Comparative Planetology: Terrestrials
While each of the terrestrial planets (the Moon can be considered one of these) has its individual characteristics, many of their differences may be understood in the context of their cooling history: Smaller objects cool more quickly than large objects. The smallest terrestrial objects, the Moon and Mercury, froze solid relatively quickly. As a result, their surfaces date to the time of this formation and preserved on these solid surfaces is a history of events dating from very early times in the solar system. On the other hand, the largest terrestrial planet, Earth, is still in the process of cooling, with heat flowing outward from the hot core, driving convection in the mantle and producing the plate tectonic phenomena at the surface.
All the terrestrial planets have a similar crust‐mantle‐core structure (see Figure 1). There is a general trend for density to increase with the size of the object (even when corrected for compressional effects), indicating that a greater proportion of iron and nickel is present in progressively larger planetary cores. Mercury, however, is an exception to this rule; its iron core is far larger than expected for such a small planet.