Meteoroids

Meteoroids are small solid extraterrestrial bodies which travels in the space. Their size ranges from sand- to boulder-sized particle of debris in the Solar System. Meteoroids originate from various sources. Most Meteoroids come from Asteroids that break apart by impacts with other Asteroids while others come from the Moon, Comets and from the Planet Mars. Up to 4 billion Meteoroids fall to Earth everyday. But most of them are too tiny to do any noticeable harm. Most Meteoroids, travelling at five times the speed of sound or more, burn up in the Upper Atmosphere, but a large one's may survive its fiery plunge and reach the surface as a solid body. When a Meteoroid hits the Earth's atmosphere and air friction causes the Meteoroid to melt or vaporize or explode, it is known as Meteor. Meteors are also known as 'Shooting Stars' or 'Falling Stars'. They are called 'Shooting Stars' because the friction between the fast-moving Meteor and the gas in the Earth's atmosphere produces intense heat causing the Meteors to glow with heat and usually making a fiery trail as they fall. This glowing and fiery trail gives the Meteors, an appearance of a 'Falling' or 'Shooting Star'. This glowing phase usually occurs 50 to 68 miles (80 to 110 kilometres) above the Earth. This phenomena is visible in a clear sky as a streak of light. If a Meteor reaches the ground and survives impact, then it is called a Meteorite. A Meteorite can make a hole, or crater on the surface of the Earth when it hits it. More than 24,000 Meteorites are known to have landed on Earth, but only 34 are believed to have originated on Mars. Meteorites are typically named after the town, post office or other geographical landmark nearest the recovery sight. Many Meteors appearing seconds or minutes apart are called a Meteor Shower. Meteors that do not appear to belong to Showers are called Sporadic. A Fireball is any Meteor that is brighter than Venus (magnitude -4). A Sonic Boom often follows a Fireball. Micrometeorite is a tiny particle of Meteoric Dust, one that drifts through the Earth's atmosphere to the ground without becoming Incandescent. They probably originate in Comets. In Greenland, people dig up Meteorites and use the iron in them to make tools.

Classification and Types of Meteoroids / Meteorites

The system of classification used for Meteorites is a dynamic one. There is no consensus among researchers as to what hierarchy of classification terms is most appropriate.

Classification based on Composition

1.  Stony Meteorites -  Stony Meteorites are the most abundant of the 3 Meteorite groups and come closest to resembling Earth rocks in their appearance and composition. Commonly referred to as Stones, they are mostly made up of silicate minerals. They are also known as Aerolites.

Stony Meteorites are divided into 2 main groups:

• Chondrites -  Meteorites or Stones with 'Chondrules' (granules) are known as Chondrites. Chondrites are composed of plagioclase, pyroxene and olivine.

• Achondrites - Meteorites or Stones without 'Chondrules' (granules) are known as Achondrites . They are more like Earth's igneous rocks than are the Chondrites.

2.  Iron Meteorites -  Iron Meteorites are characterized by the presence of two nickel-iron alloy metals: kamacite and taenite. They are commonly known as to as Irons. They are also known as 'Siderolites'.

Depending upon the percentage of nickel to iron, these subdivisions are classified as:

• Hexahedrites -  They are composed almost exclusively of the nickel-iron alloy kamacite, and are lower in nickel content (5%-6%). When etched, Hexahedrites do not display a Widmanstätten pattern, but they often do show Neumann lines which actually parallel lines that cross each other at various angles, and are indicative of impact shock on the parent body.

• Octahedrites - They are composed primarily of the nickel-iron alloys: taenite - high nickel content (7% -12%), and kamacite - low nickel content. These are the most common type of Iron Meteorite. They exhibit a unique structural feature called the 'Widmanstatten Pattern' when etched with a weak acid. This unique crystal pattern is the result of the combination of the two nickel-iron minerals kamacite and taenite being present in approximately equal amounts.

• Ataxites - They are composed mainly of the alloy taenite, and also contain plessite, troilite, and microscopic lamellae of kamacite. They have no visible Widmanstätten pattern. Ataxites are the most nickel-rich Meteorites known, they usually contain over 18% nickel. It is because of the the high nickel content that they do not develop a Widmanstätten Pattern, because in this case kamacite can be exsolved from taenite only at such a low temperature (below about 600°C) where diffusion is already too slow.

3.  Stony-Iron Meteorites - Commonly referred to as Stoney-Irons since they are approximately 50% nickel-iron and 50% silicate minerals.  They are also known as 'Siderites' or 'Mixtures'.     

Stony-Iron Meteorites are divided into:

• Lodranites - These are composed of equal amounts of nickel-iron, olivine and pyroxene.

