  Dissociation of gas hydrates below 240 K leads to the formation of a
metastable form of water ice, so called cubic ice (Ic). Through its defective
nature and small particle size the surface film composed of such material is
incapable of creating any significant diffusion barrier. Above 160 K, cubic ice
gradually transforms to the stable hexagonal (Ih) form on laboratory time
scales. The annealing, coupled with a parallel decomposition of gas hydrates,
accelerates as temperature rises but already above 190 K the first process
prevails, transforming cubic stacking sequences in-to ordinary Ih ice within a
few minutes. Remaining stacking faults are removed through very slow isothermal
annealing or after heating up above 240 K. The role of the proportion of cubic
stacking on the decomposition rate is discussed. A better understanding of the
dissociation kinetics at low temperatures is particularly im-portant for the
critical evaluation of existing hypotheses that consider clathrates as a
potential medium that actively participate in geological processes or is able
to store gases (e.g. CH4, CO2 or Xe) in environments like comets, icy moons (i.
e. Titan, Europa, Enceladus) or on Mars. Here, we present kinetics studies on
the dissociation of CO2 clathrates at isothermal and isobaric conditions
between 170 and 190K and mean Martian surface pressure. We place special
attention to the formed ice and demonstrate its influence on the dissociation
rates with a combination of neutron diffraction studies (performed on D20 at
ILL/Grenoble) and cryo-SEM. More detailed crystallo-graphic information has
been acquired via a flexible stacking-fault model capable of revealing the time
evolution of the defect structure of ice Ic in terms of stacking probabilities
and crystal size.
