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While comets may be the best obtainable source of the original material from which the solar system formed, they have been subjected to many changes during the past 4.5 billion years, Jet Propulsion Laboratory scientist said.
In his paper, "How Pristine Are Cometary Nuclei?" presented before the 1986 fall meeting of the American Geophysical Union, Dr. Paul Weissman said the key words are "best obtainable."
To interpret the record contained in the comet nuclei, he said, scientists must consider how much they have been modified since their formation in the solar nebula 4.5 billion years ago.
He identified some of the possible processes as collision with other bodies during formation, heating by short-lived radioactive material, accretion of interstellar gases, erosion by interstellar dust, polymerization or changes in the surface structure by cosmic ray impacts, and heating from the gravitational influences of planets.
He said comets were considered the "best" source of primitive solar system material because they would likely provide samples of volatile and non-volatile materials of the solar nebula. The word "obtainable," he said, recognizes that we are limited to those short-period comets whose returns can be accurately predicted and can be reached with current technology.
Volatile materials would include gases and water molecules, and non-volatile materials would include more solid substances.
But short-period comets are less than ideal, Weissman said, because they have made repeated passages near the sun resulting in heating of their surfaces and interiors.
"This has driven off the more volatile materials and modified those that remain," he said.
In addition, he said, many of the known short- period comets would no longer have the original amorphous ice in their nuclei because their central temperatures would be above 140 Kelvin (-207 F). At that temperature ice becomes crystalline.
It would be better to examine long-period comet from the Oort cloud, theoretical zone where comets orbit about light year or so from the sun. But even those comets would have undergone some modification, he said, from early heating, collision and cosmic ray bombardment.
Recent studies have determined that cosmic rays from the center of the galaxy could completely change the carbon-bearing molecules of the comet surfaces as well as molecules several meters below the surface.
Weissman said also that investigators have recently suggested that short-period (sun orbits of less than 200 years) comets may come from an inner Oort cloud just beyond the orbit of Neptune.
"A variety of evidence has suggested the existence of an extended ring or belt of comets beyond the planetary region and extending out as far, and merging with, the distant spherical Oort cloud surrounding the solar system," Weissman said in his paper.
But because short-period comet may have spent part of its past in the inner solar system, returned to the Oort cloud and subsequently evolved back into the planetary region second time, "it is impossible to say where in the solar nebula it formed, and where it has been since the time of formation."
Weissman's study was funded by the NASA Planetary Geology and Geophysics Program.