A meteorite is a natural object originating in outer space that survives impact with the Earth's surface.
Common objects falling from the sky are rain drops, snow flakes, and hail. Less often meteors are stones and chunks of metal.
A megacryometeor is a very large chunk of ice ... sometimes called huge hailstones, but do not need to form in thunderstorms.
Rocks found on the Moon have also been determined to be meteoritic; i.e., not of lunar origin, have survived impact with the lunar surface, probably came from "outer space". This suggests a more general definition of meteorite is needed.
Astronomy[edit | edit source]
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids. When a meteoroid enters the atmosphere, frictional, pressure, and chemical interactions with the atmospheric gasses cause the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star.
Widmanstätten patterns, also called Thomson structures, are unique figures of long nickel-iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellæ. Commonly, in gaps between the lamellæ, a fine-grained mixture of kamacite and taenite called plessite can be found.
Radiation[edit | edit source]
Martian meteors are thought to be from Mars because they have elemental and isotopic compositions that are similar to rocks and atmosphere gases analyzed by spacecraft on Mars.
The image at right is of the Mackinac Island meteorite, discovered on Mars by the NASA Opportunity rover on October 13, 2009.
At top left is "the first meteorite of any type ever identified on another planet. The pitted, basketball-size object is mostly made of iron and nickel. Readings from spectrometers on the rover determined that composition. Opportunity used its panoramic camera to take the images used in this approximately true-color composite on the rover's 339th martian day, or sol (Jan. 6, 2005). This composite combines images taken through the panoramic camera's 600-nanometer (red), 530-nanometer (green), and 480-nanometer (blue) filters."
Comparison of the two meteorites shown here suggests that the left one is a much more recent fall.
Meteorites have been determined to occur on Earth, the Moon, and Mars.
Radiation in a general sense is an entity, source, or object moving fast relative to local entities, sources, or objects that are or appear relatively motionless such as the ground, an atmosphere (although the gases within may move fast), or a nearby mountain.
The larger the entity, source, or object that is radiating, the larger is the impact, or impact crater, and perhaps the surviving pieces, which can still be called a meteorite.
Objects larger than a molecule, that have been radiated, can be termed meteors. A meteor, or meteoroid, could range continuously to larger particle sizes to a maximum on the order of a galaxy cluster.
Planetary sciences[edit | edit source]
"Willamette Meteorite (Tomanowos iron meteorite) [is] another massive iron meteorite that slammed into the earth and left no crater, yet is the largest meteorite in the USA!"
"Cape York iron meteorites are separate lumps of iron but have been grouped together as fragments of the same iron meteorite, as they are found around the same location. The iron pieces known as the Women and the Dog were found about 25 meters from each other on the mainland and Ahnighito was found on an island. They were found above the ground and with no visible crater around them, even for the largest one called Ahnighito."
Cape York is in Savissivik, Northwwest Greenland. Ahnighito (the Tent) weighs 31 metric tons; the Woman, weighs 3 metric tons; the Dog, weighs 400 kilograms, Savik I 3.4 tons, Savik II 7.8 kg, and Agpalilik about 15 tons.
"The meteorite lay on an ice-free slope 500 m from the shore and was partly covered with gneiss boulders. There was no crater and no crushing of rocks discovered."
Minerals[edit | edit source]
Mineral astronomy is the use of various astronomical techniques to locate and identify minerals and mineral deposits, especially on astronomical rocky objects.
At right is a thin-section image of a slice through the Allende meteorite. The Allende meteorite "lit up Mexico's skies in 1969 [and] scattered thousands of meteorite bits across the northern Mexico state of Chihuahua. ... Panguite [a titanium dioxide mineral] is believed to be among the oldest minerals in the solar system, which is [estimated to be] about 4.5 billion years old. Panguite belongs to a class of refractory minerals that could have formed only under the extreme temperatures and conditions present in the infant solar system."
Theoretical meteorites[edit | edit source]
Def. a metallic or stony object or body that is the remains of a meteor is called a meteorite.
Here's a couple of theoretical definitions:
Def. any natural object radiating through a portion or all of the Earth's or another natural, astronomical object's atmosphere is called a meteor.
These are usage notes:
- Such an object may be as small as an electron or much larger.
- Astronomical objects that are atoms, nuclei, or subatomic particles are part of cosmic-ray astronomy.
- Astronomical objects larger than atoms, nuclei, or subatomic particles that are fast-moving relative to perceived, almost motionless objects, radiating through another natural object's atmosphere or gaseous environment are also here referred to as meteors.
- These can be a high-velocity star moving through the interstellar medium or a larger object moving through an intergalactic medium.
- At the extreme a meteor can be a galaxy cluster moving relative to apparently stationary clusters in its neighborhood of the universe.
Def. for a meteor that strikes another entity, source, or object, which appears relatively motionless, and leaves behind a rock, that rock is called a meteorite.
Meteoroids[edit | edit source]
A meteoroid is a suggested term for a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters the Earth's atmosphere (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite.
