Fulgurites (from the Latinfulgur, meaning "lightning")
are natural tubes, clumps, or masses of sintered, vitrified, and/or
fused soil, sand, rock, organic debris and other sediments that
sometimes form when lightning discharges into ground. Fulgurites are
classified as a variety of the mineraloidlechatelierite. When lightning strikes a grounding substrate, upwards of 100 million volts (100 MV) are rapidly discharged into the ground. This charge propagates into and rapidly vaporizes and melts silica-rich quartzosesand, mixed soil, clay, or other sediments. This results in the formation of hollow, branching assemblages of glassy, tubes, crusts, and vesicular masses.
Because of the high temperature differential between the core of a
fulgurite and the surrounding soil, many fulgurites show evidence of
progressive crystallization: in addition to glasses, many are partially protocrystalline or microcrystalline.
Fulgurites have no fixed composition because their chemical composition
is determined by the physical and chemical properties of whatever
material is being struck by lightning.
Fulgurites are formed when lightning strikes the ground, fusing and vitrifying mineral grains. The primary SiO2 phase in common tube fulgurites is lechatelierite, an amorphous silica glass. Because their groundmass is generally amorphous in structure, fulgurites are classified as mineraloids.
Material properties (color, surface texture) of fulgurites vary
widely, depending on bulk composition, interface dynamics, and trace
elements. Most natural fulgurites fall on a spectrum from colorless
(transparent), to white, to black. Iron oxide is a common impurity that
can result in deep brownish-green coloration. Lechatelierite
similar to fulgurites can also be produced via controlled (or
uncontrolled) arcing of artificial electricity into a medium. Downed high voltage power lines have produced brightly-colored lechatelierites, due to copper or other materials from the power lines themselves.
Brightly-colored lechatelierites resembling fulgurites are usually
synthetic and reflect the incorporation of synthetic materials.
However, lightning can strike man-made objects, resulting in colored
fulgurites.
The interior of Type I (sand) fulgurites normally is smooth or
lined with fine bubbles, while their exteriors are coated with rough
sedimentary particles or small rocks. Other types or fulgurites are
usually vesicular, and may lack an open central tube; their exteriors
can be porous or smooth. Branching fulgurites display fractal-like self-similarity and structural scale invariance
as a macroscopic or microscopic network of root-like branches, and can
display this texture without central channels or obvious divergence from
morphology of context or target (e.g. sheet-like melt, rock
fulgurites). Fulgurites are usually fragile, making the field collection
of large specimens difficult.
Fulgurites can exceed tens of centimeters in diameter and can penetrate deep into the subsoil, sometimes occurring as far as 15 m (49 ft) below the surface that was struck. Or they may form directly on sedimentary surfaces.
One of the longest fulgurites to have been found in modern times was a
little over 4.9 m (16 ft) in length, and was found in northern Florida. The Yale UniversityPeabody Museum of Natural History displays one of the longest known preserved fulgurites, approximately 4 m (13 ft) in length. Charles Darwin in The Voyage of the Beagle recorded that tubes such as these found in Drigg, Cumberland, UK reached a length of 9.1 m (30 ft). The Winans Lake fulgurite[s] (Winans Lake, Livingston County, Michigan),
extended discontinuously throughout a 30 m range, and arguably includes
the largest reported fulgurite mass ever recovered and described: its
largest section extending approximately 16 ft (4.88 m) in length by 1 ft
in diameter (30 cm).
Fulgurites have been classified by Pasek et al. (2012) into five types related to the type of sediment in which the fulgurite formed, as follows:
Type I - sand fulgurites with tubaceous structure; their central axial void may be collapsed
Type II - soil fulgurites; these are glass-rich, and form in a wide
range of sediment compositions, including clay-rich soils, silt-rich
soils, gravel-rich soils, and loessoid;
these may be tubaceous, branching, vesicular, irregular/slaggy, or may
display a combination of these structures, and can produce exogenic
fulgurites (droplet fulgurites)
Type III - caliche or calcic
sediment fulgurites, having thick, often surficially glazed granular
walls with calcium-rich vitreous groundmass with little or no
lechatelierite glass; their shapes are variable, with multiple narrow
central channels common, and can span the entire range of morphological
and structural variation for fulguritic objects
Type IV - rock fulgurites, which are either crusts on minimally
altered rocks, networks of tunneling within rocks, vesicular outgassed
rocks (often glazed by a silicide-rich
and/or metal oxide crust), or completely vitrified and dense rock
material and masses of these forms with little sedimentary groundmass
Type V - [droplet] fulgurites (exogenic fulgurites), which show
evidence of ejection (e.g. spheroidal, filamentous, or aerodynamic), related by composition to Type II and Type IV fulgurites
Significance
Paleoenvironmental indicator
The
presence of fulgurites in an area can be used to estimate the frequency
of lightning over a period of time, which can help to understand past
regional climates. Paleolightning
is the study of various indicators of past lightning strikes, primarily
in the form of fulgurites and lightning-induced remanent magnetization
(LIRM) signatures.
