Solar nebula temperatures

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. 

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... 

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 discovery of homochirality on a planet such as Mars could be an excellent biomarker and strengthen the argument for life on Mars. With an EE in the solar nebula there should be an EE on the surface of Mars of order 9 per cent but remains of ancient life on Mars would show a greater excess. The interchange of enantiomers occurs naturally in a process called racemisation and for the most labile amino acid, aspartic acid, the half-life for the racemisation is 800 years at 300 K in 800 years, half of the non-biotic aspartic acid would racemise and the EE would go to zero. In dry conditions, however, the half-life is much longer, perhaps as large as 5 x 104 years at 300 K. Extrapolation of the racemisation rate to 215 K, the equatorial temperature of Mars, extends the half-life further to 3 x 1012 years and to 1027 years at 150 K, Martian polar temperatures. Hence, discovery of a considerable EE in the Martian soil would be a strong indicator of ancient Martian life. 

The volatile-trapping mechanism has a further problem associated with the temperature. Very volatile molecules such as N2, CO and CH4 are not easily trapped in laboratory ice simulation experiments unless the ice temperature is 75 K, which is somewhat lower than the estimated Saturnian subnebula temperature. This has led to the suggestion that the primary source of nitrogen within the Titan surface ices was NH3, which became rapidly photolysed to produce H2 and N2 upon release from the ice. The surface gravity is insufficient to trap the H2 formed and this would be lost to space. However, the origin of methane on Titan is an interesting question. Methane is a minor component of comets, with a CH4/CO ratio of clCT1 compared with the present atmospheric ratio of > 102. The D/H ratio is also intermediate between that of comets and the solar nebula, so there must be an alternative source of methane that maintains the carbon isotope ratio and the D/H isotope ratio and explains the abundance on Titan. 

Wark DA (1986) Evidence for successive episodes of condensation at high temperature in a part of the solar nebula. Earth Planet Sci Lett 77 129-148... 

The temperature of 50% condensation of a given element in the Solar Nebula defined by Wasson (1985) is 1037 K for Cu and 660 K for Zn. The much more volatile character of Zn with respect to Cu conditions the relative abundances of the two elements among the dififerent classes of chondrites. Copper concentrations vary from 80 to 120 ppm in carbonaceous and ordinary chondrites (Newsom 1995). In contrast, Zn concentrations decrease from 310 ppm in the volatile-rich Cl to 100 ppm in CO and CV, and to 50 ppm in ordinary chondrites. McDonough and Sun (1995) estimate the Cu and Zn content of the Bulk Silicate Earth to be 30 and 55 ppm, respectively. 

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. 

Because of the isotopic variability and the high cosmic abundance of oxygen, oxygen isotopes are very useful for meteorite classification. Below the condensation temperature of silicates and above the condensation temperature of ices, approximately 25% of the oxygen in the solar nebula is predicted to have occurred in condensed solids, with the remainder in gaseous molecules. Chondrites provide samples of the condensed oxygen in the early solar system. 

We should also address the question of whether Cl chondrites represent the complete, low-temperature condensate from the solar nebula. Their bulk composition is consistent with such a model. However, Cl chondrites contain among the highest abundances of presolar grains that are not destroyed by aqueous alteration (the mineralogy of Cl chondrites is almost entirely due to such alteration). This suggests that it is more likely that Cl chondrites formed from representative samples of the dust inherited from the Sun s parent... 

Discuss the evolution of ideas about temperatures in the solar nebula, and how that relates to the formation of CAIs. 

C60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in circumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature-low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented. 

Noble gas clathrates will not now form on the Earth, as can be seen from the air pressure decomposition temperatures in Table 2.7. They might, however, form in cooler regions of the primitive solar nebula (see Limine Stevenson, 1985). Sill and Wilkening (1978) note that for pressures in a plausible model nebula, pure ice clathrates of Ar, Kr, and Xe could form at 40, 45, and 62 K, respectively. 

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

Observations show that protoplanetary disks are dynamic, evolving objects, whose density and temperature structures change with time due to the transport of mass and angular momentum. Chondritic meteorites and comets record a dynamic history of our own solar nebula as they contain materials that formed in a wide variety... 

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