PLATO - 5 • Planetary atmospheres

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 1 Mercury • Smallest planet ! 0.38 Earth radii

! 0.055 Earth masses

! 0.39 AU orbit (eccentric)

! 350K surface temperature (ranges from 100K-700K)

! Slow 59 day rotation (2/3 orbital period)

! No atmosphere

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 2 Surface

Dominated by impact craters • Scarp • Evidence of geological activity: ! Some craters are flooded with lava cuts across craters ! “Scarp” cliff: Crustal fracture • Large “Caloris Basin”: ! Relatively smooth Plateau

! Result of major impact

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 3 Interior

Very high density Partly liquid • Iron-Nickel core ! Mostly iron core Rock (Silicates) • Rotation irregularities: ! Interior is partly molten

! Suggests presence of some Sulfur (see Mars)

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 4 The Moon • Nearest celestial object ! 0.27 Earth radii Apollo 11

! 0.012 Earth masses (only 60% density of Mercury)

! Phase locked rotation near

! 220 K surface temperature (average; from 70K - 390K)

! No atmosphere

! Water ice in polar craters?

far

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 5 Maria • Lunar Highlands Lunar Maria

light: calcium & aluminum oxide dark: basalt

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 6 Formation of Maria • Late massive impacts ! Flooded Lunar low-lands

! Erased low-lying craters

! Left dark basalt surface • Few impact craters in Maria ! Flood must have happened ~3.5 billion years ago.

! Ideal site for lunar landings

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 7 Lunar Geology • Rilles: ! Cracking of cooling surface

! Lava flows • No sign of current geological activity

! Moon has cooled

! Similar to Mercury, moon is geologically dormant

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 8 Internal Structure • Determined from seismographs left by Apollo missions

! Asymmetric interior

! Earth-facing side thinner

! Easier to flood Mantle (poor in iron) ⇒ Maria only on Earth- facing side

! Note the small To Earth Crust Crust Iron core ~150 km thick ~65 km thick

Maria (on Earth-facing side)

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 9 Lunar Formation • Composition: ! Surface rock very similar to Earth

! Much smaller iron content • Impact ejection theory ! Earth hit by Mars-size object at ~ 4.5 billion years

! Obliterated crust

⇒ Moon formed from crustal debris in orbit around Earth

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 10 Comparative Planetology

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 11 Liquid Cores • Why do Earth and Venus show geological activity? • Terrestrial planets formed from colliding solid planetesimals

! Lots of kinetic energy

! Turned into thermal energy = lots of heat

⇒ All Planets interiors were initially liquid

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 12 Heat Retention • Planets are born hot • But all things cool by radiation • How long does it take a planet to cool?

3 E 3 kT n 4πR nkR t = thermal = 2 × × 3 = R cool L 4πR2 σT 4 σT 3 ∝ ×

! Bigger planets take longer to cool

! This explains why Earth and Venus are most active today

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 13 Radioactive Heating • Recall: ! Rock contains small amounts of radioactive material, such as 238U • Each radioactive decay releases: ! Nuclear decay products

" fast particles with lots of kinetic energy = thermal energy

" photons = electromagnetic energy • The energy goes into heat ! Planets stay hot longer

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 14 Comparative Planetology

Now we understand...

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 15 Atmospheres How can we understand the differences in the atmospheres of terrestrial planets?

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 16 Atmospheres

Psurface ~ 90 atm. Psurface = 1 atm. Psurface = 0.01 atm.

• Why are these atmospheres so different? • Where do they come from?

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 17 Earth’s Atmosphere • Mass: ! 10-6 of total Earth mass • Composition

! 78.8% Nitrogen (N2) Mesosphere

! 20.9% Oxygen (O2)

! 0.9% Argon

! 0.04% Carbon Dioxide

! 0.002% Neon

! 0.0005% Helium

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 18 Hydrostatic Equilibrium • Stratification: ! Denser at lower altitudes

! Higher pressure at lower altitudes • Why is this? ! Gravity pulls down

! Pressure pushes in all directions

! But: Lower pressure at higher altitudes, so downward force from above weaker than upward force from below

! Net pressure force = gravity (equilibrium)

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 19 Evaporation • Remember: ! Thermal velocity is only an average

! Some particles move faster, some slower

! Fastest particles can escape!

