Cometary materials

Radiation chemical processes involving cosmic and UV irradiation The extremely low density of material in interstellar space (ISM gas and ISM nuclei), which could affect the cometary material in the course of millions of years... 

The interstellar dust was shown to contain quinone derivatives as well as oxygen-rich condensed aromatic compounds the quinones were present in both hydrated and carboxylated form. Very little nitrogen was present in the compounds detected. The cometary material, however, contained condensed nitrogen heterocycles. Hardly any oxygen was detected in the solid phase of the cometary dust it possibly evaporates from the tail of the comet in the form of water or oxidized carbon compounds. The authors assume that these analytical results could lead to a reconsideration of the current biogenesis models (Kissel et al 2004 Brownlee, 2004). 

The Rosetta mission with its planned landing on a comet, with analysis of cometary material (see Sect. 3.2), should provide more information on the occurrence of chiral molecular species in the cosmos (Adam, 2002). The GC-MS apparatus installed in the robotic lander RoLand is also able to separate and analyse chiral organic molecules (Thiemann and Meierhenrich, 2001). 

Extraterrestrial origins of life Terrestrial origins of life Life was delivered to the Earth (or any planet) by meteorites of cometary material, leading to the idea of panspermia The molecules of life were built on Earth, perhaps in the primordial soup or little warm pool... 

Repeating the calculation at 170 K gives K2 = 32.41, which is considerably larger. At the colder temperature the reaction is strongly in favour of the reactants. This suggest that any source of oxygen atoms from water photolysis of cometary material would not form at the surface but the higher altitude layers with different chemistry may indeed form. Unfortunately, as we have seen, the formation of O2 from water photolysis is too slow by the simplest mechanisms. 

Conservative estimate based on possible cumulative input calculated assuming a flux of 10 kg of cometary material during first Ga (10 years) of Earth s history. If comets contain of the order of 15 wt% organic material, and if 10% of this material survives, it will comprise approximately 10 kg yr average flux via comets during the first 10 years. 

The main source of oxygen in prebiotic planetary atmospheres is ablation of cometary material. The primary source of various oxygen species in all atmospheric photochemical reactions is water. Water molecules are easily photolyzed by the solar Lyman aline (121.6nm) and on irradiation within the water UV absorption bands (140-190 nm).The main products are hydroxyl radicals, oxygen atoms in their ground (3P) and excited ( D) states, and hydrogen atoms [14],... 

The model is certainly a very crude approximation of the initial composition of protoplanetary dust before it becomes subject to the numerous metamorphism processes that determine the composition and structure of the solids in small and big bodies of a planetary system, but no better model is presently known. Continued examination of IDPs and cometary material will probably improve the situation in the near future. 

One of the cometary probes to have embarked is on a sample-return mission [270], and MS will certainly play a part in the analysis of the material obtained. This will not, however, be the first occasion on which cometary material has reached the Earth. One of the most intriguing, albeit contentious [271-274], reports of the last few years has been the mass-spectroscopic identification of interstellar fullerenes, such as C60, as fossil molecules in the geologic strata associated with the supposed Cretaceous/Tertiarymeteoritic [275] ( dinosaur killer ) and Permian/Triassic cometary [276] ( the Great Dying ) impacts. Similar results are also reported for the Murchison and Allende meteorites, and for material obtained from the ancient ( 2 billion-year-old) Sudbury impact feature... 

Solid bodies of extraterrestrial material that penetrate the atmosphere and reach the Earth s surface are called meteorites. Other extraterrestrial materials include micrometer-sized interplanetary dust particles (IDEs) collected in the lower stratosphere and polar ices. Most meteorites and IDEs are fragments of asteroids, but some IDEs may represent cometary material and some meteorites are fragments of the planets Mars and Earth s moon. Meteorites recovered following observed falls are called/a// those which cannot definitely be associated with observed falls are called finds. Meteorites are given names based on the location where they were recovered (e.g., the Allende meteorite fell in Allende, Mexico). Meteorites recovered in Antarctica and the deserts of Australia and northern Africa are given names and numbers, because numerous samples are found in the same locations. Fragments thought to be of the same meteorite fall, which, in Antarctica or hot deserts, may have different numbers or even names because they were found in different locations, are called... 

Comets are rich in volatile elements, but they probably delivered no more than 10% of Earth s volatile inventory. There are several reasons for this. Comets have a very low impact probability with Earth over their dynamical lifetime ( 10 Levison et al., 2000), limiting the amount of cometary material that Earth could have accreted. In addition, if most of Earth s water was acquired from comets, it seems likely that Earth s noble gas abundances would be higher than observed by several orders of magnitude (Zahnle, 1998). Einally, water measured spectroscopically in comets differs isotopically from that of seawater on Earth, with the cometary D/H ratio being greater by a factor of 2 (Lunine et al., 2000). 

Meteoritic materials that enter the atmosphere as small particles do not have the atmospheric strength filter that hampers the survival of cometary rocks. Small cometary dust particles survive atmospheric entry, and studies of interplanetary dust collected in the stratosphere have provided significant insight into the nature of cometary materials. Particles of 10 xm size decelerate at altitudes near 100 km where the ram pressures are only 0.02 kPa (Brownlee, 1985), many orders of magnitude lower than that experienced by conventional meteorites. Dust samples are also the largest source, by mass, of meteoritic materials. Over 3 X 10" t of interplanetary particles smaller than 300 xm diameter impact the Earth each year (Love and Brownlee, 1995). 

Ip and Fernandes [101] calculated that 6 x 1024 to 6 x 1025 g of cometry material could have been delivered to Earth at the time of the formation of the great Oort Cloud of comets. This amount is equivalent to 4-40 times the present mass of the oceans, assuming about 50% of the cometary mass is ice. Owen and Bar-Nun [102] examined the ability of amorphous ice formed at temperatures below 100K to trap ambient gases. By comparison of the compositions of gases trapped by ice with the compositions of the interstellar medium, comets, and planetary atmospheres, Owen and Bar-Nun [102] concluded that icy comets delivered a considerable fraction of the volatiles to the inner planets. Owen [83] emphasized that the potential supply of cometary materials is more than adequate. 

Altwegg, K. Balsiger, H. and Geiss, J. In The Composition and Origin of Cometary Materials, Editors Altwegg, K Ehrenfreund, P. Geiss, J. Huebner, W. Kluwer Academic Pub., Dordrecht, 1999, p 3. 

Likwan Cheng received a M.E. degree (1988) from Cornell University and a Ph.D. (1998) from Northwestern University. From 1988-1991, he was a staff member of the Laboratory for Planetary Studies at Cornell University, where he synthesized an analog of cometary material that has since become a widely used property standard in the astronomy community. He is currently a postdoctoral associate in Argonne National Laboratory s Environmental Research Division, where he has studied mineral-water interactions using X-rays. 

Mass spectrometry is ideally suited to the investigation of planetary atmospheres and cometary material in terms of both elemental and isotopic abundances. Mass spectrometers have been an integral component of space probe instrumentation because they are mechanically robust, have a low power consumption, yet are sensitive and versatile. 

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