The many moons of the giant gas planets in the outer Solar System continue to fascinate us. Recent space missions to the Jupiter and Saturn systems have revealed their extraordinary diversity. Several moons appear to have rocky cores wrapped in icy shells that show tantalising signs of volcanic activity. The Jovian satellite Europa and Saturn’s largest moon Titan have particularly caught the attention of planetary scientists, who even speculate whether life could evolve there – perhaps in seas hidden beneath their frozen exteriors.
Of course, the moons’ icy mantles are not made of merely water but also include other small, hydrogen containing molecules such as ammonia and methane. To understand the subsurface geophysics and chemistry requires a laboratory study of the compounds likely to form over a range of cold, high-pressure conditions.
Neutrons offer an excellent way of characterising these materials. A neutron beam readily scatters off the array of lightweight atoms to give a diffraction pattern characteristic of their arrangement and bond lengths. However, to get a clearer signal, the hydrogen is replaced by its heavier isotope, deuterium.
High-pressure structures
Dominic Fortes at University College London and colleagues have been using the High Resolution Powder Diffractometer (HRPD) at ISIS to study several ices thought to be significant to the geology of Titan and other moons. Compared with similar instruments in the world, HPRD offers the best combination of resolution and intensity.
One of the first materials explored was ice II. Frozen water can form several crystalline structures, or polymorphs, depending on temperature and pressure (see also page 19). Ice II exists over a range of moderately high pressures and at temperatures below about –130°C. It is thought that the largest icy moons, Titan, Ganymede and Callisto, could have ice II mantles hundreds of kilometres thick. However, to develop a good geological model, more data are needed about the behaviour of ice II. Overcoming significant technical challenges, the team was able to confirm its structure and measure its thermal expansivity and strength.
The researchers also analysed the crystal structure of methanol monohydrate at -115°C, which may be a volcanic constituent on icy moons. They then went on to explore the structure of ammonia dihydrate over a range of high pressures and low temperatures. It exists in several polymorphs, four of which have so far been investigated using the OSIRIS and POLARIS diffractometers as well as the HPRD. These studies will go towards helping planetologists decide whether deep oceans really could exist below Titan’s ice-cold surface.
Dominic Fortes (University College London), andrew.fortes@ucl.ac.uk
Research date: December 2006
Further Information
The incompressiblity and thermal expansivity of D2O ice II determined by powder diffraction, AD Fortes et al, J. Appl. Cryst. 38 (2005) 312.
The crystal structure of methanol monohydrate (CD3OD.D2O) at 160 K from powder neutron diffraction, AD Fortes, Chem. Phys. Lett. (2006), Chem. Phys. Lett. 431 (2006) 283.
The high-pressure phase diagram of ammonia dihydrate, AD Fortes et al., High Pressure Research 27 (2007) 201.
