Terrestrial Planets: Rock and Iron

Last: 3. Revolution of the Spheres Next: 5. Giant Planets: Hydrogen and Helium

Although different on the outside, Mercury, Venus, Earth, Mars, and our Moon all have a good deal in common. The main factor determining their status is their size: big planets cool more slowly.



    A Closer Look 6.1: Comparative Data for the Terrestrial Planets ...   p. 112
    A Closer Look 6.2: Density   p. 113
  6.1 Earth: There's No Place Like Home   p. 112
  6.1a The Earth's Interior   p. 112
  6.1b Continental Drift   p. 114
  6.1d The Earth's Atmosphere   p. 117
  6.2 The Moon   p. 121
  6.2a The Moon's Appearance   p. 121
  6.2b The Lunar Surface   p. 121
  6.2c The Lunar Interior   p. 126
  6.3c Mercury from Mariner 10   p. 130
  6.4d Why is Venus So Incredibly Hot?   p. 135
  6.5b Mars's Surface   p. 141
  6.5c Mars's Atmosphere   p. 144

Internal Structure

Internal structure of Earth   Internal structure of Moon

Although the Earth and Moon appear very different, their internal structures display a family resemblance. Both have molten cores, thick mantles and thin outer crusts. The other terrestrial planets are probably similar.

Probing Planetary Interiors: Seismology

A primary (P) wave
What Are Seismic Waves? [UPSeis]
A secondary (S) wave
What Are Seismic Waves? [UPSeis]
  Probing Earth with earthquakes
How Do I Read a Seismogram? [UPSeis]

The Earth's molten core creates shadow zones where seismic waves of one kind or another cannot reach.

Careful monitoring of earthquakes has given us a detailed picture of the Earth's interior.

Structure of the Earth

The core of the Earth is composed of iron and other heavy elements (density ~8 gm ⁄ cm3). In contrast, the mantle and crust are composed of lighter rocks (density ~3 gm ⁄ cm3).

When the Earth formed, the heavy elements sank to the center. Planets with this kind of structure are said to be differentiated.

  Internal structure of Earth
Earth's Interior [Views of the Solar System]

Escape of Internal Heat

Deep inside a planet, heat is generated by the decay of naturally radioactive atoms like uranium and thorium. This warms the interior, and the hotter it becomes the more heat escapes into space. Eventually a balance is reached where heat escapes at the same rate it is produced. This flow of heat keeps the surface active.

There is more radioactive material under each square meter of a big planet's surface, so there must be more heat escaping per square meter. As a result, big planets tend to be more active geologically than small ones.

In a very small planet, the heat produced can escape by conduction -- the same process which allows heat to flow along a bar of iron. But in larger planets conduction is too slow, and convection takes over to carry heat from the interior to the surface.


Simulation of Convection
Mantle convection [H. Schmeling]
  Simulation of Convection
The Earth as a Heat Engine [Columbia]

When a fluid is heated from the bottom and cooled at the top, convection develops: warm fluid rises to the top, cools, and sinks back to the bottom. This transports heat from hot to cold much more efficiently than conduction.

Magnetic Fields

Earth's magnetic field is created in its molten outer core by a combination of (1) rotation, (2) convection, and (3) electrical conduction. Mercury, despite its slow rotation, also produces a magnetic field; Mars apparently did when it was young and active.   The Earth and a bar magnet
Magnetic Field of the Earth [hyperphysics]
The Earth's magnetic field can reverse direction in response to changes in the convective flow within its outer core. On average, this happens once every 250,000 yr, but the last reverse was almost three times that long ago.   Earth's field between and during reversal
Earth's Inconstant Magnetic Field [NASA]

Mantle Convection in the Earth

Mantle Convection with Surface Plates
Mantle Convection with Surface Plates [S. Zhong]

Although the mantle is not fluid, it is `soft' enough to slowly flow and thus carry heat by convection. This has several interesting consequences.

Planetary Tectonics

Tectonic processes create large-scale features (such as continents) on the surfaces of planets. Some examples include:

All of these are the result of heat escape from planets. The Moon may lack tectonic features precisely because it is too small to have generated much internal heat.

Crustal Plates on Earth

Crustal plate boundaries
Evidence for Plate Tectonics [Univ. Tennessee]

Plates and Plate Boundaries

Three types of plate boundary
Plate tectonics [Wikipedia]

Plates are pushed around by convective motion of the mantle they float on.

Continental Drift

Breakup of Pangaea [Exploring Earth]

One well-known consequence of plate tectonics on Earth is continental drift. The present arrangement of continents on Earth's surface is temporary, and over many millions of years the continents have continuously shifted, collided, separated, and reformed.

Uplift of Tharsis on Mars. I

Mars lacks mobile plates like those responsible for plate tectonics on Earth. Instead, long-lived plumes of upwelling mantle material may push the crust up by as much as 8 km, producing the Tharsis Bulge -- an enormous `blister' -- on the surface of Mars.   Formation of Tharsis bulge
Mantle Convection [Views of the Solar System]

Uplift of Tharsis on Mars. II

Mars topographics map
Flat Topography Map of Mars [Views of the Solar System]
  Valles Marineris
Valles Marineris [Views of the Solar System]

Convulsive Resurfacing of Venus

Topographic map of Venus
Venusian Topography [Views of the Solar System]

Compression Scarps on Mercury

Cratered surface with scarp
Large Faults on Mercury [Views of the Solar System]

Volcanic Features

Volcanic activity at plate boundaries
Plate tectonics [Wikipedia]

On Earth, most volcanic activity is associated with plate tectonics -- although `hot spot' volcanoes like those responsible for Hawaii are an exception. Other terrestrial planets don't have plates, and their volcanic activity follows different patterns.

