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

Causes of Geologic Activity Heating of Interior Accretion and differentiation when planets were young Radioactive decay is most important heat source today

Cooling of Interior Convection transports heat as hot material rises and cool material falls (outer core; inner mantle) Conduction transfers heat from hot material to cool material (lithosphere) Radiation sends energy into space (surface & atmosphere)

Role of Size is Dominant Smaller worlds cool off faster and harden earlier Moon and Mercury are now geologically “dead” Mars lasted longer; Venus may still be active Earth is VERY active

Surface Area to Volume Ratio Gives Cooling Time Heat content depends on volume Loss of heat through radiation depends on surface area Time to cool depends on surface area divided by volume Larger objects have smaller ratio and cool more slowly

Why do some planetary interiors create magnetic fields? Iron filings follow the magnetic field lines of a bar magnet 10/22/09 START HERE

Sources of Magnetic Fields Motions of charged particles create magnetic fields Electromagnets via currents in coil of wire (usually amplified by magnetic material, like iron) Permanent magnets: electron spins act as currents in iron or nickel

Sources of Magnetic Fields in Planets A world can have a magnetic field if charged particles are moving inside 3 requirements: Molten interior Convection Moderately rapid rotation Earth has molten iron outer core Outer planets have metallic hydrogen Stars have ionized H

Practice Question Answers True: a CCD is more linear and preferred over film. False: Jupiter is 11.2 Earth radii but 318 Earth masses, not about 100. True: oldest rocks on Earth ~4 Gyr, oldest on moon from Apollo ~4.4 Gyr False: While most large telescopes are reflectors, they spend most of their time taking spectra, not pictures. False: The earth’s magnetic field is generated in its liquid outer core. (Mantle is rocky and plastic.) False: Twice the wavelength means 1/2 the energy: E = hf = hc/ True: liquids, solids & dense gases give continuum thermal spectrum

More practice answers True: this is stimulated emission B: 293 K is earth or body temp. and thermal emission peaks in IR D: 500 atoms after 1 half-life of 30 yrs, 250 after 2, 125 after three half-lives, or 90 years E: angular momentum conservation means flattening, gravity means condensation, collisions meant extra flattening as vertical energy is lost. A: getting above atmosphere means less turbulence and less absorption by water vapor D: LA/LB=(RA/RB)2(TA/TB)4= 22(1/2)4=4/16=1/4

What processes shape planetary surfaces? Their surfaces are amazingly different, yet same forces act!

Key Processes that Shape Surfaces Impact cratering Impacts by asteroids or comets Volcanism Eruption of molten rock onto surface Tectonics Disruption of a planet’s surface by internal stresses Erosion Surface changes made by wind, water, or ice

Impact Cratering Most cratering happened soon after solar system formed: the heavy bombardment era Craters are about 10 times wider than object that made them Small craters greatly outnumber large ones Areas with many craters are old; those with few were “repaved”

Impact Craters: Classical Tycho (Moon) Barringer Meteor Crater (Arizona) Rim at edge of shock; rebound makes peak in center

Impact Craters on Mars: Evidence for Liquid Water in the Past “standard” crater impact into icy ground eroded crater Cratering History Movie

Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface Molten rock is called lava after it reaches the surface It solidifies to create volcanoes

Lava Properties & Volcano Types Runny lava makes flat lava plains Slightly thicker lava makes broad shield volcanoes Thickest lava makes steep stratovolcanoes

Outgassing Volcanism also releases gases from Earth’s interior into atmosphere: in the past for other terrestrial planets We’ll talk more about this later

Tectonics Convection of the mantle creates stresses in the crust called tectonic forces Compression forces make mountain ranges (Appalachian Mts on Earth on left) Valley can form where crust is pulled apart (Ceraunius Valleys on Mars on right)

Plate Tectonics on Earth Earth’s continents slide around on separate plates of crust Plate Tectonics Applet Tectonics: Mantle Convection

Erosion Erosion is a general term for weather-driven processes that break down or transport rock Processes that cause erosion include Glaciers Rivers Wind (which demands an atmosphere!)

Erosion by Water Colorado River continues to carve Grand Canyon Land has been uplifted, but river manages to wear it down

Erosion by Ice Glaciers carved the Yosemite Valley They covered most of the Northern US and Europe in recent ice ages

Erosion by Wind Wind wears away rock and builds up sand dunes Frequent on Earth, also seen on Mars

Erosional Debris Erosion can create new features by depositing debris Again, frequent on Earth, but remnants seen on Mars too

The Earth as a Planet You gotta love it, it's our pretty home! You gotta know something about it too! (At least to pass this class.)

The Earth is (nearly) a Sphere and it Rotates Proofs (other than Greek): Masts of ships visible before their decks. Cicumnavigation in 1522! Distances corresponding to degree of latitude change; bulges near equator. Newton realized that a rotating object has extra support perpendicular to its axis: Re = 6.378136 x 106 m Rp = 6.356753 x 106 m ELLIPTICITY = (Re -Rp )/Re = 1/298.3 = 0.0033528. If the earth were “mushier”, the same rotation speed would yield a higher ellipticity.

More Proofs of Earth’s Rotation FOUCAULT PENDULUM --- fixed plane of oscillation, with the Earth rotating underneath it. Satellites in POLAR ORBIT see different sections every 90 minutes as the Earth rotates under the satellite's FIXED path.