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In order to better understand physical and chemical processes taking place within cometary comae, it is desirable to know the detailed spatial distribution of both the dust and the various gas species in a comet. In recent years it has become clear that a significant fraction of the ``dust'' coma is composed of material which is neither inert nor unchanging. Rather, direct and indirect evidence of the existence of volatile grains as an additional source of cometary gas comes from the discoveries of CHON particles and collimated gas jets in Comet Halley. Analysis of radial brightness profiles of 14 comets by Baum, Kreidl, and Schleicher (1992) indicated that the majority of profiles were steeper than the canonical 1/$\rho$, consistent with fading of the grains, perhaps caused by darkening or shrinking of the grains due to the sublimation of ices. In order to further investigate this phenomenon, we have an on--going program at Lowell Observatory of narrow--band CCD imaging of ``bright'' comets. In this paper, we will show that Comets P/Brorsen--Metcalf (1989 X) and Austin (1990 V) both have spatial profiles consistent with grain fading. Moreover, the deviation from a 1/$\rho$ profile for Comet Austin clearly evolves during the month of imaging. Possible causes of this evolution, such as declining production rates due to increasing heliocentric distance or effects due to the change in phase angle, will be presented.
This research was supported by NASA, NSF, and the Research Corporation.
