UCL Planetary Ices Group

Research

Our research focuses on the structure and behaviour of the constituent materials of icy moons, in order to better understand their thermal and geological evolution. The icy Galilean satellites of Jupiter, Europa, Ganymede and Callisto, have diverse and complex histories which can be partly attributed to differing degrees of internal differentiation. The icy moons of Saturn are currently being explored by the Cassini/Huygens mission, and vary greatly in size, with the largest, Titan, being bigger than our own Moon.

We use theoretical calculations and experiments to determine the physical properties of likely icy moon constituents. By using ab initio simulations, and utilising the expertise of the Crystallography and Mineral Physics group, we are able to determine the equation of state for various high-pressure polymorphs of ice and other constituents. These simulations compliment experimental work, but are also capable of simulating pressures not achievable in the laboratory. We also conduct experimental work which involves studying phase changes at different pressures and temperatures. The results from both these methods are then incorporated into models of planetary evolution. We are modelling the interiors of the icy moons on a global-scale by using parameterised and finite-element methods, but also on a smaller-scale by studying localised processes such as cryovolcanic processes.

The page below highlights some of the research conducted on likely icy-moon constituents, with links to more detailed information on Dr. Dominic Fortes' research page.

Europa

Above: Europa, as seen by the Galileo spacecraft. (Courtesy NASA/JPL-Caltech)

WATER ICE

The form of water ice we all know and love (which is called ice Ih) consists of water molecules bound together into hexagonal rings; these rings are joined together to form sheets, and these sheets are stacked in such a way that every fourth sheet lies directly over the first (in other words the stacking is AB-AB-AB, or hexagonal close packing). Moreover, the water molecules are free to flip around, and the orientation of the molecules is what we would call disordered; just because a molecule in one part of the crystal is arranged a certain way, doesn't mean an equivalent molecule elsewhere in the crystal will have the same orientation. The crystal is hexagonal (hence the subscript 'h' after ice I) and this is why snow-flakes exhibit largely six-fold symmetry, for example.

Now, one can pack the hexagonal layers so that the repeat is ABC-ABC-ABC (cubic close packing) and this results in a crystal of cubic symmetry being formed (called ice Ic). Ice Ic may be formed by direct condensation of water vapour into the solid state - this apparently occurs in the polar stratosphere - or else it can be formed from other forms of ice (as described in a moment). This form of ice is also disordered. The disorder is ice I can be made to disappear at very low temperatures (below 100 Kelvin), but because molecular motions are so slow at such low temperatures it is rather difficult to make. If the crystal structure of ice Ih becomes ordered it is no longer crystallographically hexagonal, but the true symmetry is orthorhombic - this form of ice is called ice XI.

Why ice eleven?? When ice is subjected to hydrostatic compression, the relatively open crystal structure is unable to withstand the strain. The effect of pressure on ice is to make the bonds break, and the molecules rearrange themselves into a denser crystal structure. These different crystal structures (whilst retaining the same composition) are called polymorphs, and ice has more polymorphs than almost any other substance. Well, strictly speaking, many are polytypes. For example, ices Ih, Ic, and XI are poly types of one another since they are simply different stacking (or ordering) arrangements of the same structure. The first high-pressure polymorphs, ices II and III, were discovered in 1900 by Gustav Tammann. Subsequently, Nobel prize winner Percy Bridgman identified ices IV, V, and VI: the numbering of the phases has no relation to the P,T conditions under which they exist, it is simply a reflections of the order of discovery, hence ice XI is one of the more recently identified phases.

Above: The phase diagram of water ice, illustrating the pressure and temperature conditions under which different crystal structures of ice are stable.

AMMONIA HYDRATES

As a likely constituent of outer solar system bodies, ammonia is expected to combine with water to form ammonia hydrates, most likely ammonia dihydrate. Although the hydrates of ammonia, the dihydrate (ADH), the monohydrate (AMH), and the hemihydrate (AHH), have been known for some time, their physical properties are very poorly characterised and little is known of their behaviour at high pressures. In recent years, with advances in computational techniques, and improvements in high-pressure neutron scattering environments, it has become possible to address these questions. Thus Dominic's PhD thesis focussed on using a combination of techniques to improve our understanding of solids in the ammonia - water system, not just the hydrates, but also the end members, water ice and solid ammonia. The rationale in the context of planetary science is explained in detail in Chapter One of his thesis. Details of the work done on individual compounds is outlined below.

