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L. M. Carter (U. Illinois), N. H. Samarasinha, M. J. S. Belton (NOAO), W. H. Julian (NMSU)
We use ground-based photometry by Schleicher {\em et al.} (1998, {\em Icarus}, 132, 397) and a simple thermal model (Julian {\em et al.}, 1998, {\em Icarus}, submitted) to investigate the size and relative chemical composition of active regions on the nucleus of Comet 1P/Halley. The thermal model calculates the water production per unit area as a function of time for five active regions on the surface identified by Belton {\em et al.} (1991, {\em Icarus}, 93, 183). The OH production rates derived by Schleicher {\em et al.} (1998, {\em Icarus}, 132, 397) and the expansion velocities of gas in the coma observed by Bockele'e-Morvan {\em et al.} (1990, {\em A&A}, 238, 382) are used to obtain the total water production of the comet as a function of time. We use the data and model values in a linear least squares fitting procedure to derive the areas of the active regions.
We are now using our derived sizes with NH, CN, C2, and C3 photometry by Schleicher {\em et al.}, (1998, {\em Icarus}, 132, 397) to search for chemical heterogeneity between the active regions. Some evidence for chemical heterogeneity has already been suggested by Belton {\em et al.} (1998, {\em BAAS}, 30, 1088) based on occurrences of major outbursts and a number of H2CO observations. We will discuss the procedure for finding the sizes of individual active regions and present the results of those calculations. We will also show the method and the results for a search on possible chemical variations among active regions and implications for the formation of cometary nuclei.
This research was supported through the NSF Research Experiences for Undergraduates program.