Il Sistema Solare e l’Accrescimento dei Planetesimi La teoria di Sofronov (Introduzione all’Universo Parte VII)

La formazione del sistema solare da una nube di gas e grani di polvere: “la nebula primordiale” Almost 5 billion years ago, the actual solar system was only a cloud made of gas (mostly hydrogen and helium) and very diffuse dust grains (carbon and silicate). This cloud, called primordial nebula, had a very low density and temperature, and on the all, it had a mass that is today estimated being 1.1 masses of the actual Sun. In these conditions, a gravitational collapse occurred, making the temperature and density of the central part of the nebula augment. In other words, the nucleus of the cloud started to collapse gravitationally under its own attraction, until it reached a high enough temperature for nuclear reaction to start. At this point hydrogen started burning, and the protostar was born. At the end of this phase it is believed that 90 % of the global mass of the nebula had already been captured by the protostar.

Durante la fase a disco (100 milioni di anni), il materiale che non è stato incorporato nella protostella centrale le ruota intorno. Le particelle di polvere crescono in dimensione molto rapidamente: questo fenomeno è stato chiamato “accrescimento”. Around the protostar the material that has not been incorporated in the star rotates around it, forming a disk. During this phase, that is called disk phase and that can last up to 100 millions years, the grains of dust grow in size very rapidly (this phenomenon being called accretion).

Dopo un periodo relativamente breve (100 Dopo un periodo relativamente breve (100.000 anni) le particelle di polvere sono cresciute diventando oggetti di qualche km di diametro chiamate planetesimi. These planetesimal have a composition that depends on the region where they have formed: if in the inner parts the temperatures are higher and gas sublimate, leaving rocky planetesimals, in the outer parts of the disk we can instead find ices. At this point of the evolution, the star is embedded in a disk shape structure made of several rings of planetesimals of different composition. The accretion of planets is now possible. This accretion is due to the impacts between planetesimals that can glue together, forming growing objects with a composition that depends on the formation's place and that is still respected by the actual structure of the solar system (where, in the inner parts, wet find rocky planets, while in the outer parts, planets are gaseous). Asteroids and comets are leftover planetesimals that have not been incorporated into a planet during this period.

La faccia nascosta della luna presenta molti più crateri di quella visibile At this point of the evolution, a period of heavy bombardment is thought to have taken place (about from 4 to 3.5 millions years ago). At this phase, planets were already formed and planetesimals had time to grow into very big objets (the size of the Moon) that can have very violent impacts with the planets. Most of the craters today seen on the Moon and on satellites without atmosphere are due to that period. Under this heavy bombardment, life wasn't of course possible: on the early Earth, oceans vaporized and the fragile carbon-based molecules, upon which life is based, could not survive. In other words, the influx of interplanetary debris was so strong that the proto-Earth was far too hot for life to have formed. A questo punto dell’evoluzione, si pensa che sia iniziato un periodo di pesanti bombardamenti (da 4 a 3.5 milioni di anni fa). La maggior parte dei crateri che oggi sono visibili sulla Luna e su altri satelliti senza atmosfera sono dovuti a questo periodo.

Il Sistema Solare oggi Considerations Life on Earth began at the end of the period of heavy bombardment, 3.8 billion years ago. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier, just at the end of the period of late heavy bombardment. So the window when life could have begun is very short: just as soon as life could have formed on our planet, it did. A very interesting question about life formation is about the presence of water on Earth: if at the time of life formation there was little water and carbon-based molecules, how were these building blocks of life delivered to the Earth's surface so quickly? The answer may involve the collision of comets with the Earth, since comets contain abundant supplies of both water and carbon-based molecules. Comets are the leftover building blocks of the outer solar system formation process, and offer clues to the chemical mixture from which the giant planets formed some 4.6 billion years ago. Kuiper Belt also  holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretional phases of the solar system. In fact, according to this theory, the inner, dense parts of the pre-planetary disk condensed into the major planets much faster than the outer parts could have done(probably within a few millions to tens of millions of years). In the outer parts of the disk - and so, in the Kuiper Belt- accretion progressed much more slowly and only very small objects were formed. Curiously, it seems that the Oort Cloud objects were formed closer to the Sun than the Kuiper Belt objects. In fact, the small objects formed near the giant planets, would have been ejected from the solar system by gravitational encounters. Those that didn't escape entirely, formed the distant Oort Cloud. Small objects formed farther out, had no such interactions and remained as the Kuiper Belt objects. Il Sistema Solare oggi

Gli asteroidi attualmente popolano la fascia principale, la cui materia è considerata quella di un pianeta non nato. The asteroids today inhabiting the main belt, can be seen as the remnants of a never-born planet. In the zone of the actual main belt, as well as in the totality of the disk, there was a big number of growing planetesimals. To make two bodies stick during a collision, their relative speed must necessarily be smaller than 100 m/s otherwise the two bodies will undergo a collisional fragmentation giving birth to smaller bodies. Now, in the zone of the actual Main Belt, at the time of the heavy bombardment period, Mars was accreting at about 1.5 A.U., while at 5.2 A.U we could find Jupiter, with a mass much bigger than Mars (and of the planetesimals). The presence of Jupiter deviated a big number of planetesimals sending them in the inner zones of the solar system, making the concentration in the zone change, and the speeds of the bodies in the actual main belt zone become greater. For this reason the collisions became destructive and only the bigger embryos continued accreting (becoming the bigger asteroids).

La parola NEO sta per Near Earth Object, ovvero un corpo minore del Sistema Solare. Una prima classificazione di NEO è: NEC (Near Earth Comets) NEA, Near Earth Asteroids, costituenti la maggior parte dei NEO, ulteriormente divisi in tre famiglie: Amor, Apollo e Atens, ciascuna caratterizzata in base alle distanze del perielio, dell’ afelio e alla dimensione del semiasse maggiore dell’orbita. The word NEO stands for Near Earth Object, meaning a minor body of the solar system (or in other words a comet or an asteroid) which comes into the Earth neighborhood. A first classification of NEOs divides NEC (Near earth comets) from NEAs, Near Earth Asteroids. NEAs constitute the vast majority of NEOs and are further divided into three main families, depending on the features of their orbits. In particular they are classified in three groups (Amor, Apollo and Atens) according to their perihelion and aphelion distances and their semi major axes.

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