simple craters and complex craters. Simple craters are relatively small, with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl shape (slide #1, top). In larger craters, however, gravity causes the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20) (slide #1, bottom). The diameter at which craters become complex depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2–4 kilometers (depending on target rock properties); on the Moon, at one-sixth Earth's gravity, the transition diameter is 15–20 kilometers.

The cratering process is traditionally divided into three stages: The contact and compression stage begins when the impactor hits the ground and initiates a shock wave that travels into the target and into the impactor, compressing the target and generating shock metamorphic effects. This is followed by the excavation stage, wherein the release of the shock compression leads to mass flow that opens up the crater and ends with a relatively deep transient cavity.

Finally, the modification stage involves collapse of the steep walls of the transient crater and infilling of the crater by fall-back debris. The complete crater-forming sequence takes less time than it would to free fall from a height equivalent to the final crater diameter. In the case of Meteor Crater, the compression and excavation phases of the crater formation were over in a few seconds. Thus, impact cratering has the distinction of being the geologic process that releases the greatest amount of energy in the shortest amount of time.

The final crater expression depends on the magnitude of the event. The central peak or peak ring of the complex crater is formed as the initial (transient) deep crater floor rebounds from the compressional shock of impact. Slumping of the rim further modifies and enlarges the final crater. Complex structures in crystalline rock targets may also contain coherent sheets of impact melt overlying the shocked and fragmented rocks of the crater floor.

On the geologically inactive lunar surface, this complex crater form will be preserved until subsequent impact events alter it. On Earth, weathering and erosion of the target rocks quickly alter the surface expression of the structure; despite the crater's initial morphology, crater rims and ejecta blankets are quickly eroded and concentric ring structures can be produced or enhanced as weaker rocks of the crater floor are removed. More resistant rocks of the melt sheet may be left as plateaus overlooking the surrounding structure.

Large terrestrial impacts are of greater importance for the geologic history of the Earth than the number and size of preserved structures might suggest. Currently, about 160 structures of impact origin have been confirmed on the Earth's surface (slide #2). Precise ages are known for only about one-third of these structures. Crater ages can be determined by a variety of methods.

The more precise ones involve radiometric dating of impact melt rocks or impact glasses or biostratigraphic dating of related impact ejecta within a well-defined stratigraphic sequence. The paucity of age data reflects not only the lack of detailed studies, but, in many cases, the lack of datable material, especially for deeply eroded or subsurface structures.

This collection of slides in this report presents orbital and aerial photographic views of a selection of proven or suspected terrestrial impact structures that represent the variety in appearance of impact structures at different erosional stages on Earth. Several examples of impact structures on Earth's planetary neighbors are included to show the fundamental role impact plays in shaping planetary surfaces. These relatively well-preserved extraterrestrial craters provide an important reference for understanding the more eroded impact features on Earth.

The terrestrial structures we have chosen represent a compromise between those with the best surface expression and those that represent a diversity of age, size, and appearance as the craters are reworked by geologic processes. The freshest craters are presented first, followed by those that exhibit more ambiguity and complexity. We have included two examples of features that have the appearance of impact structures, but have not yet been shown to contain any of the diagnostic chemicals or physical markers of impact events.

However, we would like to caution against just using images or remote sensing to identify impact craters on Earth without corroborating petrographic and geochemical studies on crater rocks. Only such studies can provide confirming evidence that a geological structure is of impact origin.

NOTE: In the orbital photographs of this 40-slide presentation of terrestrial craters, north is up, unless otherwise noted. The Lunar and Planetary Institute is operated by the Universities Space Research Association under contract number NASW-4574 with
the National Aeronautics and Space Administration.



GUEST BLOGBy Wikipedia--The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named. The age of the Chicxulub asteroid impact and the Cretaceous–Paleogene geological boundary (K–Pg boundary) coincide precisely. The crater is more than 110 miles in diameter and 12 miles in depth, making the feature one of the largest confirmed impact structures on Earth; the impacting bolide (a large meteor that explodes in the atmosphere) that formed the crater was at least six miles in diameter.

Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán during the late 1970s, discovered the crater. Penfield was initially unable to obtain evidence that the geological feature was a crater, and gave up his search. Through contact with Alan Hildebrand, Penfield obtained samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.

The age of the rocks marked by the impact shows that this impact structure dates from roughly 66 million years ago, the end of the Cretaceous period, and the start of the Paleogene period. It coincides with the K-Pg boundary, the geological boundary between the Cretaceous and Paleogene.

The impact associated with the crater is thus implicated in the Cretaceous–Paleogene extinction event, including the worldwide extinction of non-avian dinosaurs. This conclusion has been the source of controversy. In March 2010, 41 experts from many countries reviewed the available evidence: 20 years' worth of data spanning a variety of fields. They concluded that the impact at Chicxulub triggered the mass extinctions at the K–Pg boundary.

