Uncompressed density and bulk planetary compositions

The bulk density of planets can be precisely determined from geodetic data. In turn, the best geophysical measure of a planet's bulk composition comes from the average uncompressed (or "zero pressure") density. However, pressure corrections to uncompressed density estimates require detailed knowledge of the internal planetary structure (i.e., details of core, mantle and crust structures), equations of state of the various materials that make up the planet (e.g. bulk moduli and their pressure derivatives) and the thermal structure of the planet.

For Earth, the uncompressed density is well constrained. In addition, the basic materials that make up planets (e.g. metallic phases, silicate minerals) and their equations of state can be reasonably inferred from experiments and from seismolo-gical studies on Earth. However, for the other terrestrial planets and moons where there are no seismological data (or very limited data, as for the Moon) the internal physical structures and thermal states are less clear.

For the Earth, Moon and Mars, there are precise moment of inertia factors (I/Mr2), which provide constraints on internal structure. For Mercury and Venus, no such data are available and accordingly estimates of uncompressed density are much more model dependent. Some reasonable assumptions, such as the planet being fully differentiated, can be made to constrain internal structure; however other important factors, such as the oxidation state of the planet (governing bulk metal/ silicate ratios), can only be made with considerably less confidence. In general, the larger the planetary body, the greater and more complex the pressure corrections and consequently the greater the uncertainties. Thus, the uncompressed density of Mercury should be better known than that of Venus.

Stacey [30] reviewed the question of the equations of state of planetary materials and estimated an internally consistent set of uncompressed densities for the terrestrial planets. His values are, in order of increasing uncertainty: Earth: 3.955 g/cm3; the Moon: 3.269g/cm3; Mars: 3.697g/cm3; Mercury: 5.017g/cm3; Venus: 3.868 g/cm3. No systematic evaluation of precision has been performed and Stacey [30] commented: "It is difficult to assign uncertainties because of unknown compositional variations and temperatures, especially for Mercury and Venus, without moment of inertia control, but they are clearly large enough to justify neglect of a crust". Accordingly, for this book, we quote uncompressed densities for the Earth (3.96 g/cm3), Moon (3.27 g/cm3) and Mars (3.70 g/cm3) at three significant figures and for Mercury (5.0 g/cm3) and Venus (3.9 g/cm3) at two significant figures. These values are slightly lower (up to 0.1 g/cm3), especially for Mercury and Venus than commonly quoted values.

On the basis of these values, a few simple conclusions can be reached. The uncompressed densities and therefore bulk compositions of the terrestrial planets vary considerably with the Moon having the lowest value (consistent with the presence of only a very small core) and Mercury the largest (consistent with an immense core, due to loss of silicate material during a late-stage giant impact). Mars almost certainly has a significantly lower (by ~6-7%) uncompressed density compared to the Earth, consistent with its volatile-rich and more oxidized state. The uncompressed densities of Venus and Earth are probably indistinguishable within uncertainties and combined with their very similar size, indicate a similar bulk composition and internal mantle/core structure.

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Responses

  • luca
    Which planet has the lowest uncompressed dennsity?
    3 months ago

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