Place in planetary processes and the geologic record
Many
high-pressure, high-temperature materials have been observed in
fulgurites. Many of these minerals and compounds are also known to be
formed by synthetic or meteoritic sources such as nuclear weapon tests, hypervelocity impacts, and cosmic dust. Shocked quartz was first described in fulgurites in 1980. Other materials, including highly reduced silicon-metal alloys (silicides), the fullereneallotropes C60 (buckminsterfullerene) and C70, as well as high-pressure polymorphs of SiO2, have since been identified in fulgurites.
Reduced phosphorus as phosphides and phosphites
has been identified through quantitative analyses of a representative
sample of 10 fulgurites recovered from most continents, in the form of schreibersite (Fe3P, (Fe,Ni)3P), and titanium(III) phosphide (TiP).
Many of these reduced compounds are otherwise rare on Earth due to the
presence of oxygen in Earth's atmosphere, which creates oxidizing
surface conditions.
In material culture
An object initially believed to be a fulgurite was found within the contents of the ash altar at the temple of Lykaian Zeus at Mount Lykaion in Greece.
However, after nearly two decades, the "fulgurite" has not yet been
analyzed or confirmed to be a fulgurite, and it has not been described
in any peer-reviewed publications. The two published reports of the
excavations at Mt. Lykaion notably omit any references to, or
description of, a "fulgurite."
Fulgurites are popular among hobbyists and collectors of natural specimens.
Two splash-form tektites, molten terrestrial ejecta from a meteorite impact
Tektites (from Greek τηκτός tēktós, "molten") are gravel-sized bodies composed of black, green, brown, or gray natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz Eduard Suess (1867–1941), son of Eduard Suess. They generally range in size from millimeters to centimeters. Millimeter-scale tektites are known as microtektites.
Tektites are characterized by:
a fairly homogeneous composition
an extremely low content of water and other volatiles
a general lack of microscopic crystals known as microlites and chemical relation to the local bedrock or local sediments
their distribution within geographically extensive strewn fields
Characteristics
Although tektites are superficially similar to some terrestrial volcanic glasses (obsidians),
they have unusual distinctive physical characteristics that distinguish
them from such glasses. First, they are completely glassy and lack any
microlites or phenocrysts, unlike terrestrial volcanic glasses. Second, although high in silica (>65 wt%), the bulk chemical and isotopic composition of tektites is closer to those of shales and similar sedimentary rocks
and quite different from the bulk chemical and isotopic composition of
terrestrial volcanic glasses. Third, tektites contain virtually no water
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The difference in water content can be used to distinguish
tektites from terrestrial volcanic glasses. When heated to their melting
point, terrestrial volcanic glasses turn into a foamy glass because of
their content of water and other volatiles. Unlike terrestrial volcanic
glass, a tektite produces only a few bubbles at most when heated to its
melting point, because of its much lower water and other volatiles
content.
Classification
On
the basis of morphology and physical characteristics, tektites have
traditionally been divided into four groups. Those found on land have
traditionally been subdivided into three groups: (1) splash-form
(normal) tektites, (2) aerodynamically shaped tektites, and (3) Muong
Nong-type (layered) tektites. Splash-form and aerodynamically shaped
tektites are only differentiated on the basis of their appearance and
some of their physical characteristics. Splash-form tektites are
centimeter-sized tektites that are shaped like spheres, ellipsoids,
teardrops, dumbbells, and other forms characteristic of isolated molten
bodies. They are regarded as having formed from the solidification of
rotating liquids, and not atmospheric ablation. Aerodynamically shaped
tektites, which are mainly part of the Australasian strewn field, are
splash-form tektites (buttons) which display a secondary ring or flange.
The secondary ring or flange is argued as having been produced during
the high-speed re-entry and ablation of a solidified splash-form tektite
into the atmosphere. Muong Nong tektites are typically larger, greater
than 10 cm in size and 24 kg in weight, irregular, and layered tektites.
They have a chunky, blocky appearance, exhibit a layered structure with
abundant vesicles, and contain mineral inclusions, such as zircon, baddeleyite, chromite, rutile, corundum, cristobalite, and coesite.
