Solar extraterrestrial

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 ... 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

Tritium has also been observed in meteorites and material recovered from sateUites (see also Extraterrestrial materials). The tritium activity in meteorites can be reasonably well explained by the interaction of cosmic-ray particles and meteoritic material. The tritium contents of recovered sateUite materials have not in general agreed with predictions based on cosmic-ray exposure. Eor observations higher than those predicted (Discoverer XVII and sateUites), a theory of exposure to incident tritium flux in solar flares has been proposed. Eor observations lower than predicted (Sputnik 4), the suggested explanation is a diffusive loss of tritium during heating up on reentry. 

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. 

Life which exists, or could exist, outside our Earth is generally known as extraterrestrial life. A distinction is also made between life (or possible life) within or outside the solar system. In spite of what is claimed in many science fiction books and films, there is no single piece of evidence for a living system outside Earth. The coming years and decades will hopefully provide clarity on the question as to whether we are really alone in the universe or not. 

There are three objects within the solar system which are the subject of research on possible extraterrestrial life, traces of life, biomolecules or their precursors ... 

Of the three extraterrestrial targets in our solar system, the Saturnian moon Titan is the least likely to provide signs of life. To quote Christopher McKay from the NASA Ames Research Center, Titan is an interesting world. For example, its organic haze layer could be an example of the prebiotic chemistry which led to life on Earth . Direct links to extraterrestrial life have not, however, yet been found, as water (one of the main preconditions for life) has not been detected on Titan, apart from traces of water vapour in the higher layers of the Titanian atmosphere (Brack, 2002). 

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 careful study of at least five different carbonaceous chondrites establishes the fact that these meteorites contain carbon compounds of extraterrestrial origin and of great significance in chemical evolution. Their presence confirms that the chemical reaction paths producing biologically important monomer molecules occur in the far reaches of our solar system. 

The adjective space in the chapter title loosely means extraterrestrial and could include planetology, the study of other solid bodies in the solar system, such as Mars, Comet Halley, or asteroid Ceres. While MS is vital to all planetary exploration, these devices function much the same way as laboratory MS, except that they are remotely operated, use less power, and are considerably more expensive. But space can also have the more restricted meaning of outside the ionosphere of any planet, but inside the solar system, which will be the area discussed in this chapter. The properties and challenges of this region are very different from the lab, although the science turns out to be often the same. 

The first concerns the origin of life on earth as we know it, of our biological world. The second considers the possibility of extraterrestrial life, within or beyond the solar system. The third question wonders why life has taken the form we know, and it has as corollary the question whether other forms of life can (and do) exist is there artificial life It also implies that one might try to set the stage and implement the steps that would allow, in a distant future, the creation of artificial forms of life. 

Isotope variations found in extraterrestrial materials have been classified according to different processes such as chemical mass fractionation, nuclear reactions, nucleosynthesis, and/or to different sources such as interplanetary dust, solar materials, and comet material. Various geochemical fingerprints point to the reservoir from which the planetary sample was derived and the environment in which the sample has formed. They can be attributed to a variety of processes, ranging from heterogeneities in the early solar nebula to the evolution of a planetary body. For more details the reader is referred to reviews of Thiemens (1988), Clayton (1993, 2004), and McKeegan and Leshin (2001). 

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

TABLE 3.8 Correction Factors for Extraterrestrial Solar Flux Values Depending on Earth - Sun Distance at Various Times of the Year... 

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 ... 

Fundamental geochemical research - also relevant for a comparison between the elemental distribution of extraterrestrial, meteoritic, solar and stellar material and of interest for the large field of cosmochemistry - was performed by Clarke at the US Geological Survey as long ago as 1889. Clarke s pioneering work, based on the analysis of materials from the Earth s crust, concerned the geochemical study of the relative abundances of the chemical elements as a function of... 

EXTRATERRESTRIAL MATERIALS. Extraterrestrial materials arc samples from other bodies in the solar system that can he studied in earth-hound laboratories. Sensitive and ever-improving analytical techniques are used lo provide information at levels of detail and sophistication that cannot be matched by telescopic or spacecraft investigations. Much of the know ledge of early solar system bodies, processes, environments, and chronology has come from the study of these samples. Extraterrestrial materials that arc availahlc for laboratory study include metcoritic materials that fall naturally lo the earth, some meteorilic material that has been... 

The solar constant (intensity of solar radiation outside the Earth s atmosphere at die mean distance between die earth and the sun) has been determined by measurements from satellites and high-altitude aircraft and is 1.353 kilowatts per square meter. This extraterrestrial radiation,... 

The data calculated from satellite readings of extraterrestrial solar flux may be accurate for cloudless and aerosol free skies. In the case of cloud covered skies the retrieval of aerosol attenuation factor becomes extremely complicated. In the case of cloudless skies, the aerosol attenuation factor q for UV flux at the earth s surface depends on the optical (and chemical properties) of he aerosol, i.e. as to whether the aerosol is absorbing or non absorbing at the wavelengths of interest. This is well depicted in figure 9 below. (Krotkov et al., 1998). 

The value of 10 is determined by molecular and particulate (cloud and aerosol) scattering, and surface reflection. A small fraction of the molecular scattering is the non-conservative Rotational Raman scattering (RRS) that partially fills the solar Fraunhofer lines in the scattered radiation, creating what is commonly known as the Ring effect [15] As a result, the ratio Iq/F, where F is the extraterrestrial solar flux, contains structure that is correlated with solar Fraunhofer lines. By separating these effects, one can write... 

In the search for extraterrestrial signatures of life, we need to distinguish among Solar System sites, extrasolar system sites, and the search for intelligent life. It is likely that within the 21st century most potential life-bearing... 

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