Possible cometary origin of heavy noble gases in the atmospheres of Venus, Earth and Mars.

Models that trace the origin of noble gases in the atmospheres of the terrestrial planets (Venus, Earth and Mars) to the 'planetary component' in chondritic meteorites confront several problems. The 'missing' xenon in the atmospheres of Mars and Earth is one of the most obvious; this gas is not hidden or trapped in surface materials. On Venus, the absolute abundances of neon and argon per gram of rock are higher even than those in carbonaceous chondrites, whereas the relative abundances of argon and krypton are closer to solar than to chondritic values (there is only an upper limit on xenon).

Gas release from comets.

Water ice was shown experimentally to retain trapped gases beyond the transformation temperature of amorphous ice to cubic ice. The amount of retained gases, which emerge during the transformation of cubic ice to hexagonal ice and when the ice evaporates, depends linearly on the thickness of the ice layer. Implications to comets are discussed.

On the temperature and gas composition in the region of comet formation.

The findings of the Giotto and Vega spacecrafts on the gas composition of comet Halley, together with an experimental study on the trapping of gas mixtures in amorphous water ice, enable estimation of the gas composition and temperature in the region of comet Halley's formation: If Halley was formed in the solar nebula by condensation of water vapor in the presence of gas, in the region of its formation the CO/CH4 ratio had to be at least 100 and the temperature about 48 K. The ice particles that formed the comet could not have condensed at a higher temperature and subsequently cool down because then the 7% CO found as a parent molecule could not have been trapped in the ice. A approximately 48 K formation implies that the ice was in amorphous form.

Trapping of gas mixtures by amorphous water ice.

Our studies on gas trapping in amorphous water ice at 24-100 K were extended, by using mixtures of CH4, CO, N2, and Ar, rather than single gases. In 1:1 gas:(water vapor) mixtures, the competition among these gases on the available sites in the ice showed that the trapping capacity for the various gases is determined not only by the structure and dynamics of the ice, but is also influenced by the gas itself. Whereas at 24-35 K all four gases are trapped in the ice indiscriminantly, at 50-75 K there is a clear enhancement, in the order of CH4 > CO > N2 > or approximately Ar.