"As of 2011 the International Astronomical Union officially defines a meteoroid as a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid is between 100 µm and 10 m across. Following the discovery and naming of asteroids below 10 m in size (e.g., 2008 TC3), Rubin and Grossman refined the Beech and Steel definition of meteoroid to objects between 10 µm and 1 m in diameter. The near-Earth object (NEO) definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).
The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors which are otherwise very difficult to observe.
The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice, to nickel-iron rich dense rocks.
"The silicate spheres are the most dominant group."
From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. ... Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit. The Earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteors were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second). Meteoroids moving through the earth's orbital space average about 20 km/s.
Meteors[edit | edit source]
A meteor is the visible path of a meteoroid that has entered the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble.
Although there are many definitions of a meteor ranging from any atmospheric phenomenon to a fast-moving streak of light in the night sky caused by the entry of extraterrestrial matter into the earth's atmosphere: A shooting star or falling star, for this resource, an alternative definition is proposed.
Def. any natural object radiating through a portion or all of the Earth's or another natural object's atmosphere is called a meteor.
Such an object may be as small as an electron or much larger.
"The distribution of photographic meteors in iron, stony, and porous meteors is given in this paper". "[A]mong all the 217 meteors for which we know the beginning there are 70 iron meteors, i. e. about 32 p. c., and 147 stony meteors, i. e. 68 p. c." The meteor streams: Perseids, Geminids, Taurids, Lyrids, κ Cygnids and Virginids, are quite stony.
"The dominant group in all cases are stony meteors."
Meteors become visible between about 75 to 120 kilometers (34 - 70 miles) above the Earth. They disintegrate at altitudes of 50 to 95 kilometers (31-51 miles). Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized.
Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs (for example The Great Daylight 1972 Fireball).
Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific single cause. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. All of the orbits passed through the asteroid belt.
Fireballs[edit | edit source]
A fireball is a brighter-than-usual meteor. The International Astronomical Union defines a fireball as "a meteor brighter than any of the planets" (magnitude −4 or greater). The International Meteor Organization (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of −3 or brighter if seen at zenith. This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude −1 at 5 degrees above the horizon would be classified as a fireball because if the observer had been directly below the meteor it would have appeared as magnitude −6. For 2011 there are 4589 fireballs records at the American Meteor Society.
At right is cell phone camera image of the green fireball over San Mateo, California, that left meteorite fragments. "The asteroid entered at a speed of 14 km/s, typical but on the slow side of other meteorite falls for which orbits were determined. ... The orbit in space is also rather typical: perihelion distance close to Earth's orbit (q = 0.987 AU) and a low-inclination orbit (about 5 degrees). ... 2012, October 17 - At 7:44:29 pm PDT this evening, a bright fireball was seen in the San Francisco Bay Area."
Bolides[edit | edit source]
Def. a fireball reaching magnitude −14 or brighter. is called a bolide.
Def. a fireball reaching an magnitude −17 or brighter is called a superbolide.
Meteor showers[edit | edit source]
The Perseid meteor shower, usually the richest meteor shower of the year, peaks in August. Over the course of an hour, a person watching a clear sky from a dark location might see as many as 50-100 meteors. Most meteors are actually pieces of rock that have broken off a comet and continue to orbit the Sun. The Earth travels through the comet debris in its orbit. As the small pieces enter the Earth's atmosphere, friction causes them to burn up.
"The Orionid meteor shower [leftover bits of Halley's Comet] is scheduled to reach its maximum before sunrise on Sunday morning (Oct. 21 ). This will be an excellent year to look for the Orionids, since the moon will set around 11 p.m. local time on Saturday night (Oct. 20) and will not be a hindrance at all ... The orbit of Halley's Comet closely approaches the Earth's orbit at two places. One point is in the early part of May producing a meteor display known as the Eta Aquarids. The other point comes in the middle to latter part of October, producing the Orionids."
"At 66 kilometers (41 miles) per second, they appear as fast streaks, faster by a hair than their sisters, the Eta Aquarids of May. And like the Eta Aquarids, the brightest of family tend to leave long-lasting trains. Fireballs are possible three days after maximum."
At right is a meteorite fragment from the October 17, 2012, green fireball over San Mateo, California, USA. "It is 63 grams, dense (feels heavy) and responds to a magnet (note: better to keep magnets away from meteorites to preserve the natural magnetic field)."
"The meteorite looks very unusual, because much of the fusion crust had come off. ... The meteorite appears to be a breccia, with light and dark parts."
The first meteorite from the San Mateo, California, fireball was looked at with a petrographic microscope and concluded it was not a meteorite. The crust appeared to be a product of weathering. The find of a second meteorite with the same crust confirms the first and second to be ordinary chondrites. The photo on the right side shows the second meteorite cut in two.
"The Leonid meteor shower peaked early Saturday (Nov. 17 ), and some night sky watchers caught a great view. The Leonids are a yearly meteor display of shooting stars that appear to radiate out of the constellation Leo. They are created when Earth crosses the path of debris from the comet Tempel-Tuttle, which swings through the inner solar system every 33 years."
