COMETARY ACTIVITY AND NUCLEUS MODELING - A NEW APPROACH

The phenomena of comet splittings with an average frequency of about o ne splitting per 100 years and comet (Chen and Jewitt, Icarus 108, 255 -271, 1994), and the restriction of cometary activity to well-defined small areas at the almost passive and mantle covered surface (Keller e t al., ESA SP-250, Vol. II, pp. 363-364, 1986) are at present driving challenges to structure and evolution of comet nuclei. Extending the p resently discussed models by incorporating lateral subsurface transpor t of sublimed volatiles, there appears the possibility that the places of sublimination are different from those of activity (the so called active areas). Then, there is no necessity to distinguish between diff erent surface properties at active and passive areas, assuming, e.g. a n uncovered icy surface at active areas. Active areas are simply the v ery local ''source sites'' where the accumulated subsurface flows from distant regions reach the surface. The pressure driven subsurface flo ws of volatiles may not only leave the comet at its surface, they may penetrate via cracks, etc. also deeply into the nucleus. There they ca n cause a further growth of cracks and also new cracks. This can be a cause for the observed regular splittings. Furthermore, actual models (Kuhrt and Keller, lcarus 109, 121-132, 1994; Skorov and, Rickman, Pla net. Space Sci. 43, 1587-1594, 1995) of the gas transport through poro us comet surface crusts can be interpreted as to give first indication s for thermodynamical parameters in heat conducting and porous comenta ry crusts which are appropriate for 1 AU conditions to-permit the temp orary existence of a layer with fluid subsurface water within these cr usts. This exciting result of the possible temporary existence of subs urface warm water in comets which approach the Sun within about 1 AU m akes a cometary subsurface chemistry much more efficient than expected hitherto. Copyright (C) 1996 Elsevier Science Ltd