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 50-50 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 + 0.5 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