Impact cratering

Space-age discoveries about the surface character of other terrestrial planets and satellites around planets throughout the solar system reveal that the early Earth must have been heavily scarred by impacts with planetesimals and minor bodies. Erosion processes and plate tectonics have obliterated most of these ancient craters, but modern evidence that major impacts may have had a significant role in shaping the evolution of life has spurred a search for large impact craters.

Overview

Impact cratering is one of the most fundamental geologic processes in the solar system. Craters have been found on the surfaces of all the solid planets and natural satellites thus far investigated by spacecraft. Mercury and the Moon, bodies whose ancient surfaces have not been reworked by subsequent geologic processes, preserve a vivid record of the role that impact cratering has played in the past. It is inconceivable that the Earth somehow escaped the bombardment that caused such widespread scarring or that it does not continue to be a target for planetesimals still roaming the solar system.

In the 1960s, only a handful of sites on Earth were accepted to be of impact origin. In the early 2020s, the number of confirmed astroblemes (circular surface features considered to have been large impact craters) reached 190 and increased annually. Additionally, many probable and possible impact features are under study. Nevertheless, an enormous discrepancy exists between the number of identified or suspected impact sites on Earth and the number that might be expected.

It is assumed that the flux of incoming bodies is the same for Earth as for the Moon. Making allowances for the fact that the Earth is the largest target of any of the terrestrial planets and that more than two-thirds of its surface is covered by water, planetologists calculate that land areas of the Earth should have been scarred by at least fifteen hundred craters ten kilometers or more in diameter. Only about half of the known astroblemes are in this size range. On a global scale, 99 percent of the predicted large impact craters seem to be missing. However, this statistic is not a valid indicator of the impact history of the Earth. Although the impact phenomenon is a geographic process, the probability of discovering impact sites is strongly modified by the geologic stability of various regions of the Earth and by the intensity of the search programs in those areas. Roughly half of the confirmed astroblemes have been found in Canada, constituting only one percent of the Earth’s surface. In part, this is owing to the stability of the Precambrian rock of the Canadian Shield, which preserves more of the crater’s features, but it also reflects a diligent research effort by Canada’s Department of Energy, Mines, and Resources. In general, the number of large impact sites found in the well-explored areas of the Earth agrees with the accepted rate of crater formation on the other terrestrial planets in the past two billion years.

The obvious difference between the surface appearances of Earth and the Moon is explained not by any difference in the rate at which impact craters have formed but in the rate at which they are destroyed. Most of the tremendous numbers of craters on the Moon are more than 3.9 billion years old, while the Earth’s oldest surviving astroblemes were formed less than two billion years ago. Studies have shown that erosion effectively removes all traces of a 100-meter (diameter) crater in only a few thousand years and that a one-kilometer-wide crater, such as the well-known Barringer meteor crater in Arizona, will disappear within a million years. Only craters with diameters greater than 100 kilometers can be expected to leave any trace after a billion years of erosion. This explains not only the absence of widespread cratering on Earth’s landscape but also the fact that, among the astroblemes known to exist, medium and large scars are more common than small ones.

Significant craters can be produced only by objects having masses of hundreds of thousands to billions of tons. The Barringer crater, 1.2 kilometers wide and 200 meters deep, is believed to have been formed by a one-million-ton planetesimal that was perhaps fifty meters in diameter. A twenty-seven-kilometer-wide astrobleme known as the Nördlinger Ries crater in Germany required an impacting body greater than one kilometer in diameter with a mass in excess of one billion tons. Planetesimals as large as these two examples are not characteristic of the vagrant meteors that wander through the solar system and occasionally streak into the Earth’s skies as shooting stars.

Most of the past impacts on Earth and the Moon appear to be attributable to a family of asteroids known as the Apollo-Amor group (after two specific members of the family). Members of this group are in orbits that graze Earth’s orbit and become subject to orbital perturbations that lead them across Earth’s path periodically. It is estimated that the average Apollo-Amor object intersects Earth’s orbit once every five thousand years, although, usually, the planet is at some other point in its orbit when this happens. The probability of a collision between Earth and any given Apollo-Amor object is small, but several studies have shown that this family contains between 750 and 1,000 asteroids larger than one kilometer in diameter. Statistical analysis suggests that such sizable bodies must collide with the Earth an average of once every 600,000 years.

