Structure and dynamics of small planets and moons

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

Abstract

The proposed research aims to understand the interior structure and evolution of smaller planetary bodies, both icy (e.g., the Jovian moons) and rocky (e.g., the planet Mercury).

Icy moons:
Orbiting the gas-giant planets are many icy moons, which vary in size from 10s of kilometres across to >2500 km, larger than the planet Mercury. The three largest icy moons are Ganymede and Callisto (orbiting Jupiter), and Titan (orbiting Saturn); they have similar radii and bulk densities, but have experienced radically different geological histories. Callisto appears not to have evolved at all, its interior is a near uniform mixture of rock and ice, and the surface geology is dominated by impact craters. Ganymede's metal, rock, and ice components have separated out to form an iron core, a rocky mantle, and a thick icy shell, which has rifted the crust and caused the eruption of liquid water - an icy equivalent to Earth's volcanic magma. Titan has also undergone internal segregation to form a dense core coated by a thick icy shell. Unlike Ganymede, Titan may still be active. The most remarkable discovery is that all of these large icy bodies have global oceans of liquid water beneath icy crusts 10-200 km thick. These oceans are possible niches for extraterrestrial life in the outer reaches of our solar system.

To understand why icy bodies of otherwise similar size and composition have led such different lives, we must construct mathematical models of the internal structure and heat flow. This modelling relies upon knowledge of how the icy layer transports heat from the core to the surface. Under the high pressures in the interior of an icy body, water-ice exists in several different crystalline forms, each with very different thermo-physical properties. In addition, there are likely to be abundant water-rich hydrates of various molecules, such as ammonia, and many soluble sulfates. These compounds often have a smaller thermal conductivity than water ice; just as a thick winter quilt will keep you warm in bed, a low-thermal-conductivity planetary crust will keep the interior much warmer than it would be otherwise, allowing subsurface oceans to stay liquid throughout geological history. For most of ices and hydrates, the physical properties we need to construct accurate models are not known at relevant pressures and temperatures. In this project we will measure properties such as the thermal expansion, thermal conductivity and specific heat capacity, supporting our measurements with computer simulation. We shall then incorporate the results into our planetary models and thus investigate the internal structure and evolution of the icy moons.

Mercury
Mercury is the target of two orbital missions, MESSENGER (current) and Bepi-Columbo (2019) both of which have instruments on board to study its internal structure, composition and magnetic field. Mercury is only slightly less dense than the Earth but is much smaller and therefore the material within its interior is not as strongly compressed. For Mercury to have such a high density, its core must be large (>40% by volume, <70% by mass) and iron-rich (~70% Fe, ~30% silicate). Mercury's small size also suggests it must have cooled more rapidly than the Earth and therefore will have a distinct chemistry and evolutionary history. The presence of a magnetic field suggests that Mercury has a molten region, although fast cooling means that this may be confined to a rather thin shell. As a result of these differences, it is possible that the dynamo that supports the magnetic field of Mercury differs substantially from the Earth's dynamo.

Understanding Mercury's interior requires us to construct geophysical models of its internal structure and evolution. To do this we must know the physical properties of the materials that make up its interior; these can be obtained through calculations based on quantum mechanics for both solid and liquid iron alloys at high pressures and temperatures.

Planned Impact

Impact on Planetary Science in general: The thermal properties of materials that we shall obtain in Project 1 and the new models of icy moons that we shall construct will be central to the work of planetary scientists throughout the world who are constructing geophysical models of icy bodies. Our results will, therefore, impact upon research into many problems requiring knowledge of the thermal history of outer solar system objects, including their possible astrobiological potential. In particular, the work will facilitate better interpretation of observations made by the Cassini-Huygens spacecraft, including measurements of the global shape and gravity field during the Primary (2004-2008) and Equinox Missions (2008-2010) and the current Solstice Mission (2010-2017). Furthermore, our results will assist in planning observations to be made by future missions to the outer solar system, including New Horizons (en route to Pluto and the Kuiper Belt), and the Europa and Ganymede Orbiters, which comprise the proposed joint ESA-NASA mission to the Jovian System, EJSM, recently selected for further funding and due to launch circa 2020. These multi-billion dollar investments in outer solar system exploration are of little use without a broad foundation of materials science upon which to build a framework to support the observations. Similarly, we expect our research in Project 2 to yield crucial new insight into the origins of magnetic fields and core formation in small terrestrial bodies. This is especially timely with MESSENGER Mercury orbital insertion recently completed and Bepi-Columbo due to be orbiting in 2019.

Impact on Materials Science: The materials science aspects of both Projects 1 & 2 provide us with plentiful scope for engagement with a wide of spectrum of physicists, chemists and biologists. For example, modelling hydrogen-bonded systems (Project 1) is now a topic of widespread interest in the chemical, pharmaceutical and materials science communities. The properties of solid and liquid metal alloys (Project 2) at extremes of temperature and pressure are, similarly, of great interest to those working in the defense and nuclear industries. Our work thus has the potential to impact on a range of wealth-creating high-technology activities. A similar potential exists for our experimental programme, during which we shall develop new and novel apparatus for: (i) X-ray diffraction at very low temperatures (thereby allowing determination of crystal structures) and (ii) measurement of thermal properties at high pressure and low temperature.

