CLATHRATE HYDRATES FORMATION IN COMETARY NUCLEI

The initial composition of current models of cometary nuclei is only based on two forms of ice: crystalline ice for long period comets and amorphous ice for short period comets. A third form of ice, i.e. clathrate hydrate, could exist within the shord period cometary nuclei, but the area of formation of this crystalline structure in these objects has never been studied. Here, we show that the thermodynamic conditions in the interior of short period comets allow the existence of clathrate hydrates in Halley Type Comets. We show that their existence is viable in the Jupiter Family Comets only when the equilibrium pressure of CO clathrate hydrate is at least one order of magnitude lower than the usually assumed theoretical value. The amount of volatiles that could be trapped in the clathrate hydrate layer may be orders of magnitude greater that the daily amount of gas released at the surface of the nucleus at perihelion. The formation and the destruction of the clathrate hydrate cages could then explain the diversity of composition of volatiles observed in comets.