Impact events involve tremendous transfers of energy from the incoming planetesimal to Earth’s surface. A projectile’s energy of motion increases linearly with its mass, but as the square of its velocity, surprisingly large craters result from relatively small bodies traveling at hypervelocities. Depending on the directions of motion of Earth and of the planetesimal, impacts on the planet may involve relative velocities as high as fifty kilometers per second. At velocities surpassing four kilometers per second, the energy of the Shock wave created by the impact is far greater than the strength of molecular adhesion for either the planetesimal or Earth. Therefore, on impact, the planetesimal acquires the properties of a highly compressed gas and explodes with a force equivalent to a similar mass of blasting powder.

The shock wave from this explosion intensely compresses the target material and causes it to be severely deformed, melted, or even vaporized. In all but the smaller impacts, the entire projectile is also vaporized. The shock wave swiftly expands in a radial fashion, pulverizing the target material and intensely altering the nature of the target rock by extreme and almost instantaneous heat and pressure. This is immediately followed by decompression and what is called a rarefaction wave that restores the ambient pressure. The rarefaction wave moves only over free surfaces, so it travels outward over the ground surface and into the atmosphere above the impact. It becomes the excavating force that lifts vast quantities of the pulverized target material upward and outward to create the crater cavity.

The rarefaction wave excavates a hole whose depth is one-third of its diameter and whose profile follows a parabolic curve, but this depression is short-lived and is, therefore, called the transient cavity. After the passage of the rarefaction wave, a large amount of pulverized target material from the walls of the transient cavity slumps inward under gravity, and some of the ejecta, lofted straight up into the atmosphere, falls back into the excavation. Together, these sources contribute to a lens-shaped region of breccia that fills the true crater’s floor and leaves a shallower, flat-floored, apparent crater as the visible scar of the impact. Apparent craters generally exhibit a depth of only one-tenth to one-twentieth of their diameters. Meanwhile, the rarefaction wave carries ejecta particles outward over the surrounding landscape, where they fall to Earth as a blanket of regolith that is distinguishable from the local target rock by the effects of shock metamorphism.

Methods of Study

Impact phenomena are rare enough on the human timescale that no crater-forming events are known to have occurred in recorded history. Owing to this passage of time and to the fact that most existing astroblemes have been severely altered by erosion, impact cratering has been studied by the unique modifications that a powerful impact shock makes in the rocks and minerals at the site. Scientists study the deformation and structural damage to buried strata by looking for the presence of certain rare elements and minerals in the sediments surrounding suspected impact sites.

Much attention has been given to the effects of the shock wave on terrestrial rocks since shock metamorphism is considered the most enduring and positive identifier of ancient astroblemes. Shock metamorphism differs from endogenic metamorphism by the scales of pressure and temperature involved and by the very short duration of the exposure to those pressures and temperatures. Endogenic metamorphism usually involves pressures of less than one gigapascal (100,000 atmospheres) and temperatures not greater than 1,250 kelvins. The pressures involved in shock metamorphism are exponentially greater, reaching several hundred gigapascals for an instant in the vicinity of the impact. Rock exposed to pressures in excess of eighty gigapascals and temperatures of several thousand kelvins is immediately vaporized. Lesser pressures and temperatures at increased distances from the point of impact produce signs of melting, thermal decomposition, phase transitions, and plastic deformation.

Pockets of melt glass up to several meters thick are commonly found in the breccia within the crater, indicating that pressures there reached forty-five to sixty gigapascals. Coesite and its denser relative, stishovite, are forms of quartz that occur naturally only at impact sites. Shatter cones, conically shaped crystals created at pressures of from two to twenty-five gigapascals, are another prominent feature of shock metamorphism and are particularly well-developed in fine-grained isotropic rock. Microscopic examination of impact-shocked porous rock reveals that quartz grains are deformed to fill the pores and interlock like jigsaw puzzle pieces. Even a considerable distance from the impact point, quartz grains tend to be elongated in the direction of the shock wave’s passage.

Theories concerning cratering dynamics can also be tested by analogy to some of the craters produced by the detonation of nuclear devices. This latter technique has adequately explained the morphology of the smaller astroblemes, those with diameters that do not exceed two to four kilometers. Larger impact events involve additional dynamics that are not mimicked by nuclear devices thus far tested. Astroblemes greater than two kilometers in diameter in sedimentary rock or four kilometers in diameter in crystalline rock display a pronounced central uplift owing to an intense vertical displacement of the strata under the center of the impact. An additional feature distinguishing complex craters is that their depths are always a much smaller fraction of their diameters than is the case with simple craters.