Volcanos on Earth. I

Steep-sided strato-volcanos like Mt. Rainier typically form where crustal plates are being forced back into the mantle.  
Mount Rainier, Washington [Views of the Solar System]

Volcanos on Earth. II

Shield volcanos like Mauna Loa are not associated with plate boundaries; they form where upward-oozing mantle material creates hot spots.

Mauna Loa, Hawaii [Views of the Solar System]

Volcanos on Mars. I

The largest known volcano in the solar system, Olympus Mons on Mars, is a shield volcano. Because Mars lacks moving plates, hot spots in the mantle create single mountains of enormous size, instead of chains as on Earth.

Olympus Mons is 24 km high and 550 km in diameter; in contrast, Mauna Loa is 9 km high, 97 km long, and 48 km wide.

Olympus Mons [Views of the Solar System]

Volcanos on Mars. II

The caldera, or crater, on the summit of Olympus Mons is 2.5 km deep and 80 km wide.  
Olympus Mons Caldera [Views of the Solar System]

Volcanos on Venus. I

Maat Mons is one of many volcanos on Venus. These volcanos may have been created by hot spots. Volcanic eruptions resurfaced virtually all of Venus just 300 to 500 million years ago; in contrast, other terrestrial planets contain features as much as 8 times older.  

Volcanos on Venus. II

Surface Photographs from Venera 9 and 10 [Views of the Solar System]

Impact Craters

Impact craters on Mercury, the Moon, and Mars

Impact craters are found on all terrestrial planets. Mercury (A) and the Moon (B) have the most, while some parts of Mars (C) are heavily cratered as well. Older surfaces are more heavily cratered; Venus and Earth have fewer craters because their surfaces are generally young.

Craters on Mercury

Mercury [Views of the Solar System]

Craters on the Moon

Far side of the Moon [Views of the Solar System]

Craters on Mars

Mariner 6's View of Mars [Views of the Solar System]

Craters on Venus

Golubkina Crater [Views of the Solar System]

Craters on Earth. I

This is one of the oldest (212 million years) and largest (70 to 100 km) craters still visible on Earth. Glaciers have eroded much of its structure.  
Manicouagan, Quebec, Canada [Views of the Solar System]

Craters on Earth. II

This is a young (49 thousand years) and rather small (1.2 km) crater.  
Barringer Meteor Crater, Arizona [Views of the Solar System]

Asteroid Impact

Asteroid Impact at the End of the Cretaceous [Exploring Earth]

65 million years ago, the impact of a modest-sized asteroid, about 10 to 15 km in diameter, temporarily changed Earth's enviroment. By the time the environment recovered, many lifeforms including dinosaurs had died out.

Impact Basins. I

Impact basin formation   Waning gibbous Moon

In the first 500 million years after the planets formed, even larger impacts were common. The Moon displays a number of circular impact basins due to gigantic events; these flooded with lava after their formation, creating the dark and rather smooth mares we know today.

Impact Basins. II

Caloris basin, on Mercury, is another product of a giant impact with features similar to the mares on the Moon. The Hellas and Argyre basins on Mars are probably also due to giant impacts.  
Caloris Basin [Views of the Solar System]


Mars [Views of the Solar System]
Earth [Views of the Solar System]
Venus [Views of the Solar System]

A planet's ability to keep an atmosphere depends on temperature and gravity. On hot planets, gas atoms move faster and more easily escape the planet's gravity. Compare the thin atmosphere of Mars with the abundant atmosphere of Earth and the smothering atmosphere of Venus.


  Earth Mars Venus
O2 21%    
N2 78% 2.7% 3.5%
CO2 0.03% 95.3% 96.5%
Ar    1.6%  
Pressure 1.0 0.006 90

Although Venus and Mars have vastly different amounts of atmosphere, both have similar compositions -- essentially, carbon dioxide with traces of other elements. Earth is unusual in having an oxygen-rich atmosphere. Without green plants, oxygen would quickly combine with other elements such as iron and carbon in Earth's crust.

Oxygen is a signature of life.

Greenhouse Effect. I

  Mercury Venus Earth Mars
125±300 475 20±20 -50±75
0.4 0.7 1.0 1.5

Other things being equal, a planet's temperature depends on its distance from the Sun. Yet Venus is hotter than Mercury! Why?

The thick atmosphere of carbon dioxide on Venus allows some solar energy to reach the surface, but effectively traps outgoing infrared energy. This warms the surface by almost 400°C! Greenhouse on Venus

Greenhouse Effect. II

On Earth the same greenhouse effect currently increases the surface temperature by about 33°C. This is just enough to keep our planet comfortable. But as burning of fossil fuels increases the amount of carbon dioxide in the atmosphere, our planet will get hotter. CO2 record on Earth

The total increase over the next 100 years could be as much as 10°C -- enough to drastically change the environment!

Water & Erosion

Anybody living in Hawaii knows that flowing water has a great influence on the landscape. But the effects of water are not confined to permanently wet locations. For example, the Grand Canyon completely dwarfs the river which carved it almost a mile deep into the desert plateau.
Grand Canyon, Arizona [Views of the Solar System]

Drainage Network on Earth

Dendritic Drainage Pattern, Yemen [Views of the Solar System]

Drainage Network on Mars

Valley Network [Views of the Solar System]

Water Erosion on Mars and Earth

Evidence for Recent Liquid Water on Mars [Views of the Solar System]

Water on Mars

Robots exploring the surface of Mars strongly support the idea that liquid water flowed on Mars at some point -- probably about 3 billion years ago. Greenhouse heating probably kept Mars warm enough for liquid water. Some of this water may still exist below the surface of the planet.


Joshua E. Barnes (barnes@ifa.hawaii.edu)
Last modified: September 14, 2006
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