! All planets slowly leak particles into space! • Rule of thumb: ! You can retain an atmosphere over the age of the solar system if the escape velocity is more than ten times the thermal velocity

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 20 Evaporation • The escape & thermal velocities of different planets:

! Mercury ! vesc=4.3 km/s!! vth=0.7 km/s

! Venus !! vesc=10.3 km/s!! vth=0.7 km/s

! Earth !! vesc=11.2 km/s!! vth=0.48 km/s

! Mars !!vesc=5.0 km/s!! vth=0.35 km/s

! Moon !! vesc=2.4 km/s!! vth=0.5 km/s • Explains why Mercury and Moon have no atmosphere • But: does it explain the presence of Earth’s & Venus’ atmosphere?

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 21 Secondary Atmospheres • Where did Earth’s atmosphere came from? A) Volcanism B) Comets C) Plants

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 22 Secondary Atmospheres • Terrestrial planets: ! Not massive enough to accrete gas from the solar nebula

! They only accreted solids • Atmosphere on terrestrial planets must have formed from solids

! This is called a “secondary atmosphere”

! Material for secondary atmosphere comes from

" Volcanism (CO2)

" Comets (H2O)

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 23 Atmospheres

Psurface ~ 90 atm. Psurface = 1 atm. Psurface = 0.01 atm.

• Earth: Mostly N2, some O2

• Mars & Venus: Mostly CO2 and a bit of N2

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 24 Comparative Planetology • Start from the assumption that Earth, Mars, and Venus started out roughly similar

! How can we understand their different atmospheres today? • Critical ingredient: ! Water

" CO2 cycle

" Fosters life (photosynthesis)

" Mild green house gas

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 25 Earth’s CO2 cycle

• Why the difference in CO2 concentration?

! Rain washes out CO2

! Calcium Carbonate deposits in ocean and on land as rock

! Photosynthesis turns residual CO2 to O2 and carbohydrates

! Tectonic and volcanic activity returns some CO2

Ozone layer absorbs UV light

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 26 Venus vs. Earth • Venus is geologically similar to Earth • Why is Venus’ atmosphere so different from Earth’s?

! CO2 concentration:

" Amount of Carbon locked away on Earth comparable to CO2 in Venus atmosphere!

⇒ Earth once had as much CO2 as Venus ⇒ Good job, plants and rain! Thanks!

! Why did this not happen on Venus?

! No liquid water on Venus - but why?

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 27 Venus vs. Earth • Why no liquid water on Venus? Here’s a theory: 1. Venus closer to Sun ⇒ started out hotter 2. Temperature so hot that most water is evaporated 3. Water vapor rises above any Ozone layer

4. Sun’s UV radiation breaks apart H2O molecules

5. Hydrogen H2 will escape into space

6. Residual O2 will combine with Carbon to make CO2 • But why the high pressure and temperature today? ! Before we figure that out, we need to talk about clouds...

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 28 Albedo • Planet atmospheres transmit some light, reflect the rest. ! Fraction of light reflected by an atmosphere is called its albedo

P incoming

Preflected A = reflected Pincoming P

P transmitted

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 29 Albedo • Planet atmospheres transmit some light, reflect the rest. ! Fraction of light reflected by an atmosphere is called its albedo • The higher the albedo of a planet ! the less Sun light reaches the ground.

! the brighter the planet appears in the sky (Venus is bright!)

Material Albedo A Planet Albedo A Snow 0.8 Mercury 0.1 Ice 0.6 Venus 0.75 soil 0.2 Earth 0.3 grass 0.25 Mars 0.15 ocean 0.1 Moon 0.12

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 30 Thermal Equilibrium • Before talking about the greenhouse effect, let’s discuss thermal equilibrium

! Equilibrium: Incoming power = outgoing power

Psolar (1-A) x Psolar Solar power Incoming power

A x Psolar Reflected power

(1-A) x Psolar Outgoing power

Clouds/surface Planet

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 31 Thermal Equilibrium • Before talking about the greenhouse effect, let’s discuss thermal equilibrium

! Equilibrium: Incoming power = outgoing power • Solve for the average surface temperature: (1 A) L 1/4 L1/4 T = − (1 A)1/4 16πσD2 ∝ D1/2 − ￿ ￿ ! Planets at larger distances to Sun are colder

! Planets around dim stars are colder

! Planets with high albedo are colder - but what about Venus?

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 32 Blackbody Radiation • Every object with non-zero temperature emits light • Blackbodies are special emitters: ! They are in equilibrium with the radiation inside them

! The radiation they emit is of a certain type - we call it thermal or blackbody radiation.

