Messengers from outer space, meteorites are the rocks that survive the fiery descent through Earth’s atmosphere. Here are some facts detailing the genesis, types, and history of these space rocks.
Was Earth Life Seeded by a Meteorite Impact?
Recent research conducted by the Centre for Planetary Science and Exploration has found evidence in an impact crater in Germany, which suggests that life on Earth may have been seeded by microbial life forms, transported by impacting meteorites. This supports thePanspermia hypothesis, which states that life exists abundantly throughout the universe and spreads through the medium of space-traveling bodies like meteors, planetoids, and comets.
The streaks of light in the nighttime sky, a.k.a. falling stars are actually meteors burning through the atmosphere, pulled by Earth’s inescapable gravity.
Interesting Facts About Meteorites
Any rocky body floating in space, with a size ranging from a few microns to 10 meters, is known as a meteoroid.
When these objects fall towards the planetary surface, making a blazing entry due to the burning caused by atmospheric friction, they are termed as meteors.
Very bright meteorites, that outshine even planet Venus, in the nighttime sky, are known as fireballs.
The part of a meteor that survives the burning on descent and arrives intact on the planetary surface is a meteorite.
Large meteors that burn exceptionally bright in the atmosphere (like a fireball) and/or impact on Earth, are known as bolides.
Asteroids are large left over pieces of matter from formation of the solar system, that could not clump together to form a planet. There is a dense swarm of about 100,000 such objects, outside the orbit of planet Mars, collectively known as the ‘Asteroid Belt’. Their sizes can range from a few meters to more than a 1000 Km.
Ranging in sizes from less than 50 µm to 2 mm, micrometeorites are extraterrestrial particles that reach our planet’s surface, after burning through the atmosphere. The bulk of extraterrestrial matter reaching our planet, is in the form of such particles.
While the total mass of meteorites reaching the planet every year is 50 tonnes, the total mass of descending micrometeorites is five times more, amounting to 2700±1400 tonnes per year.
If a meteorite is observed while falling and recovered after impact, it’s called a ‘fall’ meteorite. If it is not observed while falling, but found from any location, it’s called a ‘find.’
Most of the space debris like meteorites are fragments of parent bodies like asteroids, comets, or other large bodies like planetoids and satellites.
Almost all of these impacting bodies have an origin in the solar system, in the residual matter that didn’t accrete to form planetary bodies, including asteroids and comets.
Micrometeorites mostly originate from comet debris, left behind through the disintegration of the nucleus.
Most meteoroids that burn completely during descent, are relatively small pieces of dust and debris, left behind in the wake of a comet.
Most of the meteorites that survive the descent, have an asteroid origin. Usually, they are fragments of asteroids that are dislodged from their trajectory, by the gravitational pull of Sun and Jupiter, or through collisions.
According to NASA, among the roughly 50,000 meteorites found on the planet, till date, about 99.8% have an origin in asteroids.
Comets leave tons of dust and debris in their trail as they travel along extensive orbits around the Sun. When Earth, while revolving around the Sun, passes through some patches of this comet debris, it enters the atmosphere, causing meteor showers.
Meteor showers are periodic events. The most popular ones are ‘Perseids’ (which peak around August 12) and Leonids (which peak around 17 November). During these showers, you can observe a shooting star at the rate of 1 meteor per minute on an average.
Here is a table identifying the yearly meteor showers, visible in the Northern hemisphere, that you can look forward to. It includes information about their peak observation period and parent bodies.
|Meteor Shower Schedule
|Major Meteor Shower
|Time to Watch
|23:00 to Dawn
|60 to 200
|(196256) 2003 EH1
|21:30 to Dawn
|10 to 15
|Comet C/1861 G1
|01:30 to Dawn
|40 to 85
|21:30 to Dawn
|15 to 20
|Dusk to Dawn
|60 to 100
|22:00 to Dawn
|23:30 to Dawn
|10 to 15
|19:00 to Dawn
|60 to 120
Some meteoroids that fell on Earth had a Martian and Lunar origin. Most probably, they were jettisoned into space by the impact of other space rocks on Mars and the Moon. About 134 lunar and 132 Martian meteorites have been found till date.
