A meteorite is a solid body of natural cosmic origin that has fallen to the surface of the planet, starting at 2 mm in size. Bodies that have reached the planet’s surface and have sizes from 10 microns to 2 mm are commonly 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 in chemical composition and structure
Meteorite is mainly composed of mineral and metal components in various proportions. The mineral part is iron-magnesium silicates, the metal 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 - chondras - in the mineral matrix.
- Iron meteorites, consisting mainly of nickel iron.
- Iron stones - bodies of an intermediate structure.
In addition to the classification, taking into account the chemical composition of meteorites, there is also the principle of dividing the "heavenly stones" into two broad groups according to structural features:
- differentiated, which include only chondritis;
- undifferentiated - an extensive group including all other types of meteorites.
Chondritis - the remains of a protoplanetary disk
A distinctive feature of this type of meteorites is chondras. They are mostly silicate formations of an 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 the protoplanetary cloud.
These meteorites have never been part of large celestial bodies that have already undergone magmatic differentiation. Chondrites formed by condensation and accretion of protoplanetary matter, while experiencing some thermal effect. The substance of chondrites is quite dense - from 2.0 to 3.7 g / cm 3 - but fragile: a meteorite can be crushed by hand.
Let us consider in more detail what composition there are meteorites of this type, the most common (85.7%) of all.
Carbon Chondritis
Carbon chondrites (C-chondrites) are characterized by a high iron content in silicates. Their dark color is due to the presence of magnetite, as well as impurities such as graphite, carbon black and organic compounds. In addition, carbonaceous chondrites contain water bound in hydrosilicates (chlorite, serpentine).
According to a number of signs, C-chondritis are divided into several groups, one of which - CI-chondritis - is of exceptional interest to scientists. These bodies are unique in that they do not contain chondras. It is assumed that the material of meteorites of this group was not subjected to thermal action at all, that is, it remained almost unchanged from the time of the condensation of the protoplanetary cloud. These are the most ancient bodies of the solar system.
Organics in the composition of meteorites
In carbonaceous chondrites, organic compounds such as aromatic and saturated hydrocarbons, as well as carboxylic acids, nitrogenous bases (in living organisms they are part of nucleic acids) and porphyrins are found. Despite the high temperatures that a meteorite is exposed to when passing through the Earth’s atmosphere, hydrocarbons are retained due to the formation of a melting crust, which serves as a good heat insulator.
Most likely, these substances are of abiogenic origin and testify to the processes of primary organic synthesis even under the conditions of a protoplanetary cloud, taking into account the age of carbonaceous chondrites. So the young Earth already at the very early stages of its existence had the source material for the emergence of life.
Ordinary and enstatitis chondritis
The most common chondrites are common (hence their name). In addition to silicates, these meteorites contain nickel iron and bear traces of thermal metamorphism at temperatures of 400-950 ° C and shock pressures up to 1000 atmospheres. The chondras of these bodies often have an irregular shape; they contain debris. Common chondrites include, for example, the Chelyabinsk meteorite.
Enstatite chondrites are characterized by the fact that the iron in them is contained mainly in metal form, and the silicate component is rich in magnesium (mineral enstatite). The meteorites of this group have fewer 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 visited small protoplanetary bodies in which the processes of differentiation of the bowels did not pass.
Differentiated meteorites
Let us now turn to the consideration of what types of meteorites are distinguished by chemical composition in this vast group.
Firstly, these are stone achondrites, secondly, iron-stone and, thirdly, iron meteorites. What unites them is that all representatives of these groups are fragments of massive bodies of an asteroid or planetary size, the bowels of which underwent differentiation of matter.
Among differentiated meteorites, there are both fragments 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, it is a silicate meteorite. In composition and structure, achondrites are close to terrestrial and lunar basalts. Of great interest is the group of HED meteorites, presumably originating from the Vesta mantle, which is considered to be the preserved protoplanet of the earth group. They are similar to ultrabasic rocks of the upper mantle of the Earth.
Iron-stone meteorites - pallasites and mesosiderites - are characterized by the presence of silicate inclusions in the matrix of nickel iron. Pallasites got their name in honor of the famous Pallasov iron found in the 18th century near Krasnoyarsk.
Most iron meteorites have an interesting structure - “Widmannstetten figures” formed by nickel iron with different nickel contents. Such a structure was formed under conditions of slow crystallization of nickel iron.
The 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 origin of planetesimals - the embryos of future planets. Radioisotope dating of chondrites shows that their age exceeds 4.5 billion years.
As for the differentiated meteorites, they demonstrate to us the formation of the structure of planetary bodies. Their substance has distinct signs of melting and recrystallization. Their formation could occur in different parts of the differentiated parental body, which subsequently underwent complete or partial destruction. This determines what the chemical composition of meteorites is, what structure was formed in each particular 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, iron-stone meteorites. Their composition indicates an incomplete separation of light silicate and heavy metal components of the ancient protoplanet.
In the processes of collision and crushing of asteroids of different types and ages in the surface layers of many of them, accumulation of mixed fragments of various origins could occur. Then, as a result of a new collision, such a “composite” fragment was knocked out of the surface. An example is the Kaidun meteorite, containing particles of several types of chondrites and metallic iron. So the history of meteorite matter is often very complex and confusing.
Currently, much attention is paid to the study of asteroids and planets using automatic interplanetary stations. Of course, it will contribute to new discoveries and a deeper understanding of the origin and evolution of such witnesses of the history of the solar system (including our planet as well), such as meteorites.