A meteorite is a solid body of natural cosmic origin that has fallen to the surface of the planet, with a size of 2 mm or more. Bodies that have reached the surface of the planet and have sizes from 10 microns to 2 mm are usually called micrometeorites; smaller particles are cosmic dust. Meteorites are characterized by different composition and structure. These features reflect the conditions of their origin and allow scientists to more confidently judge the evolution of the bodies of the solar system.
Types of meteorites by chemical composition and structure
Meteoritic matter is mainly composed of mineral and metal components in various proportions. The mineral part is iron-magnesium silicates, the metal part is represented by nickel iron. Some meteorites contain impurities that determine some important features and carry information about the origin of the meteorite.
How are meteorites divided by chemical composition? Traditionally, there are three large groups:
- Stone meteorites are silicate bodies. Among them are chondrites and achondrites, which have important structural differences. So, chondrites are characterized by the presence of inclusions - chondrules - in the mineral matrix.
- Iron meteorites,consisting predominantly of nickel iron.
- Ironstone - bodies of intermediate structure.
In addition to the classification, which takes into account the chemical composition of meteorites, there is also the principle of dividing "heavenly stones" into two broad groups according to structural features:
- differentiated, which include only chondrites;
- undifferentiated - an extensive group that includes all other types of meteorites.
Chondrites are the remains of a protoplanetary disk
A distinctive feature of this type of meteorites is chondrules. They are mostly silicate formations of elliptical or spherical shape, about 1 mm in size. The elemental composition of chondrites is almost identical to the composition of the Sun (if we exclude the most volatile, light elements - hydrogen and helium). Based on this fact, scientists came to the conclusion that chondrites were formed at the dawn of the existence of the solar system directly from a protoplanetary cloud.
These meteorites have never been part of large celestial bodies that have already undergone magmatic differentiation. Chondrites were formed by condensation and accretion of protoplanetary matter, while experiencing some thermal effects. The substance of chondrites is quite dense - from 2.0 to 3.7 g / cm3 - but fragile: a meteorite can be crushed by hand.
Let's take a closer look at the composition of meteorites of this type, the most common (85.7%) of all.
Carbonaceous chondrites
For carbonaceouschondrites (C-chondrites) are characterized by a high content of iron in silicates. Their dark color is due to the presence of magnetite, as well as impurities such as graphite, soot and organic compounds. In addition, carbonaceous chondrites contain water bound in hydrosilicates (chlorite, serpentine).
According to a number of features, C-chondrites are divided into several groups, one of which - CI-chondrites - is of exceptional interest to scientists. These bodies are unique in that they do not contain chondrules. It is assumed that the substance of meteorites of this group was not subjected to thermal impact at all, that is, it remained practically unchanged since the time of condensation of the protoplanetary cloud. These are the oldest bodies in the solar system.
Organics in meteorites
Carbonaceous chondrites contain such organic compounds as aromatic and saturated hydrocarbons, as well as carboxylic acids, nitrogenous bases (in living organisms they are part of nucleic acids) and porphyrins. Despite the high temperatures experienced by a meteorite as it passes through the earth's atmosphere, hydrocarbons are retained by the formation of a melting crust that serves as a good heat insulator.
These substances, most likely, are of abiogenic origin and indicate the processes of primary organic synthesis already in the conditions of a protoplanetary cloud, given the age of carbonaceous chondrites. So the young Earth already at the earliest stages of its existence had the source material for the emergence of life.
Ordinary andenstatite chondrites
The most common are ordinary chondrites (hence their name). These meteorites contain, in addition to silicates, nickel iron and bear traces of thermal metamorphism at temperatures of 400–950 °C and shock pressures of up to 1000 atmospheres. The chondrules of these bodies are often irregular in shape; they contain detrital material. Ordinary chondrites include, for example, the Chelyabinsk meteorite.
Enstatite chondrites are characterized by the fact that they contain iron mainly in the metallic form, and the silicate component is rich in magnesium (enstatite mineral). This group of meteorites contains less volatile compounds than other chondrites. They underwent thermal metamorphism at temperatures of 600-1000 °C.
Meteorites belonging to both of these groups are often fragments of asteroids, that is, they were part of small protoplanetary bodies in which the processes of subsurface differentiation did not take place.
Differentiated meteorites
Now let's turn to the consideration of what types of meteorites are distinguished by chemical composition in this large group.
Firstly, these are stone achondrites, secondly, iron-stone and, thirdly, iron meteorites. They are united by the fact that all representatives of the listed groups are fragments of massive bodies of asteroid or planetary size, the interior of which has undergone differentiation of matter.
Among differentiated meteorites are found asfragments of asteroids, and bodies knocked out from the surface of the Moon or Mars.
Features of differentiated meteorites
Achondrite does not contain special inclusions and, being poor in metal, is a silicate meteorite. In composition and structure, achondrites are close to terrestrial and lunar bas alts. Of great interest is the HED group of meteorites, thought to originate from the mantle of Vesta, which is thought to be a preserved terrestrial protoplanet. They are similar to the ultramafic rocks of the Earth's upper mantle.
Stony-iron meteorites - pallasite and mesosiderite - are characterized by the presence of silicate inclusions in a nickel-iron matrix. Pallasites got their name in honor of the famous Pallas iron found in the 18th century near Krasnoyarsk.
Most iron meteorites have an interesting structure - "widmanstetten figures", formed by nickel iron with different nickel content. Such a structure was formed under conditions of slow crystallization of nickel iron.
History of the substance of "heavenly stones"
Chondrites are messengers from the most ancient era of the formation of the solar system - the time of accumulation of pre-planetary matter and the birth of planetesimals - the embryos of future planets. Radioisotope dating of chondrites shows that their age exceeds 4.5 billion years.
As for differentiated meteorites, they show us the formation of the structure of planetary bodies. Themthe substance has distinct signs of melting and recrystallization. Their formation could take place in different parts of the differentiated parental body, which subsequently underwent complete or partial destruction. This determines what chemical composition of meteorites, what structure formed in each case, and serves as the basis for their classification.
Differentiated celestial guests also contain information about the sequence of processes that took place in the bowels of the parent bodies. Such, for example, are iron-stone meteorites. Their composition testifies to the incomplete separation of the light silicate and heavy metal components of the ancient protoplanet.
In the processes of collision and fragmentation of asteroids of different types and ages, the surface layers of many of them could accumulate mixed fragments of various origins. Then, as a result of a new collision, a similar “composite” fragment was knocked out from the surface. An example is the Kaidun meteorite containing particles of several types of chondrites and metallic iron. So the history of meteoritic matter is often very complex and confusing.
Currently, much attention is paid to the study of asteroids and planets with the help of automatic interplanetary stations. Of course, it will contribute to new discoveries and a deeper understanding of the origin and evolution of such witnesses to the history of the solar system (and our planet as well) as meteorites.
