0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Radius (Earth's radius = 1)

Figure 8. Density-radius relationship of terrestrial planets.

It is possible that the high density of Mercury (if it is indeed exceptionally high) could be attributed to extreme degassing and loss of water of crystallization from its rocky substance to a great depth below the surface. Since Mercury apparently keeps one face perpetually toward the Sun, the surface-rock temperature on the sunny face may reach about 700°C (Newburn, 1961). Temperatures of this order of magnitude, accompanied by high vacuum, can effect loss of water of crystallization from some common rock minerals and thus produce increased density. Another hypothesis is that the rock materials of which Mercury is composed were degassed before their aggregation into a planet.

Data on other terrestrial-type bodies in the solar system, for example, the large satellites of Jupiter and Saturn, are not sufficiently reliable to warrant their inclusion in Figure 8.

If the above empirical relationship is accepted as a working hypothesis

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