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Unlocking the Story of the Early Solar System (1Dec95) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 44, December 1995) Comets falling into the inner Solar System from distant space may be telling us stories about the distant time of the solar system's formation. They are, in fact, of fundamental importance to planetary science because their nuclei may yet preserve in deep freeze not only the primitive material out of which the solar system formed, but physical traces of the various conditions throughout the primitive solar nebula. However, just as the geologist must contend with the continual weathering and processing of ancient geological strata, the planetary astronomer must understand how the comets have been altered by their travels through the warm heart of the solar system. Karen Meech (Hawaii) has been conducting a long-term program to understand fully the extent and nature of aging effects on the different populations of comets. The core of this program involves monitoring and then modeling the light curve behavior of comets over a large range of heliocentric distance, r, contrasting the behavior of periodic comets that have spent considerable time in the inner Solar System to comets new to the inner Solar System (the Oort Comets). This program has been ongoing since 1985, and has involved the use of the KPNO Schmidt, 2.1-m and 4-m telescopes, the CTIO 0.9-m, 1.5-m and 4-m telescopes, as well as other facilities on Mauna Kea, at ESO, in Russia and the HST. Meech has shown that the Oort comets systematically possess dust comae out to large heliocentric distances, whereas, with the exception of the outburst of P/Halley, the periodic comets do not. In addition, the shape of the Oort comet light curves is shallower, which in combination with Finson-Probstein dust modeling, shows that the Oort comets remain active at temperatures much colder than the regimes for water-ice sublimation. Figure 1 shows a sequence of images of P/Neujmin 1 from r = 3.8 to 12.3 AU during which time there is no apparent activity. In contrast, Figure 2 shows the Oort comet, Shoemaker 1984f (C/1984 K1) from r = 4.9 to 11.8 AU, and the difference in the amount of coma is striking. Figure 3 shows composite light curves for 5 Oort comets (solid lines) and P/Halley (filled circles and solid line at bottom of figure representing the behavior of the bare nucleus) showing the difference in their heliocentric light curves. All the other periodic comets in the program are much fainter and steeper than P/Halley. Because the observations of the Oort comets at large r show that the nuclei are not unusually large, Meech infers that either they have larger percentages of active surface area, or that they have different compositions, with more volatile ices responsible for the activity. [Figures not included] Figure 1: Images of Comet P/Neujmin Figure 2: Images of Comet Shoemaker 1984E Figure 3: Cometary Light Curves With the two comet classes showing clear physical differences, the question of whether this is an evolutionary effect, or the result of different formation locations, becomes important. Recent solar nebula model formation scenarios for comets indicate that they may form in more massive disks than previously believed, and that frictional heating as interstellar grains settle down to the disk mid-plane may cause sublimation of the original volatile materials. The water vapor will then recondense to amorphous water ice in the presence of other more volatile gases at temperatures dependent upon the heliocentric distance. Nebula models and laboratory experiments suggest that comets, such as the periodic comets which form in the Kuiper Belt at 100 AU, should trap large amounts of these other volatiles, whereas the Oort comets, forming closer to the sun in the Uranus-Neptune region between 20-30 AU, should have 2-3 orders of magnitude less trapped volatile material. This suggests that the periodic comets should initially be more active than their Oort counterparts. The fact that exactly the opposite is seen in this study is strongly suggestive that we are seeing the effects of aging in the periodic comets, and that the Oort comets best represent the pristine remnants of the early solar system.
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