Terrestrial Planetary Geology: Basic Processes & Earth

What are terrestrial planets like on the inside? Planets (inc. Moon) to scale, with typical surface features

Earth’s Interior Core: Highest density; nickel and iron Mantle: Moderate density; silicon, oxygen, etc. Crust: Lowest density; granite, basalt, etc. Other terrestrial planets have similar layers

Terrestrial Planet Interiors Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like

Differentiation Gravity pulls high-density material to center Lower-density material rises to surface Material ends up separated by density This generates heat inside planet

Lithosphere A planet’s outer layer of cool, rigid rock is called the lithosphere: crust + outer mantle It “floats” on the warmer, softer rock that lies beneath: most of the mantle is “plastic”-- the rock slowly deforms

Strength of Rock Rock stretches when pulled slowly but breaks when pulled rapidly--like Silly Putty but MUCH slower The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere if bigger than about 300 km in diameter Rapid shear, like an earthquake or impact breaks rock

Seismic Waves Let Us Know What’s Inside a Planet P (primary) waves push matter back and forth (longitudinal like sound waves) S (secondary) waves shake matter side to side (transverse, like water waves)

SEISMIC WAVES REVEAL INTERIOR SEISMOGRAPHS detect EARTHQUAKES, VOLCANOS, and EXPLOSIONS at varied distances. Long surface (L) waves travel fastest, but are not very useful as they don't probe the interior. P-waves, PRIMARY, (push-pull waves) are COMPRESSIONAL, LONGITUDINAL waves. Propagate through liquids as well as solids. VP = function of (compressibility; composition, T, P) S-waves, SECONDARY, (side-slip) are SHEAR, TRANSVERSE waves. CANNOT propagate through liquids (OUTER core). VS = (a new) function of (rigidity; composition, T, P) We have some seismic measurements for the Moon too: seismometers left by Apollo astronauts

Seismic Wave Paths Both S and P waves can be detected from earthquakes on the same side of the earth, ONLY P waves are detected on the opposite side of the earth “shadow zone”: region where no S (absorbed) & only weak P (refracted) waves are detected. Tells us the outer core of the earth is LIQUID

Seismographic measurements have found for Earth Boundaries between: inner/outer core; outer core/mantle; composition changes in mantle (600 km); oil and natural gas deposits in crust. Seismic measurements can even detect excess rotation of earth’s inner core, upwellings and sideways motions in the mantle. MODERN SEISMOLOGY CAN GIVE A 3-D PICTURE, CHANGING IN TIME, OF THE EARTH'S INTERIOR! --- Tomography There is a good analogy with 3-D images of people's interiors from MRI or CAT scans.

Thought Question What is necessary for differentiation to occur in a planet? a) It must have metal and rock in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

Thought Question What is necessary for differentiation to occur in a planet? a) It must have metal and rock in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c