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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 36:
The Terrestrial Planets in Comparison:

Key Ideas:

A Comparison of the Terrestrial Planets

Surfaces & Interiors depend on size:


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

Terrestrial Planet Interiors

Small Bodies:

Large Bodies:

Terrestrial Planet Atmospheres

Mercury & Moon:
No atmospheres
Gravity is too low to retain all but trace gases
Hot, heavy, dry CO2 atmosphere
Warm, light, moist, N2 & O2 atmosphere
Cold, thin, dry CO2 atmosphere

Atmosphere Formation

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

Primordial Atmosphere formation:

Evolution of Planetary Atmospheres

All of the terrestrial planets, including Mercury, started out with substantial primordial atmospheres consisting primarily of CO2 and H2O.

Their subsequent evolution was driven by a number of factors:

The Greenhouse Effect

Makes surface temperatures warmer than they would be with no atmosphere
Earth 255 K 285 K
Venus 280 K 750 K
Mars 214 K 220 K
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)

Mercury's Atmosphere

Mercury is too hot for liquid water:

Mercury has a low surface gravity:

Result is no atmosphere to speak of in the current epoch.

Venus' Atmosphere

Venus is also too hot for liquid water:

Venus is big enough to retain an atmosphere:

Result is a very dry, super hot, heavy atmosphere.

Earth's Atmosphere

Earth is cool enough for liquid water:

CO2 chemistry in water:

Result is a light, warm, moist N2 & O2 atmosphere.

Mars' Atmosphere

Might have been warm enough for liquid water during the first Gyr:

As Mars cools:

Result is a thin, cold, dry, CO2 and N2 atmosphere.

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

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:

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