Origin, Evolution, Heat Accretion of Earth from solar nebula Geophysical differentiation of Earth Moon formation Magnetics and the core geodynamo Heat budget of our planet

Accretion The Crab Nebula © NASA The Solar Nebula (painting) © W.K. Hartmann PSI

The original composition Except for volatile elements, carbonaceous chondrites have the same elemental composition as the Sun and presumably as the bulk Earth.

Present distribution of elements Comparing bulk Earth to crust

Physical differentiation Fe: 35%; O: 30%; Si: 15%; Mg: 13%: olivine! Scenarios... Earth forms cool and homogeneous, heats up due to radiogenic element decay and so differentiates... Earth forms from already partially differentiated planetesmals..., heats up and continues further differentiations... This is the preferred model. Physical differentiation of Earth... Where is all the Fe? It has mostly sunk into the core! Essentially density driven: denser compositions sink to the deepest interior.

What mobilizes differentiation? Heat! Original sources of heat: Gravitational potential energy of accretion There would have been enough energy in the original accretion to vapourize the entire Earth had it not been re-radiated away during the window of time of accretion. Decay of assembled radionucleides Short-lived radionucleides may have been quite rich in the mix of nebular dusts, especially if accretion of the Solar System followed quickly upon the supernoval explosions that produced the elemental mix. Physical and chemical differentiation itself Differentiation is really just reorganization of the Earth toward equilibrium; disequilibrium entropy releases heat. Formation of the Moon: the “Big Whack”

The “Big Whack” Another source of heat: the Moon forms... within about 40million years of initial accretion. © W.K. Hartmann PSI

This scenario “fits” data... The Earth has a large iron core, but the Moon does not. Fe had already largely sunk into core. Earth has a mean density of 5.5 gm/cm3; Moon, 3.3 gm/cm3. Moon has less iron. The Moon has the same 18O, 17O, 16O composition as the Earth. Mars and the asteroids differ. Earth's Moon is “large”. The other terrestrial planets have only small moons or none at all.

Archean paleomagnetics The core, or part of the core is liquid and circulating with enough vigour to drive the geodynamo; it has been actively generating field for at least 3.5 billion years. Valet, J-P., Time variations in paleomagnetic intensity, Reviews of Geophysics, 41-1, 2003.

Geodynamo

Core 10 http://geomag.usgs.gov/movies/movies/index.php?type=corefield&format=flash

The interior of the Earth is hot! Hot hot? Heat budget... The interior of the Earth is hot! Hot hot? ~ 1500K at base of lithosphere ~ 2800K – 3400K at base of mantle ~ 4500-5000K at inner-core boundary ~ 5000-5500K at centre

Heat again... Why hot? Rapid heating following a cold accretion by decay of short-lived radionuclei (26Al26Mg) Residual gravitational potential energy from original accretion (perhaps as much as ~40%). The mantle is convecting. Continuing internal heating from K, U, Th decay. Heat enters from the core beneath. The geodynamo requires a heat drive. Core freezing, chemical differentiation and 40K decay or even U/Th (fission?) in the inner core.