Microtektites, the fourth group of tektites, are less than 1 mm
in size. They exhibit a variety of shapes ranging from spherical to
dumbbell, disc, oval, and teardrop. Their colors range from colorless
and transparent to yellowish and pale brown. They frequently contain
bubbles and lechatelierite inclusions. Microtektites are typically found
in deep-sea sediments that are of the same ages as those of the four
known strewn fields.
Microtektites of the Australasian strewn field have also been found on
land within Chinese loess deposits, and in sediment-filled joints and
decimeter-sized weathering pits developed within glacially eroded
granite outcrops of the Victoria Land Transantarctic Mountains,
Antarctica.
A very rare aerodynamically shaped Australite – Shallow Bowl
Occurrence
Since
1963, the majority of known tektites have been known to occur only
within four geographically extensive strewn fields: the Australasian,
Central European, Ivory Coast, and North American. As summarized by Koeberl,
the tektites within each strewn field are related to each other with
respect to the criteria of petrological, physical, and chemical
properties, as well as their age. In addition, three of the four strewn
fields have been clearly linked with impact craters using those same
criteria. Recognized types of tektites, grouped according to their known strewn fields, their associated craters, and ages are:
Comparing the number of known impact craters versus the number of
known strewn fields, Artemieva considered essential factors such as the
crater must exceed a certain diameter to produce distal ejecta, and that
the event must be relatively recent. Limiting to diameters 10 km or more and younger than 50 Ma, the study yielded a list of 13 candidate craters, of which the youngest eight are given below:
Povenmire and others have proposed the existence of an additional
tektite strewn field, the Central American strewn field. Evidence for
this reported tektite strewn field consists of tektites recovered from
western Belize in the area of the villages of Bullet Tree Falls, Santa
Familia, and Billy White. This area lies about 55 km east-southeast of
Tikal, where 13 tektites, two of which were dated as being 820,000 years
old, of unknown origin were found. A limited amount of evidence is
interpreted as indicating that the proposed Central American strewn
field likely covers Belize, Honduras, Guatemala, Nicaragua, and possibly
parts of southern Mexico. The hypothesized Pantasma Impact Crater in
northern Nicaragua might be the source of these tektites.
Age
The ages of tektites from the four strewnfields have been determined using radiometric dating methods. The age of moldavites, a type of tektite found in the Czech Republic, was determined to be 14 million years, which agrees well with the age determined for the Nördlinger Ries crater (a few hundred kilometers away in Germany) by radiometric dating of Suevite (an impact breccia
found at the crater). Similar agreements exist between tektites from
the North American strewnfield and the Chesapeake Bay impact crater and
between tektites from the Ivory Coast strewnfield and the Lake Bosumtwi
Crater. Ages of tektites have usually been determined by either the
K-Ar method, fission-track dating, the Ar-Ar technique, or combination
of these techniques.
Origins
Terrestrial source theory
A simple, spherical splash-form Indochinite tektite
The overwhelming consensus of Earth and planetary scientists is that
tektites consist of terrestrial debris that was ejected during the
formation of an impact crater.
During the extreme conditions created by an hypervelocity meteorite
impact, near-surface terrestrial sediments and rocks were either melted,
vaporized, or some combination of these, and ejected from an impact
crater. After ejection from the impact crater, the material formed
millimeter- to centimeter-sized bodies of molten material, which as they
re-entered the atmosphere, rapidly cooled to form tektites that fell to
Earth to create a layer of distal ejecta hundreds or thousands of
kilometers away from the impact site.
A moldavite tektite
The terrestrial source for tektites is supported by well-documented
evidence. The chemical and isotopic composition of tektites indicates
that they are derived from the melting of silica-rich crustal and sedimentary rocks, which are not found on the Moon. In addition, some tektites contain relict mineral inclusions (quartz, zircon, rutile, chromite,
and monazite) that are characteristic of terrestrial sediments and
crustal and sedimentary source rocks. Also, three of the four tektite
strewnfields have been linked by their age and chemical and isotopic
composition to known impact craters. A number of different geochemical
studies of tektites from the Australasian strewnfield concluded that
these tektites consist of melted Jurassic sediments or sedimentary rocks
that were weathered and deposited about 167 My
ago. Their geochemistry suggests that the source of Australasian
tektites is a single sedimentary formation with a narrow range of
stratigraphic ages close to 170 Mya more or less. This effectively
refutes multiple impact hypotheses.
