Cosmic petrology and the planetary evolution of the Solar System
Cosmic petrology, whose origin was triggered by astronomic discoveries at the turn of the twenty-first century, now plays a pivotal role in the ‘compositional' interpretation of the results of extraterrestrial observations. Cosmic petrology is able to play this role owing to the experience gained in studying meteorites and, in particular, chondrites, which display preserved traces of their two-stage evolution. The latter relates these meteorites to the planetary evolutions of the Solar System. This evolution started with the origin of the giant planets via the accretion of water–hydrogen icy (‘cometary') planetesimals simultaneously with the accumulation of the Sun's mass. They underwent contraction with the release of energy sufficient for their complete melting and subsequent differentiation into giant fluid envelopes and chondritic cores. The iron–silicate differentiation of the latter brought about the magnetic fields of the giant planets. The Sun evolved even further and, upon reaching its stellar state, began actively to affect the surrounding planetary system and induced the dissipation of the dense interplanetary nebula in space. This process gave rise to the rapid rotation of the giant planets. The influence of the Sun on the giant planets caused the surface migration of hydrogen, a process accompanied by the acceleration of the rotation and, as a consequence, the separation of satellites under the effect of centrifugal forces. This glaringly manifests the deceleration of the planetary evolution with increasing distance from the Sun. This effect was at a maximum on the near-Sun giant planets, which have lost their fluid envelopes, so that their cores were transformed into independent terrestrial planets simultaneously with the loss of their satellite systems. The preserved relics of these systems are the Moon of the Earth, and Phobos and Deimos of Mars. They provide evidence of the very old age of the near-Sun terrestrial protoplanets, in contrast with the relatively young ages of satellites of the giant planets belonging to the Jovian group. The Moon is one of the oldest known satellites in the Solar System. Volcanic activity on the Moon has an age of 4.6–3.2 Gyears, whereas its analogue Io (a satellite of Jupiter) is now characterized by the culmination of its volcanic activity. Volcanic events on the satellites of the giant planets and on the terrestrial planets are some of the most conspicuous manifestations of their endogenic activity, which is caused by the fluid state of these molten cores, generating magnetic fields. This activity was lost by planets when their consolidation was completed. The duration of the endogenic activity of planets was predetermined by their protoplanetary evolution in the form of the cores of their parent giant planets. In this sense, the Earth is a unique planet, whose endogenic activity has already lasted for 4.6 Gyears, whereas the Earth's core is now less than 50% consolidated. This makes the Earth different in a major way from the rest of the terrestrial planets (Mercury, Venus and Mars), whose endogenic evolution terminated at a primitive stage because of the loss of their fluid components as a result of complete consolidation, which was coupled with the loss of their magnetic fields. There are good reasons to believe that the Earth had completely differentiated under the tremendous pressure of the fluid shell of its parental planet (Proto-Earth), and this predetermined the huge reserves of fluid components stored in its liquid core. At the same time, the differentiation of the other terrestrial planets was associated with the loss of the fluid shells of their parental protoplanets. The effect of parent protoplanets was even weaker in the states of the satellite planets, which lost their fluid shells and differentiated in space vacuum, a process that was responsible for the low reserves of fluid components in their molten iron cores and, correspondingly, their relatively short-lived endogenic activity (1.4 Gyears). The endogenic activity of iron–stony planets is caused by their protoplanetary evolution, which was associated with the concentration of fluid components in their molten cores, whereas the planetary stage itself was accompanied only by the loss of fluid components coupled with endogenic activity up to the complete consolidation of the planets. Traces of these two evolutionary stages can also be discerned in more primitive chondritic planets, whose orbits are situated around the terrestrial planets and lie between the orbits of Mars and Jupiter. Because of the greater distances between these planets and the Sun, they did not have sufficient time to differentiate and develop their rigid silicate shells. Because of this, these planets underwent explosive break-up and gave rise to the asteroid belt (‘chondritic belt'), the source of meteorites. The two-stage character of the evolution of the planets can also be definitely inferred for chondrites. During the early protoplanetary stage, the isotopically anomalous evolution of chondritic melts proceeded as chondrites differentiated into chondrules and matrices, in which diamond nuclei with abundant inclusions of fluid components were formed. This testifies that they were produced under the huge pressures of the fluid shells of the giant planets. Chondrites crystallized mostly later, during the planetary stage proper, under a relatively low pressure. In contrast with the protoplanetary stage, this was not associated with the anomalous fractionation of isotopes.
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Document Type: Research Article
Affiliations: Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432, Russia
Publication date: 01 December 2005