Solar system, elements

Beginnings of cosmochemistry (and geochemistry) Philosophical foundations Meteorites and microscopy Spectroscopy and the compositions of stars Solar system element abundances Isotopes and nuclear physics Space exploration and samples from other worlds New sources of extraterrestrial materials Organic matter and extraterrestrial life ... 

Table 1 Composition of the mantle of the Earth assuming average solar system element ratios for the whole Earth.

Chemical equilibrium calculations predict the distribution of each element between its gaseous, solid, and liquid compounds as a function of temperature, pressure, and bulk elemental composition. These calculations are often called condensation calculations because they show the stable phases that condense out of a cooling gas with solar system elemental abundances. However, chemical equilibrium calculations are path independent because the Gibbs energy is a state function, i.e., its differential dG is an exact (or perfect) differential. Thus, the results of chemical equilibrium calculations apply equally well to heating or cooling of a solar composition system. 

Motivations to Study Solar System Elemental Abundances... 

Although much public concern is related to the dispersal of man-made radioisotopes, there are, in fact, a considerable number of sources of natural radioactivity in most soils. Some of these ultimately trace their origin to the stellar nuclear synthesis events that produced the basic elemental makeup of the solar system. Elements in U and Th decay chains (Table 1-3) fall in this class, along with a short list of lighter long lived radioisotopes t, Re, Ta, Lu, Dy,... 

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Element Solar system Mean CI chondrite, ppb Orgued, ppb Element Solar system Mean CI chondrite, ppb Orgued, ppb... 

In order to understand the Earth s character as a planet, it also is helpful to have an understanding of how the elements in our solar system were formed. Chapter 2 starts with the Big Bang theory and continues with how very small grains eventually came together and accreted to form the beginnings of what would eventually become the Earth and other planets, about 4.5 X 10 years ago (4.5 Gyr). The initial processes of the Earth s evolution involved heat... 

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

Our planet Earth contains significant amounts of elements all the way up to Z = 92. This indicates that our solar system resulted from the gravitational collapse of a cloud of matter that included debris from second-generation stellar supemovae. Thus, our sun most likely is a third-generation star. The composition of a third-generation star includes high-Z nuclides, but the nuclear reactions are the same as those in a second-generation star. 

The geological sciences are involved in studying the naturally occurring materials of the earth and solar system (i) to understand the fimdamental processes of crustal formation on earth and solar system evolution, and (2) to evaluate the crustal materials of potential economic value to man. Prior to the 1930 s, analyses were carried out exclusively using classical analytical techniques, with detection limits on the order of o.oi-o.i % (mass fraction). The number of elements contained in any sample could be as extensive as the periodic table, but very few of these could be determined. The development of instrumental techniques revolutionized the analysis of geochemical samples, beginning in the 1930 s. 

According to present-day concepts, our solar system was formed from a huge gas-dust cloud several light years across in a side arm of the Milky Way. The particle density of this interstellar material was very low, perhaps 108-1010 particles or molecules per cubic metre, i.e., it formed a vacuum so extreme that it can still not be achieved in the laboratory. The material consisted mainly of hydrogen and helium with traces of other elements. The temperature of the system has been estimated as 15 K. 

The two rare earth elements niobium (Nb) and tantalum (Ta) were the main subject of study in the investigation referred to. Both elements have very similar properties and almost always occur together in our solar system. However, the silicate crust of the Earth contains around 30% less niobium (compared to its sister tantalum). Where are the missing 30% of niobium They must be in the Earth s FeNi core. It is known that the metallic core can only take up niobium under huge pressures, and the conditions necessary for this may have been present on Earth. Analyses of meteorites from the asteroid belt and from Mars show that these do not have a niobium deficit. 

The planet Jupiter occupies a special position in the solar system. It is the largest and heaviest planet, with a mass of 1/1,047 that of the sun. Jupiter consists almost solely of hydrogen and helium with a ratio similar to that found in the sun itself He H 1 10. Small amounts of some heavier elements are present, such as B, N, P, S, C and Ge. The density of Jupiter has been calculated as 1,300 kg/m3. Its atmosphere can be divided into three zones (starting from the outermost) ... 

Table 3.2 The commonness of elements (log n) in the solar system, in the sun and in carbonaceous chondrites of type Cl, with respect to hydrogen log n(H) = 12, i.e., n( ) = 1012 (Unsold and...

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 interstellar medium is thus a chemically diverse medium fed nearly all of the chemical elements by supernova explosions. Conditions in the interstellar medium produce a cocktail of molecules that ultimately find themselves back on the surface of planets during the formation of the new star and solar system. Does the interstellar medium seed life with molecules from space The nature of interstellar medium chemistry might then add credibility to the formation of life in many places within the Universe and act as a panspermia model for the origins of life. 

Figure 1.4 shows the local Galactic abundances of isobars, based on a combination of elemental and isotopic determinations in the Solar System with data from... 

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