! The hotter a blackbody, the more radiation it emits.

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 33 Blackbody Radiation • The blackbody spectrum 3 106 nm ! Has a peak at λ = × max T ! This is called the “Wien Law”

! So: Hotter things are “bluer”, colder things “redder”

!max 15,000 K star 108 the Sun (5,800 K)

3,000 K star 106 human (310 K) 104

Intensity (relative) 102

101 102 103 104 105 wavelength (nm)

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 34 Blackbody Radiation • The blackbody spectrum 3 106 nm ! Has a peak at λ = × max T ! This is called the “Wien Law”

! So: Hotter things are “bluer”, colder things “redder”

cool burner hot burner

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 35 Blackbody Radiation • The higher the temperature, the more radiation • A blackbody of temperature T , the flux F at the surface of the object (say, a star) is

4 FBB = σSBT

-8 -2 -4 ! Stefan-Boltzmann constant: "SB=5.67x10 Wm K • If the surface area of the object is A, the power is

L = A σ T 4 BB · SB

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 36 The Greenhouse Effect

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 37 The Greenhouse Effect • How does a greenhouse work? ! Glass is a filter: Transmits visible light but blocks infrared light

" Radiation from the Sun is in the visible

" Radiation from the ground up is in the infrared

! Heat is trapped! Energy gets in, but can’t get out.

incoming Blackbody Sun light high spectrum with enough greenhouse Temp. greenhouse glass gas filter

Blackbody Intensity spectrum without higher Temp. greenhouse low Temperature Wavelength ground

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 38 The Greenhouse Effect • How does a greenhouse work? ! Glass is a filter: Transmits visible light but blocks infrared light

" Radiation from the Sun is in the visible

" Radiation from the ground up is in the infrared

! Heat is trapped! Energy gets in, but can’t get out.

! Temperature rises until as much energy escapes as is transmitted from the Sun (equilibrium: in = out)

" Higher temperature means more flux (Wien law)

" Higher temperature means shorter wavelength (Stephan- Boltzman law), less filtering

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 39 The Greenhouse Effect • Some gases act like glass: ! They are transparent to visible light but block infrared light

! Examples: Visible radiation " CO2 (comletely transparent in visible)

" Water vapor

" Methane

" Ozone

! The concentration of any of these gases in Earth’s atmosphere is small, so Earth is not a good greenhouse.

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 40 Venus • Venus is perfect illustration of the greenhouse effect

! Very large CO2 concentration due to lack of liquid water

! Temperature now so high that liquid water evaporates

" Exacerbates greenhouse effect

" H2O destroyed by Sun’s UV radiation

" Thus, no water, no O2

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 41 Runaway Greenhouse Effect

• If CO2 content of Earth’s atmosphere rises drastically 1. Temperature goes up because of greenhouse effect 2. Ocean water evaporates more rapidly 3. Water is a greenhouse gas, so temperature rises more 4. Once water vapor reaches upper atmosphere:

" UV light dissociates (destroys) water into H2 and O2

" H2 escapes into space and is lost

" No H2O, so CO2 can no longer be washed out of atmosphere

! This runaway greenhouse effect is irreversible

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 42 Comparative Planetology • Comparison:

Surface temp. Semi major Actual surface Greenhouse Planets Albedo without axis (AU) temp. warming greenhouse Venus 0.72 0.72 228K 750K 523K Earth 1 0.36 254K 287K 33K Mars 1.52 0.25 216K 221K 5K • Why does Mars have such a weak greenhouse effect? ! Mars had liquid water in the past

⇒ Must have had moderate green house effect, like Earth

! But: Tectonic activity is over, so no more CO2 release

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 43 Planets: Greenhouse Effect

Comparison: • Solar wind

• Why does Mars have such a weak greenhouse effect? ! Mars had liquid water in the past

⇒ Must have had moderate green house effect, like Earth

! But: Tectonic activity is over, so no more CO2 release

" Ice-house: Freeze-out of Carbon, Mars lost atmosphere

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 44 Planets: Greenhouse Effect

Comparison: • Solar wind

• So Mars might have been more habitable if it were big enough:

! Bigger planets stay hotter

! Hotter planets maintain tectonics/geological activity

! That replenishes CO2 in the atmosphere

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 45 The Jovian Planets

Cassini image of Jupiter, Io, and Io’s shadow (NASA/JPL)

PLATO 2011: Planets - Foreign Worlds (5) Atmospheres, Jovian Planets 46