summary by Nate Conway
Dust-sized particles dominate the majority of the “empty” space of the solar system and give rise to many observational phenomena. Zodiacal light, for example, is the pyramid-shaped glow extending from the horizons at night for specific times of the year. This yellow glow is due to sunlight reflecting off of interplanetary dust between the Earth and the Sun, most particles on the order of microns. Larger particles are pulled into the planets or Sun, or pushed out of the solar system by solar radiation pressure; however, the supply of these micrometeoroids is replenished by fragmentation action in the asteroid belt and the dusty tails of comets close to the ecliptic plane. Similarly, a fainter light called the gegenscheinis due to backscattered sunlight from dust beyond the Earth’s orbit. This dust was studied for years before collected via balloons in the upper atmosphere and from the ocean floor; the Earth is predicted to collect up to 100,000 tons of dust annually.
Meteoroids refer to the bodies while in space; meteorites refer to the solids surviving the atmospheric fall, and the lights in the sky from heating these solids are meteors. The average speed of these particles are 26 miles per second, heating up to 2000 F. Over 25 million meteors are potentially observable by the naked eye each day, and the use of small telescopes or binoculars raises this to 400 billion. Sizes of meteoroids range from thousandths of an inch to pea-sized, averaging the size of a sand grain; the smaller meteoroids, surprisingly, have better chances of surviving atmospheric heating and potentially cause noctilucent clouds to condense. Sporadic meteors may approach Earth from any angle and, due to the Earth’s orbital speed, can approach with a relative velocity from 8 to 44 miles per second. The best time to view meteors is between midnight and dawn, due to meteoroids crossing Earth’s orbit and meteoroids striking head-on.
Meteor showers and their periodicity arise from periodic comets, whose dust lies along the parent comet’s orbit. If the path crosses the Earth’s orbit, like Comet Halley, there is a high chance of two annual meteor showers. A trick of perspective causes these showers to seem to originate from a radiant point, which categorizes the showers by the constellation they appear to emit from. Older comets have their dust evenly spread throughout the orbit, while new comets tend to have bunches of dust which cause high-count meteor showers such as the 33-year Leonid showers with six-figure estimates. Some showers have died out and yield low counts, such as the Draconids, while others are extremely reliable, such as the Perseids. Larger meteoroids manifest fireballs in the atmosphere.
Historical records of fireball events and meteoritic finds detail the long journey in understanding their nature. Ancient societies gave the meteorites religious significance, such as the stone at the temple at Delphi, anointed with oil daily due to the association with Kronos or the possible meteoritic origin of the holy stone of Mecca. The Ensisheim meteorite fall in France, 1492 was considered by the Germans to be divine intervention against the French. Early theories of meteoritic origins included thunderstorms, but excluded cosmic origins. Even the tide of the Enlightenment actually worked against meteoritics, since most observations were colored with supernatural meaning and were thus tossed completely. The first US fall occurred in 1807 over Weston, Connecticut, but scientists were still skeptical. Over time, European scientists gained progress, mainly from Ernst Friedrich Chaldni, whose connection between fireballs and meteorites included a cosmic origin in 1794, but was not believed until numerous falls in 1803. Chemical analyses revealed in stony and iron meteorites a non-terrestrial origin due to the association of nickel and iron not seen on Earth. Early conclusions pointed to the Moon, until the discovery of the asteroid belt truly opened up solar system origins.
Fireball sightings such as one in Oregon, 1987, share similar descriptions: sonic booms/rumbling, a bright flash, a cloud left in its wake, and “whomping” sounds. While the Oregon meteorite has yet to be found, this sighting is far from unique. All meteoroids are subjected to the high-temperature atmospheric heating, and the light is seen for hundreds of miles while sound is limited to 30 miles from impact, but all of this is due to the loss of kinetic energy in the meteoroid. The light emitted doesn’t occur until an altitude of 60-90 miles, where the atmosphere is dense enough to produce enough air resistance for ionization of the surrounding air. Elements of the meteoroid and the surrounding air contribute to the coloration of the light. The sound of the fireball follows the explosion by a minute or two, and separation into pieces can cause rumbling like thunder; a commonly reported hissing noise is still not understood, but the “whomping” noise is likely the irregularly shaped meteoroid spinning in descent. A dust trail is formed from the vaporization, more from stony than iron meteoroids, and can hang in the air for hours before scattered and brought down as micrometeorites. Higher-velocity meteoroids experience much greater mass loss than lower velocities (24 vs 12 miles per second), and most meteoroids lose all cosmic velocity in the atmosphere before experiencing free fall, more so with shallower entry angles. However, heavier objects experience less dampening and retain a fraction of their initial speed. Impact craters depend on the speed of the object and nature of the ground. Stony meteorites commonly shatter on hard surfaces, but iron meteorites are often intact, as is the ground beneath them. 10- to 100-ton meteoroids are rare, and they frequently do not survive impact, with the largest known meteorite being in Namibia, more than 60 tons in a 5 ft deep crater.
