FEATURE ARTICLE
How Were the Comets Made?
Explaining the composition of these 4.5 billion-year-old relics may require scientists to revise their models of the primitive solar nebula
Joseph A. Nuth III
Age-Dating Comets
The trend toward increasing crystallinity in the older nebulae has important implications for future studies of the chemical evolution of the solar nebula. Comets that contain only amorphous silicates must have formed early in nebular history, whereas those containing a large fraction of crystalline grains formed much later, perhaps as the last gases of the nebula were dispersing into space. We might expect other chemical components in comets to show similar evolutionary trends with age. For example, as the solar nebula evolved, primitive species (such as CO, CO2, N2) from the parent molecular cloud would be increasingly converted into complex organic molecules, such as hydrocarbons, alcohols and amines. In this scenario, the fraction of complex organic molecules in a comet should be positively correlated with the fraction of crystalline silicate dust it contains.
The possibility of placing comets into a chronological order opens up some interesting possibilities. We are far from understanding how the simple organic ices found in the cores of molecular clouds combined to form the complex organic molecules present in a comet. Modeling the chemistry of a comet on the basis of telescopic observations of its dusty and gaseous emanations (in its coma, or "head") is a very complex affair. Molecules evaporate at various rates from a surface that is far from homogeneous, and many of the complex chemical species are destroyed by ultraviolet light from the sun. More important, some of the most interesting compounds—high-molecular-weight amino acids, sugars and proteins—would not be present in sufficient quantities in the coma to be observed. The only feasible way to detect these materials is by means of a sample-return mission to a passing comet.
However, in order to provide a complete record of the processes in the solar nebula we would want to select comets that were formed at different stages of nebular evolution. This is where the spectral differences between amorphous silicate grains and crystalline grains should come in handy. If our proposed comet chronology is correct, we may be able to assess the relative age of a comet while it is still some distance from the Earth merely by observing its infrared spectrum. In the near future we could thus target specific comets for sample return missions and so study various stages in the chemical evolution of the solar nebula. We would then begin to understand the origin of the molecules that gave a "jump-start" to life on Earth.
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