In 1978, geophysicists Antonio Camargo and Glen Penfield were working for the Mexican state-owned oil company Petróleos Mexicanos, or Pemex, as part of an airborne magnetic survey of the Gulf of Mexico north of the Yucatán peninsula. Penfield's job was to use geophysical data to scout possible locations for oil drilling. In the data, Penfield found a huge underwater arc with "extraordinary symmetry" in a ring 40 miles across.

He then obtained a gravity map of the Yucatán made in the 1960s. A decade earlier, the same map suggested an impact feature to contractor Robert Baltosser, but he was forbidden to publicize his conclusion by Pemex corporate policy of the time. Penfield found another arc on the peninsula itself, the ends of which pointed northward. Comparing the two maps, he found the separate arcs formed a circle, 111 miles wide, centered near the Yucatán village Chicxulub; he felt certain the shape had been created by a cataclysmic event in geologic history.

Pemex disallowed release of specific data but let Penfield and company official Antonio Camargo presented their results at the 1981 Society of Exploration Geophysicists conference. That year's conference was underattended and their report attracted scant attention. Coincidentally, many experts in impact craters and the K–Pg boundary were attending a separate conference on Earth impacts. Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact.

He knew Pemex had drilled exploratory wells in the region. In 1951, one bored into what was described as a thick layer of andesite about 4,200 ft. down. This layer could have resulted from the intense heat and pressure of an Earth impact, but at the time of the borings it was dismissed as a lava dome — a feature uncharacteristic of the region's geology. Penfield tried to secure site samples, but was told such samples had been lost or destroyed. When attempts at returning to the drill sites and looking for rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work.

At the same time, scientist Luis Walter Alvarez put forth his hypothesis that a large extraterrestrial body had struck Earth and, unaware of Penfield's discovery, in 1981 University of Arizona graduate student Alan R. Hildebrand and faculty adviser William V. Boynton published a draft Earth-impact theory and sought a candidate crater. Their evidence included greenish-brown clay with surplus iridium containing shocked quartz grains and small weathered glass beads that looked to be tektites.

Thick, jumbled deposits of coarse rock fragments were also present, thought to have been scoured from one place and deposited elsewhere by a kilometres-high tsunami resulting from an Earth impact. Such deposits occur in many locations but seem concentrated in the Caribbean basin at the K–Pg boundary.

So when Haitian professor Florentine Morás discovered what he thought to be evidence of an ancient volcano on Haiti, Hildebrand suggested it could be a telltale feature of a nearby impact. Tests on samples retrieved from the K–Pg boundary revealed more tektite glass, formed only in the heat of asteroid impacts and high-yield nuclear detonations.

In 1990, Houston Chronicle reporter Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater. Hildebrand contacted Penfield in April 1990 and the pair soon secured two drill samples from the Pemex wells, stored in New Orleans. Hildebrand's team tested the samples, which clearly showed shock-metamorphic materials.

A team of California researchers including Kevin Pope, Adriana Ocampo, and Charles Duller, surveying regional satellite images in 1996, found a sinkhole (cenote) ring centered on Chicxulub that matched the one Penfield saw earlier; the sinkholes were thought to be caused by subsidence of the impact crater wall. More recent evidence suggests the actual crater is 190 miles wide, and the previous ring is in fact an inner wall of it.

The Chicxulub impactor had an estimated diameter of 10 km (6.2 mi) and delivered an estimated energy equivalent of 100 teratons of TNT (4.2×1023 J).[21] By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×1017 J),[22] making the Chicxulub impact 2 million times more powerful. Even the most energetic known volcanic eruption, which released an estimated energy equivalent of approximately 240 gigatons of TNT (1.0×1021 J) and created the La Garita Caldera,[23] delivered only 0.24% of the energy of the Chicxulub impact.

The impact would have caused some of the largest megatsunamis in Earth's history. A cloud of super-heated dust, ash and steam would have spread from the crater as the impactor (meteor) burrowed underground in less than a second. Excavated material along with pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon re-entry, boiling the Earth's surface and possibly igniting wildfires; meanwhile, colossal shock waves would have triggered global earthquakes and volcanic eruptions.

The emission of dust and particles could have covered the entire surface of the Earth for several years, possibly a decade, creating a harsh environment for living things. The shock production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect. Over a longer period, sunlight would have been blocked from reaching the surface of the Earth by the dust particles in the atmosphere, cooling the surface dramatically. Photosynthesis by plants would also have been interrupted, affecting the entire food chain. A model of the event developed by Lomax et al. (2001) suggests that net primary productivity (NPP) rates may have increased to higher than pre-impact levels over the long term because of the high carbon dioxide concentrations.

In February 2008, a team of researchers led by Sean Gulick at the University of Texas at Austin's Jackson School of Geosciences used seismic images of the crater to determine that the impactor landed in deeper water than was previously assumed. They argued that this would have resulted in increased sulfate aerosols in the atmosphere. According to the press release, that "could have made the impact deadlier in two ways: by altering climate (sulfate aerosols in the upper atmosphere can have a cooling effect) and by generating acid rain (water vapor can help to flush the lower atmosphere of sulfate aerosols, causing acid rain)."