When tracking a meteorite fall, the initial reports can be key, but not individually; eyewitness accounts can give good details on the observation point but usually lack precision in distance and direction. Data shows a peak in meteorite falls during May and June, suggesting some concentration of meteoroids at that point of Earth’s orbit. When viewing hours of the day, a similar peak is seen at 3 p.m., with a minimum at 3 a.m.; this is likely due to the relative velocity of the meteoroid to the Earth, which is higher from midnight to noon and more likely to vaporize in the atmosphere. Two observations of the meteor’s path can be used to determine the true path; two reports of the extinction point help determine the height of the path. Combining multiple accounts of the fall post-extinction, one can determine a rough elliptical area of of impact; several observations pre- and post-impact can yield crucial information about the meteorite. Ownership of the meteorite is complicated; private land gives ownership to the landowner and public (federal) land gives ownership to the government. Contacting the appropriate people can assist in arranging terms of ownership, rights to a sample, or at least a reward for discovery. Pricing of meteorites per gram can radically vary, depending on demand, type, freshness, and fragmentation, and some can be worth multiple times their weight in gold, such as Murchison.
Plenty of stories surrounding meteorite showers exist, and such falls frequently display fragmentation between 7 and 17 miles in altitude. The shared trajectory of meteorites raises the question of fragmentation or separate meteoroids entering the atmosphere as a group. Meteor showers tend towards an elliptical distribution along the ground, with the larger specimens at the far end and smaller specimens at the other due to atmospheric braking and momentum. The 19th century had numerous showers in Europe, including the 1803 L’Aigle and 1868 Pultusk falls, the latter of which produced thousands of pea-sized meteorites; the 1864 Orgueil fall revealed water-bearing phases and amino acids. Iowa experienced multiple falls in the 1800s, yielding huge meteorites and, in the case of Estherville in 1879, a complex ownership history and several marble-sized iron inclusions. The 20thcentury yielded fewer showers, with the 16,000 small meteorites from the 1912 Holbrook, Arizona shower and the enormous iron meteorites of the 1947 Sikhote-Alin shower. Allende, Mexico experienced a 1969 shower that carried the now-famous CV3 carbonaceous chondrites containing aluminum-calcium-titanium inclusions; the same year, Murchison, Australia had a fireball yielding CM2 carbonaceous chondrites containing water. Some reports of meteorites striking populated areas were recorded, although most of these caused no harm to people (but there are reports of dead animals). Most either collide with houses or vehicles, while one meteorite with low velocity bruised a woman’s hip in Alabama, 1954.
Siberia experienced two great meteorite falls in the 1900s which yielded crucial knowledge about meteorite impacts for the future. The first event in 1908 near the Tunguska River Basin manifested as an enormous fireball, the explosion of which sent flames high into the sky and damaging shockwaves for miles around, detectable by London barographs hours later. The night sky was bright for several days as dust dispersed and settled. 13 years later, an investigation was opened by the Soviet Academy of Sciences to collect eyewitness accounts, with an excavation party sent over 5 years later. Eventually, miles and miles of fallen trees, all in a radial direction, pointed towards a basin filled with crater-like holes which were assumed to hold meteorites. Sadly, when the second expedition came, no magnetic readings were present, and following expeditions’ excavations found no meteorites at all. Eventually, they concluded that the object vaporized above the ground, as evidenced by the meteoritic dust found in soil samples. Early predictions settled on a comet which exploded 5 miles up, raining down dust and ice. Later studies and meteoritic knowledge stated that a comet wouldn’t get that far, and predicted a stony meteorite as the culprit, decimated by pressure differences and the sudden change in momentum hitting the lower atmosphere. Further sample analysis points towards this origin, as well as similar impacts viewed on Venus.