• Mesosiderites - These are composed of mainly of plagioclase and pyroxene silicates in the form of heterogeneous aggregates intermixed with the metal alloy.

• Pallasites - These consists of a matrix of nickel-iron with embedded grains of olivine. It is a differentiated silicate-rich Meteorite.

Classification based on Grouping

Anomalous Meteorites - Anomalous Meteorites are members of well-established groups that are different enough in some important property to merit distinction from the other members.

Ungrouped Meteorites - Ungrouped Meteorites are those which do not fit into any known group, although they may fit into a clan or class.

Characteristics and Physical Features of Meteoroids

• Most Meteoroids burn up in the Atmosphere - Scientists estimate that 1,000 tons to more than 10,000 tons of Meteoritic material falls on the Earth each day. Vast majority of Meteoroids which collide with Earth, burn up in the upper atmosphere. Only large Meteors survive their trip through Earth's atmosphere. Most Meteors glow for only a few seconds before they burn up.

• Glowing streak of Meteoroids - When a Meteor enters the Earth's Atmosphere, it glows. The brightness of the glow varies, appearing to emit sparks or flares, and sometimes leaves a luminous train that lingers after its flight has ended. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Most Meteoroids that cause Meteors are about the size of a pebble. They become visible between 65 and 120 km above the Earth. They disintegrate at altitudes of 50 to 95 km. Meteors have roughly a fifty percent chance of a daylight, or near daylight, collision with the Earth as the Earth orbits in the direction of roughly west at noon.

• Meteor Showers -  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. Several Shooting Stars or Meteors can be seen per hour on any given night. When the number of Meteors seen increases dramatically, these are known as Meteor Showers. In fact, some Meteor Showers occur annually or at rather regular intervals. The number is greater in autumn and winter. The number always increases after midnight and is usually greatest just before dawn. Some of the the most famous Meteor Shower are the 'Perseids' which peak around August 12th every year. Meteor showers are usually named after a star or constellation which is close to the radiant (the position from which the meteors appear to come). Many of the Meteor showers are associated with Comets. The 'Leonids' are associated with 'Comet Tempel-Tuttle'; 'Aquarids' and 'Orionids' with 'Halley', and the 'Taurids' with Encke.

• Meteoric Dust - Most Meteoroids are destroyed 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 catalysing chemical reactions in the upper atmosphere.

• Varied Orbit - Meteoroids travel around the sun in many different orbits, some clustering in streams often associated with a parent Comet, others apparently sporadic. Meteoroids that orbit together are called stream component and these are probably Comet remnants.

• Varied Velocity - Meteoroids travel around the sun in various velocities. The fastest ones move at about 42 km per second (26 miles per second) through space in the vicinity of Earth's orbit. The Earth travels at about 29 km per second. If a Meteoroids meets the Earth's atmosphere head-on, the combined speed may reach to about 71 km per second.

• Varied Composition - Meteoroids range in composition from fragile snowball-like objects with a density of about a quarter that of ice, to nickel-iron rich dense rocks.

• Varied Colour - The visible light produced by a Meteor may take on various hues, depending on the chemical composition of the Meteoroid, and its speed through the atmosphere. As layers of the Meteoroid are stripped off and ionized, the colour of the light emitted may change according to the layering of minerals. Some of the possible colors and the compounds responsible for them are: orange/yellow (sodium); yellow (iron); blue/green (copper); lilac (potassium); and red (silicate).

Size - The largest individual iron Meteor has a mass of about 54,000 kg. The stones are much smaller, the largest falling in Norton County, Kansas having a mass of about 1,000 kg.

Some of the Major Meteorites / Meteoroids

Chassigny.
Shergotty.
Nakhla.
Lafayette.
Governador Valadares.
Zagami.
ALHA 77005.
Los Angeles 001.
Northwest Africa 2737.
Y000593.

Origin and Evolution of Meteoroids

It is not clear as to where do Meteoroids originate. However 16 Meteorites found in Antarctica are considered to have originated on the Planet Mars. Gases trapped in these Meteorites match the composition of the martian atmosphere as measured by the Viking spacecraft ,which landed on Mars in the mid-1970s. Several Achondrites sampled from Antarctica since 1981 have conclusively been shown to have originated from the Moon based on compositional matches of lunar rocks obtained by the Apollo missions of 1969-1972. Radiometric dating of Chondrites places them at the age of 4.55 billion years, which is the approximate age of the Solar System. They are considered pristine samples of early Solar System matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration. Their composition provides clues to their origins. They may share a common origin with the Asteroids. Some Meteoritic composition is similar to the Earth and Moon and some is quite different. Some evidence indicates an origin from Comets.