Although the formation of and widespread distribution of tektites
is widely accepted to require the intense (superheated) melting of
near-surface sediments and rocks at the impact site and the following
high-velocity ejection of this material from the impact crater, the
exact processes involved remain poorly understood. One possible
mechanism for the formation of tektites is by the jetting of highly
shocked and superheated melt during the initial contact/compression
stage of impact crater formation. Alternatively, various mechanisms
involving the dispersal of shock-melted material by an expanding vapor
plume, which is created by a hypervelocity impact, have been used to
explain the formation of tektites. Any mechanism by which tektites are
created must explain chemical data that suggest that parent material
from which tektites were created came from near-surface rocks and
sediments at an impact site. In addition, the scarcity of known strewn
fields relative to the number of identified impact craters indicate that
very special and rarely met circumstances are required for tektites to
be created by a meteorite impact.
Nonterrestrial source theories
Aerodynamically shaped australite, the button shape caused by ablation of molten glass in the atmosphere
Though the meteorite impact theory of tektite formation is widely
accepted, there has been considerable controversy about their origin in
the past. As early as 1897, the Dutch geologist Rogier Diederik Marius Verbeek (1845–1926) suggested an extraterrestrial origin for tektites: he proposed that they fell to Earth from the Moon. Verbeek's proposal of an extraterrestrial origin for tektites was soon seconded by the German geologist Franz E. Suess. Subsequently, it was argued that tektites consist of material that was ejected from the Moon
by major hydrogen-driven lunar volcanic eruptions and then drifted
through space to later fall to Earth as tektites. The major proponents
of the lunar origin of tektites include NASA scientist John A. O'Keefe, NASA aerodynamicist Dean R. Chapman, meteorite and tektite collector Darryl Futrell, and long-time tektite researcher Hal Povenmire.
From the 1950s to the 1990s, O'Keefe argued for the lunar origin of
tektites based upon their chemical, i.e. rare-earth, isotopic, and bulk,
composition and physical properties.
Chapman used complex orbital computer models and extensive wind tunnel
tests to argue that the so-called Australasian tektites originated from
the Rosse ejecta ray of the large crater Tycho on the Moon's near side.
O'Keefe, Povenmire, and Futrell claimed on the basis of behavior of
glass melts that the homogenization, which is called "fining", of silica
melts that characterize tektites could not be explained by the
terrestrial-impact theory.
They also argued that the terrestrial-impact theory could not explain
the vesicles and extremely low water and other volatile content of
tektites. Futrell also reported the presence of microscopic internal features within tektites, which argued for a volcanic origin.
At one time, theories advocating the lunar origin of tektites
enjoyed considerable support as part of a spirited controversy about the
origin of tektites that occurred during the 1960s. Starting with the
publication of research concerning lunar samples returned from the Moon,
the consensus of Earth and planetary scientists shifted in favor of
theories advocating a terrestrial impact versus lunar volcanic origin.
For example, one problem with the lunar origin theory is that the
arguments for it that are based upon the behavior of glass melts use
data from pressures and temperatures that are vastly uncharacteristic of
and unrelated to the extreme conditions of hypervelocity impacts.
In addition, various studies have shown that hypervelocity impacts are
likely quite capable of producing low volatile melts with extremely low
water content.
The consensus of Earth and planetary scientists regards the chemical,
i.e. rare-earth, isotopic, and bulk composition evidence as decisively
demonstrating that tektites are derived from terrestrial crustal rock,
i.e. sedimentary rocks, that are unlike any known lunar crust.
In 1960, another nonterrestrial hypothesis for the origin of tektites was proposed by the Russian-born mathematician Matest M. Agrest,
who suggested that tektites were formed as a result of nuclear blasts
produced by extraterrestrial beings. He used this as an argument to
support his paleocontact hypothesis.
Meteoroids are significantly smaller than asteroids, and range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as micrometeoroids or space dust. Most are fragments from comets or asteroids, whereas others are collision impactdebris ejected from bodies such as the Moon or Mars.
When a meteoroid, comet, or asteroid enters Earth's atmosphere at a speed typically in excess of 20 km/s (72,000 km/h; 45,000 mph), aerodynamic heating
of that object produces a streak of light, both from the glowing object
and the trail of glowing particles that it leaves in its wake. This
phenomenon is called a meteor
or "shooting star". A series of many meteors appearing seconds or
minutes apart and appearing to originate from the same fixed point in
the sky is called a meteor shower. If that object withstands ablation from its passage through the atmosphere as a meteor and impacts with the ground, it is then called a meteorite.
