Terrestrial Planets Earthlike Worlds of Rocks and Metals

Earth: Model Planet Mass and radius give mass/volume = bulk density, about 5.5 times water Key to composition, internal structure, verified by seismic waves Metals bulk density about 8, rocks about 3; earth about metals/rocks

Density Layers Core (metals) Mantle (dense rocks) Crust (less dense rocks) Partially or fully melted to separate by density (differentiation)

Internal Energy Heat now at surface about 0.1 watt per square meter Internal energy stored from formation (by accretion) plus radioactive decay => larger in past Infrared from surface escapes to space, lost forever

Energy Outflow Volcanism: Molten material, gases rise to surface; adds to crust and atmosphere Tectonics: Any motions of the crust; plate tectonics involve large-scale motions

Age of Earth Radioactive dating: Decay of isotopes with long half-lives Gives elapsed time since rock last melted and solidified (remelting resets clock) Oldest rocks about 4 Gy Gy for earth’s formation => about 4.5 Gy for earth’s age

Relative Ages Oldest regions of crust: Central regions of continents (few Gy) Youngest regions of crust: Seafloor (few hundred My) –Upwelling of materials from mantle by convection –Constantly renewed –Migration of continents

Mercury: Surface Cratered highlands (4 Gy old) Large impact basins, plains with few craters (3 Gy old) Ratio volcanic/cratered terrain about 0.3; same as moon’s ratio => evolution similar to moon’s “Dead” planet now

Venus: Surface Highlands: Volcanic and local (not global!) tectonic rises Lowlands: Undulating lava plains Ratio volcanic/cratered about 4; similar to earth’s ratio; surface evolved as much as earth’s

Mars: Surface Lowlands: Cratered southern hemisphere (wind erosion now; water erosion in past)) Highlands: Volcanic regions in northern hemisphere (2 Gy old) Ratio volcanic/cratered about 0.7; between moon and earth

Interiors Moon: Rocky core; cool Mercury: Large cold metal core, thin rocky mantle Mars: Small metal core, large rocky mantle Venus: Large hot metal core; interior much like earth’s

Comparative Evolution Mass matters! More mass, greater internal energy from formation, radioactive decay More mass, greater size (volume), ratio mass/surface area less, lower rate of heat loss, longer evolution

Evolution Formed by accretion of smaller bodies; melted, differentiated Crust solidified; cratered by impacts; basins formed (filled by volcanism, water on earth) Loss of internal energy: End of evolutionary life