Topic 5: The Terrestrial Planets and Terrestrial Worlds

Reading: Earth and the Terrestrial Planets, Chapter 7

We next consider the Terrestrial planets with the goal of understanding why planets which presumably had similar origins based on the Nebular theory (roughly the same distance from the Sun, roughly the same sizes and masses, roughly the same interior compositions, and probably the same initial atmospheres), turned out so differently. In particular, we want to understand why the current conditions on the surfaces of Mercury, Venus, Earth, and Mars are so different. Their current atmospheric conditions bear strongly on our thoughts about how prevalent life will be in our Galaxy, the Milky Way galaxy.
There is something known as the Habitable Zone in our Solar System where astronomers think planets may develop conditions suitable for life. To help us pin down this region, we use Life As We Know It (LAWKI), that is, life as defined by life we find on Earth, to guide us. Crucial for LAWKI are liquid oceans. This need leads to definitions for the Habitable Zone in our Solar System. With defined ideas about Habitable Zones, we may search for extra-Solar life in more efficient ways.

PROPERTIES OF THE TERRESTRIAL PLANETS^a

Property

Mercury

Venus

Earth

Mars

Moon

Io

M/M_E

0.055

0.825

1

0.107

0.0123

0.0149

Radius

2,439 km

6,051 km

6,371 km

3,390 km

1,738 km

1,815 km

Temperature

333 F (average)

867 F

59 F

-85 F (average)

-13 F (average)

Atmosphere (%)

H,He,O,Na

CO₂,96%;N₂,4%

N₂,79%;O₂,21%

CO₂,95%;N₂,3%

He,Ne,Ar,O

Pressure

10^-14 bars

90 bars

1 bar

0.0065 bars

3x10^-15 bars

Density,ρ(g/cm³)

5.44

5.25

5.51

3.93

3.34

3.55

Interior (%)

Fe,70%;O,Si,Mg

Fe;Ni;O;Si;S;Mg

Fe,35%;O,3%;Si,15%;Mg,13%

Si;Fe;O;Mg

similar to Earth, but different mix

Magnetic Field, B^b

3x10^-7 Tesla

...

3x10^-5 Tesla

...

...

^aWe make a more loose definition of what we mean by a Terrestrial planet. We define a Terrestrial planet as one whose density is larger than around 3 grams per cubic centimeter (g-cm^-3) with a diameter greater than around 2,000 kilometers. Our definition is loose and so the Terrestrial planets include at least the objects Mercury , Venus , Earth , the Moon , Mars , and the Galilean moon of Jupiter Io.
^bFor a sense of these magnetic fields, a typical refrigerator magnet has a strength of 0.001 Tesla. Among the strongest magnetic fields are those in MRI machines. They typically have field strengths of 1.5 Tesla.

EVOLUTION OF TERRESTRIAL PLANETS
The way in which the Terrestrial planets evolve can be understood through consideration of:

I. The structure and evolution of their interiors
II. How and why their surface features change with time
III. The origin and evolution of their atmospheres

I. INTERIORS OF THE TERRESTRIAL PLANETS
We now begin with our discussion of the interiors of the Terrestrial planets. We are primarily concernted with why the atmospheres are so different today. It turns out that the interior structures of the planets have much to do with this issue. Whether the current interiors of the planets are hot or cold strongly affects why Venus, Earth, and Mars turned out so differently. For example:

Whether a planet has a hot or cold interior affects whether the planet displays an active geology which, interestingly, is crucial for the path the evolution of the Earth's atmosphere followed.

Whether the interior is hot or cold affects whether a planet has a magnetic field which, surprisingly, played a major role in the way the atmosphere of Mars evolved.

Whether a planet or a moon has a hot or cold interior affects the way in which we think about the moons of Jupiter and Saturn and whether they can support liquid oceans and so their likelihood as places where LAWKI may happen.

We examine the Earth and Moon because these two objects will bracket the types of behavior exhibited by the Terrestrial planets. The Earth is the most massive Terrestrial planet and the Moon is one of the lowest mass Terrestrials. Furthermore, these two objects are the most well-studied of the Terrestrial planets.

How do we learn things about the interiors of planets?
Well, we do not probe the deep interiors of planets directly, e.g., by drilling holes directly into them. The deepest mine in the world, the Mponeng gold mine in South Africa, reaches a depth of 3.16 to 3.84 kilometers in the Earth (the radius of the Earth is 6,378 kilometers). The mine reaches about 0.05-0.06 % of the radius of Earth into the Earth. This, proportionately, is thinner than the skin of an apple. Suppose a typical apple has a radius of 1.5 inches or so (3.75 cm). Its skin has a thickness of something like 0.01 inches (0.03 cm). The skin is thus about 0.7 % of the apple's radius.
Most everything we know about the interior of the Earth has been learned through indirect means.

PROBES OF THE INTERIORS OF TERRESTRIAL PLANETS

Density tells us about the chemical compositions of the interiors of the planets
Seismology tells us about the mechanical structure of the interiors of the planets
Magnetic Fields tells us about electrical and other properties in the interiors of the planets; to maintain a planetary magnetic field requires a conductor that can flow (like a liquid) and rotation

Topic 5: The Terrestrial Planets and Terrestrial Worlds

Reading: Earth and the Terrestrial Planets, Chapter 7

Property

Mercury

Venus

Earth

Mars

Moon

Io