Cosmic rays[edit | edit source]
Micrometeorite is often abbreviated as MM. Most MMs are broadly chondritic in composition, meaning "that major elemental abundance ratios are within about 50% of those observed in carbonaceous chondrites." Some MMs are chondrites, (basaltic) howardite, eucrite, and diogenite (HED) meteorites or Martian basalts, but not lunar samples. "[T]he comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation." From the number of different asteroidal precursors, the approximate fraction in MMs is 70 % carbonaceous. "[T]he carbonaceous material [is] known from observation to dominate the terrestrial MM flux." The "H, L, and E chondritic compositions" are "dominant among meteorites but rare among micrometeorites."
"Ureilites occur about half as often as eucrites (Krot et al. 2003), are relatively friable, have less a wide range of cosmic-ray exposure ages including two less than 1 Myr, and, like the dominant group of MM precursors, contain carbon."
Liquid objects[edit | edit source]
Def. a flammable liquid ranging in color from clear to very dark brown and black, consisting mainly of hydrocarbons is called petroleum.
Rocky objects[edit | edit source]
A rock is a naturally occurring solid aggregate of one or more minerals or mineraloids.
Def. a solid, homogeneous, crystalline chemical element or compound that results from natural inorganic processes is called a mineral.
Alpha-quartz (space group P3121, no. 152, or P3221, no. 154) under a high pressure of 2-3 gigapascals and a moderately high temperature of 700°C changes space group to monoclinic C2/c, no. 15, and becomes the mineral coesite. It is found in extreme conditions such as the impact craters of meteorites.
Def. a high-temperature (above 1470°C) polymorph of α-quartz with cubic, Fd3m, space group no. 227, and a tetragonal form (P41212, space group no. 92) is called cristobalite.
Def. a polymorph of α-quartz formed by pressures > 100 kbar or 10 GPa and temperatures > 1200 °C is called stishovite.
Def. a polymorph of α-quartz formed at an estimated minimum pressure of 35 GPa up to pressures above 40 GPa with a orthorhombic space group Pmmm no. 47 is called seifertite.
Def. a polymorph of α-quartz formed at temperatures from 22-460°C with at least seven space groups for its forms with tabular crystals is called tridymite.
Def. a substance that resembles a mineral but does not exhibit crystallinity is called a mineraloid.
Def. a small, round, dark glassy object, composed of silicates is called a tektite.
Def. any natural material with a distinctive composition of minerals is called a rock.
Shocked quartz is associated with two high pressure polymorphs of silicon dioxide: coesite and stishovite. These polymorphs have a crystal structure different from standard quartz. Again, this structure can only be formed by intense pressure, but moderate temperatures. High temperatures would anneal the quartz back to its standard form. Stishovite may be formed by an instantaneous over pressure such as by an impact or nuclear explosion type event.
So far several of the polymorphs of α-quartz formed at high temperature and pressure occur with rock types away from meteorite impact craters.
Def. full of, or abounding in, rocks; consisting of rocks is called rocky.
A division of astronomical objects between rocky objects and gaseous objects (including gas giants and stars) may be natural and informative. This division allows moons like Io to be viewed as rocky objects like Earth as part of rocky object science rather than as a natural satellite around a gaseous object like Jupiter, or a plasma object like a coronal cloud.
Astronomical objects that radiate, reflect, or fluoresce may range in size from individual atoms or subatomic particles to rocky objects. A rocky object may be radiation, radiated, or irradiated.
Bombs[edit | edit source]
Def. "distinctively shaped [natural] projectiles ... which acquired their shape essentially before landing" are called bombs.
Volcanic bombs are thrown into the sky and travel some distance before returning to the ground.
Def. a bomb "ejected from a volcanic vent" is called a volcanic bomb.
"Confirmation of the source of excess argon comes from step-heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and one glass aliquot prepared from the matrix of a volcanic bomb."
Sopečná puma, též sopečná bomba, je těleso, které vzniká při explozi vulkánu. Jedná se o pyroklastický materiál, který je během exploze vymrštěn do okolí vulkánu.
The second image at right is a volcanic bomb found in a shield volcano near Kladno. The rock hammer is a size gauge.
Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight.
The third image at right is an "[a]ccretionary lava ball comes to rest on the grass after rolling off the top of an ‘a‘a flow in Royal Gardens subdivision. Accretionary lava balls form as viscous lava is molded around a core of already solidified lava."
Volcanic bombs cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they do not have grains making them extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight.
Volcanic bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m in diameter up to 600 m from the vent.
Volcanic bombs are known to occasionally explode from internal gas pressure as they cool, but ... explosions are rare ... Bomb explosions are most often observed in 'bread-crust' type bombs.
Ribbon or cylindrical bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name "ribbon bombs". These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.
Spherical bombs also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.
Spindle, fusiform, or almond/rotational bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shaped; the spinning theory behind these bombs' development has also given them the name 'fusiform bombs'. Spindle bombs are characterised by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.
