Solar System origin

Presolar grains exhibit large isotopic anomalies not only in their major elements, but also in many minor elements. Isotopic ratios vary over many orders of magnitude, indicative of contributions from different types of stellar sources, namely evolved stars, novae, and SN explosions. Isotope anomalies are also seen in objects with Solar System origin, which, however, are much smaller than those in presolar grains. For example, the calcium-aluminum-rich inclusions (CAIs), the earliest... 

There is convincing observational evidence that the placental interstellar medium (ISM) from which the solar system originated was a dense molecular cloud (Wasserburg et al., 1982 1979). In fact, the recent evidence of the presence of short-lived nuclei in meteorites requires that the free-fall time scale for gravitational collapse (tft) be less than or comparable to the mean lifetime of Al ( 10 yrs), i.e. ttt 4.10 / /n < 10 yrs, which requires nn lOVcc, a value typical of molecular clouds. Since molecular clouds are observed to be a major feature in our galaxy, they constitute a most reliable starting point for the processes that will eventually lead to the formation of stars and planetary systems (Falk and... 

The solar nebula was the rotating, flattened disk of gas and dust from which the solar system originated —4.6 Gyr ago. Much of the motivation... 

To the extent that this high precision, high accuracy result represents the absolute age of crystallization of CAls generally, it provides a measure of the age of formation of the solar system since several lines of evidence, in addition to the absolute Pb-Pb ages, indicate that CAls are the first sohd materials to have formed in the solar nebula (for a review, see Podosek and Swindle (1988)). In fact, it is the relative abundances of the short-lived radionuclides, especially A1, which provides the primary indication that CAls are indeed these first local materials. Other evidence is more circumstantial, e.g., the prevalence of large stable isotope anomalies in CAls compared to other material of solar system origin (see Chapter 1.08). We will return to the issue of antiquity of CAls when we examine the distribution of short-hved isotopes among different CAl types. 

The quantitative comparison of various shortlived radionuclide systems with each other and with Pb-Pb chronology has only been made possible by new data obtained during the last decade, or in many cases, the last few years. Over this same time period, evidence for the decay of several important new short-lived isotopes in the early solar system has been discovered. The record of now-extinct isotopes in early solar system materials is becoming sufficiently well defined to allow construction of a plausible timeline and scenario for solar system origin. 

Origin of the solar system Origin of earth -Origin of the moon -... 

A solar system origin could explain the anomalously high abundance of nanodiamonds relative to other types of presolar grains in meteorites (Hoppe and Zinner, 2000). The recent detection by the Infrared Space Observatory (ISO) of nanodiamonds formed in situ within the accretion disks of young stars confirms that nanodiamonds could indeed have formed in the inner solar system (Van Kerckhoven et al., 2002). 

The broad outlines of Earth history during the Hadean are starting to become visible. The solar system originated 4.57 Ga (Allegre et al., 1995). The accretion of small bodies in the solar nebula occurred within lOMyr of the birth of the solar system (Lugmaier and Shukolyukov, 1998). The Earth reached its present mass between 4.51 Ga and 4.45 Ga (Halliday, 2000 Sasaki and Nakazawa, 1986 Porcelli et ah, 1998). The core formed in <30 Ma (Yin et al., 2002 Kleine et aL, 2002). The early Earth was covered by a magma ocean, but this must have cooled quickly at the end... 

Current evidence indicates that all major bodies in our solar system originated about the same time, approximately 4.6 Ga ago. The oldest rocks taken from the surface of the Moon and meteorites found on Earth are about 4.5 Ga old. The Moon, Mercury, Venus, and Mars have cratered surfaces that appear to be the result of the same type of meteoric activity that produced craters on Earth. The early atmosphere on Earth was probably similar to those found today on nearby planets on which life did not evolve. Hence, we can use information about those planets to help us infer the nature of the conditions on Earth under which life presumably evolved. 

Primarily because of their rather normal isotopic composition, it is generally assumed (but in no way certain) that Q-gases (and hence Q) are of local, possibly Solar System, origin, unlike the gases hosted by the presolar circumstellar phases being discussed further below. 

