skip navigation
Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 36:
Worlds in Comparison:
The Terrestrial Planets

Key Ideas:

A Comparison of the Terrestrial Planets

Surfaces & Interiors depend on size:

Atmospheres:

The Habitable Zone


The Terrestrial Planets

Large Bodies:

Small Bodies:


Evolution of Planetary Surfaces

The evolution of planetary surfaces is driven by three main processes: Volcanism & tectonism are driven by the internal structure of the planets:

Small Planet Surfaces

Surfaces of the small terrestrial bodies, Mars, Mercury, & the Moon, are distinguished from the Earth & Venus by being old and relatively inactive: Crustal Shaping:

Large Planet Surfaces

The surfaces of the large terrestrial planets are younger and active than those we see on the small terrestrial planets.

Younger Surfaces:

Active "tertiary" crusts:


Evolution of Planetary Interiors

Internal heating & subsequent cooling drives the evolution of planetary interiors:

First Stage: Differentiation (heat of formation)

Second stage: Volcanism

How fast the evolution proceeds depends on the Cooling Time for the planet, determined by the heating and cooling balance.

Heating and Cooling

Start with the total amount of internal thermal energy in the planet:
The total energy scales like the volume times the temperature, and since planets are roughly spherical, volume scales like radius cubed.

Hot planets cool by radiation losses from their surfaces:

This is the planet's surface area multiplied by the amount of energy radiated per unit area from a Blackbody of temperature T given by the Stefan-Boltzmann Law.

In the absense of any additional sources of heating, the Cooling Time is the total energy divided by the energy loss rate:

In words: This should square with our everyday experience: babies get cooler faster on a cold day than football players, for example.

Terrestrial Planet Interiors

Small Bodies:

Large Bodies:


Planetary Atmospheres

During formation, the terrestrial planets were molten from impacts with planetesimals

Primordial Atmosphere Formation:

All of the terrestrial planets started out with substantial CO2, H2O vapor, and N2 atmospheres.

Evolution of Planetary Atmospheres

The evolution of a planetary atmosphere is driven by three major factors:

Greenhouse Effect

Planetary Gravity Chemistry of CO2 and H2O

The Greenhouse Effect

Makes surface temperatures warmer than they would be with no atmosphere
  Without
Atmosphere
With
Atmosphere
Water
Earth 255 K 285 K Liquid
Venus 280 K 750 K Vapor
Mars 214 K 220 K Ice
Data are from Bruce Jakowsky's article Atmospheres of the Terrestrial Planets in The New Solar System, 4th Edition, Beatty, Peterson, & Chaikin, eds, (1999, Cambridge University Press).

Runaway Greenhouse Effect

The Greenhouse effect helps make terrestrial planets warmer than they would be without atmospheres, but it can be an unstable process.

If the amount of solar heating increases (e.g., the Sun gets brighter), this leads to an increase in the planet's surface temperature (air, ground, and any oceans).

Normally, if air has too much water vapor, it saturates and the water vapor condenses and begins to rain out as a liquid, removing that much water vapor as a greenhouse gas.

However, as the air gets warmer, it can also hold more water vapor before becoming saturated and raining. Get it too warm, and you can start a positive feedback loop:

and you start getting a Runaway Greenhouse Effect.

Eventually, the temperature get so high that you begin to evaporate the oceans, releasing the dissolved CO2. This adds to the greenhouse warming, increasing the temperature further.

After the oceans evaporate, the carbonaceous rock in the dried out oceanic crust starts to decompose, releasing its huge load of CO2 into the super-heated atmosphere, increasing greenhouse absorption and increasing the temperature further.


Atmospheric Retention

The other factor in determining what kind of atmosphere a planet will have is its ability to hold onto its atmosphere.

More massive planets have a larger Escape Speeds, given by

escape speed
The more massive the planet, the faster something must be moving to escape from the planet into space.

Similarly, in a hot gas, the temperature is related to the speeds of the individual atoms and molecules in the gas. The average thermal speed of an atom or molecule is given by

mean thermal speed
Here at a given Temperature, Big molecules (bigger mass, m) move slower.

Putting these two together:

Thus the ability of a planet to hold onto a particular molecule in gas form depends on its mass and radius (which determines the escape speed) and its temperature (which depends on how far it is from the Sun and whether or not its atmosphere has a greenhouse effect operating).

Other factors can decrease the ability of a planet to retain gases, like no magnetic fields to shield against the Solar Wind, or being sufficiently warm that the atmosphere is extended (like Earth or Mars).


Mercury's Atmosphere

Mercury is too hot for liquid water: Mercury has a low surface gravity: Result: Mercury has no atmosphere today.

Venus' Atmosphere

Venus is also too hot for liquid water: Venus is big enough to retain an atmosphere: Result: Venus has a very dry, super hot, heavy CO2 atmosphere today.

Earth's Atmosphere

Earth is cool enough for liquid water: CO2 chemistry in water: Result: Earth has a light, warm, moist N2 & O2 atmosphere today.

Mars' Atmosphere

Mars might have been warm enough for liquid water during its first Gyr: As Mars cooled: Result: Mars has a dry, cold, thin CO2 atmosphere today.

Present-day Atmospheres

The composition of present-day terrestrial planet atmospheres can be summarized as follows:
  Earth Venus Mars
CO2 0.035% 96% 95%
N2 77% 3.5% 2.7%
H2O 1% 0.01% 0.007%
Ar 0.93% 0.007% 1.6%
O2 21% trace trace

The Habitable Zone

What would happen if we moved the Earth closer to the Sun?

What would happen if we moved the Earth away from the Sun?

In between, where water can be liquid at normal atmospheric pressure, is called the Habitable Zone.
The Sun's Habitable Zone today
Current Habitable Zone of the Sun
[Click on the image for a full-size version]
There are two estimated ranges for the Habitable Zone in our Solar System:
Conservative: 0.95-1.4AU
Optimistic: 0.85-1.7AU
The more conservative estimate is based on the assumption that a runaway greenhouse effect starts at a lower temperature, and that catastrophic freeze-out occurs just before the orbit of Mars.

The more optimistic estimate has a higher temperature found closer in, and that the greenhouse effect helps keep a heavier atmosphere like Earth's warmer further away from the Sun.


Lessons for a Future Earth

As the Sun ages, it slowly gets brighter.

In ~1 Gyr, the Sun will be ~10% brighter:

In ~3.5 Gyr, the Sun will be ~40% brighter:


Return to [ Unit 6 Index | Astronomy 161 Main Page ]
Updated: 2007 November 11
Copyright Richard W. Pogge, All Rights Reserved.