The Terrestrial Planets
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In this chapter you will encounter the five inner "worlds" of our solar system. A common theme connects them - heat! You will also engage in comparative planetology - the study of planets by noting similarities and differences.
These five worlds - Mercury, Venus, Earth, Moon and Mars are the Terrestrial planets of our solar system and are characterized by the following basic properties:
| Mass | Compared to the outer planets the inner 4 plus the moon are small but there is a hierarchy within the Terrestrial planets: Mercury and the Moon are small, Mars is mid-sized and Venus and Earth are roughly equal in size and the largest of this grouping |
| Rocky or metallic composition | The inner planets consist of rocky and metallic compounds which can be understood in terms of the Solar Nebula Theory. Only these elements were able to condense into solids at the high temperatures of the inner part of the Solar Nebula |
| Differentiated structure | The inner planets exhibit a differentiated structure with crustal material that is different from core material. The larger bodies (Earth and Venus) have hot, active centres while the smaller bodies have essentially "cooled-off". Internal heat (presence or absence) will turn out to be a very significant factor in determining large scale properties of the planet. |
| Evidence of surface bombardment | All of the inner planets show evidence of a violent past! All are cratered and suggest that they were subjected to bombardment by smaller bodies in the early history of the solar system. |
| Atmospheres | Mercury and Moon have no appreciable atmosphere, Mars a very thin atmosphere and Venus and Earth more substantial atmospheres. Understanding the formation of atmospheres will be an important task for the sections that follow. |
| Table 17.1 Summary of properties of the inner planets | |
Example 17.1 Why is "heat" a common, connecting idea for the Terrestrial planets?
Solution: There are two essential aspects to this: 1) Heat differentiation in the solar nebula determined where materials would condense. The inner part of the solar nebula was so hot that only rocky materials were able to condense. Hence the inner planets are rocky. 2) After formation, the larger the body the more heat energy it contained and this, as you will see in future sections, will have a strong influence on how the planet evolves over time. Small objects like the Moon and Mercury have cooled off and do not display active geology.
To summarize the common properties of the inner 5 major "worlds" in our solar system
Chp 20.1