Sodium sulfate decahydrate (mirabilite) may be an important rock-forming mineral in the outer solar system, comprising a large fraction of the mantles of Jupiter's icy moons; it is also an important evaporite mineral on Earth. In order to model accurately a more compositionally realistic icy moon mantle, we need to fill in the gaps in our knowledge of the thermoelastic properties of this substance. We can do this by making crystallographic measurements upon deuterated mirabilite at high pressures and low temperatures using powder neutron diffraction.