An estimated 25 million meteoroids, micrometeoroids and other space debris enter Earth's atmosphere each day, which results in an estimated 15,000 tonnes of that material entering the atmosphere each year.
Meteoroids
Meteoroid embedded in aerogel; the meteoroid is 10 µm in diameter and its track is 1.5 mm long
In 1961, the International Astronomical Union (IAU) defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". In 1995, Beech and Steel, writing in the Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid would be between 100 µm and 10 m (33 ft) across.
In 2010, following the discovery of asteroids below 10 m in size, Rubin
and Grossman proposed a revision of the previous definition of
meteoroid to objects between 10 µm and one meter (3 ft 3 in) in diameter
in order to maintain the distinction.
According to Rubin and Grossman, the minimum size of an asteroid is
given by what can be discovered from Earth-bound telescopes, so the
distinction between meteoroid and asteroid is fuzzy. Some of the
smallest asteroids discovered (based on absolute magnitudeH) are 2008 TS26 with H = 33.2 and 2011 CQ1 with H = 32.1 both with an estimated size of one m (3 ft 3 in).
In April 2017, the IAU adopted an official revision of its definition,
limiting size to between 30 µm and one meter in diameter, but allowing
for a deviation for any object causing a meteor.
Almost
all meteoroids contain extraterrestrial nickel and iron. They have
three main classifications: iron, stone, and stony-iron. Some stone
meteoroids contain grain-like inclusions known as chondrules and are called chondrites. Stony meteoroids without these features are called "achondrites", which are typically formed from extraterrestrial igneous activity; they contain little or no extraterrestrial iron.
The composition of meteoroids can be inferred 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. From these trajectory measurements, meteoroids
have been found to have many different orbits, some clustering in
streams often associated with a parent comet,
others apparently sporadic. Debris from meteoroid streams may
eventually be scattered into other orbits. 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 study of meteorites also gives insights into the composition of non-ephemeral meteoroids.
In the Solar System
Most meteoroids come from the asteroid belt, having been perturbed by the gravitational influences of planets, but others are particles from comets, giving rise to meteor showers. Some meteoroids are fragments from bodies such as Mars or our moon, that have been thrown into space by an impact.
Meteoroids travel around the Sun in a variety of orbits and at
various velocities. The fastest move at about 42 km/s (94,000 mph)
through space in the vicinity of Earth's orbit. This is escape velocity
from the Sun, equal to the square root of two times Earth's speed, and
is the upper speed limit of objects in the vicinity of Earth, unless
they come from interstellar space. Earth travels at about 29.6 km/s
(66,000 mph), so when meteoroids meet the atmosphere head-on (which only
occurs when meteors are in a retrograde orbit such as the Eta Aquariids, which are associated with the retrograde Halley's Comet) the combined speed may reach about 71 km/s (160,000 mph). Meteoroids moving through Earth's orbital space average about 20 km/s (45,000 mph).
On January 17, 2013 at 05:21 PST, a one meter-sized comet from the Oort cloud entered Earth atmosphere over California and Nevada. The object had a retrograde orbit with perihelion at 0.98 ± 0.03 AU. It approached from the direction of the constellation Virgo
(which was in the south about 50° above the horizon at the time), and
collided head-on with Earth's atmosphere at 72 ± 6 km/s
(161,000 ± 13,000 mph) vapourising more than 100 km (330,000 ft) above ground over a period of several seconds.
Collision with Earth's atmosphere
When meteoroids intersect with Earth's atmosphere at night, they are likely to become visible as meteors. If meteoroids survive the entry through the atmosphere and reach Earth's surface, they are called meteorites. Meteorites are transformed in structure and chemistry by the heat of entry and force of impact. A noted 4-metre (13 ft) asteroid, 2008 TC3,
was observed in space on a collision course with Earth on 6 October
2008 and entered Earth's atmosphere the next day, striking a remote area
of northern Sudan. It was the first time that a meteoroid had been
observed in space and tracked prior to impacting Earth. NASA
has produced a map showing the most notable asteroid collisions with
Earth and its atmosphere from 1994 to 2013 from data gathered by U.S.
government sensors.
World map of large meteoric events (also see Fireball below)
A meteor, known colloquially as a shooting star or falling star, is the visible passage of a glowing meteoroid, micrometeoroid, comet or asteroid through Earth's atmosphere, after being heated to incandescence by collisions with air molecules in the upper atmosphere,
creating a streak of light via its rapid motion and sometimes also by
shedding glowing material in its wake. Although a meteor may seem to be a
few thousand feet from the Earth, meteors typically occur in the mesosphere at altitudes from 76 to 100 km (250,000 to 330,000 ft). The root word meteor comes from the Greekmeteōros, meaning "high in the air".