Most meteorites are 4.55 billion years old; almost the same age as that of the solar system. Some may even be older than the Earth. Most of them are named after their place of fall.
The earliest reported observation of a meteorite in human history, is in the Parian chronicle, dating back to 1478 BC, which mentions the fall of a ‘Fireball’ over the island of Crete.
In 1794, a German physicist and musician, Ernst Florens Friedrich Chladni, now known as the father of meteoritics and acoustics, was the first to suggest the extraterrestrial origin of meteorites, through his published book. Of course, he was mocked at that time, for this suggestion. However, within a decade, after further work by British chemist Edward Howard and French scientist Jean-Baptiste Biot, the theory was widely accepted.
A falling meteor can travel at a speed, ranging from 11 to more than 100 Km per second.
Every day, about 4 billion meteoroids fall on Earth. Majority of them are very tiny and therefore, do not cause much harm.
Usually, meteorites light up at an altitude of about 120 Km above the planet’s surface and most of them burn completely at an altitude of 70 Km.
The odds of dying from a meteorite or asteroid impact are 1 in 3,000, compared to the odds of dying from a car accident, which are about 1 in 90.
About 15,000 metric tons of meteorites rain down on Earth every year.
It is estimated that about 500,000 fireballs enter Earth’s atmosphere every year. There were about 3556 reported fireball sightings in 2013.
Statistics reveal that about 500 meteorites, with sizes comparable to a marble, to more than the size of a football, fall down on our planet’s surface every year. Among these, about 5 to 6 are likely to be found.
About 44 tonnes (44,000 kilograms or 48.5 tons) of meteoritic material falls to the planetary surface every day.
Meteorite Types and Composition
Recovered meteorites are classified into four main categories which are ‘Chondrites’ (the most common), ‘Achondrites’, ‘Iron’, and ‘Stony iron’ meteorites.
The name ‘Chondrites’ has an origin in round sphere-like features called ‘Chondrules’ on these meteorites. They are made up of the minerals known as pyroxene and olivine. Chondrites are about 4.55 billion years old and are the types of rocks from which our own planet formed. They are relics from the solar system’s ancient past, created due to rapid heating, followed by rapid cooling. They are primarily made up of iron-nickel alloys, olivine, and pyroxene and are further classified into Carbonaceous (Cl, CM, CO, CV, CK, CR, CH), Ordinary (H, L, LL), Kakangari, Rumuruti, and Enstatite chondrites (EH, EL). Freshly fallen meteorites tend to have a burnt fusion crust around them. Most can be identified with a metal detector, due to their iron and nickel content. They also tend to be heavier than terrestrial rocks with peculiar features like chondrules. Consult a local geologist who can run some lab tests to confirm whether it is indeed a meteorite.
Certain chondrite meteorites fallen from outer space contain traces of amino acids and organic matter, bolstering the theory that life may have been seeded on Earth.
Achondrites are relatively younger meteorites of the igneous rock type and are pieces of asteroids. They tend to be primarily composed of olivine, pyroxene, and iron-nickel alloys. They are similar in composition to chondrites, except for the chondrules, that tend to be missing. Some have been known to originate from Moon and Mars.
The absence of chondrules may be a result of their remelting under heat, followed by crystallization. They are further classified into Primitive (Acapulcoites, Winonaites, Lodranites, and Branchinites), Enstatite, Aubrites, Angrites, Ureilites, HED (Howardites, Eucrites, Diogenites, Dunite), Lunar (Breccias, Basalts, Gabbros), and Martian (Shergottites, Nakhlites, and Chassigny) asteroids.
Iron meteorites are fragments of asteroids, primarily composed of iron-nickel alloys. They are magnetic by nature and can be identified by peculiar marks called the Widmanstätten patterns or Thomson structures, that can be seen after cutting and polishing. They are a result of the object having undergone slow cooling, from an initial high-temperature state. Structurally, these types are further classified into Hexahedrites and Octahedrites. These meteorites can be identified by features called regmaglypts, that resemble clear thumbprints, made in clay. They are easily attracted to magnets due to their iron and nickel composition.
