Here,I will discuss the planet that we live on and other bodies like it, namely the three other terrestrial planets Mercury,Venus, and Mars, and also the Moon. To the astronomers of the IAU, the Moon is just a satellite, but its composition and internal structure place it among the terrestrial planets from the perspective of a geologist or geophysicist. Figure 3 shows these five at the same scale, and Table 3 lists some relevant data. Within this group, Mercury and the Moon have effectively no atmosphere. Venus has only slightly lower size, mass, and density than the Earth, so gravity at its surface is only slightly less than on the Earth. However, its atmosphere is considerably denser. Mars is larger than Mercury but less dense. These two effects offset each other so that their surface gravities are very similar, but being colder, Mars has been able to hold on to a thin but respectable atmosphere. The Moon has the lowest surface gravity of all – about one-sixth of the Earth’s – which is why Moon-walkers bound around so strangely. Mean surface temperatures obscure wide variations with latitude and, in some cases, between day and night. For example, the hottest daytime temperature on Mercury exceeds 400 °C, whereas at dawn after a long Mercurian night the temperature is below –180 °C.
Table 3 Basic data for the terrestrial planets
3.Top: from left to right, Mercury, Venus, Earth, Moon, and Mars,shown at the same scale. Bottom: the much larger giant planetsJupiter, Saturn, Uranus, and Neptune, with the terrestrial planetsinserted to the same scale
Cores
Terrestrial planets are distinguished by having rocky exteriors,made largely of silicate minerals. However, their densities are too great for them to be rocky throughout, and it is believed that each has an iron-rich core at its centre. No planet’s core can be seen or sampled directly, but there are several independent lines of evidence. Density is one, showing that the interior must be denser than rock even allowing for internal compression at high pressure,and analyses of the trajectories of orbiting spacecraft show that density increases symmetrically about each planet’s centre.Chemical models for what is likely to happen inside a rocky planet suggest that there is insuffi cient oxygen for all the iron to be oxidized and bound up in silicate minerals. Thus, if the interior had ever been molten this would have allowed metallic iron,which is denser than rock, to sink towards the centre. This is an example of a processes called differentiation.