Millions of meteors occur in Earth's atmosphere daily. Most
meteoroids that cause meteors are about the size of a grain of sand,
i.e. they are usually millimeter-sized or smaller. Meteoroid sizes can
be calculated from their mass and density which, in turn, can be
estimated from the observed meteor trajectory in the upper atmosphere.
Meteors may occur in showers,
which arise when Earth passes through a stream of debris left by a
comet, or as "random" or "sporadic" meteors, not associated with a
specific stream of space debris.
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. The
atmospheric velocities of meteors result from the movement of Earth
around the Sun at about 30 km/s (67,000 mph), the orbital speeds of meteoroids, and the gravity well of Earth.
Meteors become visible between about 75 to 120 km (250,000 to
390,000 ft) above Earth. They usually disintegrate at altitudes of 50 to
95 km (160,000 to 310,000 ft).
Meteors have roughly a fifty percent chance of a daylight (or near
daylight) collision with Earth. Most meteors are, however, observed at
night, when darkness allows fainter objects to be recognized. For bodies
with a size scale larger than 10 cm (3.9 in) to several meters meteor
visibility is due to the atmospheric ram pressure
(not friction) that heats the meteoroid so that it glows and creates a
shining trail of gases and melted meteoroid particles. The gases include
vaporised meteoroid material and atmospheric gases that heat up when
the meteoroid passes through the atmosphere. Most meteors glow for about
a second.
History
Although
meteors have been known since ancient times, they were not known to be
an astronomical phenomenon until early in the nineteenth century. Prior
to that, they were seen in the West as an atmospheric phenomenon, like
lightning, and were not connected with strange stories of rocks falling
from the sky. In 1807, Yale University chemistry professor Benjamin Silliman investigated a meteorite that fell in Weston, Connecticut.
Silliman believed the meteor had a cosmic origin, but meteors did not
attract much attention from astronomers until the spectacular meteor
storm of November 1833.
People all across the eastern United States saw thousands of meteors,
radiating from a single point in the sky. Astute observers noticed that the radiant, as the point is now called, moved with the stars, staying in the constellation Leo.
The astronomer Denison Olmsted made an extensive study of this storm, and concluded that it had a cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted the storm's return in 1867, which drew the attention of other astronomers to the phenomenon. Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved to be correct. With Giovanni Schiaparelli's success in connecting the Leonids (as they are now called) with comet Tempel-Tuttle,
the cosmic origin of meteors was now firmly established. Still, they
remain an atmospheric phenomenon, and retain their name "meteor" from
the Greek word for "atmospheric".
Fireball
A fireball is a brighter-than-usual meteor. The International Astronomical Union (IAU) defines a fireball as "a meteor brighter than any of the planets" (apparent 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.
Fireballs reaching apparent magnitude −14 or brighter are called bolides.
The IAU has no official definition of "bolide", and generally considers
the term synonymous with "fireball". Astronomers often use "bolide" to
identify an exceptionally bright fireball, particularly one that
explodes. They are sometimes called detonating fireballs.
It may also be used to mean a fireball which creates audible sounds. In
the late twentieth century, bolide has also come to mean any object
that hits Earth and explodes, with no regard to its composition
(asteroid or comet). The word bolide comes from the Greek βολίς (bolis) which can mean a missile or to flash. If the magnitude of a bolide reaches −17 or brighter it is known as a superbolide. A relatively small percentage of fireballs hit Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs. Such an event happened in broad daylight over North America in 1972. Another rare phenomenon is a meteor procession, where the meteor breaks up into several fireballs traveling nearly parallel to the surface of Earth.
A steadily growing number of fireballs are recorded at the American Meteor Society every year. There are probably more than 500,000 fireballs a year, but most will go unnoticed because most will occur over the ocean and half will occur during daytime.
Effect on atmosphere
"Ionization trail" and "Dark flight (astronomy)" redirect here.
A meteoroid of the Perseids
with a size of about ten millimetres entering the earth's atmosphere in
real time. The meteorid is at the bright head of the trail, and the
ionisation of the mesosphere is still visible in the tail.
The entry of meteoroids into Earth's atmosphere produces three main
effects: ionization of atmospheric molecules, dust that the meteoroid
sheds, and the sound of passage. During the entry of a meteoroid or
asteroid into the upper atmosphere, an ionization trail is created, where the air molecules are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time.
