Solar nebula

According to one theory earth and the other planets were formed almost 5 billion years ago from the gas (the solar nebula) that trailed behind the sun as It rotated Being remote from the sun s core the matter in the nebula was cooler than that in the in tenor and therefore it contracted accumulating heavier elements and becoming the series of planets that now circle the sun... 

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

The fractionation of these refractory elements is beheved to be the result of relative efficiencies of incorporation of condensed sohds rich in early high temperature phases into the meteorite parent bodies at different times and locations in the solar nebula. The data are taken from Reference 3. 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed, or usually assumed to be identical to that of the sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the sun s interior by nuclear reactions. The composition of the sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy, and accordingly the relative abundances of the elements in the sun are referred to as "cosmic abundances." Although the cosmic abundance pattern is commonly seen in other stars there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as in the case with neutron stars. The... 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

Carbonaceous solids also reach Earth in the form of organic and icy materials that condensed in the cold outer regions of the solar nebula and also as organic materials preserved in interstellar grains. 

Aqueous chemistry is one of the oldest forces of change in the solar system. It started less than 20 million years after the gases of the solar nebula began to coalesce into solid objects.2 Water is also the most abundant volatile molecule in comets. On the earth, the oceans alone contain about 1.4 x 1021 kilograms or 320,000,000 cubic miles of water. Another 0.8 x 1021 kilogram is held within the rocks of the earth s crust, existing in the form of water of hydration. The human... 

Fig. 2.2 The state of the incipient solar system during the T Tauri phase of the young sun. The central region around the sun was blown free from the primeval dust cloud. Behind the shock front is the disc with the remaining solar nebula, which contained the matter formed by the influence of the solar wind on the primeval solar nebula. From Gaffey (1997)... 

Fig. 2.3 According to the homogeneous accretion model (a), iron-containing material (black) and silicate-containing material (colorless) condensed out at the same time, i.e., the proto-Earth consisted of a mixture of the two. The concentration of iron in the Earth s core took place later. According to the heterogeneous model (b), the iron condensed out of the primeval solar nebula first, while the silicates later formed a crust around the heavy core. From Jeanloz (1983)...

Only the lightest gases, such as hydrogen and helium, could easily escape the gravitational field of the Earth. In contrast to earlier assumptions, it is now believed that the young Earth probably had either no atmosphere at all or only a very thin one, since the proportion of the primeval solar nebula from which the terrestrial planets were formed consisted mainly of non-volatile substances. 

The chondrules contained in the chondrites contain olivine, pyroxene, plagiok-lase, troilite and nickel-iron they can make up 40-90% of the chondrites. Chondrules are silicate spheroids, fused drops from the primeval solar nebula. Because of their differing constitution, chondrites are further subdivided one group in particular is important for the question of the origin of life, and has thus been intensively studied—that of the carbonaceous chondrites. 

Carbonaceous chondrites (C-chondrites) account for only 2-3% of the meteorites so far found, but the amount of research carried out on them is considerable. C-chondrites contain carbon both in elemental form and as compounds. They are without doubt the oldest relicts of primeval solar matter, which has been changed only slightly or not at all by metamorphosis. C-chondrites contain all the components of the primeval solar nebula, apart from those which are volatile they are often referred to as primitive meteorites . 

The Sun formed some 4.5 Gyr ago (Gyr is a Gigayear or 109 years) from its own gas cloud called the solar nebula, which consisted of mainly hydrogen but also all of the heavier elements that are observed in the spectrum of the Sun. Similarly, the elemental abundance on the Earth and all of the planets was defined by the composition of the solar nebula and so was ultimately responsible for the molecular inventory necessary for life. The solar system formed from a slowly rotating nebula that contracted around the proto-sun, forming the system of planets called the solar system. Astronomers have recently discovered solar systems around... 

The matter that made up the solar nebula from which the solar system was formed already was the product of stellar birth, aging and death, yet the Sun is 4.5 billion years old and will perhaps live to be 8 billion years but the Universe is thought to be 15 billion years old (15 Gyr) suggesting that perhaps we are only in the second cycle of star evolution. It is possible, however, that the massive clouds of H atoms, formed in the close proximity of the early Universe, rapidly formed super-heavy stars that had much shorter lifetimes and entered the supernova phase quickly. Too much speculation becomes worrying but the presence of different elements in stars and the subsequent understanding of stellar evolution is supported by the observations of atomic and molecular spectra within the light coming from the photosphere of stars. 

The volatile materials would have vaporised from the surface of the planetesimals once the temperature reached 160 K below this temperature water sticks to silicate surfaces and condenses, ultimately freezing into ice. The new gaseous material is swept away from the planetesimals by the solar wind of particles, leaving bare planetesimals too small to acquire and maintain an atmosphere. The temperature gradient and location within the solar nebula are then important to the ultimate nature and composition of the planets themselves and interplanetary debris. 

Another feature of meteorites that proves to be important is the calcium-aluminium inclusions (CAIs), which, as the name suggests, show regions of enhanced Ca and Al. These micron- to centimetre-sized particles are some of the oldest objects known and have a similar temperature history. They probably formed at temperatures in the region 1700-2400 K and so are close to the centre line of the solar nebula. Although it is hard to be sure about the origin of these objects, there is agreement on their age based on radioisotope dating. 

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

Concepts Solar nebula The collapsing giant molecular cloud that leads to the formation of a star, specifically our Sun with its associated debris in the form of meteorites, meteors and comets... 

The period of emergence of life on Earth is constrained to be between the period 4.0-3.7 Gyr ago, for which there is no fossil record. Urey postulated that all of the planets formed from the same solar nebula and so the early Earth should have an atmosphere with a composition the same as that of Jupiter (known at the time),... 

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