Stony iron types are composed of a mixture of iron and silicate materials, embedded within each other. They are primarily classified into Pallasites and Mesosiderites.
Another rare type is an anomalous primitive achondrite, also known as metachondrite, whose composition differs significantly from other achondrites. They are further chemically classified into CV, CR, En, and H type.
Impact History of Meteorites
Till February 2010, there have been roughly 1,086 witnessed falls, while there have been about 38,660 documented ‘Finds’.
The largest intact meteorite found on the planet is the Hoba meteorite. It weighs about 66 tons (60 metric tonnes). It is located near Grootfontein, in Namibia. Interestingly, it has no impact crater due to the low velocity at which it fell, about 80,000 years ago. It contains about 86% iron and about 16% nickel, with trace levels of other metals like cobalt.
About approximately 4200 to 4700 years ago, fragments of a large iron meteorite crashed in Chaco and Santiago del Estero provinces, that lie about 621 miles, towards the Northwest of Bueonos Aires, in Argentina. Collectively, they are known as the Campo del Cielo. Among the recovered fragments that together weigh more than 100 metric tons, the largest piece is the El Chaco, weighing about 37 metric tons (tonnes). It is the second largest meteorite after Hoba.
Some of the other massive meteorites that have graced our planet’s surface are Ahnighito found in Cape York, Greenland (31 tonnes), the one found at Bacubirito, in Mexico (22 tonnes), Agpalilik, in Greenland (20 tonnes), and the one at Mbosi in Tanzania (16 tonnes).
About 50,000 years ago, a nickel-iron meteorite, extending about 50 meters, fell in the Northern Arizona desert, creating the Barringer crater (a.k.a. Arizona Meteor Crater), which is about 1,200 m in diameter and 170 m deep. As seen in the photograph above, it is one of the most well-preserved impact sites in the world.
Allan Hills A81005, named after the location in Antarctica where it was found, was the first meteorite of lunar origin, to be found on our planet. The search was conducted by the ANSMET (Antarctic Search for Meteorites) team from USA.
Allan Hills 84001, a meteorite recovered by the same team, was discovered to be of Martian origin. Claims of microbial life fossils found embedded in this rock, providing telltale signs of the origin of extraterrestrial life on Mars, generated a lot of media interest. The claims are yet to be satisfactorily proved.
The 100 Kg Murchison meteorite, a carbonaceous chondrite, was recovered from Murchison in Victoria, Australia, where it fell in September 28, 1969. Interestingly, the rock was found to contain amino acids including alanine, pseudoleucine, isovaline, glycine, glutamic acid, diaminto acids, as well as complex alkanes. It provides irrefutable evidence that organic matter of non-terrestrial origin, exists in space. This provides credibility to the claim that life on Earth may have been seeded by space rocks.
The largest meteorite in USA was found at Willamette, Oregon, weighing 15.5 tonnes. Its surface is pitted and the space rock is primarily made up of iron (91%) and nickel (7.6%).
Some of the notable ‘fireball’ events in recent history occurred at Peekskill, New York (1992), Bone in Indonesia (2009), Southwestern US (2009), and Chelyabinsk Oblast in Russia (2013).
On September 15, 2007, a 3 meter wide chondritic meteorite fell near Carancas, in Peru, creating a 13 m wide crater.
On February 15, 2013, a meteor with an initial diameter of about 17 to 20 meters and initial mass of 11,000 tonnes exploded about 25 Km to 30 Km over Chelyabinsk, in Russia. Over 1500 people were injured due to the resulting shock wave of the blast that shattered glass and other brittle materials.
One of the first reported events of an extraterrestrial object strike, causing a human injury, was the grapefruit-sized Sylacauga meteorite crashing. The rock fell through the house roof of Ann Elizabeth Hodges on November 30, 1954, badly bruising the woman, on one side of the body. It is now also known as the Hodges meteorite.
Recent research conducted by the University of Bristol has revealed that most of the precious metals on Earth, including gold were brought to the planet by many billion billion tons of asteroids. This enrichment was a result of asteroids raining down, about 200 million years after the planet’s formation.
About more than 23,000 meteorites have been discovered by teams working in Antarctica. These include rocks of Martian and Lunar origin.
