More Bolide Impact Information

Impact Glass

Glassy material produced by a complete or partial fusion of target rock by the heat generated from the impact of a large meteorite and occurring in and around the resulting crater; see tektite and shock metamorphism.

Impact Melt

During an impact event, a compression wave spreads outward from the point of impact. Engulfed by the shock wave, the colliding meteoroid and the surface rocks are melted and partially vaporized; see impact crater, tektite, and shock metamorphism.


Impactite

Slag-like glassy material ranging from glassy and vesicular to finely crystalline that was produced by complete or partial fusion of a host rock by sudden, powerful energy release caused by the impact of a large bolide. Impactites consist of both shocked and unshocked mineral grains and rock fragments and contain many spherules of iron-nickel specks. The rock is also known to collectors as Libyan Desert Glass; see impact crater, tektite, and shock metamorphism. Real World Example: Impactites can be found at the Ries Crater, Bavaria, Germany, as well as the Henbury Crater in central Australia and the Wabar Crater in the Rub’al-Khali, Saudi Arabia.

Shock Metamorphism

Process of irreversible chemical or physical alteration in rocks, minerals, and crystals by shock waves generated by bolide impact, extremely powerful volcanic explosion, or by the detonation of high-explosive or nuclear devices. These ultra-high pressure changes include: heterogeneous vesicular fused glass, planar features in quartz, planar features, and bent twins in feldspar, shatter cones, shock lamellae in mineral grains, kink bands in biotite flakes, quartz, and feldspars that have been completely transformed into diaplectic glass — glass formed at high-pressure in the solid-state in the feldspar minerals, especially plagioclase; the shock wave can break down the structure of the mineral, changing parts of it into glass that is isotropic, uniform in all directions — and creation of a high-pressure form of quartz called stishovite, a mineral that has never been found anywhere on Earth but around impact sites. Also known as impact metamorphism.

Shoemaker, Eugene M.

American geologist (1928-1997) whose passion was astrogeology; his contributions to that field and the study of bolides, impact craters, lunar science, asteroids, and near-Earth comets are legendary.[1] After graduating from California Institute of Technology in 1947 with a B.S. in geology at age 19, a year later he earned an M.S. and began working for the USGS. His first assignment was exploring for uranium deposits in Colorado and Utah. Those studies brought him geographically and intellectually in close proximity to the many volcanic features and the major impact structure on the Colorado Plateau in the western United States, specifically Hopi Buttes and Barringer (Meteor) Crater, which he became convinced was the result of a bolide impact. After being assigned by the USGS to study cratering at the Nevada nuclear weapons test site in 1956, he took a leave and entered the PhD program at Princeton University. In the period 1957-1960, he engaged in dissertation research that is today acknowledged as the classic study of the structure and mechanics of meteorite impact by comparing nuclear craters at the Nevada Test Site with the similar Barringer (Meteor) Crater in north-central Arizona, whose meteorite impact origin he established scientifically once and for all. That research — including the discovery of coesite (a high pressure form of silica created during bolide impacts) with Edward Chao — immediately became the definitive benchmark on basic bolide impact cratering. Immediately after that Shoemaker demonstrated the impact origin of the lunar crater Copernicus; mapped the Copernicus region using the principles of terrestrial geologic field mapping, thereby originating mapping methods that are still in use; established a lunar stratigraphic timescale; proved (with Edward Chao) the impact origin of the previously enigmatic Ries crater in Germany; created the Astrogeologic Studies Group within the USGS that was destined to take the lead in lunar studies and became its director; and gained international recognition for himself and his ideas at geological and astronomical conferences in Denmark and Russia. All that was accomplished before President John F. Kennedy established in May 1961 the landing of a man on the Moon as the goal of Project Apollo. It was a research focus that Shoemaker continued throughout his life, both through exploration of the Earth, specifically in Australia, and the planets by remote sensing and mapping. Without Shoemaker’s pioneering research, little would be known about the Moon’s geology or, probably, much else about its nature. At NASA Headquarters during 1962 and 1963 with high intensity and enthusiastic determination he successfully lobbied for the addition of a vigorous scientific program featuring geology to Apollo’s land-and-return goal. Although Addison’s disease[2] kept him from a lifelong dream of traveling to the Moon, Shoemaker initiated and vigorously promoted the intensive geologic training of the astronauts that made them capable scientific observers. He was a major investigator of the imaging by unmanned Ranger and Surveyor satellites which, before any Apollo landing, revealed the nature of the Moon’s cover of soil and broken rock, which he named the regolith. He led the field geology support teams for Apollos 11 and 12 but, intensely disappointed with NASA’s lagging commitment to science, returned to academic life as Professor and Chairman of Caltech’s Division of Geology and Planetary Sciences from 1969 to 1972. He culminated his lunar studies in 1994 with new data on the Moon from Project Clementine, for which he was the science-team leader. Tragically, Shoemaker was killed in a car accident in July 1997 in Australia while investigating meteor impact sites with his wife and professional colleague, Carolyn (an internationally famous astronomer, the discoverer of 32 comets, more than 800 asteroids and co-discoverer of the Shoemaker-Levy 9 comet). In a fitting tribute conceived by a former student, Eugene Shoemaker’s ashes were placed on-board the Lunar Prospector spacecraft, which successfully reached a polar mapping orbit around the Moon. After completing its scientific mission, the spacecraft impacted the Moon, scattering Shoemaker’s ashes across the lunar surface. During his lifetime Shoemaker’s pioneering research was recognized by at least 25 awards, including election to the National Academy of Sciences in 1980; the Day Medal of the Geological Society of America, awarded in 1982; the National Medal of Science, presented personally by President George H. W. Bush in 1992; and the American Geophysical Union’s Bowie Medal, awarded in 1996. Elected an AGU fellow in 1971, he served as President of the Planetary Sciences Section (1968-1970).

