EXPERIMENTAL AND COMPUTATIONAL STUDIES OF PLANETARY ICES AND THEIR ROLE IN THE THERMAL EVOLUTION OF ICY MOONS

The solar system's three largest icy moons, Ganymede and Callisto (which orbit Jupiter) and Titan (which orbits Saturn) are nearly identical in terms of their size and density. However, Callisto has experienced no geological activity, Ganymede was active for the first quarter of its history, and Titan is probably still active today. By contrast, the small saturnian moon Enceladus has vigorous ice volcanism occurring at its south pole. The motivation behind the proposed research is to understand why these icy moons have evolved so differently. Understanding the way such objects have evolved gives information on the initial conditions under which they, and the other planets, formed. It is also useful in assessing the potential of icy bodies as habitats for extraterrestrial life. In the case of the three large icy moons, it is likely that each consists of a rocky core, roughly the same size as our own Moon, overlain by a mantle of ice combined with other substances such as ammonia hydrates, methane hydrates, and 'salt' hydrates (including magnesium sulfate or ammonium sulfate). Over long periods, heat inside these moons comes from the decay of radioactive elements in the core, and from tidal heating. Most planetary objects experience tides in some form; on the Earth we are familiar with ocean tides, but tides also occur in solid objects and the resulting deformation releases heat. Geological activity at the surface is driven by these heat sources; an example of this is Jupiter's rocky moon Io where tidal heating dominates the surface heat flow, making it the most volcanically active body in the solar system. In the icy bodies, the way in which heat flows to the surface from the core, and the extent to which tides contribute, depends on the composition and structure of the icy layer. If the icy layer is rich in ammonia or salts then the melting point of the mixture is lower than if the layer were pure ice (this is the same principle behind spraying rock-salt on icy roads), and it is possible for a liquid layer to exist below the surface. The presence of a liquid layer has the effect of concentrating all of the tidal heating into the shell above the ocean, rather than throughout the mantle. The thermal evolution of a planetary body is calculated using a mathematical model, which in turn yields predictions about the internal structure and surface geology that can be tested against observations. In this case, I will construct models of the icy moons with different initial compositions and tidal heating inputs, and calculate how each would evolve with time. For example, a model of Titan with a water-ice shell would be very different from one with a methane hydrate shell because heat flows through the two materials differently. To build the models I also need to know how the substances in the model behave at high pressure (up to 20,000 times atmospheric pressure on Earth), because ices and hydrates alter their structure at high pressure, giving them very different physical properties. For many of the materials I wish to include in the model, we do not know how they behave at high pressure. So a large proportion of this work will be devoted to carrying out experimental studies at high pressures. I will do this work at national laboratories here in the UK and in the Russian Federation. In addition, I will use computer simulations based on quantum mechanics to calculate some of the desired properties. Together, experimental and computational studies can achieve far more than they could alone. As the work progresses it will become possible to refine the models using new materials data, and make predictions that will be tested against observations of Jupiter's moons by the Galileo space-craft, and of Saturn's moons by the Cassini space-craft. This modelling will effectively 'weed out' the structures and compositions that fail to match the observations, and so help us to understand the conditions under which they formed.