A second fireball occurred in 1947 over the Sikhote-Alin mountains with an entrance similar to the Tunguska fall, and teams set out within 9 days to investigate the region. The expedition yielded over a hundred craters and eventually thousands of meteorite fragments. The craters revealed that the larger masses made the bigger craters but took extensive damage, leaving smaller specimens behind, while smaller masses made smaller craters but were otherwise intact. The meteorites themselves completed the story: the larger craters held fragments that had no fusion crusts but looked like shrapnel torn apart from impact. More traditional meteorites with fusion crusts and cavities were found in smaller craters, and more material was embedded in nearby trees. Overall, the extinction position was estimated to me 3.6 miles up (lower than usual), the distribution eclipse was only 1.2 miles long, and the reliable observations modeled the orbit of the meteoroid out to the asteroid belt.
The misnamed Meteor Crater in Arizona, nearly a mile wide and 60 feet deep, was first recorded in 1871, with shepherds discovering the surrounding iron in 1886. In 1891, mineral dealers began to look at the crater due to reports of iron meteorites, not connecting the two causally; those who were looking into lunar crater origins investigated here to determine volcanic or meteoritic origins. Initial findings pointed towards volcanic, explaining away the meteorites around the rim either as coincidence or assisting a steam explosion from a volcanic vent. Daniel Moreau Barringer claimed the land in 1903 for the potential stores of iron meteorites (with nickel) in the crater. The aquatic history of the region gave layers of strata around the crater, with a fossil layer on top followed by two distinct layers of limestone. Barringer’s first investigation in 1905 found limestone and meteorites scattered throughout the rim and surrounding plains with the distribution centered on the crater, with no signs of volcanism.
Skeptics dismissed the findings, so Barringer wrestled with sandstone aquifers and continued drilling, finding nothing in the center. 10 years later, he began looking towards the southern rim rather than the center, based off the round craters a bullet makes at low angles. Scientists began theorizing that the probable size and speed of a meteorite making this crater would vaporize, spelling bad news for those seeking samples. 1920 had increasing numbers of found fragments, until 1923 when the drills jammed on a hard object and the later 1928 excavation had more flooding from aquifers; further studies predicted more of the vaporization hypothesis and, with the aid of the stock market crash, prevented Barringer from any further investigation. Modern studies of the area yielded mixed results until in 1957, Eugene Shoemaker found impact breccia and shock-melted glass beneath the crater floor, convincing scientists of a preserved meteoritic impact crater.
Discovery of separately-treated fragments and fused silica bombs helped form a model for this (and most) meteorite explosion craters. A low-angle, low speed meteoroid sheds smaller fragments in flight, and the main body impacts the ground, sending out shock waves that compress and melt rock below it before waves travel back upwards into the meteorite, spitting out fragments before the superheating causes an explosion, all within one second. The explosion removes the surrounding layers of rock and inverts their order, depositing debris on the rim before the meteoritic debris settles out of the air. Since then, craters have been tested for authenticity, and few have passed. Barring meteorites, a crater may be determined authentic through either the presence of impactite quartz (coesite or stishovite) or shatter cones formed out of surrounding rock that bear radial geometry due to the shock waves from the point of impact. Meteor Crater was the first identified meteorite crater, and 15 more were confirmed by midcentury.
Weathering, erosion, and plate movement have obscured numerous early impact craters identified on Earth, recently counted to be near 150 in total. Craters may either be “simple” with a small bowl shape and flat floor or “complex” with large diameters, central uplifts, or concentric rings. The United States features craters in Odessa, Texas which contained the standard fragments in smaller craters alongside the main, empty crater which still contained impactites. Astroblemes, or “star wounds,” are impact craters which hold no sign of meteorites today, either due to vaporization at impact or years of elemental exposure, yet demonstrate other traits of impact. The Sierra Madera astrobleme in Texas is a complex crater yielding shock-formed quartz and shatter cones. Australia features five craters; the Henbury craters feature various sizes and one elliptical crater, likely a fusion of two smaller craters, of which 3 hold no meteorites, implying explosive origins. The Boxhole crater iron meteorites are similar to those from Henbury, while the Wolf Creek crater lacks pure iron meteorites. Dalaranga, Australia’s smallest crater, features weathered stony-iron meteorites, while Mount Darwin, the largest Australian crater, yields impactite-like glass rather than meteoritic samples.
