PLATO - 4 - UW Madison Astronomy Departmentheinzs/Homepage/PLATO_2011... · PLATO 2011: Planets - Foreign Worlds (4) The terrestial planets, Composition • Earth’s composition

PLATO 2011: Planets - Foreign Worlds (4) The terrestial planets,

PLATO - 4• The terrestrial planets, planetology

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Disk Formation• Why does the contracting cloud form a disk?

! As matter rotates faster and faster, it feels more and more centrifugal force, resisting gravity

! Centrifugal force eventually balances gravity - Kepler orbit!

" Centrifugal force is always away from rotation axis

" Gravity always points to center

" Net force: matter is pulled towards mid-plane

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Star & planet formation• link

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http://lasp.colorado.edu/education/outerplanets/solsys_star.php

http://lasp.colorado.edu/education/outerplanets/solsys_star.php


Formation of the Sun• This explains:

! Uniform sense of rotation

! The fact that planets orbit in a disk

• What about the formation of the Sun?

! Center of rotation: natural place for matter to collect

! The proto-Sun formed from the lowest angular momentum material that sank to the center of the disk.

! Friction moves disk material inward, adding mass to the Sun

! This process is called “accretion”

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Phase Transitions

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Freezing (liquid!solid) Deposition (gas!solid)

Melting (solid!liquid) Sublimation (solid!gas)Evaporation (liquid!gas)

Condensation (gas!liquid)


Composition• Element: ! Sun! Earth! Condensation Temp.!

Hydrogen ! 71.1%! 0.0033%! 180K (H2O)

Helium! 27.4%! 2x10-8 %! 3K

Oxygen ! 0.65%! 30.1%! 1300K (Silicates), 180K (H2O)

Carbon! 0.25%! 0.045%! 80K (CH4)

Iron! 0.14%! 32.1%! 1400K

Neon! 0.12%! 4x10-10 %! 9K

Nitrogen! 0.08%! 0.0004%! 130K (NH3)

Magnesium! 0.07%! 13.9%! 1300K (Silicates)

Silicon! 0.06%! 15.1%! 1300K (Silicates)

Sulfur ! 0.04%! 2.9%! 700K (FeS)

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This must have been the composition of the cloud the solar system formed from


Composition• Element: ! Sun! Earth! Condensation Temp.!

Iron! 0.14%! 32.1%! 1400K

Oxygen ! 0.65%! 30.1%! 1300K (Silicates), 180K (H2O)

Magnesium! 0.07%! 13.9%! 1300K (Silicates)

Silicon! 0.06%! 15.1%! 1300K (Silicates)

Sulfur ! 0.04%! 2.9%! 700K (FeS)

Hydrogen ! 71.1%! 0.0033%! 180K (H2O)

Nitrogen! 0.08%! 0.0004%! 130K (NH3)

Carbon! 0.25%! 0.045%! 80K (CH4)

Helium! 27.4%! 2x10-8 %! 3K

Neon! 0.12%! 4x10-10 %! 9K

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Composition• Earth’s composition is closely related to the

condensation temperature of matter.

• What is the link?

! Planets must form in the outer disk, away from center

! Data show: Planets form from elements that easily condense/freeze into solids (dust particles, ice crystals)

⇒ Planets are formed from dust or ice particles that collide, stick together and grow bigger and bigger

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Composition• Why didn’t all disk matter

condense?

! Radiation from proto-Sun heated the gas.

! Temperature was highest closest to the Sun

! Close in, only silicates and iron condensed to rock

! Further out, water and ammonia condensed to ice

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Copyright © The McGraw-Hill Companies Inc.


Composition

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Copyright © The McGraw-Hill Companies Inc. Permission required for reproduction or display

PLATO 2011: Planets - Foreign Worlds (4) The terrestial planets, 11

Hierarchical Planet Formation1. Planet seeds form by collisions of microscopic dust (=rock) and

ice particles in the outer parts of the proto-Solar disk

2. Colliding particles stick together, making bigger particles. We call these bigger “particles” planetesimals


1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-Solar disk


Hierarchical Planet Formation





An artist’s impression of planetesimals in the early Solar System



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3. Bigger planetesimals sweep up more particles than smaller ones

4. Some planetesimals grow massive enough to attract other particles by gravity (gravitational focusing)

Without gravitational focusingWith gravitational focusing


Hierarchical Planet Formation1. Planet seeds form by collisions of microscopic dust (=rock) and

ice particles in the outer parts of the proto-Solar disk



4. Some planetesimals grow massive enough to attract other particles by gravity (gravitational focusing)

5. The more particles they attract, the more massive they become, the more they attract, the more ... runaway growth!

6. Planets continue to collide and grow until all material is either used up or pushed out of the Solar System by the Solar Wind

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Question:• Which planets would you expect to grow faster?

