Terrestrial Planets: Rock & Iron - Astronomy 110: SURVEY OF ASTRONOMY - The Earth and Other Planets 2. The Moon, Mercury, Venus & Mars 3 ...
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Astronomy 110: SURVEY OF ASTRONOMY
5. Terrestrial Planets: Rock & Iron
1. The Earth and Other Planets
2. The Moon, Mercury, Venus & Mars
3. Our Living Planet
Although very different on the outside, Mercury, Venus, Earth, Mars, and our Moon all have a good deal in common. The main factor determining their status is their mass: larger planets cool more slowly and thus stay active longer.
1. THE EARTH AND OTHER PLANETS
a. Evolutionary history
b. Internal structure
Evolutionary History
How did this
variety arise?
Many Craters Volcanos Volcanos Many Craters Volcanos
Smooth Plains “Continents” Continents! Highlands Some Craters
High Cliffs Few Craters Oceans Smooth Plains Riverbeds?
Evolutionary History: Deep Similarities Terrestrial planets differentiated: high-density metal sank downward, while low-density rock floated upward. To differentiate, the planets must have melted completely.
Evolutionary History: Sources of Heat 1. Accretion of planetesimals — impacts release gravitational energy 2. Internal radioactivity: 40K, 238U, 232Th — decaying atoms release nuclear energy 3. Differentiation — sinking core releases gravitational energy
Evolutionary History: Size Makes a Difference
Large planets stay hot — why?
A
same area A
A
Evolutionary History: Size Makes a Difference Large planets stay hot — why? Which wedge has more volume? This one! Which has more radioactivity? This one! Which produces more heat? This one! Which surface releases more heat? This one!
Evolutionary History: Size Makes a Difference Large planets stay hot — why? More volume per unit of surface. More heat outflow through unit of surface.
Evolutionary History: Cooling Off Smaller planets have cooled more, and therefore are solid to greater depths. Near-solid region is called the lithosphere.
Internal Structure
Lower mantle (plastic)
Rock (Si/Mg/O/Al/...)
Outer core (molten)
Fe/Ni/Co & O/S?
Inner core (solid)
Iron/Nickel/Cobalt
Earth’s Interior & Plate TectonicsInternal Structure: Earthquakes as a Probe
P and S Waves Moving Through Earth
Earthquakes produce two Liquid outer core lets
different types of waves: P-waves through but
S-waves and P-waves. reflects S-waves.Internal Structure: Mantle Convection
Heat is carried by convection —
slow circulation of semi-solid rock.
Convective Heat TransferInternal Structure: Convection Simulation
Mantle Convection
Cold Downwellings Warm Upwellings
Elapsed time: about 200 Myr.Internal Structure: Magnetic Field
S
N
The Earth’s magnetic field To be a dynamo, core must:
resembles a bar magnet’s, but (1) conduct electricity, (2)
the Earth is not a bar magnet! rotate, (3) have convection.Earth’s Magnitosphere
Earth’s magnetic field deflects charged particles from
the Sun and funnels them towards the magnetic poles.
Wikipedia: MagnitosphereInternal Structure: Magnetic Field Reversals
Convection in outer core is unstable — Earth’s field can
reverse polarity as flow pattern changes.
Before last reversal During last reversal
These reversals happen roughly every 300,000 yr and
take a few thousand yr.Internal Structure: Magnetic Field
Which planets have magnetic fields?
yes — strong! none; spins not now; weak; none;
too slowly too cold? slow spin cold
Magnetic fields may help planets retain atmosphere.2. THE MOON, MERCURY, VENUS & MARS
a. The Moon & Mercury
b. Venus: a greenhouse planet
c. Mars: dead or just chill?Internal Structure and Surface Activity Which planets are still geologically active? Which have more heat outflow?
The Moon & Mercury Both exhibit craters and smooth plains.
Formation of the Moon by Giant Impact
Moon-forming impact
Mars-sized planet (Thea) hits
Earth about 4.5 Gyr ago.
Moon forms from debris:
This explains why Moon is poor in metals and volatiles.Impact Cratering • occurs on any body with a solid surface • impact vaporizes projectile and comparable mass of target • expanding vapor blasts material in all directions circular crater • craters typically 10 times diameter of projectile
Crater Structure
Small impacts produce simple bowl-
shaped craters.
Impact Cratering
Impact Cratering
Large impacts produce craters with
terraced rims and central peaks.
Impact Cratering
Impact CrateringImpact Cratering: Late Heavy Bombardment
Evidence from Earth and
Moon suggests a peak in
cratering rate about 3.8
to 4 Gyr ago.
This would have affected
other planets as well.
Time (Gyr ago) A Cool Early EarthImpacts: Formation of Lava Plains 4 Gyr ago, a heavily cratered surface. A giant impact creates a huge crater. A few 100 Myr later, lava fills crater. The smooth lava plains of the Moon and Mercury are a relic of the late heavy bombardment.
Mercury vs. the Moon
• Fewer craters — more buried
by lava floods?
• Giant cliffs — a tectonic relic of
global shrinkage?1. Why do some regions of the Moon and Mercury have more craters than others? A. These regions were subject to fewer impacts. B. Lava flooding buried the craters which were there. C. Volcanic activity created more craters in these regions.
Venus: a Greenhouse Planet
Venus: Just Passing By
Venus Unveiled
The surface is hidden
by thick clouds . . . but can be mapped by radar.Venus: Surface Features
Shield volcanos Lava domes
Fractured crust (tectonic)
Impact crater (rare) “Corona” (mantle plume)Venus: Surface Features
• Craters are rare because entire surface
was “repaved” about 750 Myr ago.
