  Crystal faceting can emerge via two broad physical mechanisms: anisotropic
attachment kinetics on growing crystals and anisotropic surface energies on
near-equilibrium crystals. For the case of the ice/vapor system, anisotropic
attachment kinetics is the dominant faceting mechanism, while the possible
occurrence of equilibrium faceting has been debated for many decades. In this
investigation we examine ice/vapor faceting at low supersaturations over the
temperature range -15C<T<0C, where evidence of a roughening transition has been
previously reported. Our findings indicate that a comprehensive attachment
kinetics model can explain all the experimental data to date, while assuming an
essentially isotropic surface energy (which is supported by other
considerations). Specifically, our kinetic model naturally explains the
observed disappearance of prism faceting on slowly growing ice crystals in
vacuum at T>-2C, thus suggesting that snow crystal faceting is caused by
anisotropic attachment kinetics even at extremely slow growth rates.