A) Inner planets

B) Outer planets

C) They should all grow at the same rate

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Inner vs. Outer Planets• Why are the outer planets more massive?

! They grew faster because they could accumulate not just rock, but also ice particles

! Hydrogen and Oxygen were very common, making water ice a good “food source” for a growing planet

! This explains the large water content of objects in the outer solar system

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Inner vs. Outer Planets• Why are outer planets mostly Hydrogen and Helium?

! Thermal velocity Escape velocity

! Hydrogen and Helium particles are very light and thus very fast. They are not bound to small (terrestrial) planets.

! Massive cores in the outer solar system had much higher escape velocities and were able to hold on to H and He.

⇒ Massive outer planets could attract H and He gas from the disk and hold on to it, making them even more massive

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vthermal =

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1. Planet seeds form by collisions of microscopic dust (=rock) and ice particles in the outer parts of the proto-solar disk

2. Colliding particles stick together, making bigger particles. We call these bigger particles “planetesimals”


4. Some planetesimals grow massive enough to attract other particles by gravity

The more particles they attract, the more massive they become, the more they attract, the more ... runaway growth!

Gas giants suck up all the matter surrounding them, eventually starving themselves of any more mass to add. Growth stops.

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Gap Formation

Phil Armitage

Planet clears a gap



The Age of the Solar System• When did the Solar System form?

• Best direct evidence: radiometric dating.

! Many atomic nuclei are unstable to fission (splitting).

! This happens randomly

• Example: Uranium 238, which occurs naturally in rock, decays into Lead via a complicated decay chain.

! Half life of 238U: 4.5 billion years.

! That means, on average, after 4.5 billion years, half of original 238U has decayed into Lead and is now gone.

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• Newly formed Zircon (ZrSiO4) crystals contain some Uranium instead of Zr, but no Lead

The Uranium Clock

Photo: R. Lavinsky


• Newly formed Zircon (ZrSiO4) crystals contain some Uranium instead of Zr, but no Lead

• 238U and 235U decay into stable 206Pb and 237Pb, respectively

• Measure ratio Pb/U

• Determine age of Zircon crystal:

The Uranium Clock

age = 6.5× 109 yrs ln� 206Pb

238U+ 1

�


The Age of the Solar System• Many other kinds of clocks available, e.g.:

! 40Potassium"40Argon (half life: 1.3 billion years)

! 14Carbon"14Nitrogen (half life: 5730 years)

• All available age measurements of the oldest rocks, lunar samples, meteorites:

! The solar system is about 4.5 billion years old

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Geology• Learn about planet formation and evolution from

studying the crusts of terrestrial planets

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Earth’s Surface• Mostly: Water

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Earth’s Surface• Canyons: Erosion by water

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Earth’s Surface• Limestone (Calcium Carbonate): Fossil deposits

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Earth’s Surface• Volcanism

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Earth’s Surface• Mountain ranges: Plate tectonics

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Earth’s Surface• Asteroid impacts craters (rare)

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Earth’s Surface• Lithosphere: Crustal plates, about 20-70km thick

! Volcanism

! Plate tectonics: Plates move ~ cm / year

" Mountain ranges

" Earth quakes

! Very few impact craters visible on Earth

• Mostly covered by water (up to 11km deep)

! Erosion

! Deposits

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Question:

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• Which of these surfaces is youngest?

A) Mercury

B) Mars

C) Earth


• Impact craters

! Are mostly old: planetesimal impact

• Geological processes:

" Tectonic activity

" Volcanism

" Erosion

! Act as erasers

" Planetary equivalent of face lift

" No craters = young surface

Geological Activity

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Earth’s Surface• Lithosphere: Crustal plates, about 20-70km thick

! Volcanism

! Plate tectonics: Plates move ~ cm / year

" Mountain ranges

" Earth quakes

! Very few impact craters visible on Earth

• Mostly covered by water (up to 11km deep)

! Erosion

! Mineral and fossil deposits

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Earth’s Surface• Young, constantly reshaped

! Few impact craters

! Surface erosion by water erases surface features quickly

! Volcanism generates new surface features

! Mountain ranges created over millions of years by plate tectonics (compare to Earth’s age of 4.5 billion years)

• The two main effects:

! Water

! Plate tectonics

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Question

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• What happens when you heat a liquid from below?