• Volcanic features are found everywhere,
indicating ongoing volcanic activity.
• Tectonic features are unlike
those on Earth; no plates!Venus: Why So Hot? Greenhouse effect! Volcanos supply CO2 CO2 traps infrared radiation (heat)
Venus: Atmosphere Properties: — 96% CO2; traces of N2, SO2, H2SO4, etc. — about 90 times mass of Earth’s atmosphere! Why no H2O? (should be released by volcanos) — Solar UV radiation breaks up H2O — Hydrogen is stripped away by solar wind — Oxygen reacts with surface rocks
2. What keeps the interior of Venus hot? A. Decay of radioactive elements. B. Volcanic activity. C. Solar radiation trapped by a CO2 greenhouse.
3. What keeps the surface of Venus hot? A. Decay of radioactive elements. B. Volcanic activity. C. Solar radiation trapped by a CO2 greenhouse.
Mars: Dead or Just Chill?
Normal weather Global dust storm
Polar capMars: Surface Topography
Olympus
Mons
Smooth Lowlands
Valles
Marineris
Tharsis
Bulge
Hellas
Cratered Highlands
Flat Topography Map of Mars
Evidence for a complex and varied geological history.Olympus Mons:
hot-spot vulcanism
Olympus MonsValles Marineris: tectonic stresses
Meandering Riverbed? Crater counts suggest an age of 2 to 3 Gyr.
Evidence for Surface Water in Mars’s Past Layered (sedimentary?) rock in eroded crater (right). Surface feature resembling sedimentary rock (below).
Water on Mars Today The North and South polar caps contain enough water to make an ocean 15 m deep . . . More water may exist as permafrost elsewhere on the planet. Water on Mars Liquid water cannot exist on surface under present conditions — it evaporates due to low pressure.
Water on Ancient Mars Mars had a more massive CO2-rich atmosphere in the distant past. The greenhouse effect probably warmed the planet above freezing. Water on Mars
Loss of Atmosphere 3 Gyr ago, Mars was protected With the field gone, from the solar wind by magnetic the atmosphere was field generated internally. stripped away.
Summary: Surface Processes Impact Cratering: — widespread on all planets at early times — older surfaces have more craters deduce age Volcanism: — more significant on larger planets (more heat!) Tectonics: — global stress plays some role on Venus & Mars Erosion: — important on Mars, not on Venus!
3. OUR LIVING PLANET a. Plate tectonics b. Atmosphere and climate
Impact Craters Age: 49,000 yr
on Earth
1.2 km
Barringer Meteor Crater, Arizona
Age: 212 Myr
Terrestrial Impact Craters
Impact melt breccia, Chicxulub
70 km
Manicouagan, Quebec, CanadaVolcanoes
on Earth
Wikipedia: Mount Fuji Mauna Loa Volcano, HawaiiErosion
on Earth
Grand Canyon, ArizonaPlate Tectonics Earth’s crust is a collection of slowly-moving plates.
Plate Tectonics: Earthquakes
Evidence for Plate Tectonics
The plate boundaries are often sites of earthquakes.Plate Tectonics
Plates float on denser mantle material.
Wikipedia: Plate Tectonics
Mantle convection drives plate motion.
Plates are formed at ocean ridges and continental
rifts, and destroyed in subduction zones.Plate Tectonics: Age of Sea-Floor
Evidence for Plate Tectonics
Ages confirm picture of plate formation at ocean ridges.Plate Tectonics: Continental Drift
Breakup of PangaeaAtmosphere
VenusBy Earth Mars
Venus Earth Mars
CO2 96.5% 0.038% 95.3%
N2 3.5% 78% 2.7%
O2 21%
Ar 0.93% 1.6%
pressure 90 1.0 0.006Atmosphere: O2 Cycle Most O2 is underground, but exchange between atmosphere and biosphere is the major pathway. Wikipedia: Oxygen Cycle Biological processes are the key sources and sinks of O2: 6 CO2 + 6 H2O + energy C6H12O6 + 6 O2 Amount of O2 is kept fairly constant by competition between sources and sinks.
Atmosphere: CO2 Cycle
CO2 in air
falls as
acid rain
Carbon and oxygen cycle dissolves
minerals
together over short times. returned by
volcanos
forms
carbonates
Over longer times, carbon subducted
cycles deep underground. underground
This regulates Earth’s climate (very slowly):
hot more rain less CO in air weaker greenhouse
too 2
cold less more stronger
Plate tectonics is necessary to return buried carbon!Climate Cycles
Small regular changes in
Earth’s orbit and tilt vary
the amount of sunlight
the northern continents
receive.
This triggers an ice-age
every 105 yr (roughly).
Time (kyr) Wikipedia: Milankovitch CyclesCO2 and Ice Ages 1. During an ice age, surface ice covers the oceans. 2. CO2 from volcanic outgassing builds up. 3. High CO2 produces rapid warming, ending ice age.
Climate and CO2
Peaks in temperature match peaks in CO2
CO2 concentration and temperature track each other.
Now, humans activity is increasing CO2 concentration.Global Warming
Models with CO2
Data
Models without CO2
Warming has dramatic
Climate models including CO2 effects near the poles.
can account for warming.
Models predict 3° to 5° C increase by end of century.You can also read