Above: The inferred phase diagram of ammonia dihydrate at high-pressure.

SALT HYDRATES

Interactions between primitive chondritic materials and water in the interiors of many icy moons during their formation is believed to have resulted in leaching of salts such as MgSO 4 , Na 2 SO 4 , and Na 2 CO 3 . These should form highly hydrated minerals such as epsomite (MgSO 4 .7H 2 O), Fritzsche's salt (MgSO 4 .12H 2 O), mirabilite (Na 2 SO 4 .10H 2 O), and natron (Na 2 CO 3 .10H 2 O). As described in the section on water ice, it is vital that we know which polymorphs are stable under the pressure and temperature conditions extant in the salt hydrate mantle, and that we know the relevant physical properties of this polymorphs for input into planetary models.

In the case of all of the substances listed above, we have very little information indeed on their behaviour at high pressure or low temperatures. It is the gola of the Planetary Ice Group to correct this knowledge deficit. Dominic began work on this class of planetary minerals by carrying out ab initio calculations of the bulk properties of epsomite, and subsequently,in late 2003, collected a fantastic data set on HRPD (RB 14491), concluding in March 2004 with the measurement of the thermal expansivity at 1.4, 3.0, and 4.5 kbar from 50-290 K.

Above: A surface plot depicting the pressure - volume - temperature equation of state for Epsomite.

SULFURIC ACID HYDRATES

The binary system sulfuric acid - water is interesting for several reasons. There are a number of hydrate species known or believed to exist; there are the well known mono-, di-, and tetrahydrates, a suspected trihydrate, and a very poorly characterised 6½- and a higher hydrate that may be an 8-, 9-, 9½-, or even 10-hydrate!! These hydrates are likely to occur in the stratosphere and may provide sites for condensation of high-altitude ice clouds that can significantly affect the Earth's climate, particularly after volcanic eruptions which deposit large quantities of sulfur into the upper atmosphere. Moreover, sulfuric acid hydrates are thought to exist on the surface of Jupiter's moon Europa, on the basis of Galileo spectroscopic data. The source of the sulfur may be Io, or perhaps sulfuric acid solutions erupted from a subsurface ocean. In the latter case it is plausible that the crust of Europa constains a considerable proportion of sulfuric acid hydrates as major rock-forming minerals.

This being the case, it is very important to better characterise the higher hydrates of sulfuric acid, and to clarify the exact nature of the highest hydrate.

In May 2005, we carried out a neutron diffraction experiment (RB 20047) upon three different sulfuric acid - water compositions using the High Resolution Powder Diffractometer (HRPD) at ISIS. We initially attempted to flash freeze the solution in a liquid-nitrogen-cooled mortar and then cryogrind the solid. However, we found that the solution formed a very viscous syrupy mass that was impossible to grind, so we resorted to loading the solution into standard vanadium cans with silica wool to help crystallise a sufficiently random sample. As it turned out, all of the compositions we loaded formed a glass to begin with that only crystallised after warming to 150-160 K, and this had the effect of producing rather coarse oriented specimens.

Our three solutions had compositions corresponding to D 2 SO 4 .3D 2 O, D 2 SO 4 .6½D 2 O, and D 2 SO 4 .9D 2 O. In the first case, we formed crystals of sulfuric acid tetrahydrate and were able to collect good data (the first neutron data on this or any other sulfuric acid hydrate) at 217 K and 4 K. The acid-rich residue appears to have persisted as a glass throughout the experiment.

$Sulfuric acid diffraction pattern$

Above: The neutron diffraction pattern of sulfuric acid tetrahydrate at 4.2 K as seen by the 90 degree detectors on HRPD, and fitted with the existing X-ray derived structural model for the deuterated species.

This page last modified 29 October, 2007 by [Peter Grindrod]