Cow pie bombs are formed when highly fluid magma falls from moderate height; so the bombs do not solidify before impact (they are still liquid when they strike the ground). They consequently flatten or splash and form irregular roundish disks, which resemble cow-dung.
Bread-crust bombs are formed if the outside of the lava bombs solidifies during their flights. They may develop cracked outer surfaces as the interiors continue to expand.
Cored bombs are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock or, in rare cases, bits of lava formed earlier during the same eruption.
Natural bombs may produce impact craters and deform rock strata.
"[I]ron oxide melts can exist in nature [as indicated] by describing a volcanic bomb composed of magnetite from El Laco." "[E]xistence of ballistic volcanic bombs composed of radiating porous aggregates of magnetite crystals in some of the orebodies, demonstrates that apatite iron ores can form directly from a melt."
"The sulfur isotope ratio as well as the sulfur content were found to be uniform within a single unit of lava flow and a volcanic bomb."
"Volcanic bombs with a distinctive shape are produced by post-impact mechanical rounding processes while traveling at high speed down the slopes of the scoria cone of the Pacaya Volcano in Guatemala. The name “cannonball bombs” is proposed for bombs formed by this mechanism."
"There have been recorded in all periods of historic time ... showers of one kind or another of animals and plants or their products -- showers of hay, of grain, of manna, of blood, of fishes, of frogs, and even of rats. ... so many wonderful things occur in nature that negation of any observation is dangerous; it is better to preserve a judicial attitude and regard all (authentic) information that comes to hand as so much evidence, some of it supporting one side, some the other, of a given problem."
"Mr. A. N. Caudell, of the United States Bureau of Entomology ... relates that at his former home in Oklahoma, on one occasion after a brief shower during an otherwise dry and hot period, numerous earthworms were found on the seat of an open buggy standing in the yard."
"[T]he statement by the famous French scientist, Francis Castlenau, ... he had seen fishes rain down in Singapore in such numbers that the natives went about picking them up by the basketful".
"By the tornado at Beauregard, Miss., April 22, 1883, the solid iron screw of a cotton press, weighing 675 pounds, was carried 900 feet."
"In the tornado at Mount Carmel, Ill., June 4, 1877, a piece of tin roof was carried 15 miles and a church spire 17 miles."
Iron meteorites[edit | edit source]
Iron meteorites are meteorites that consist overwhelmingly of nickel–iron alloys.
While they are fairly rare compared to the stony meteorites, comprising about 5.7% of witnessed falls, they have historically been heavily over-represented in meteorite collections.
Iron meteorites account for almost 90% of the mass of all known meteorites, about 500 tons.
The Hoba iron meteorite shown on the lower right left no observable crater. This meteorite is the largest meteorite ever found and the largest piece of iron ever found. It is specifically an ataxite, which contains a significant fraction of nickel (about 84% iron and 16% nickel, with traces of cobalt), but not in taenite. It was estimated to weigh 66 tons when initially discovered.
On the left is apparently another iron meteorite discovered in a desert that left no crater.
On the lower left is a piece of the ataxite Chinga meteorite. Ataxites exhibit no Widmanstätten patterns upon etching.
"The Mundrabilla iron meteorite and meteorite irons found around the Nullarbor Plain in Western Australia may have all been found above or just below the surface."
The main mass of the Mundrabilla meteorite weighs 12.4 tonnes and is shown third down on the right.
"There are a number of large Mundrabilla iron meteorites but there are also the much smaller and strangely shaped Mundrabilla meteorite irons."
"In April 1966, two large masses estimated to be 10-12 tons and 4-6 tons [second on the left], later named Mundrabilla, were found approximately 200 yards (ca. 183 m) apart and described by the finders R. B. Wilson and A. M. Cooney."
"The 12.4 tonne main mass (recently accurately weighed) of the Mundrabilla meteorite shower is the largest meteorite yet found in Australia. In all, some 22 tonnes of fragments of this ancient meteorite shower have been recovered."
"Mundrabilla meteorite irons are strange little pieces of iron, twisted into odd shapes."
Fourth on the right is perhaps the third largest piece of the Mundrabilla meteorite.
The fourth image on the left is another piece of the Mundrabilla meteorite still covered in some of the sediment and partial weathering produced after its fall.
One small fragment of the Mundrabilla iron meteorite looks like it is thumb-printed. It is the fifth image down on the right.
The last fragment here of the Mundrabilla iron meteorite on the left down to the fifth is about 9 cm on edge and has a hole in it.
As indicated in the first image on the left, all or nearly all, iron meteorites are magnetic. Passage through the Earth's magnetic field and the natural electric field of the Earth may cause these iron meteorites to slow down sufficiently so as to land without a crater.
Achondrites[edit | edit source]
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies. As a result, achondrites have distinct textures and mineralogies indicative of igneous processes.
Achondrites account for about 8% of meteorites overall, and the majority (about two thirds) of them are HED meteorites, originating from the crust of asteroid 4 Vesta. Other types include Martian, Lunar, and several types thought to originate from as-yet unidentified asteroids other than Vesta. These groups have been determined on the basis of e.g. the Fe/Mn chemical ratio and the 17O/18O oxygen isotope ratios, thought to be characteristic "fingerprints" for each parent body.