Training of skilled personnel: The project will result in the training of two PDRAs, both working, at macro- and micro-scales respectively, on computer simulation methods; this is an area which has been identified as having a national shortfall of trained researchers. The importance of computer-enabled research is recognised by the UK Research Councils; for example, the International Review of Research Using High Performance Computing in the UK commissioned by EPSRC reported that "Computation has now become essential for the advancement of all research across science and engineering". Career destinations of those who have worked previously with the PI and Co-Is at UCL on planetary materials include employment both in the UK and abroad, split between academic research on planets, academic research in materials science and careers in promoting the public understanding of science.

Educational and Societal Impact: as evidenced by the Cassini-Huygens mission, the planets and moons of our solar system continue to attract wide public interest. Together with "big physics" (e.g. the LHC at CERN) they provide one of the best routes by which scientists can engage with both young and old. For the old, this can be viewed as enhancing the cultural life of the nation; for the young it is a way of achieving the national goal of increasing the number of trained scientists and technologists.

Publications

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Fortes A (2017) Structure, thermal expansion and incompressibility of MgSO 4 ·9H 2 O, its relationship to meridianiite (MgSO 4 ·11H 2 O) and possible natural occurrences in Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials

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Fortes A (2014) Structure, hydrogen bonding and thermal expansion of ammonium carbonate monohydrate in Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials

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Fortes AD (2014) Crystal structure of magnesium selenate hepta-hydrate, MgSeO4·7H2O, from neutron time-of-flight data. in Acta crystallographica. Section E, Structure reports online

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Fortes AD (2015) Crystal structures of spinel-type Na2MoO4 and Na2WO4 revisited using neutron powder diffraction. in Acta crystallographica. Section E, Crystallographic communications

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Fortes AD (2013) MgSO4·11H2O and MgCrO4·11H2O based on time-of-flight neutron single-crystal Laue data. in Acta crystallographica. Section C, Crystal structure communications

 
Description We have investigated the crystal structures (i.e. the atomic arrangement) and other physical properties, such as the thermal expansion and the incompressibility (how the volume of the substance changes with pressure), of several of the materials that are thought to be rock-forming minerals in the icy bodies of the outer solar system. This information is essential if accurate models of the interiors of these bodies are to be produced.
In particular, we have examined the behaviour of: (i) the high-pressure phases of water ice, ice III and ice V and (ii) several different sulfates. We have determined the response of ice III and ice V to pressure and temperature over a wide range of conditions. In the sulfates, we have identified a number of changes with pressure in the crystal structure of epsomite (MgSO4.7H2O). These occur in the range 14-25 kbars and so could result in the mantle of an icy moon having a layered structure. The transition at about 20 kbars has now been shown to result from the loss of two water molecules, so as to produce a new high-pressure form of MgSO4.5H2O (whose crystal structure has been determined) + ice. We have also measured the response of mirabilite (Na2SO4.10H2O) to both pressure and temperature in the range 0 - 5.5 kbar and 150 - 270 K. The crystal structures of meridianiite (MgSO4.11H2O) and its related compound MgCrO4.11H2O have been determined, as have the structure of MgSeO4.7H2O, MgSeO4.9H2O and MgSeO4.11H2O. The purpose of studying these chromium and selenium compounds is to give us insight into the possible behaviour of the naturally-occurring sulfur compounds under different conditions of temperature and pressure.
Finally, we have determined the structure and thermal expansion of a member of another category of icy material, ammonium carbonate monohydrate.

We have also developed a major new piece of apparatus - a cold-stage for X-ray powder diffraction that is capable of operating in the temperature range from 40 to 310 Kelvin (-233 to 37 C). This stage, which was constructed for us by Oxford Cryosystems, is unique in that it allows samples to be loaded into it at temperatures as low as 80 K (-193 C). It thus allows us to prepare materials at very low temperatures (e.g. by quenching a solution into liquid nitrogen at around -196 C ) and examine them in our laboratories, maintaining them at low temperature throughout the process; previously it was impossible for us to do this as samples could be loaded into our X-ray diffraction system only at room temperature. A complementary pressure vessel in which samples may be compressed to 20 kbars and then rapidly cooled and recovered into liquid nitrogen has been developed and is working well; the system has been used in the determination of the crystal structure of the new high-pressure form of MgSO4.5H2O and in the preparation of samples of ice III for neutron diffraction experiments at ISIS. We are now using this apparatus (in parallel with neutron diffraction at the STFC ISIS Facility) to determine the difference in behaviour of the thermal expansion of the high-pressure phases of H2O and D2O water ice; this is important both in terms of fundamental understanding of the physics and chemistry of the water molecule and also because H2O is dominant in the natural system, whereas experimental studies by neutron diffraction are normally carried out on D2O.
Exploitation Route Our improved material property measurements can be used by workers modelling the interiors of icy bodies.
Our new X-ray cold stage may well be of interest to a wide range of materials scientists.
Similarly our newly-constructed high pressure apparatus, with sample recovery into liquid nitrogen may have applications in a number of fields.
Sectors Agriculture, Food and Drink,Chemicals,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Description This award required the construction of a specialised stage for low-temperature X-ray powder diffraction. This was designed and built for us by Oxford Cryosystems Ltd. who are now marketing the apparatus to other customers.
Impact Types Economic