Tektite[3]

Group of silica-rich glassy objects that, although still poorly understood, are today thought by most scientists to be melt products of terrestrial rocks formed by hypervelocity impacts of large, extraterrestrial objects. They have no crystal structure and superficially resemble obsidian in appearance and chemical composition. However, several things distinguish these objects from obsidian. Primarily, they have a very low water content, a low alkali content, and they always contain lechatelierite (pure fused silica glass).They also often contain coesite (a highly dense silica polymorph), nickel-iron spherules, and baddeleyite (a zircon oxide mineral produced at very high temperatures during shock metamorphism), which lend evidence to a meteorite impact origin. In addition, many of the tektites found exhibit aerodynamic shapes, as if they were formed during flight. Relict mineral inclusions often yield information about the tektite parent material. Tektite sizes range from less than one millimeter to chunks 10-20 centimeters in width, with most being a centimeter or so in size, weighing a few grams, and are jet-black to dark green, although some have a yellowish tint.

Author’s Note: Tektites exhibit a wide array of sizes, shapes, and surface features. For example, splash forms include spheres, teardrops, dumbbells, discs, ablated forms also known as buttons, and chunks known as Muong Nong types that display a layered structure and are found primarily in Southeast Asia, hence the name. Real World Examples: Tektites are found in broad geographic bands, or strewnfields. Four of the major strewnfields are in Australia (where millions have been scattered over an area the size of Texas), Ivory Coast, Czechoslovakia, and North America. Strewnfields include tektites, which are found on land, and microtektites, which are microscopic tektites that have been found in deep-sea sediments.

Historical Background: Human association with tektites goes back to prehistoric times, when they were used as implements and ornaments. Tools made of tektites date to circa 4,000-6,000 BCE. After the Iron Age (500 BCE) tektites were frequently worn as good luck charms or as religious objects. The first reference in scientific literature appeared in 1788, when they were described as a type of terrestrial volcanic glass. In 1900, the famous geologist Eduard Suess coined the term tektite from the Greek tektos, meaning melted or molten. He was convinced that tektites were of an extraterrestrial origin and believed their shapes were caused by sculpturing due to high velocity air flow. He believed them to be glass meteorites and, because his work was highly read, people began referring to tektites as such. However, that idea was later rejected when no meteorites were found with compositions similar to that of tektites and when no evidence of cosmic ray exposure was found in tektites. The lack of cosmic ray exposure also led to the idea that tektites could not have evolved outside of the Earth-Moon system, since it indicated that the tektites’ time in space had to be less than 900-90,000 years, which is not long enough for anything outside an Earth-Moon system to travel to Earth. In 1917, the Austrian meteoriticist, Friedrich Martin Berwerth, discovered that tektites were similar chemically to certain sedimentary rocks, the first hint of terrestrial origins. The return of lunar materials from the Apollo missions in the late 1960s provided evidence that tektites are compositionally unrelated to lunar materials, which convinced the majority of scientists that tektites are terrestrial in origin and result from bolide impacts. See impact crater and Eugene M. Shoemaker for additional related information.

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[1] Sources: http://wwwflag.wr.usgs.gov/USGSFlag/Space/Shoemaker/GeneObit.html

http://cfa-www.harvard.edu/~marsden/SGF/shoemaker.html http://www.agu.org/inside/awards/geneshoemkr.html

[2] “. . . an endocrine or hormonal disorder that occurs in all age groups and afflicts men and women equally. The disease is characterized by weight loss, muscle weakness, fatigue, low blood pressure . . . and occurs when the adrenal glands do not produce enough of the hormone cortisol and, in some cases, the hormone aldosterone.” Source: National Institute of Diabetes and Digestive and Kidney Diseases http://www.niddk.nih.gov/health/endo/pubs/addison/addison.htm

[3] Source: D. M. Schneider: www.uark.edu/campus-resources/metsoc/techweb.htm