ASTR1030 Lecture 31: Star Birth, Planet Formation, and Terrestrial Planets - Prof. Robert , Study notes of Astronomy

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ASTR1030 - FALL, 2008. LECTURE 31; PAGE 1

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ASTR1030 - FALL, 2008. LECTURE 31; PAGE 2

Nebular Theory

• Collapse of an Interstellar Cloud• Star Birth• Formation of Planetesimals• Formation of Planets• Solar Wind: Clearing Away the Gas• Leftovers• Bombardment

Radioactive Dating

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 4

Interiors of the Terrestrial Planets

By composition: • Core: Mostly metals.• Mantle: Rock• Crust: The scum that floats to the top. By rigidity: • Lithosphere: The rigid outer section; the crust and part of the mantle.

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 5

Cooling of Terrestrial Planets

Basic Processes ofCooling •^

Convection:

Hot material

flows carrying thermalenergy with it.

-^

Conduction

: Heat flows

within a material. Thematerial does not move.

-^

Eruption

: In my mind, a

form of convection

-^

Radiation

: Conduction

and convection move heatto the surface. Ulti-mately, the energyescapes the planet byradiation, mostly in theinfrared.

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 7

Planets

Mercury • Cratering*• Volcanism• Tectonics *• Erosion Venus • Impact cratering• Volcanism ***• Tectonics **• Erosion ** Earth • Impact cratering• Volcanism **• Tectonics ***• Erosion ***

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 8

Planetary Atmospheres

Venus: • 0.72 AU from the sun.• 0.72 planetary albedo.• 117 days per revolution.• Atmosphere: 96% CO

, 3.5% N 2

2

.

• 230 Kelvin

expected

surface temperature.

• 740 Kelvin

actual

surface temperature.

Earth: • 1.0 AU from the sun.• 0.36 planetary albedo.• ~1 day per revolution.• Atmosphere: 77% N

2

, 21% O

2

, 1% Argon, some H

2

O.

• 250 Kelvin

expected

surface temperature.

• 288 Kelvin

actual

surface temperature.

Mars: • 1.52 AU from the sun.• 0.25 planetary albedo.• ~1 day per revolution.• Atmosphere: 95% CO

, 2.7% N 2

2

, 1.6% Argon.

• 218 Kelvin

expected

surface temperature.

• 223 Kelvin

actual

surface temperature.

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 10

What Determines a Planet’s Surface Temperature

• How far is the planet from to the Sun?• How much sunlight does it absorb?• How fast does it rotate?• Atmosphere, atmosphere, atmosphere! The Greenhouse Effect Energy enters Earth in visible light. Energy escapes Earth in IR light. Partialabsorption of the IR light warms the Earth’s surface. Greenhouse Gasses: • CO

2

(Carbon Dioxide); CH

4

(Methane); H

2

O (Water)

Calculating Temperatures R

= Planetary radius.

F

sun

= Solar radiance. (1360 W/m

2

for Earth)

α

= Planetary albedo. (0.36 for Earth)

σ

= Stefan-Boltzmann constants (5.7 x 10

W/m

2 K

4 ).

T

= Planetary surface temperature.

g

h^

= Effective infrared transmission factor (greenhouse effect).

(g

h^

= 1 - No greenhouse gasses;

g

h^

~ 0 - Complete greenhouse effect) T

F

sun

α

(^

σ

g

h

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 11

Stability

Snowball Earth

• The albedo of seawater is 0.05 (almost black) and the albedo of ice can be as

high as 0.90 (almost totally reflecting). Ice formation has positive feedbackand can lead to run-away freezing.

• If the surface gets colder -> more ice -> reflects more sunlight -> gets even

Unstable - Positive Feedback colder -> even more ice ->...

Quasi-Stable

Stable - Negative Feedback

The Carbon-Silicate Cycle

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 13

Ozone

Important Point: The Ozone problem is understood. • Ozone (O

3

)is a primary absorber of ultraviolet radiation.

• CFC’s (chlorofluorocarbons) emitted into the air over the past 50 years have

increased the amount of chlorine in the Ozone layer.

• Chlorine can act as a catalyst in breaking down ozone.• International treaties should halt the production of CFC’s - and the ozone

layer should start recovering by 2050.

1979

1998

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 14

Mars

Basics: • Distance from Sun: 1.52 AU. (1.38 - 1.65 AU)• Orbital Period: 1.88 years.• Rotation Period: 1.03 Earth days.• Radius 0.533 R

E

.

• Mass: 0.11 M

E

.

• Surface gravity 0.38 g.• Axis tilt: 24

o

.

• Planetary albedo: 0.25.• Atmosphere: 95% CO

2

, 2.7% N

, 1.6% Argon. 2

• Surface temperature: 223 Kelvin.• Moons: Phobos and Diemos.

Crustal Dichotomy Argyre Crater

Hellas Crater

Isidis Crater

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 16

Giant Planets

• Jupiter• Saturn• Uranus• Neptune

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 17

Interiors

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 19

Composition

Banded Structure Jupiter’s Surface Composition: • Ammonia (NH

3

• Water (H

2

O)

• Ammonium Hydro sulfide (NH

4

SH)

• Methane (CH

4

Color: It is believed the color comes from Sulfur compounds.

ASTR1030 - FALL, 2008. LECTURE 31; PAGE 20

Moons

Io

Europa

Ganymede

Callisto

Titan

Triton