There is a "precise mixing required to create oxygen-18" in "silica grains".
"[S]ilica (SiO2) grains in the primitive carbonaceous chondrites LaPaZ 031117 and Grove Mountains 021710 ... are characterized by moderate enrichments in 18O relative to solar, indicating that they originated in Type II supernova ejecta."
The second image at right is a 175g individual of the Millbillillie meteorite shower, a eucrite achondrite that fell in Australia in 1960. This specimen is approx. 6 centimeters wide. Note the shiny black fusion crust with flow lines. The chip at lower right allows one to see the light-gray interior. The orange staining at top is a result of weathering, as these stones were not recovered until many years after they fell.
Chondrites[edit | edit source]
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body.
Prominent among the components present in chondrites are the enigmatic chondrules, millimeter-sized objects ..., most [of which] are rich in the silicate minerals olivine and pyroxene. Chondrites also contain refractory inclusions (including Ca-Al Inclusions), ... particles rich in metallic Fe-Ni and sulfides, and isolated grains of silicate minerals. The remainder of chondrites consists of fine-grained (micrometer-sized or smaller) dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions.
At right is a piece of the NWA 869 meteorite. "Chondrules and metal flakes can be seen on the cut and polished face of this specimen. NWA 869 is a ordinary chondrite (L4-6). ... The cut surface is about 65mm at it's widest point."
Most meteorites that are recovered on Earth are chondrites: 86.2% of witnessed falls are chondrites, as are the overwhelming majority of meteorites that are found. There are currently over 27,000 chondrites in the world's collections. The largest individual stone ever recovered, weighing 1770 kg, was part of the Jilin meteorite shower of 1976. Chondrite falls range from single stones to extraordinary showers consisting of thousands of individual stones, as occurred in the Holbrook fall of 1912, where an estimated 14,000 stones rained down on northern Arizona.
The second image is of the Bruderheim (or Bruederheim) meteorite. "The Bruederheim Meteorite fell in 1960 ... Chondrules, spherical to subspherical minerals or mineral aggregates, form the most conspicuous textural features. The chondrules are in a fine to medium crystalline groundmass. This groundnass is seriate textured with the smallest grains being about 0.01 mm. in diameter. The larger grains of the groundmass are as much as 0.3 mm. in diameter. They are generally anhedral and have forms that range fron angular to subrounded." The Bruederheim meteorite is approximately 39.55 % SiO2, 13.89 % FeO+Fe2O3 as FeO, and 24.69 % MgO by weight.
Cryometeorites[edit | edit source]
Hail is a form of solid [water] precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hailstone. Unlike graupel, which is made of rime, and ice pellets, which are smaller and translucent, hailstones – on Earth – consist mostly of water ice and measure between 5 and 200 millimetres (0.20 and 7.87 in) in diameter.
The METAR reporting code for hail 5 mm (0.20 in) or greater is GR, while smaller hailstones and graupel are coded GS. ... Hail has a diameter of 5 millimetres (0.20 in) or more. Hailstones can grow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb).
Unlike ice pellets, hailstones are layered and can be irregular and clumped together.
A cross-section through a large hailstone shows an onion-like structure. This means the hailstone is made of thick and translucent layers, alternating with layers that are thin, white and opaque.
The image at left shows a blanket of hail precipitated on the ground at Cerro El Pital, El Savador. "Cerro El Pital se encuentra a 12 kilómetros de La Palma, con una altura de 2730 msnm es el punto más alto del territorio Salvadoreño. Es una montaña en medio de un bosque nebuloso que suele tener una temperatura aproximada de 10 ºC. El 13 de abril de 2004, las temperaturas bajaron tanto que el cerro fue cubierto por una escarcha de hielo que causó conmoción entre los pobladores, atribuyendo el fenómeno a una supuesta "nevada"."
The third image at right shows a hailstone that fell at Washington, D. C., on May 26, 1953, that was 4 in in diameter and weighed 7 oz.
In the fourth image at right is the largest hailstone ever recovered in the United States as of June 22, 2003. This hailstone fell in Aurora, Nebraska. It has a 7-inch (17.8 cm) diameter and an approximate circumference of 18.75 inches.
The next hailstone image is one, approximately 133 mm (5 1/4 inches) in diameter, that fell in Harper, Kansas on May 14, 2004.
After 2003, another record-setting hailstone fell in Vivian, South Dakota, on July 23, 2010. Its diameter is 8 inches with a weight of 1 pound 15 ounces.
Terminal velocity of hail, or the speed at which hail is falling when it strikes the ground, varies by the diameter of the hail stones. A hail stone of 1 cm (0.39 in) in diameter falls at a rate of 9 metres per second (20 mph), while stones the size of 8 centimetres (3.1 in) in diameter fall at a rate of 48 metres per second (110 mph). Hail stone velocity is dependent on the size of the stone, friction with air it is falling through, the motion of wind it is falling through, collisions with raindrops or other hail stones, and melting as the stones fall through a warmer atmosphere.
