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

Many CratersSmooth Plains

High Cliffs

Volcanos“Continents”Few Craters

VolcanosContinents!

Oceans

Many CratersHighlands

Smooth Plains

VolcanosSome Craters

Riverbeds?

Evolutionary HistoryHow did this variety arise?

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

2. Internal radioactivity: 40K, 238U, 232Th

3. Differentiation

— impacts release gravitational energy

— decaying atoms release nuclear energy

— sinking core releases gravitational energy

Evolutionary History: Size Makes a Difference

Large planets stay hot — why?A

A

same area A

Evolutionary History: Size Makes a Difference

Which wedge has more volume? This one!

Which has more radioactivity? This one!

Which produces more heat? This one!

Large planets stay hot — why?

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

Earth’s Interior & Plate Tectonics

Inner core (solid)Iron/Nickel/Cobalt

Outer core (molten)Fe/Ni/Co & O/S?

Lower mantle (plastic)Rock (Si/Mg/O/Al/...)

Earthquakes produce two different types of waves:S-waves and P-waves.

Liquid outer core lets P-waves through but reflects S-waves.

P and S Waves Moving Through Earth

Internal Structure: Earthquakes as a Probe

Internal Structure: Mantle Convection

Heat is carried by convection — slow circulation of semi-solid rock.

Convective Heat Transfer

Mantle Convection

Internal Structure: Convection Simulation

Cold Downwellings Warm Upwellings

Elapsed time: about 200 Myr.

Internal Structure: Magnetic Field

The Earth’s magnetic field resembles a bar magnet’s, but the Earth is not a bar magnet!

To be a dynamo, core must: (1) conduct electricity, (2) rotate, (3) have convection.

N

S

Earth’s Magnitosphere

Wikipedia: Magnitosphere

Earth’s magnetic field deflects charged particles from the Sun and funnels them towards the magnetic poles.

Internal Structure: Magnetic Field Reversals

Before last reversal During last reversal

Convection in outer core is unstable — Earth’s field can reverse polarity as flow pattern changes.

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 too slowly

not now; too cold?

weak; slow spin

none; 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

Mars-sized planet (Thea) hits Earth about 4.5 Gyr ago.

This explains why Moon is poor in metals and volatiles.

Moon-forming impact

Moon forms from debris:

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

Impact CrateringImpact Cratering

Small impacts produce simple bowl-shaped craters.

Large impacts produce craters with terraced rims and central peaks.

Impact Cratering

Impact Cratering

Impact Cratering: Late Heavy Bombardment

Time (Gyr ago) A Cool Early Earth

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.

Impacts: Formation of Lava Plains

4 Gyr ago, a heavily cratered surface. A few 100 Myr later, lava fills crater.A giant impact creates a huge 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: Just Passing By

The surface is hidden by thick clouds . . .

Venus Unveiled

but can be mapped by radar.

Venus: a Greenhouse Planet

Impact crater (rare)

Shield volcanos Lava domes

“Corona” (mantle plume)

Fractured crust (tectonic)

Venus: Surface Features

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?

Volcanos supply CO2 CO2 traps infrared radiation (heat)

Greenhouse effect!

Venus: Atmosphere

— 96% CO2; traces of N2, SO2, H2SO4, etc.

Properties:

— 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 cap

Mars: Surface Topography

Flat Topography Map of Mars

Cratered Highlands

Smooth Lowlands

Hellas

Tharsis Bulge

Olympus Mons

Valles Marineris

Evidence for a complex and varied geological history.

Olympus Mons

Olympus Mons:hot-spot vulcanism

Valles 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

Water on Mars

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.

Liquid water cannot exist on surface under present conditions — it evaporates due to low pressure.

Water on Mars

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.

Loss of Atmosphere

3 Gyr ago, Mars was protected from the solar wind by magnetic field generated internally.

With the field gone, the atmosphere was stripped away.

Impact Cratering:— widespread on all planets at early times— older surfaces have more craters ⇒ deduce age

Summary: Surface Processes

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

Barringer Meteor Crater, Arizona

Manicouagan, Quebec, Canada

Impact Craterson Earth

Terrestrial Impact Craters

Impact melt breccia, Chicxulub

1.2 km

Age: 49,000 yr

70 km

Age: 212 Myr

Volcanoeson Earth

Mauna Loa Volcano, HawaiiWikipedia: Mount Fuji

Erosionon Earth

Grand Canyon, Arizona

Plate Tectonics

Earth’s crust is a collection of slowly-moving plates.

Plate Tectonics: Earthquakes

The plate boundaries are often sites of earthquakes.

Evidence for Plate Tectonics

Wikipedia: Plate Tectonics

Plate Tectonics

Mantle convection drives plate motion.

Plates are formed at ocean ridges and continental rifts, and destroyed in subduction zones.

Plates float on denser mantle material.

Evidence for Plate Tectonics

Plate Tectonics: Age of Sea-Floor

Ages confirm picture of plate formation at ocean ridges.

Breakup of Pangaea

Plate Tectonics: Continental Drift

Atmosphere

VenusBy Earth Mars

Venus Earth MarsCO2 96.5% 0.038% 95.3%N2 3.5% 78% 2.7%O2 21%Ar 0.93% 1.6%

pressure 90 1.0 0.006

Wikipedia: Oxygen Cycle

Atmosphere: O2 Cycle

Biological processes are the key sources and sinks of O2:6 CO2 + 6 H2O + energy ⇆ C6H12O6 + 6 O2

Most O2 is underground, but exchange between atmosphere and biosphere is the major pathway.

Amount of O2 is kept fairly constant by competition between sources and sinks.

Atmosphere: CO2 Cycle

Carbon and oxygen cycle together over short times.

Over longer times, carbon cycles deep underground.

CO2 in airfalls as

acid rain

dissolves minerals

forms carbonates

subducted underground

returned by volcanos

This regulates Earth’s climate (very slowly):

too ⇒ rain ⇒ CO2 in air ⇒ greenhousehotcold

moreless

lessmore

weakerstronger

Plate tectonics is necessary to return buried carbon!

Climate Cycles

Time (kyr)Wikipedia: Milankovitch 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).

CO2 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

CO2 concentration and temperature track each other.

Peaks in temperature match peaks in CO2

Now, humans activity is increasing CO2 concentration.

Global Warming

Climate models including CO2

can account for warming.

Data

Models with CO2

Models without CO2

Warming has dramatic effects near the poles.

Models predict 3° to 5° C increase by end of century.