Small, sand-grain
sized meteoroids are entering the atmosphere constantly, essentially
every few seconds in any given region of the atmosphere, and thus
ionization trails can be found in the upper atmosphere more or less
continuously. When radio waves are bounced off these trails, it is
called meteor burst communications. Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail. Most meteoroids burn up when they enter the atmosphere. The left-over debris is called meteoric dust
or just meteor dust. Meteor dust particles can persist in the
atmosphere for up to several months. These particles might affect
climate, both by scattering electromagnetic radiation and by catalyzing
chemical reactions in the upper atmosphere. Meteoroids or their fragments may achieve dark flight after deceleration to terminal velocity. Dark flight starts when they decelerate to about 2–4 km/s (4,500–8,900 mph). Larger fragments will fall further down the strewn field.
Colours
A meteor of the Leonid meteor shower, the photograph shows the meteor, afterglow, and wake as distinct components
The visible light produced by a meteor may take on various hues,
depending on the chemical composition of the meteoroid, and the speed of
its movement through the atmosphere. As layers of the meteoroid abrade
and ionize, the colour of the light emitted may change according to the
layering of minerals. Colours of meteors depend on the relative
influence of the metallic content of the meteoroid versus the
superheated air plasma, which its passage engenders:
Sound generated by a meteor in the upper atmosphere, such as a sonic boom, typically arrives many seconds after the visual light from a meteor disappears. Occasionally, as with the Leonid meteor shower of 2001, "crackling", "swishing", or "hissing" sounds have been reported, occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras.
Theories on the generation of these sounds may partially explain
them. For example, scientists at NASA suggested that the turbulent
ionized wake of a meteor interacts with Earth's magnetic field, generating pulses of radio waves. As the trail dissipates, megawatts of electromagnetic power could be released, with a peak in the power spectrum at audio frequencies. Physical vibrations
induced by the electromagnetic impulses would then be heard if they are
powerful enough to make grasses, plants, eyeglass frames, the hearer's
own body (see microwave auditory effect), and other conductive materials vibrate.
This proposed mechanism, although proven to be plausible by laboratory
work, remains unsupported by corresponding measurements in the field.
Sound recordings made under controlled conditions in Mongolia in 1998
support the contention that the sounds are real.
A meteor shower is the result of an interaction between a planet, such as Earth, and streams of debris from a comet or other source. The passage of Earth through cosmic debris from comets and other sources is a recurring event in many cases. Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951, and by breakup. Each time a comet swings by the Sun in its orbit,
some of its ice vaporizes and a certain amount of meteoroids will be
shed. The meteoroids spread out along the entire orbit of the comet to
form a meteoroid stream, also known as a "dust trail" (as opposed to a
comet's "dust tail" caused by the very small particles that are quickly
blown away by solar radiation pressure).
The frequency of fireball sightings increases by about 10–30% during the weeks of vernal equinox. Even meteorite
falls are more common during the northern hemisphere's spring season.
Although this phenomenon has been known for quite some time, the reason
behind the anomaly is not fully understood by scientists. Some
researchers attribute this to an intrinsic variation in the meteoroid
population along Earth's orbit, with a peak in big fireball-producing
debris around spring and early summer. Others have pointed out that
during this period the ecliptic is (in the northern hemisphere) high in
the sky in the late afternoon and early evening. This means that
fireball radiants with an asteroidal source are high in the sky
(facilitating relatively high rates) at the moment the meteoroids "catch
up" with Earth, coming from behind going in the same direction as
Earth. This causes relatively low relative speeds and from this low
entry speeds, which facilitates survival of meteorites.
It also generates high fireball rates in the early evening, increasing
chances of eyewitness reports. This explains a part, but perhaps not all
of the seasonal variation. Research is in progress for mapping the
orbits of the meteors to gain a better understanding of the phenomenon.
Notable meteors
1992—Peekskill, New York
The Peekskill Meteorite was filmed on October 9, 1992 by at least 16 independent videographers.
Eyewitness accounts indicate the fireball entry of the Peekskill
meteorite started over West Virginia at 23:48 UT (±1 min). The fireball,
which traveled in a northeasterly direction, had a pronounced greenish
colour, and attained an estimated peak visual magnitude of −13. During a
luminous flight time that exceeded 40 seconds the fireball covered a ground path of some 430 to 500 mi (700 to 800 km). One meteorite recovered at Peekskill, New York,
for which the event and object gained their name, had a mass of 27 lb
(12.4 kg) and was subsequently identified as an H6 monomict breccia
meteorite.
The video record suggests that the Peekskill meteorite had several
companions over a wide area. The companions are unlikely to be recovered
in the hilly, wooded terrain in the vicinity of Peekskill.