A megacryometeor is a very large chunk of ice which, despite sharing many textural, hydro-chemical and isotopic features detected in large hailstones, is formed under unusual atmospheric conditions which clearly differ from those of the cumulonimbus cloud scenario (i.e. clear-sky conditions). They are sometimes called huge hailstones, but do not need to form in thunderstorms. Jesus Martinez-Frias, a planetary geologist at the Center for Astrobiology in Madrid, pioneered research into megacryometeors in January 2000 after ice chunks weighing up to 6.6 pounds (3.0 kg) rained on Spain out of cloudless skies for ten days.
Graupel ... also called soft hail or snow pellets) refers to precipitation that forms when supercooled droplets of water are collected and freeze on a falling snowflake, forming a 2–5 mm (0.079–0.197 in) ball of rime.
Strictly speaking, graupel is not the same as hail or ice pellets, although it is sometimes referred to as small hail. However, the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.
The frozen droplets on the surface of rimed crystals are hard to resolve and the topography of a graupel particle is not easy to record with a light microscope because of the limited resolution and depth of field in the instrument. However, observations of snow crystals with a low-temperature scanning electron microscope (LT-SEM) clearly show cloud droplets measuring up to 50 μm (0.00197 in) on the surface of the crystals. The rime has been observed on all four basic forms of snow crystals, including plates, dendrites, columns and needles. As the riming process continues, the mass of frozen, accumulated cloud droplets obscures the identity of the original snow crystal, thereby giving rise to a graupel particle.
Graupel commonly forms in high altitude climates and is both denser and more granular than ordinary snow, due to its rimed exterior. Macroscopically, graupel resembles small beads of polystyrene. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and layers of 20–30 cm (7.9–11.8 in) present a high risk of dangerous slab avalanches. In addition, thinner layers of graupel falling at low temperatures can act as ball bearings below subsequent falls of more naturally stable snow, rendering them also liable to avalanche. Graupel tends to compact and stabilise ("weld") approximately one or two days after falling, depending on the temperature and the properties of the graupel.
Ice pellets (also referred to as sleet by the United States National Weather Service) are a form of precipitation consisting of small, translucent balls of ice. Ice pellets are usually smaller than hailstones and are different from graupel, which is made of rime, or rain and snow mixed, which is soft. Ice pellets often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.
Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between −2 and −8 °C (28.4 and 17.6 °F).
Snow is precipitation in the form of flakes of crystalline water ice that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless subjected to external pressure. Snowflakes come in a variety of sizes and shapes. Types that fall in the form of a ball due to melting and refreezing, rather than a flake, are known as hail, ice pellets or snow grains.
Mercury[edit | edit source]
"NWA 7325 is actually a group of 35 meteorite samples discovered in 2012 in Morocco. They are ancient, with Irving and his team dating the rocks to an age of about 4.56 billion years."
"NWA 7325 has a lower magnetic intensity — the magnetism passed from a cosmic body's magnetic field into a rock — than any other rock yet found, Irving said. Data sent back from NASA's Messenger spacecraft currently in orbit around Mercury shows that the planet's low magnetism closely resembles that found in NWA 7325, Irving said."
"NWA 7325 has olivine in it that is insanely magnesium-rich. Iron and magnesium are two elements that are almost always found together in rocks; the ions they make have the same size and charge so they happily occupy the same positions in crystal lattices. It's weird to have a rock that is so dominantly magnesium-rich. Mercury's surface rocks are known (thanks to MESSENGER) to be unusually low in iron."
"NWA 7325's oxygen isotope ratios do not match any known meteorites from any other planet-size body. In fact, they're not particularly similar to much of anything that we've measured oxygen isotope ratios for."
"The ratios of Al/Si (0.224) and Mg/Si (0.332) plus the very low Fe content of NWA 7325 are consistent with the compositions of surface rocks on Mercury , but the Ca/Si ratio (0.582) is far too high. However, since NWA 7325 is evidently a plagioclase cumulate (and presumably excavated from depth), it may not match surface rocks on its parent body. The abundance of diopside rather than enstatite might be consistent with some earlier spectral observations of Mercury ."
"[I]t's about 23 times harder to get a rock from Mercury to Earth than it is from Mars to Earth. Given that we've got more than 70 known Mars meteorites in our collections, that means we ought to have found 3 (give or take a couple) Mercury meteorites by now."
Venus[edit | edit source]
While there is little or no water on Venus, there is a phenomenon which is quite similar to snow. The Magellan probe imaged a highly reflective substance at the tops of Venus's highest mountain peaks which bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide (galena).
Earth[edit | edit source]
In the image at left is an aerial view of the Barringer Meteor Crater about 69 km east of Flagstaff, Arizona USA. Although similar to the aerial view of the Soudan crater, the Barringer Meteor Crater appears angular at the farthest ends rather than round.