Wabar Crater Field in Saudi Arabia was said to feature a large iron meteorite, but expeditions in 1932 failed to find the mass described, though shifting dunes revealed the 4,500 lb meteorite in 1965. South America featured elongated craters in Argentina formed by low entry angles and a more traditional complex crater in Brazil with shock-induced quartz and shatter cones. Canada offers many authenticated simple and complex craters, the major sites formed in Precambrian granites and gneiss dating from 5 million to 800 million years old, which sadly removes the likelihood of intact meteorites. Many of these craters are filled wither with water or sediment, making the finds difficult unless assisted by an aerial view. The number and scale of these discoveries offer new insight into the violent history of the Earth as recently as the 1960s and new technologies offer increasing numbers of found astroblemes.
Identifying a meteorite is commonly (incorrectly) reduced down to “heavy with a black crust.” Already, this biases finds away from stony meteorites, which are the much more common type of meteorite. Meteoritic samples in museums are a bit idealized; most meteoritic specimens on Earth have experienced weathering and oxidation. If a meteoroid is non-spherical, it will often orient itself in atmospheric descent to form a conical shape, with ablation pushing material to the back of the object. Stony meteorites have smoother surfaces than iron meteorites after this process, but breaks up in the atmosphere more often. Irons have more variability in shape, frequently bearing pits across the surface from ablation of lower melting-point materials, and can have twisted or distorted edges.
Fusion crusts (usually black) form when the meteorite is at high speeds, varying from sub-millimeter to several millimeters in thickness, and is thinnest on the front of the meteorite and is thickest toward the back, due to ablation. If broken up before the retardation point, a thinner secondary fusion crust can still form on the exposed surface. Iron meteorites give thin crusts, and can show flow structures during the fall or fine fractures due to rapid cooling. Open exposure inevitably destroys the crust, either from physical weathering or chemical reactions such as rusting on the outside (and inside) as well as deposition of calcium carbonate. An odd property of ferrous chloride (lawrencite) causes iron-bearing meteorites to rapidly decompose even under pristine display cases, eating away at the iron structure. Several objects can be mistaken for rusted meteoritic fragments, such as common desert magnetite and hematite or iron artifacts from old mining tools and bomb casings. These and desert rocks with a crust-like coating of manganese can be differentiated from meteorites by quickly scratching off the surface. Volcanic basalt can also be an imposter due to cavities similar to iron meteorite pits.
The classification system used to group meteorites looks at structure and mineralogy. Rock-forming minerals containing the ion make up silicates that make up the Earth’s crust and exist throughout the meteorites. In stony meteorites, olivines make up 40-50% of the material with , sometimes with Ca substitutions. Pyroxenes make up 15-25%, similar to olivine but with chains, and 2-25% belongs to nickel-iron minerals. Iron sulfide, iron oxides, and feldspar occur as secondary phases, making up 10-15% of the meteorite. Stony meteorite structure is split into chondritic and achondritic. Chondrites make up 85% of all meteorite falls and contain small spherical chondrules embedded in the matrix and overall nearly match the Sun in elemental composition. Formed before the rest of the meteorite, chondrules are composed of olivines and pyroxenes, with respective bands and radial structures. Petrologic type is used to group chondrites by chondrule texture and thermal metamorphism: type 1 is void of chondrules, type 2 has sparse chondrules, types 3-6 have increasing abundances, becoming indistinguishable up to type 7, which is completely melted.
Chemically, the ordinary chondrites are sorted by iron content: H-chondrites have high iron amounts with high free iron proportions, L-chondrites have high total iron but low free iron, and LL-chondrites have lower iron amounts overall. These contain differing amounts of oxygen related to the available free iron; enstatite (E) chondrites are oxygen-depleted and the pyroxene has exclusively magnesium, but are still grouped similar to ordinary chondrites with high (H) and low (L) free iron. Carbonaceous chondrites have a wide range of compositions and petrologic types, showing nearly no thermal metamorphism but evidence of aqueous alteration through water-bearing minerals. CI chondrites are 20% water, 3-4% carbon and bear no chondrules; CM2 chondrites show sparse chondrules, slightly lower carbon, and only 10% water. CV chondrites range from types 3-5 with abundant chondrules and the presence of calcium-aluminum inclusions (CAIs); CO carbonated have smaller chondrules, but much more in proportion to the matrix than CV. Historical analyses of the carbonaceous chondrites yielded organic material, most importantly amino acids. After several experiments were ruled to be contaminated, 74 amino acids (55 not of Earth) were discovered by 1985 via carefully controlled systems.