2009—Bone, Indonesia
A large fireball was observed in the skies near Bone, Indonesia
on October 8, 2009. This was thought to be caused by an asteroid
approximately 10 m (33 ft) in diameter. The fireball contained an
estimated energy of 50 kilotons of TNT, or about twice the Nagasaki atomic bomb. No injuries were reported.
2009—Southwestern US
A large bolide was reported on 18 November 2009 over
southeastern California, northern Arizona, Utah, Wyoming, Idaho and
Colorado. At 00:07 local time a security camera at the high altitude W.
L. Eccles Observatory (9,610 ft (2,930 m) above sea level) recorded a
movie of the passage of the object to the north.
Of particular note in this video is the spherical "ghost" image
slightly trailing the main object (this is likely a lens reflection of
the intense fireball), and the bright fireball explosion associated with
the breakup of a substantial fraction of the object. An object trail
can be seen to continue northward after the bright fireball event. The
shock from the final breakup triggered seven seismological stations in
northern Utah; a timing fit to the seismic data yielded a terminal
location of the object at 40.286 N, −113.191 W, altitude 90,000 ft
(27 km). This is above the Dugway Proving Grounds, a closed Army testing base.
2013–chelyabinsk Oblast, Russia
The Chelyabinsk meteor was an extremely bright, exploding fireball, known as superbolide, measuring about 17 to 20 m (56 to 66 ft) across, with an estimated initial mass of 11,000 tonnes, as the relatively small asteroid entered Earth's atmosphere. It was the largest known natural object to have entered Earth's atmosphere since the Tunguska event in 1908. Over 1,500 people were injured mostly by glass from shattered windows caused by the air burst approximately 25 to 30 km (80,000 to 100,000 ft) above the environs of Chelyabinsk,
Russia on 15 February 2013. An increasingly bright streak was observed
during morning daylight with a large contrail lingering behind. At no
less than 1 minute and up to at least 3 minutes after the object peaked
in intensity (depending on distance from trail), a large concussive
blast was heard that shattered windows and set-off car alarms, which was
followed by a number of smaller explosions.
2019–Midwestern, United States
On November 11, 2019, a meteor was spotted streaking across the skies of the Midwestern United States. In the St. Louis
Area, security cameras, dashcams, webcams, and video doorbells captured
the object as it burned up in the earth's atmosphere. The superbolide meteor was part of the South Taurids meteor shower.
It traveled east to west ending its visible flight path somewhere over
the US state of South Carolina becoming visible once again as it entered
the earth's atmosphere creating a large fireball. The fireball was
brighter than the planet Venus in the night sky.
A meteorite is a portion of a meteoroid or asteroid that survives its
passage through the atmosphere and hits the ground without being
destroyed. Meteorites are sometimes, but not always, found in association with hypervelocity impact craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites. Geologists use the term, "bolide", in a different sense from astronomers to indicate a very large impactor. For example, the USGS
uses the term to mean a generic large crater-forming projectile in a
manner "to imply that we do not know the precise nature of the impacting
body ... whether it is a rocky or metallic asteroid, or an icy comet
for example".
Meteoroids also hit other bodies in the Solar System. On such stony bodies as the Moon or Mars that have little or no atmosphere, they leave enduring craters.
Frequency of impacts
The diameter of the largest impactor to hit Earth on any given day is
likely to be about 40 centimeters (16 inches), in a given year about
four metres (13 ft), and in a given century about 20 m (66 ft). These
statistics are obtained by the following:
Over at least the range from five centimeters (2.0 inches) to
roughly 300 meters (980 feet), the rate at which Earth receives meteors
obeys a power-law distribution as follows:
where N (>D) is the expected number of objects larger than a diameter of D meters to hit Earth in a year. This is based on observations of bright meteors seen from the ground and space, combined with surveys of near-Earth asteroids.
Above 300 m (980 ft) in diameter, the predicted rate is somewhat
higher, with a two kilometres (one point two miles) asteroid (one
million-megaton TNT equivalent) every couple of million years — about 10 times as often as the power-law extrapolation would predict.
Impact craters
Meteoroid collisions with solid Solar System objects, including the Moon, Mercury, Callisto, Ganymede, and most small moons and asteroids,
create impact craters, which are the dominant geographic features of
many of those objects. On other planets and moons with active surface
geological processes, such as Earth, Venus, Mars, Europa, Io, and Titan, visible impact craters may become eroded, buried, or transformed by tectonics over time. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth. Molten terrestrial material ejected from a meteorite impact crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.