At around this time, there was much scientific debate about the theory of the origin of species proposed by Charles Darwin (1809-1882), a theory which was to change the world. Darwin himself was very cautious about making statements on biogenesis. It was still too early to answer such questions, because neither results from the science of cell biology nor an extensive knowledge of our planet, the solar system and the cosmos were available. 

The question of the origin of life on Earth leads directly to the question of the formation of our planet, of the solar system and of the universe. The ancient philosophers, as we have seen, attempted to answer such questions, but the models which we discuss and argue about today were proposed by scientists only in the last century. 

Binzel et al. (1991) give an account of the origin and the development of the asteroids, while Gehrels (1996) discusses the possibility that they may pose a threat to the Earth. The giant planets, and in particular Jupiter, caused a great proportion of the asteroids to be catapulted out of the solar system these can be found in a region well outside the solar system, which is named the Oort cloud after its discoverer, Jan Hendrik Oort (1900-1992). Hie diameter of the cloud has been estimated as around 100,000 AU (astronomic units one AU equals the distance between the Earth and the sun, i.e., 150 million kilometres), and it contains up to 1012 comets. Their total mass has been estimated to be around 50 times that of the Earth (Unsold and Baschek, 2001). 

In 1994, a unique incident occurred the impact of the Shoemaker-Levy comet on the Jovian atmosphere. Die strong gravitational field of Jupiter caused the comet to break up before it could enter the atmosphere, and the parts of the comet crashed separately into the atmosphere one after the other. This unique spectacle was observed by many observatories and also by the Galileo spacecraft and the Hubble telescope. It led to the discovery of yet another phenomenon the most intensive aurora effects in the solar system, observed at Jupiter s poles. Astronomers assume that the energy for these comes from the planet s rotation, possibly with a contribution from the solar wind. This process differs from that of the origin of the aurora on Earth, where the phenomenon is caused by interactions between the solar wind and the Earth s magnetic field. 

In August 2006, the International Astronomical Union redefined the term planet and decided that the former ninth planet in the solar system should be referred to as a dwarf planet with the number 134340. The dwarf planet Pluto and its moon, Charon, are the brightest heavenly bodies in the Kuiper belt (Young, 2000). The ratio of the mass of the planet to that of its moon is 11 1, so the two can almost be considered as a double planet system. They are, however, quite disparate in their composition while Pluto consists of about 75% rocky material and 25% ice, Charon probably contains only water ice with a small amount of rocky material. The ice on Pluto is probably made up mainly of N2 ice with some CH4 ice and traces of NH3 ice. The fact that Pluto and Charon are quite similar in some respects may indicate that they have a common origin. Brown and Calvin (2000), as well as others, were able to obtain separate spectra of the dwarf planet and its moon, although the distance between the two is only about 19,000 kilometres. Crystalline water and ammonia ice were identified on Charon it seems likely that ammonia hydrates are present. 

It has recently been suggested that the comets also went through a number of subtle, but important, evolutionary processes in the Oort cloud and the Kuiper belt. Thus, their present nature is probably not the original one, as was previously thought (Stern, 2003). The assumption that the material which comets contain is in the same state as it was when the solar system was formed must be revised or modified. The evolutionary mechanisms to which they were subjected are likely to have changed their chemical composition. 

Cronin J (1998) Clues from the origin of the Solar System meteorites. In Brack A (Ed.) The Molecular Origins of Life. Cambridge University Press, p 119-146 Cronin J, Pizzarello S (2000) Orig Life Evol Biosphere 30 209 Dalgamo A (1991) Nature 353 502 Delsemme (1984) Orig Life Evol Biosphere 14 51... 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

There is no one correct theory for the origin of life on Earth or any habitable planet, although many have been presented. The current set of ideas is summarised in Figure 1.5. Aside from the theory of creation, which seems particularly hard to test, the testable theories of the origins of life divide into two extraterrestrial or panspermia, the theory that life was seeded everywhere somewhat randomly and terrestrial, that life originated de novo on Earth or other habitable planets around other stars. The theories of terrestrial origin are more favoured but the recent discovery of habitable planets and life within any solar system suddenly makes panspermia more likely. 

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. 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

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