A starting point to understanding the inner structure of an icy satellite
like Titan, is a full understanding of the phase diagram of H2O-ice.
Methane clathrates are also important but H2O-ice is a more general and
abundant system. The low temperature phases are perfectly crystalline but
the hydrogen sub-lattice disorders with increasing T before melting. The
molecular phases consist of one single hydrogen-bond network, while the
high pressure ones consist of two interpenetrating networks. A common low
temperature phenomenon is amorphization under pressure or decompression.
The reconstruction of one single into two interpenetrating networks, or
vice-versa, is not straightforward and occurs in steps, with perhaps more
than one amorphous and metastable phases intervening before the full
reconstruction occurs. We wish to understand two things: 1) the
phase boundaries between the low temperature (i.e., ~70 K at 0 GPa, ~300K
at 10 GPa), high pressure phases, including hydrogen disordered phases.
These boundaries are still unknown. 2) Since metastability is the norm in
H2O-ice, rather than the exception, we wish to predict energy barriers and
transition rates, for structural transitions. Transition rates are
important to determine the lifetime of metastable phases, which might be
very abundant in these objects. To address the first problem, i.e., the
ordering of H and "empty sites", we will use techniques similar to those
used to investigate solid solutions. For the second problem we need an
effective technique to search for the "transition path" and energy
barriers. A key issue will be to include quantum mechanical tunneling in
these low-temperature processes. (Truhlar, Wentzcovitch, and Baroni)
