Galileo's observations of the moons of Jupiter and the phases of Venus in 1610 led to the wide acceptance of the idea that the Sun is at the center of the solar system as proposed by Nicolaus Copernicus. In the 18th century, when the heliocentric system was firmly established, several scientists, philosophers and mathematicians proposed that the Sun and the planets originated by the gravitational collapse of a nebula of dust and gas. The nebular model has been refined with many modern scientific observations, but it remains the most accepted hypothesis for the formation of the solar system. Chondrites, the most primitive meteorites, provide clues about how the clouds of dust condensed to form the objects that exist today in the solar system.
What are Chondrites?
Chondrites are the oldest rocks that have been analyzed by scientists. Radioisotope dating shows that these rocks originated before the planets were formed, approximately 4,650 million years ago. Chondrites are meteoroids or asteroids that did not completely melt. They contain small grains and inclusions that preserve the minerals and components that were present in the early solar system. Chondrites have approximately the same chemical composition as the surface of the Sun. There are four main types of chondrites:
Nebulas, such as the Horsehead Nebula, are sometimes called star nurseries because it is thought that the gravitational collapse of portions of these galactic dust clouds results in the formation of new stars and planets similar to our solar system.
What are chondrules?
Chondrules are tiny mineral beads that are generally smaller than a grain of rice and comprise about eighty percent of the weight of chondritic meteorites. Chondrules are spherical and contain a variety of small mineral crystals. It is thought that chondrules acquired their spherical shape when clumps of stellar dust were heated and partially melted. Some chondrules have a layered structure, indicating that the accretion and flash melting process was repeated multiple times. The time of formation of chondrules ranges from 4567.32 to 4564.71 million years ago. Chondrules made of higher melting point minerals formed earlier than those consisting of lower melting point materials during a brief span of 3 million years.
Mechanisms for chondrule formation
Chondrules have been made experimentally in the laboratory by aiming high-energy lasers at various minerals to melt them. Chondrules of different types are created depending on the temperature, cooling rate and chemical composition. Rapid crystallization from a supercooled melt produces spherules with rimmed, excentro-radial, barred and glassy textures similar to those of many meteoritic chondrules.
Many theories have been proposed for the formation of chondrules in the solar nebula, and this is a topic with many dissenting opinions. Chondrules have a great diversity of microstructures, and this indicates that there were many ways in which these spherules were created, but they all involved a flash of heat high enough to melt rock, from about 1200°C to 1400°C. The chondrules that have an internal crystalline structure either cooled at a slow rate in order for the mineral crystals to grow, or conversely, crystallized rapidly from a supercooled spherule of molten mineral. The chondrules that have an igneous shell underwent surface melting that did not fuse the interior, so they had to be exposed to a brief period of high temperature. Some chondrules seem to have originated from an initially porous particle that was subsequently melted. This would happen if small clumps of dust and volatile ices, i.e., minuscule comets, were heated to evaporate the volatile components leaving a porous matrix that would later be compressed; the chondrule would then develop an igneous shell after being heated briefly at a high temperature.
This is a list of a few of the processes that have been proposed for the formation of chondrules in the primordial solar nebula and around the protoplanets:
The chondrules within chondritic meteorites tell the story of the formation of the Solar system. The conditions were complex and varied depending on the distance of the chondrules from the evolving Sun and protoplanets. Much more data is needed before scientists can reach a consensus about the conditions that formed these primordial condensation products of our solar nebula.