Planetary Atmospheres. Evolution of Terrestrial Planets After the condensation and accretion phases...

Planetary AtmospheresPlanetary Atmospheres

Evolution of Terrestrial Planets

After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.)

Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface Flooding – water, lava, and gases trapped inside the

planetcome to the surface and cover the terrain. Erosion – surface features are destroyed due to running

water, atmosphere, plate tectonics, and geologic motion.

What has the Most Craters?

What has the Most Craters?

a) Mercury

Mercury

Venus Earth Mars

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Why is the Earth’s Core Hot?

c) Heat from radioactivity

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Accretion

Contraction

Radioactivity

What is the Density of Kuiper Belt Objects?

b) similar to water.



Which Heating Source Doesn’t Decline?

c) Heating by the Sun





Accretion Solar Contraction

Radioactivity

At What Wavelength Can You Observe Planet Formation?

d) in the infrared part of the spectrum

The Atmospheres of the Solar System

Planet Atmosphere

Mercury none

Venus thick CO2

Earth N2, O2, [CO2]

Mars thin CO2

Titan N2, CH4, NH3

Jupiter, Saturn, Uranus, Neptune H2, He, CH4, NH3

The Atmosphere of Jovian Planets

Jovian planets are similar to the Sun. Due to their smaller mass, their central pressures and temperatures are not great enough to fuse hydrogen. They are thus cooler, so hydrogen can react with other atoms to form molecules.

H + H H2 4H + C CH4 3H + N NH3

Since the inside of Jupiter is hot (due to the pressure), while the cloud tops are cool, the composition of the atmosphere changes with depth.

The fast rotation rate of the Jovian planets also drives strong currents and storms, similar to the trade winds and hurricanes on Earth.

The Structure of Jupiter’s Atmosphere

The inside of Jupiter is extremely hot and, in fact, Jupiter shines (in the infrared) by gravitational contraction.

Jupiter’s Trade Winds

Jupiter’s equator is moving faster than the poles (it has farther to go in a day). This drives a network of very strong winds and storms.





Formation of an Atmosphere

When a terrestrial planet enters its flooding stage, gases trapped inside the during formation (or created as a result of radioactive decay) will be outgassed. These gases include

H2, He, H2O, N2, CO2, and probably CH4 and NH3

As the planet cools, water vapor condenses out of the atmosphere and falls as rain. Oceans form. But will the planet be able to keep this atmosphere?

Temperature versus Gravity

Escape velocity: the speed a particle must have to escape the gravity of a body and not come back

Temperature: the average kinetic energy of an atom or molecule

The kinetic energy of an atom or molecule depends both on its speed, and its mass. Light particles move quickly; heavy particles move slowly.

It’s easier for a body to hold onto heavy gases than light gases.

The Masses of Gases

Atom # protons # neutrons Total weight

H 1 0 1

He 2 2 4

C 6 6 12

N 7 7 14

O 8 8 16

Molecule Weight Molecule Weight

H2 2 He 4

CH4 16 NH3 17

N2 28 O2 32

CO2 44

Mercury versus Titan

Mercury and Titan are both low-mass bodies. But … Mercury is close to the Sun, so it is hot. Its gravity is not

strong enough to keep its gases from escaping into space. Titan is in the outer solar system and is cold. The

molecules are moving slowly, so the moon can hang onto its atmosphere (except for the lightest gases of H2 and He).

Titan’s LakesThe atmospheric pressure at the surface of Titan is about twice that on Earth. In fact, Titan’s surface is not all solid -- it has lakes (of liquid methane and ammonia).



The Atmosphere of Mars

The composition of Mars’ atmosphere is determined by The mass of the planet. Since Mars is only about 0.1 M, it

does not have the gravity to hold onto H2 and He. It can barely hold onto N2.

Proximity to the Sun. Gases such as CH4 and NH3 are destroyed by ultraviolet light. Mars’ atmosphere is not thick enough to shield itself from ultraviolet photons.

Chemistry. Oxygen (O2) reacts with almost anything (i.e., minerals in rocks), so it cannot stay free.

Consequently, Mars’ atmosphere is primarily CO2 with a little bit of N2.

Carbon-Dioxide and Mars

Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes seasons, just like the Earth. In winter at the pole, CO2 freezes out and becomes dry ice. In summer, this ice evaporates and becomes part of the atmosphere. This cycle produces strong winds and dust storms.



Carbon-Dioxide and Mars

Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes seasons, just like the Earth. In winter at the pole, CO2 freezes out and becomes dry ice. In summer, this ice evaporates and becomes part of the atmosphere. This cycle produces strong winds and dust storms.

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Venus and Earth

The similarities: The planets have similar masses (0.82 M versus 1.0 M)

The planets have similar compositions (density 4.2 vs. 5.5) The planets’ distances from the Sun are similar (0.72 A.U.

versus 1.0 A.U.) Neither planet can hold onto light gases (H2 and He)

Neither planet can keep large amounts of CH4 and NH3 in its atmosphere (due to ultraviolet light from the Sun)

The main difference: The Earth’s temperature is between 50° C and +50° C,

while Venus’ temperature is +470° C

Properties of Carbon-Dioxide

CO2 has two interesting properties:

CO2 dissolves into liquid water (H2O) to create H2CO3 (carbonic acid). Carbonic acid (i.e., the fizz in soda) then reacts with any number of minerals. For instance

H2CO3 + Ca H2 + CaCO3 (limestone)

The result is that, if liquid water is around, CO2 will be removed from the air, and locked up in rocks.