Vredefort Crater, located in Free State Province, South Africa, is the largest known impact crater on our planet, with a diameter extending up to 300 km. It was created by a 5 Km to 10 Km asteroid, that struck about 2.023 billion years ago.
The Siljan crater (now a lake), with an estimated diameter of 52 Km, is the largest impact crater in Europe, created about 377 million years ago.
Here’s a table that lists most of the major impact craters around the world.
|Impact Craters Around the World
|Northern Ontario, USA
|Chesapeake Bay Crater
|Chesapeake Bay, USA
(Double Impact Crater)
|26 Km/36 Km
|Nordlinger Ries Crater
The Dynamics of a Meteor Impact
When meteoroids enter the Earth’s atmosphere, they generate intense heat through friction, leading to compression of the surrounding air and the creation of a shock wave. Due to the increase in heat associated with friction, it also glows and radiates heat. Considering that it strikes the surface at a speed of a few kilometers per second, it packs in quite a bit of momentum at strike point, creating huge craters in the process.
The impact leads to compression of exposed rock, creating a depression, whose depth is dependent on the collision force. Glass materials called Tektites are usually formed, due to the extreme heat generated by the impact. They extend a few centimeters in size.
Ejecta containing rock fragments and dust is thrown out in the atmosphere. If the impact energy is too high, a partial amount of the rock might even vaporize. This vaporized and expelled dust will eventually settle down to form a blanket around the crater.
The rock that is directly exposed to the impact will shatter and fragment into dust or even melt if the impact is powerful enough, creating a type of rock called breccia.
Shock waves of compressional and expansive nature will travel around the crater, ultimately leading to the creation of central uplifted region. Fractures in the surrounding region might occur, with deposition of ejecta in concentric circles.
The Connection of Meteorite Impact and Mass Extinctions on Earth
About 66 million years ago, at the end of the Mesozoic era, at a point in time, often referred to as the Cretaceous-Paleogene (K-Pg) boundary, a cataclysmic event wiped out about 75% of the entire planetary species, including the non-avian dinosaurs, who were until then, the apex predators. This mass extinction is now believed to have been orchestrated by the impact of a 10 Km size asteroid, that created the 180 Km-diameter Chicxulub Crater, near the Yucatan peninsula in Mexico.
The energy released through the explosion is estimated to be about 100 teratonnes of TNT. This explosion energy was about a billion times the energy output of atomic bombs dropped on Hiroshima and Nagasaki.
Evidence suggests that this cataclysmic impact ignited a series of natural disasters that eventually led to the mass extinction of many species of plants and animals on the planet. The sulfuric-acid aerosols released into the upper levels of the atmosphere, due to the impact, must have blocked out sunlight for at least a year, inhibiting photosynthesis. Firestorms, acid rains, megatsunamis, exposure to infrared radiation and extreme greenhouse effect further exacerbated the disaster, turning this event into a global catastrophe.
Considering the threat of a apocalyptic collision of a space object on Earth, in the near future, the Torino Scale is devised to measure the potential threat of a future collision event. It is a scale ranging from 0 to 10, with increasing values that indicate the degree of impact certainty and explosion magnitude.
The Relation Between Meteorite Size and Impact Frequency
Statistically, larger the meteorite, lower is the probability of its impact. Our planet is constantly bombarded with micrometeorites, but the falling of larger meteorites is comparatively rarer and less probable.
As illustrated by the graph presented above, a 1 micron diameter meteorite hits the planet every 30 microseconds, a 1 mm diameter object hits every 30 seconds, a 1 meter diameter object might hit once in a year. But cataclysmic, mass-extinction-causing impacts like Chicxulub, with meteorite diameter exceeding 10 Km, might occur only once in a 100 million years.
Since meteorites are rare space rocks, with an exotic mineral composition and structure, there is a huge market out there for finds. Online auction sites like eBay have specimens popping up, all the time.
The imprints of these space travelers on our planet are indelible and they have played a significant role in molding its destiny, for millions of years. The observation of near-Earth objects for probable future impact events and taking deterrent actions to prevent them, are the biggest challenges for humanity, the sole inheritors of this planet.