In their 1991 paper, Hildebrand, Penfield, and company described the geology and composition of the impact feature. The rocks above the impact feature are layers of marl and limestone reaching to a depth of almost 3,300 ft. These rocks date back as far as the Paleocene. Below these layers lie more than 1,600 ft of andesite glass and breccia. These andesitic igneous rocks were only found within the supposed impact feature, as is shocked quartz. The K–Pg boundary inside the feature is depressed to 2,000 to 3,600 ft. compared with the normal depth of about 1,600 ft. measured 3.1 miles away from the impact feature.

Along the edge of the crater are clusters of cenotes or sinkholes, which suggest that there was a water basin inside the feature during the Neogene period, after the impact. The groundwater of such a basin would have dissolved the limestone and created the caves and cenotes beneath the surface. The paper also noted that the crater seemed to be a good candidate source for the tektites reported at Haiti.

In September 2007, a report published in Nature proposed an origin for the asteroid that created Chicxulub Crater. The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný, argued that a collision in the asteroid belt 160 million years ago resulted in the Baptistina family of asteroids, the largest surviving member of which is 298 Baptistina. They proposed that the "Chicxulub asteroid" was also a member of this group. The connection between Chicxulub and Baptistina is supported by the large amount of carbonaceous material present in microscopic fragments of the impactor, suggesting the impactor was a member of a rare class of asteroids called carbonaceous chondrites, like Baptistina.

According to Bottke, the Chicxulub impactor was a fragment of a much larger parent body about 110 miles across, with the other impacting body being around 40 miles in diameter.

In 2011, new data from the Wide-field Infrared Survey Explorer revised the date of the collision which created the Baptistina family to about 80 million years ago. This makes an asteroid from this family highly improbable to be the asteroid that created the Chicxulub Crater, as typically the process of resonance and collision of an asteroid takes many tens of millions of years.

In 2010, another hypothesis was offered which implicated the newly discovered asteroid P/2010 A2, a member of the Flora family of asteroids, as a possible remnant cohort of the K/Pg impactor.

The Chicxulub Crater lends support to the theory postulated by the late physicist Luis Alvarez and his son, geologist Walter Alvarez, that the extinction of numerous animal and plant groups, including dinosaurs, may have resulted from a bolide impact (the Cretaceous–Paleogene extinction event). Luis and Walter Alvarez, at the time both faculty members at the University of California, Berkeley, postulated that this enormous extinction event, which was roughly contemporaneous with the postulated date of formation for the Chicxulub crater, could have been caused by just such a large impact.

This theory is now widely accepted by the scientific community. Some critics, including paleontologist Robert Bakker, argue that such an impact would have killed frogs as well as dinosaurs, yet the frogs survived the extinction event.

Gerta Keller of Princeton University argues that recent core samples from Chicxulub prove the impact occurred about 300,000 years before the mass extinction, and thus could not have been the causal factor.

The main evidence of such an impact, besides the crater itself, is contained in a thin layer of clay present in the K–Pg boundary across the world. In the late 1970s, the Alvarezes and colleagues reported that it contained an abnormally high concentration of iridium.

Iridium levels in this layer reached 6 parts per billion by weight or more compared to 0.4 for the Earth's crust as a whole; in comparison, meteorites can contain around 470 parts per billion of this element. It was hypothesized that the iridium was spread into the atmosphere when the impactor was vaporized and settled across the Earth's surface amongst other material thrown up by the impact, producing the layer of iridium-enriched clay.

In recent years, several other craters of around the same age as Chicxulub have been discovered, all between latitudes 20°N and 70°N. Examples include the disputed Silverpit crater in the North Sea and the Boltysh crater in Ukraine. Both are much smaller than Chicxulub, but are likely to have been caused by objects many tens of metres across striking the Earth. This has led to the hypothesis that the Chicxulub impact may have been only one of several impacts that happened nearly at the same time. Another possible crater thought to have been formed at the same time is the larger Shiva crater, though the structure's status as a crater is contested.

The collision of Comet Shoemaker–Levy 9 with Jupiter in 1994 demonstrated that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet should collide with a planet. Comets undergo gravitational interactions with the gas giants, and similar disruptions and collisions are very likely to have occurred in the past. This scenario may have occurred on Earth at the end of the Cretaceous, though Shiva and the Chicxulub craters might have been formed 300,000 years apart.

In late 2006, Ken MacLeod, a geology professor from the University of Missouri, completed an analysis of sediment below the ocean's surface, bolstering the single-impact theory. MacLeod conducted his analysis approximately 2,800 miles from the Chicxulub Crater to control for possible changes in soil composition at the impact site, while still close enough to be affected by the impact. The analysis revealed there was only one layer of impact debris in the sediment, which indicated there was only one impact. Multiple-impact proponents such as Gerta Keller regard the results as "rather hyper-inflated" and do not agree with the conclusion of MacLeod's analysis, arguing that there might only be gaps of hours to days between impacts in a multiple impact scenario (cf. Shoemaker-Levy 9) which would not leave a detectable gap in deposits.

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