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Interiors large bodies (e.g. planets) have all undergone significant changes (in surfaces and interiors) since their formation. except for the Earth and Moon, all of the data we obtain on interiors are based completely on “remote” observations: Density magnetic field moment of inertia. Seismic data (Earth and moon only). The bulk density of an object is simply its mass divided by its volume (ρ=M/V) the density of an element depends on the pressure of its environment. the pressures inside planets must mean that their bulk densities are greater than the densities of their components at 1 atmosphere. Rock is deformable and can move: even more so under partial melting conditions Internal structure obeys the hydrostatic equilibrium equation The equation of state (relating density, pressure and temperature) for solids is generally complex. But in practice T and density do not vary much. Moment of Inertia and Gravity Field The gravity field of a planet, i.e. the gravitational force it exerts on an external body, depends on the planet’s internal mass distribution the main factors affecting the gravity field are a planet’s shape, internal mass distribution and rotation. Measurements of the gravitational field of a planet, usually from space, can be mapped and deviations from a uniform, homogeneous sphere can be determined. This led to the discovery of mascons on the moon. These are large lava flows that filled ancient impact basins. The lava is denser than the surface rock, but has not been able to sink back into hydrostatic equilibrium Find that the centre of mass is offset from the geometric centre of the moon and Mars Shows the mass distribution is not symmetric The moment of inertia of a body is simply its resistance to rotation – analogous to mass which is a body’s resistance to “straight line” motion. The moment of inetria for a sphere can be written as: I kMR2 , where M is mass, R is radius, I is the moment of inertia and k is a constant. For a homogeneous sphere k=0.4 For a hollow shell k=2/3 For a point mass k=0 as central concentration in a body increases, k=I/MR2 decreases. Magnetic Fields Magnetic fields are lines of force, with both magnitude and direction. Record of Earth’s magnetic field can be determined from rocks When rock crystallizes in the presence of a magnetic field, the magnetic elements of rock are frozen in alignment Earth’s field is continually changing strength, and position (changes about 0.1% per year) Even changes direction, every few hundred thousand years A strong magnetic field present in the early solar system would decay in around 10,000 years, so planetary magnetic fields must be due to something else. Dynamo theory: A conductive fluid moving through an external magnetic field induces electrical currents in the fluid that produce their own magnetic field. This produces a feedback loop which increases the field strength Planetary magnetic fields required: A large volume of conductive fluid (e.g. molten iron) Rapid rotation Seismology The study of waves transmitted through bodies waves caused by quakes, impacts, volcanic eruptions or surface explosions Vibrations are transmitted in all directions away from the site of origin Can be detected on “the other side” The speed and direction of a wave depends on the medium it travels through Pressure (P-) waves are longitudinal waves that compress matter along the direction of motion Can pass through solid or liquid matter Shear (S-) waves move the material up and down, in a direction perpendicular to the direction of motion. Can only pass through solid matter Both types of waves can be reflected or refracted at a boundary layer where the composition changes. Earth’s Interior Crust: Thinner (5-10 km) oceanic crust is mostly basaltic (rapidly cooling) while continental crust (20-60 km) is mainly granitic, emblematic of much recycling. mean age of crustal rock is ~1.5Gyr – much younger than the Earth itself. Mantle: Beneath the crust is a thick layer of denser material which is mostly in a plastic state. made of silicate rocks rich in elements which are denser than those in the crust. The temperature difference between lower and upper mantle is sufficient to drive convection, which moves the crustal plates above it. Core: rich in iron and other metals such as nickel, sulphur and cobalt. Outer core is liquid and it is from here that the circulation producing Earth’s magnetic field is driven. Inner core is solid because of its higher density. Sources of Heating Accretion will heat a planetesimal by conversion of gravitational potential energy Energy is only likely to be sufficient for melting for bodies with radii >500 km. Differentiation is a source of heat in terrestrial planets As heavier elements sink to the core in a fluid interior, releasing more gravitational potential energy Radioactivity is an important heat source Short-lived isotopes (like 26Al) can provide a lot of heat at early times. Longer-lived isotopes (like 40K) produce more energy over a longer timescale. Tidal forces are important in some moons (esp. Io) and may have been important in the past, for the Earth-Moon system. Energy is transported to the surface via convection, conduction and radiation. For Earth, the solar heat flux at the surface is ~20,000 times larger than the energy being leaked from the interior The giant planets have stronger heat sources, and generate more heat internally than they receive from the Sun Probably because they are still finishing their slow gravitational collapse. Terrestrial Planets: interiors Moon: Lunar moment of inertia and small, ancient magnetic field suggest small core that formed 3500-4500 Myr ago. Highly-fractured surface (<25 km) suggests moon started out with Earth-mantlelike composition and then differentiated. Small, so cooled quickly. Lithosphere thickened to 1000-km depth: no plate tectonics Mercury: high uncompressed density suggests a large iron core with rcore~.75 RMercury magnetic field implies presence of a molten outer core? recent libration (wobble in the rotation) measurements show planet cannot be solid throughout magnetic field produced in a thin outer core? Mars: significant amounts of sulfur in Mars’ core lowers melting T so core is at least partly fluid means core radius ~0.5RMars variations in Mars’ gravity field measured by Global Surveyor reveal internal density fluctuations crust varies from ~20km in north to ~50km in south Venus: Expect similar radiogenic heating history to Earth Condensation: expect less volatiles and sulfur than Earth. Core may have less FeS. Smaller size, internal pressure: solid core may be smaller than Earth. Might explain weak magnetic field. No evidence for plate tectonics, large crustal motions etc. Icy Moons of outer planets Low mean densities and condensation theory suggests these moons are mainly icy Tidal heating is dominant effect in interior structure Callisto Outermost of Jupiter’s moons, and lowest in density Too far for significant tidal heating Surface: unbroken panorama of craters, no large-scale fracture systems Suggests Callisto has highest ice content. Never-melted, undifferentiated interior Ganymede Largest Jupiter satellite: highly differentiated Magnetic field implies active interior, differentiated core May be subject to some tidal heating Europa Interior has been heated enough to resurface with smooth, young, bright ice plains Some tidal heating: allows water to erupt and resurface Io High density and active volcanism: no ice Initial Io may have been deficient in ice: high temperature in proto-Jupiter nebula Gas Giants Jupiter Has a strong enough gravitational field that it retained almost all elements, in solar-nebula proportions Therefore dominated by hydrogen High temperature and pressure means the hydrogen forms a liquid/gaseous mush At higher pressures, interior becomes convecting, liquid metallic hydrogen (with free electrons) Probably has a solid, silicate and metal core of about 15 Earth masses Saturn Similar, but pressures, temperatures and densities are lower than for Jupiter. Core of about 17 Earth masses, covered with ices. Above that is a liquid hydrogen ocean. Uranus and Neptune Denser than Saturn and Jupiter, and richer in heavy elements Maybe unable to retain as much H and He due to smaller size Also may have accreted material more slowly (due to larger orbit) and the gas got dispersed before they finished forming.