A) Wave motion

B) Rolling motion

C) No motion

D) Shifting motion


Convection

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• Hot liquid rises, cold liquid sinks


• Continents move ~ 2 cm/yr

! Measured with GPS

! Explains fossil records

• Cause: Convection

! Mantle heated by core

Plate Tectonics

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Earth’s Interior Structure• Earth’s crustal plates rest on boiling layers of semi-

rigid and liquid molten rock and an iron core.

! Plate tectonics recycles crust every few 100 million years

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Solidinner core~6500K

Liquidcore~4000K

Semi-rigidmantle~1500K

Solidcrust~300K


Seismology• Like an ultrasound of the Earth’s interior

! Only P-waves travel through liquid

! S-waves cannot travel through liquid

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P wave

S wave

P waves and S waves

P waves and S waves

wavelength

only P waves


Differentiation• Why is all the Iron in the core?

! Iron atoms are heavy

! Gravity pulls heavy things downward

! Iron sinks, rock floats

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Earth’s Magnetic Field• Fairly strong

! Dipole field (it has a North and South pole)

! Magnetic axis not aligned with rotation axis

! Field reverses every few hundred thousand years

• Note for future reference:

! Earth rotates rapidly

! Earth’s interior is liquid and exhibits convection

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Venus• Earth’s evil twin

! 0.95 Earth radii

! 0.82 Earth masses

! 0.72 AU orbit

! 735K surface temp.

! 90x Earth’s pressure

! Slow retrograde rotation (243 days)

! No water

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Venus

• Venera 13 view (shortly before the probe disintegrated under Venus’ intense heat and pressure)

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Venus

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• Topographic map of Venus

! Only two continents (Ishtar and Aphrodite)


Venus: Volcanic Activity• Maat Mons Volcano

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“Currently” Active Volcanos

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Idunn Mons (courtesy of Venus Express)

Active within the past 250,000 yrs


• Pancake domes

! Rock “blisters”

• Fractures

! Cracks in crust

Venus: Volcanic Activity

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Venus: Geology• Geological activity:

! Most geological features volcanic in origin

! Little tectonic activity (few mountain ranges)

! Few impact craters

! No magnetic field

• Difference to Earth:

! Venus interior is hot, but likely less liquid

! Because in absence of water, lava is probably more viscous

! Slow rotation

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Mars• Most Earth-like planet

! 0.53 Earth radii

! 0.1 Earth masses

! 1.5 AU orbit (eccentric)

! 230K surface temp.

! 1% Earth’s pressure

! 24.6 hr rotation

! 25° axis tilt (seasons)

! Convincing evidence for water

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CO2 ice caps


Mars• NASA rovers

! Sojourner

! Spirit

! Opportunity

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Mars Topography

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Valle Marineris

Olympus Mons

Tharsis Bulge Hellas Planitia

Fairly heavily cratered

SpiritOpportunity

Pathfinder

Viking 2

Viking 1

Phoenix


Mars Geology• Evidence for past Volcanism

! Olympus Mons, 2.6 x higher than Mauna Kea

! Southern hemisphere elevated

! Few craters on slopes of Olympus Mons

! Volcanos now dormant

• No mountain ranges

! No plate tectonics

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Brooks/Cole Publishing 2001


Evidence for Water• Mars Odyssey:

! Lack of neutrons and gamma rays

⇒ Hydrogen present in crust

• Flow features:

! Islands

! Erosion into craters

! Water channels

! Molten ice around craters

! Sedimentary rock formations

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“Islands”

Erosion: Water seepage

Channel

Sediments (Opportunity)Mars Odyssey (seen by Mars Global Surveyor)


Mars Geology• Structure:

! Weak magnetic field

! Sun produces tidal bulge

⇒ Mars’ interior is partially molten

• Composition:

! Higher Sulfur content (condensation temperature 700K)

⇒ Lower density than Earth

⇒ That explains why interior is molten

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PLATO - 4 - UW Madison Astronomy Departmentheinzs/Homepage/PLATO_2011... · PLATO 2011: Planets - Foreign Worlds (4) The terrestial planets, Composition • Earth’s composition