Photographic imaging of Earth from space has revealed some young and well-preserved astroblemes in remote and poorly explored areas of Earth, such as the Sahara Desert. More important has been the satellite’s ability to reveal structures that still preserve a faint but distinct circularity when seen from orbit, although, at ground level, they are so eroded that their circularity has escaped detection. One of the largest astroblemes yet discovered was detected from landsat satellite images in this way. Modern imaging technologies, including advanced radar and sonar mapping, promise to extend the capabilities of space surveillance and remote sensing in recognizing possible impact sites.

Context

The degree to which the Earth is in danger of being struck by a massive planetesimal began to be appreciated about the middle of the twentieth century. In 1980, a team led by Nobel Prize-winning physicist Luis Alvarez announced dramatic evidence suggesting that an asteroid impact that occurred sixty-five million years ago created such planetary stress that it explained a mysterious massive extinction of life forms known to have occurred on the Earth at that time. At sites all around the world, researchers discovered that clay deposits at the boundary layer between the Cretaceous and Tertiary periods contained up to one hundred times the normal abundance of the metal iridium, which is rare in Earth’s crustal rocks but 1,000 to 10,000 times more abundant in the makeup of many asteroids. This Cretaceous-Tertiary boundary layer is coincident with the point at which fully 70 percent of the life forms then existing on the Earth, including the dinosaurs, became extinct. Further study has also revealed that this same sediment layer is rich in shock-metamorphosed quartz grains, known only to occur naturally from impact explosions.

Debate continues about whether an asteroid impact was the primary cause of the mass extinctions at the close of the Cretaceous period or merely the final factor. Still, there is general agreement that a colossal impact occurred at that time. The volume of material in the boundary sediments suggests that the planetesimal was perhaps ten kilometers in diameter and would have created a crater of as much as 200 kilometers in width. An astrobleme in the Gulf of Mexico near Belize, called the Chicxulub Crater, closely fulfills these criteria. Many scientists accept it as the impact site for the K-T (German for Cretaceous-Tertiary) event. Meanwhile, several other iridium spikes (abnormally high metal concentrations) have been found in the sedimentary beds, coinciding with other recognized mass extinctions.

Early in the twenty-first century, several researchers put forward candidate craters to mark an impact event dated to the time paleontologists often call the Great Dying. At the boundary between the Permian and the Triassic (the P-T boundary), which also marks the end of the Paleozoic and the start of the Mesozoic era, life on Earth was very nearly exterminated. A conservative estimate is that 95 percent of all species died then. Life rebounded, and the dinosaurs went on to rule the Earth until they, too, were wiped out catastrophically. Of the various craters proposed to have resulted from a P-T boundary impact event, the one that appears most likely to be correct (if any of them are correct) is a crater located in Antarctica, buried, unfortunately, under 1.5 kilometers of ice. What provides extra confidence that this crater could result from a P-T boundary impact event is the fact that the Siberian Traps are located at its antipode. Energy from the impact would have undergone antipodal focusing through the Earth’s core to ravage the area on the planet 180° away from the impact site. The Siberian Traps experienced tremendous amounts of volcanic activity around 248 million years ago, during the P-T boundary and the Great Dying. This scenario remains controversial but, if true, would represent an even larger impact event than the accepted K-T boundary event that gave rise to the Chicxulub Crater.

Three related discoveries suggest that impact cratering may not be entirely random regarding its distribution through time. Paleontologists David Raup and J. John Sepkoski, Jr. showed evidence, based on a rigorous analysis of the marine fossil record, that mass extinctions appear to occur with regularity every twenty-six million years. Independently, the team of Walter Alvarez (also a member of the team that discovered the K-T iridium anomaly) and Richard Muller have found evidence that the ages of major known terrestrial astroblemes seem to be periodically distributed at intervals of roughly twenty-eight million years. For some time, researchers have sought a mechanism that could account for the numerous polarity reversals in Earth’s magnetic field over geologic history, and some have suggested that significant impact events may be the cause. Several studies have reported an apparent fine-scale periodicity in Earth’s magnetic field reversals with a thirty million-year cycle. Although the intervals are not in perfect agreement, they are very close, considering the difficulty of dating extinctions and astroblemes' exact ages.

These discoveries suggest that there may be an undiscovered member of the solar system that moves in such a way as to periodically disrupt the Oort Cloud, the cloud of comets believed to exist on the fringes of the solar system, causing a barrage of planetesimals to descend upon the inner planets. Although the existence and location of such a body remain speculative and controversial, it has been characterized as a dwarf companion star of the Sun and is called Nemesis.

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