Experiments in the NH3-H2O system in the [0,1 GPa] pressure range - implications for the deep liquid layer of large icy satellites

International audienceWe have experimentally studied the peritectic's melting temperature of a 25 wt% ammonia water solution in the [0, 1 GPa] pressure range by using a high pressurelow temperature membrane sapphire anvil cell with nitrogen cooled cryostat. Our experimental results are in agreement with those presented by Hogenboom et al. [1997] in the [0, 0.4 GPa] pressure domain where the peritectic's melting temperature follows a plateau at 180 K. The present experimental work fills the lack of data between 0.4 and 0.7 GPa. It also suggests that the peritectic's melting temperature is much higher than that between 0 and 0.4 GPa and does not vary very much with pressure at least up to 1 GPa. This implies the existence of two new high pressure phases in the ammonia-water system. The behaviour of the peritectic's melting curve above 0.5 GPa might play an important role in the evolution of a deep liquid layer in the interiors of the largest icy satellites including Titan, Ganymede and Callisto. We show that the cooling of the deep liquid layer may have involved the crystallization of an ammonia bearing phase at higher temperature than previously expected. If the initial fraction of ammonia relative to ices is greater than 11 weight per cent, taking the partial ammonia depletion into account may lead to a final liquid layer thinner than previously inferred