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

Lead Research Organisation: University College London
Department Name: Earth Sciences

Abstract

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.

Publications

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Coustenis A (2008) TandEM: Titan and Enceladus mission in Experimental Astronomy

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Fortes A (2007) The high-pressure phase diagram of ammonia dihydrate in High Pressure Research

 
Description Characterised the properties of a large range of icy materials and water-rich inorganic minerals that are necessary to model and understand internal processes in outer solar system bodies.
Most notably the prevalence of dissociation, where water-rich materials release some water from their crystal structure to form ice and new hydrates.
Exploitation Route Implementation of material-property data in planetary models developed by the community.
Sectors Other

 
Description STFC PPAN Cosmic Visions
Amount £14,989 (GBP)
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 01/2008 
End 12/2008
 
Description Computational structure prediction of ammonia hydrates 
Organisation University of Cambridge
Department Cavendish Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution I have contributed to the interpretation of the computational results and I have carried out the neutron scattering measurements at the Institut Laue Langevin, France
Collaborator Contribution These workers have contributed quantum mechanical calculations of the thermodynamic stability of candidate high-pressure ammonia hydrate crystal structures. The results have been used in conjunction with high-pressure neutron scattering studies to confirm the structure and measure the elastic properties of ammonia hydrates.
Impact Fortes, A. D., E. Suard, M. -H. Lemée-Cailleau, C. J. Pickard, & R. J. Needs (2009): Crystal structure of ammonia monohydrate phase II. Journal of the American Chemical Society 131(37), 13508-13515 (doi:10.1021/ja9052569) Fortes, A. D., E. Suard, M. -H. Lemée-Cailleau, C. J. Pickard, & R. J. Needs (2009): Equation of state and phase transition of deuterated ammonia monohydrate (ND3·D2O) measured by high-resolution neutron powder diffraction up to 500 MPa. Journal of Chemical Physics 131(15), article 154503 [10 pages] (doi:10.1063/1.3245858) Griffiths, G., R. J. Needs, C. J. Pickard, & A. D. Fortes: Crystal structure of ammonia dihydrate II. To be submitted to the Journal of Chemical Physics Fortes, A. D., C. J. Pickard, R. J. Needs, and E. Suard, M. -H. Lemée-Cailleau (2010): The crystal structure of ammonia monohydrate phase II - a new semi-clathrate. Institut Laue Langevin Science Highlight, ILL 2009 Annual Report. Griffiths, G., R. Needs, C. Pickard, A. Misquitta, and A. Fortes (2010): Crystal structures of the ammonia hydrates from computational searches (poster). ?k conference, Berlin, Germany, September 12 - 16th 2010, Symposium 12, #P215. Institut Laue Langevin experimental reports, 5-24-384, 5-24-404, 5-24-423.
Start Year 2008
 
Description Europa/Jupiter System Mission (EJSM) planning 
Organisation University College London
Department Mullard Space Science Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution I worked on the science case for a proposed intrumented penetrator, and contributed to knowledge of expected surface and interior conditions on Europa.
Collaborator Contribution Development of ideas concerning the structure and evolution of Jupiter's icy satellites and techniques for studying their surfaces and interiors.
Impact Gowan, R. A., A. Smith, A. D. Fortes, & 40 other co-authors (2011): Micro-penetrators for in-situ sub-surface investigations of Europa. Advances in Space Research 47, in press (doi:10.1016/j.asr.2010.06.026)
Start Year 2009
 
Description Phase diagram mapping of salt hydrates at high pressure 
Organisation Institute for High Pressure Physics
Department Condensed Matter Laboratory
Country Russian Federation 
Sector Public 
PI Contribution Our group have made complementary neutron scattering studies of the high-pressure phase fields observed in the ultrasonic experiments.
Collaborator Contribution These workers have made measurements of the density and ultrasonic sound speed through samples of compressed salt hydrates.
Impact Gromnitskaya, E. L., O. F. Yagafarov, A. G. Lyapin, V. V. Brazhkin, & A. D. Fortes (2010): Ultrasonic study of epsomite (MgSO4·7H2O) under pressure. High Pressure Research 30(1), 51-54 (doi:10.1080/08957951003588860) Gromnitskaya, E. L., O. F.Yagafarov, A. G. Lyapin, V. V. Brazhkin, I. G. Wood, M. G. Tucker, & A. D. Fortes,: The high-pressure phase diagram of epsomite (synthetic MgSO4·7H2O and MgSO4·7D2O), from ultrasonic and neutron powder diffraction measurements. To be submitted to American Mineralogist in 2011 Wood, I. G., A. D. Fortes, and L. Vocadlo (2006): Polymorphism of epsomite (MgSO4.7H2O) up to 50 kbar from powder neutron diffraction (poster). American Geophysical Union Fall Meeting, San Francisco, December 11 - 15th 2006. Eos Trans. AGU, 87(52), Fall Meet. Suppl. Abstract MR21B-0020. Fortes, A. D., L. Vocadlo, I. G. Wood, P. M. Grindrod, H. E. A. Brand, E. L. Gromnitskaya, A. G. Lyapin, and O. F. Yagaforov (2007): Experimental and computational studies of planetary ices. American Geophysical Union Fall Meeting, San Francisco, December 10 - 14th 2007. Abstract MR11A-08 Fortes, A. D., (2008): Instability of hydrates in planetary interiors (invited oral) 2nd ISSI-Europlanet Workshop, November 17 - 21nd 2008, Bern, Switzerland Gromnitskaya, E. L., O. F. Yagafarov, A. G. Lyapin, V. V. Brazhkin, and A. D. Fortes (2009): Ultrasonic study of epsomite (MgSO4·7H2O) under pressure. 47th EHPRG International Conference, Paris (France), September 6 - 11th 2009. ISIS Facility Experimental reports RB 910226, 920237, 1010008/9
Start Year 2007
 
Description Titan and Enceladus Mission (TandEM) planning 
Organisation European Space Agency
Country France 
Sector Public 
PI Contribution I led a team which developed an instrument concept for a proposed Titan lander or balloon system.
Collaborator Contribution Development of ideas concerning the structure and evolution of Saturn's icy satellites, and techniques for their future exploration
Impact Coustenis, A., & 154 co-authors (2009): TandEM; Titan and Enceladus mission. Experimental Astronomy: Astrophysical Instruments & Methods, 23(3), 893-946 (doi:10.1007/s10686-008-9103-z) Fortes, A. D., I. G. Wood, D. P. Dobson, & P. F. Fewster (2009): An icy mineralogy package (IMP) for in-situ studies of Titan's surface. Advances in Space Research 44(1), 124-137 (doi:10.1016/j.asr.2008.11.024)
Start Year 2008
 
Description Primary school visits in New Zealand 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Classes of 10-20 students attended presentations

NA
Year(s) Of Engagement Activity 2007
 
Description UCL Science Centre Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact approx. 200 students and teachers attended lectures, followed by ~ 1 hour of informal questions and answers

NA
Year(s) Of Engagement Activity 2007,2008,2011,2013