CO2 is a greenhouse gas. It is transparent to optical light, but it absorbs infrared light. So sunlight can make it through CO2, but the heat it brings cannot get out.

Runaway Greenhouse Effect

Venus and Earth both started out with similar atmospheres. But because Venus is slightly closer to the Sun …

Venus was a bit warmer, and had a bit less liquid water With less liquid water, less CO2 dissolved away

With more CO2 in the atmosphere, the greenhouse effect was more effective

The warmer temperature caused more water to evaporate With even less liquid water, even less CO2 dissolved away

As all the water evaporated and the temperature increased, outgassing of greenhouse gases (CO2 and CH4) became easier. CO2 was “baked out” of the rocks

Ultraviolet light destroyed the CH4, NH3, and H2O in the atmosphere, leaving a thick atmosphere of CO2

The Atmosphere of Earth

Venus and Earth both started out with similar atmospheres. But because the Earth is slightly farther away from the Sun …

Earth was a bit cooler, and had a bit more liquid water With more liquid water, more CO2 dissolved away

With less CO2 in the atmosphere, the greenhouse effect was less effective

With more liquid water and a comfortable environment, photosynthetic life developed

Photosynthesis removed even more CO2 from the atmosphere, replacing it with O2. (When dinosaurs lived, there was 5 times more CO2 in the air!)

Lightning plus atmospheric O2 created ozone, which shielded the Earth from ultraviolet light. Water molecules in the atmosphere survived longer (along with life)

Today on Earth and Venus

A small change in the conditions now can lead to large changes later on!

There is a Tide …There is a Tide …

The Earth-Moon System

Tides are the difference in gravity from one side of a body to the other. On Earth, the tides draw the water out towards/away from the Moon.

But the Earth is constantly rotating, pulling the tidal bulge out of alignment. As a result, the water is continually moving in the opposite direction of the Earth’s rotation.

Tidal FrictionThe movement of the water on Earth has two effects:

It slows down the Earth’s rotation. When dinosaurs roamed the Earth, a day was 22 hours long.

It pulls the Moon along a bit faster, slinging it out further from the Earth.

Tides on the Moon

The movement of the water will eventually stop the rotation of the Earth. But what about the Earth’s tidal force on the Moon? Since the Earth is about 80 times more massive than the Moon, its tidal force is 80 times greater. Tidal friction of flowing rocks (lava) has long since locked the Moon to the Earth.

Jupiter’s Moons

Jupiter is much more massive than the Earth, so the tidal effect on its moons is much greater. Recall the 4 Galilean satellites …

MoonMass

(lunar)Density

(water = 1)Distance

(1000 km)Period (days)

Io 1.21 3.5 422 1.769

Europa 0.65 3.0 671 3.551

Ganymede 2.01 1.9 1071 7.155

Callisto 1.47 1.8 1884 16.689

IoIo’s density is that of rock. It has no impact craters no visible water, and is entirely molten, except for a thin crust that is constantly being resurfaced by volcanism.



Europa

Europa’s icy crust is thin. Below the crust is (probably) liquid water. There are few (if any) impact craters on Europa.



Ganymede

Ganymede has many craters, but also a network of grooves that lie on top of the craters. These are mostly likely caused by expansion and contraction of ice layers.



Callisto

Callisto has an extremely old, icy surface covered with impact craters. It is essentially unchanged since the time it was formed.

Jupiter and the Sun

There is very little difference between Jupiter and a star. The composition of Jupiter is similar to that of a star. Jupiter formed in a mini-nebula, just like the solar nebula. During formation, Jupiter shined by gravitational

contraction, just like a star. Jupiter’s luminosity prevented light elements from

condensing on its inner moons, just like the Sun.

The only difference between Jupiter and a star is that Jupiter hasn’t been able to fuse hydrogen.

Jupiter’s Moons

The four Galilean moons of Jupiter show a range of properties:

Io is entirely molten, except for a thin crust. Volcanos are erupting all the time, covering the surface with lava.

Europa is warm enough under its surface to have liquid water.

Ganymede has rills and grooves on its surface, as if ice has been warmed and cooled.

Callisto is an old, cold moon, with no sign of evolution since it was formed.

Why the difference?

Jupiter and TidesThe tidal force of Jupiter on its moons is much stronger than the tides of the Earth-Moon system. These objects should be tidally locked to Jupiter. But …

Io, Europa, and Ganymede orbit in a 1:2:4 resonance. Io is constantly being perturbed by its neighbors.Io’s orbit is elliptical – its speed changes during its

orbit.

Io can’t become tidally locked!

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Heat and the Moons of Jupiter

As a result of Jupiter’s tides … Io is continually stressed by the tides of Jupiter. Its

interior is kept entirely molten. Europa feels some tidal stress as well. However, since it

is further away, the stress is less. Europa’s interior is probably warm enough to melt ice into liquid water.

Ganymede has been thermally stressed in the past, either by heat from Jupiter’s gravitational contraction, or by tides. The grooves in its surface are probably due to ice expansion and contraction. It is now tidally locked.

Callisto is far enough away from Jupiter to be thermally unaffected. It is a cold body.

Planetary Atmospheres. Evolution of Terrestrial Planets After the condensation and accretion phases...

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