In contrast to the pristine inner matrix of chondrites, achondrites show evidence of parent body processing similar to planetary formation with full heating and differentiation into mineral zones. On Earth, melt-formed (igneous) rocks such as basalt are formed within the mantle from plagioclase and pyroxene. Minerals cool out of the magma in sequence, forming layers at the bottom of the chamber. By definition, achondrites lack chondrules due to the extensive metamorphism and recrystallization, appearing similar to terrestrial rocks. While rare, there are a handful of achondrite types: aubrites primarily contain enstatite but lack the FeNi or iron in pyroxene of the enstatite chondrites. Diogenites are nearly pure orthopyroxene with large crystals that formed under slow cooling conditions. Eucrites mimic magma-born basalt from Earth with gas bubbles and small crystal structures rich in calcium-rich anorthite and pidgeonite, covered in a shiny black fusion crust. Howardites are brecciated mixtures of diogenites and eucrites, showing all processes in-between due to repeated fragmentation on the parent body, likely from the asteroid Vesta in the asteroid belt. Ureilites contain carbonaceous material in-between the olivine and pyroxene in the form of microscopic diamonds. The SNC subgroup features igneous rock with large crystals, metallic oxides, and water. They are dated at 1.3 billion years old, making the only likely source Martian volcanoes, where an impacting body launched the rocks towards Earth. Analyses show Martian atmosphere trapped in the achondrites, and models show a dozen potential craters on Mars with the size and volcanic proximity to create and deliver these to Earth.
While achondrites relate to the crust of a processed body, iron meteorites give insight into its core due to the differentiation of the parent mass. The current classification system looks at chemical concentrations, but the original system looked more into composition and structure based on the two main cubic nickel-iron crystals found in the meteorites: the low-nickel kamacite high-nickel taenite. Hexahedrite, formed in low-nickel meteorites, are composed of large kamacite crystals with cubic structure. By polishing and etching a face with acid, parallel lines called Neumann lines emerge which represent the twinning plane sites of the lattice disrupted by shock. Octahedrites are formed with moderate amounts of nickel, creating an intergrown phase mixture of kamacite and taenite. A similar acid treatment can reveal Widmanstätten figures, displaying kamacite and taenite plates at different orientations depending on the angle of the cut, with secondary phases such as plessite filling the gaps. While highly dependent on concentration, rate of cooling also affects the amounts of each phase formed. Classifying octahedrites places the meteorites on a 5-class scale from coarsest to finest Widmanstätten figures, with higher nickel concentrations causing thinner bands of kamacite.
Ataxites, in contrast to hexahedrites and octahedrites, have no obvious internal structure and slightly higher nickel concentrations than octahedrites. At this point, Widmanstätten figures are microscopic up until 25% nickel, at which point the meteorite is almost entirely taenite. Identifying specimens solely from visible features can fail: the expected widths of Widmanstätten bands are sometimes thicker than the samples and Josephinite, a terrestrial nickel-iron mineral, resembles an iron meteorite structure. Inclusions in Iron meteorites include the nonmagnetic bronze-colored troilite (FeS), silvery brittle Schribersite, and the nearly similar cohenite which only occurs in meteorites witl less than 7% nickel. Carbon commonly appears in meteorites as graphite nodules, often ablating during a fall, but can also appear as minute meteoritic diamonds. Occasionally, iron meteorites have silicates included, in some cases substantially changing the inner structure.
Stony iron meteorites are considered mantle material in our parent body model and are classified into 3 groups, two of which—pallasites and mesosiderites—are common enough to speak on. Pallasites are identified by their mixture of crystalline olivine inside a continuous nickel-iron network. The ratio of olivine to nickel-iron ranges from pure nickel-iron to 1:2; the nickel-iron bears octahedritic ordering and the olivine is magnesium-rich. Rounded olivine grains in the pallasites may suggest an emulsion during cooling of the parent body as olivine precipitates out of the mantle, sinking towards the core which then partially exchanges metals for silicates in the matrix. Pallasites are incredibly sensitive to oxidation. Mesosiderites display breakdown of mantle rock recombined with silicates; they are this polymict breccias with a silicate-metal ratio of roughly 1:1. These stony irons yield 7-10% nickel in a uniform granular composition similar to octahedrites with plagioclase feldspar and calcium pyroxene. The ultimate difference between pallosites and mesosiderites lie in the continuous network of nickel-iron found in pallasites versus the jagged inclusions set in the matrix of the mesosiderites. Mesosiderites also contain shock-produced silica called tridymite originating from either the parent body or the impactor causing the shock.