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5 Comets
Pages 52-63

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From page 52...
... Much of the material generally thought of as rocky, such as silicate grains, may have been in solid form prior to the formation of the protoplanetary disk. More volatile species, from ordinary water ice down through very volatile species such as CO, may have provided their ices either by condensation from the vapor or from preexisting interstellar grains.
From page 53...
... The formation of a planet was inhibited because the slow accretion to planetesimals used up all the material before one planetesimal became large enough that its gravitational cross section greatly exceeded its geometrical cross section and approached the scale of the mean separation between planetesimals. Most of these planetesimals are still present today in what is now called the Kuiper Belt, although the inner parts of this belt, say from 40 to 50 AU, have been considerably depleted by subsequent planetary perturbations.
From page 54...
... If cometary nuclei had substantially melted, there could not be any significant reservoir of amorphous ice. In fact, Prialnik et al.
From page 55...
... . The known Kuiper Belt objects and Centaurs, if they have reflectivities similar to those of known cometary nuclei, are all much larger than the known cometary nuclei.
From page 56...
... Chemical Composition Because there has never yet been any in situ sampling of a cometary nucleus, and since remote sensing of cometary nuclei has never yet shown definitively any identifiable spectral signature (there are reports of such detections but none that would be considered definitive) , current knowledge of the composition of cometary nuclei comes entirely from studies of the coma by remote sensing for many comets and by in situ measurements for only comet Halley (missions to fly through comets P/Giacobini-Zinner and P/Grigg-Skjellerup returned minimal information relevant to nuclear composition)
From page 57...
... Although the deficit is correlated with various parameters, the best interpretation is that these species are depleted in a large fraction of the comets that originally came from the Kuiper Belt but not in those that came from the Oort Cloud, thus suggesting a chemical boundary somewhere in the Kuiper Belt such that comets formed outside that boundary do not contain as much of the (unknown) parent molecules of C2 and C3.
From page 58...
... PAST AND PRESENT ENVIRONMENTAL CONDITIONS In outlining the environmental history of comets, it is convenient to separate Oort Cloud comets from Kuiper Belt comets. A comet in the Oort Cloud for 4.5 Gyr has been irradiated continuously by galactic cosmic rays without any shielding by the heliosphere.
From page 59...
... suggest that some cometary material is preserved intact in impacts of 1- to 100-m cometary nuclei, but the calculations are equivocal. Comets are also a significant contributor to interplanetary dust, both the large "dust" in meteor showers and the small dust that may be more than 50 percent cometary, of which a small but not negligible fraction arrives at Earth's surface without having undergone heating above 160 °C.
From page 60...
... In particular, the irradiation by galactic cosmic rays of most bodies outside the heliosphere, particularly including Oort Cloud comets, would destroy any preexisting life in the outermost tens of meters, and the temperatures are so low that life could not form. The low temperatures of comets in the Kuiper Belt would not allow life to form, although it is not known whether preexisting life could survive.
From page 61...
... Does the preponderance of scientific evidences indicate that there was never liquid water in or on the target The scientific evidence regarding all scales of cometary bodies that have been studied indicates that there was never liquid water in cometary nuclei.
From page 62...
... Missions that would more directly sample the deep interior of a cometary nucleus would be extremely valuable in addressing whether liquid water was ever present. Missions that return dust samples from comets to Earth will assist in determining whether or not liquid water was present on very local scales on cometary nuclei, although this depends on understanding both how much the minerals are metamorphosed on capture and how minerals metamorphose on time scales up to millions of years in the presence of water vapor.
From page 63...
... 1993. Thermal evolution of cometary nuclei by radioactive heating and possible formation of organic chemicals.


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