Meteor Crater is a meteorite impact crater approximately 43 miles (69 km) east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Because the US Department of the Interior Division of Names commonly recognizes names of natural features derived from the nearest post office, the feature acquired the name of "Meteor Crater" from the nearby post office named Meteor. The site was formerly known as the Canyon Diablo Crater, and fragments of the meteorite are officially called the Canyon Diablo Meteorite. Scientists refer to the crater as Barringer Crater in honor of Daniel Barringer, who was first to suggest that it was produced by meteorite impact.
From space the crater appears almost like a square. The image at right has a resolution of 2 meters per pixel, and illumination is from the right. Layers of exposed limestone and sandstone are visible just beneath the crater rim, as are large stone blocks excavated by the impact.
The Holsinger meteorite is the largest discovered fragment of the meteorite that created Meteor Crater and it is exhibited in the crater visitor center.
The Canyon Diablo meteorite comprises many fragments of the asteroid that impacted at Barringer Crater (Meteor Crater), Arizona, USA. Meteorites have been found around the crater rim, and are named for nearby Canyon Diablo, which lies about three to four miles west of the crater.
There are fragments in the collections of museums around the world including the Field Museum of Natural History in Chicago. The biggest fragment ever found is the Holsinger Meteorite, weighing 639 kg, now on display in the Meteor Crater Visitor Center on the rim of the crater.
Moon[edit | edit source]
Def. a meteorite that is known to have originated on the Moon" is called a lunar meteorite.
The meteorite called Allan Hills 81005 resembled some rocks brought back from the Moon by the Apollo program.
Yamato 791197 is another lunar meteorite.
About 134 lunar meteorites have been discovered so far (as of October, 2010), perhaps representing more than 50 separate meteorite falls (i.e., many of the stones are "paired" fragments of the same meteoroid). The total mass is more than 46 kg.
Lunar origin is established by comparing the mineralogy, the chemical composition, and the isotopic composition between meteorites and samples from the Moon collected by Apollo missions.
Cosmic ray exposure history established with noble gas measurements have shown that all lunar meteorites were ejected from the Moon in the past 20 million years. Most left the Moon in the past 100,000 years.
All six of the Apollo missions on which samples were collected landed in the central nearside of the Moon, an area that has subsequently been shown to be geochemically anomalous by the Lunar Prospector mission. In contrast, the numerous lunar meteorites are likely to be random samples of the Moon and consequently provide a more representative sampling of the lunar surface than the Apollo samples. Half the lunar meteorites, for example, likely sample material from the farside of the Moon.
"A felsite clast in lunar breccia Apollo sample 14321 [arrowed in the image on the left], which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta [...] indicates a pre-impact depth of 30–70 km, i.e. near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce⁎ anomalies corresponding to an oxygen fugacity +2 to +4 log units higher than that of the lunar mantle, with crystallization temperatures of 771 ± 88 to 810 ± 37 °C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9 ± 1.2 kbar, corresponding to a depth of crystallization of 167 ± 27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO2, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas."
"The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin."
The "felsite and its zircon crystallized on Earth at a modest depth of 19 ± 3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia."
Mars[edit | edit source]
Roughly three-quarters of all Martian meteorites can be classified as shergottites. "[T]he most frequent type of rock (basaltic lithologies) among all known Martian meteorites is the basaltic shergottites."
"The dominant group of Martian meteorites, shergottites, are divided into two subgroups consisting of basalts and lherzolites.
Almost 100 rocks are known that demonstrably come from the Planet Mars. Meteorite researchers and collectors generally refer to the Martian rocks as the SNC meteorites - the shergottites, the nakhlites, and the chassignites. Most of these Martian rocks are shergottites.
Shergottites are a group of Martian rocks named after the Shergotty Meteorite, the type example. The Shergotty Meteorite is a shergottite that was found and identified in 2004.
The first image at right shows a small sample 6 mm from the NWA 2373 Meteorite (NWA = "Northwest Africa"). The light brown-colored material is the outer weathered surface of the rock. The greenish and black speckled surface shows the crystal & mineral make-up of the rock itself. Mineral analysis performed by Theodore Bunch and James Wittke at Northern Arizona University has shown that NWA 2373 is composed principally of olivine, pigeonite & augite pyroxene, plagioclase feldspar glass (maskelynite), chromite, Ti-magnetite, chlorapatite, and trace amounts of other minerals. It looks like an ultramafic rock, but it's apparently a basaltic shergottite (also regarded as a picritic shergottite).
NWA 2373 is reportedly paired with the NWA 1068 Meteorite. Available isotopic dates on the NWA 1068 Meteorite show it formed 185 million years ago (late Amazonian, equivalent to Earth's Early Jurassic), and was ejected from the Martian surface about 2.2 million years ago (information based on cosmogenic isotope analysis).
Very light snow is known to occur at high latitudes on Mars.
Asteroids[edit | edit source]
Aubrites are a group of meteorites that are primarily composed of the orthopyroxene enstatite, and are often called enstatite achondrites. Their igneous origin separates them from primitive enstatite achondrites and means they originated in an asteroid. Aubrites are typically light-colored, and with a brownish fusion crust. Most aubrites are heavily brecciated.
Aubrites are primarily composed of large white crystals of the Fe-poor, Mg-rich orthopyroxene, or enstatite. Around this matrix, they have minor phases of olivine, nickel-iron metal, troilite, which indicate a magmatic formation under extremely reducing conditions. The severe brecciation of most aubrites attests to a violent history for their parent body. Since some aubrites contain chondritic xenoliths it is likely that the aubrite parent body collided with an asteroid of “F-chondritic” composition.
Comparisons of aubrite spectra to the spectra of asteroids have revealed striking similarities between the aubrite group and the main belt Nysian asteroid family. A small member of this asteroid family, 3103 Eger, exhibits a near-Earth orbit, and is very likely the parent body of the aubrites.
"The recent investigation of the orbital distribution of Centaurs (Emel’yanenko et al., 2005) showed that there are two dynamically distinct classes of Centaurs, a dominant group with semimajor axes a > 60 AU and a minority group with a < 60 AU." "[T]he intrinsic number of such objects is roughly an order of magnitude greater than that for a<60 AU".
"From the dominant group, the asteroids evolve to intersect the Earth's orbit on a median time scale of about 60 Myr." "The MB group is the most numerous group of MCs. ... 50 % of the MB Mars-crossers [MCs] become ECs within 59.9 Myr and [this] contribution ... dominates the production of ECs". MB denotes the main belt of asteroids. EC denotes Earth-crossing.
Diameters[edit | edit source]
Def. a particle classification system based on diameter is called the Wentworth scale.
Def. a particle less than 1 micron in diameter is called a colloid.
Def. a particle less than 3.9 microns in diameter is called a clay.
Def. a particle from 3.9 to 62.5 microns in diameter called a silt.
Def. a particle less than 62.5 microns in diameter is called a mud.
Def. a particle from 62.5 microns to 2 mm in diameter is called a sand.
Def. a particle from 2 to 64 mm in diameter is called a gravel.
Def. a particle from 2 to 4 mm in diameter is called a granule.
Def. a particle from 4 to 64 mm in diameter is called a pebble.
Def. a particle from 64 to 256 mm in diameter is called a cobble.
Def. a particle [or large piece of stone] greater than 256 mm in diameter that can theoretically be moved if enough force is applied is called a boulder.
Natural sciences[edit | edit source]
Def. the study of the origin, composition and structure of rock is called petrology.
Each rock has a location and an environment. These are recorded. Sometimes a sequence of events is connectable to a rock in a location.
Def. the scientific description and classification of rocks is called petrography.
Def. a section formed by a plane cutting through an object, usually at right angles to an axis is called a cross section.
Def. a laboratory preparation of a rock, mineral, soil, pottery, bones, or metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe is called a thin section.
At lower left is a thin section through a sand-sized quartz grain "from the USGS-NASA Langley core showing two well-developed, intersecting sets of shock lamellae produced by the late Eocene Chesapeake Bay bolide impact. This shocked quartz grain is from the upper part of the crater-fill deposits at a depth of 820.6 ft in the core. The corehole is located at the NASA Langley Research Center, Hampton, VA, near the southwestern margin of the Chesapeake Bay impact crater." "Very high pressures produced by strong shock waves cause dislocations in the crystal structure of quartz grains along preferred orientations. These dislocations appear as sets of parallel lamellae in the quartz when viewed with a petrographic microscope. Bolide impacts are the only natural process known to produce shock lamellae in quartz grains."
Lower right shows another thin section in plane polarized light of a shocked quartz grain with two sets of decorated planar deformation features (PDFs) surrounded by a cryptocrystalline matrix from the Suvasvesi South impact structure, Finland.
In a specimen of shocked quartz, stishovite can be separated from quartz by applying hydrogen fluoride (HF); unlike quartz, stishovite will not react.
Minute amounts of stishovite has been found within diamonds.
The major evidence for a volcanic origin for tektites "includes: close analogy between shaped tektites and small volcanic bombs, and between layered tektites and lava or tuff-lava flows or huge bombs; analogy between flanged tektites and volcanic bombs ablated by gasjets: long-time, multistage formation of some tektites that corresponds to wide variations in their radiometric ages; well-ordered long compositional trends (series) typical of magmatic differentiation; different compositional tektite families (subseries) comparable to different stages (phases) of the volcanic process."
"As with the North American microtektite-bearing cores, all the Australasian microtektite-bearing cores containing coesite and shocked quartz also contained volcanic ash, which complicated the search."
Hypotheses[edit | edit source]
- Passage through the Earth's magnetic field and natural electric field of magnetic meteors may cause deflection and a slowing down during flight such that collision with the Earth does not create a crater.
See also[edit | edit source]
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Further reading[edit | edit source]
- Cesare Emiliani (1992). "Meteorites". Planet earth: cosmology, geology, and the evolution of life and environment. Cambridge University Press. p. 152. ISBN 978-0-521-40949-0. http://books.google.com/?id=QDU7AAAAIAAJ&pg=PA152.
[edit | edit source]
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