The magma ocean
The geochemical evidence is decisive that at least half and probably the whole Moon was molten at or shortly following its accretion. This stupendous mass of molten rock is referred to as the ''magma ocean'' and a very energetic mode of origin of the Moon, such as delivered by the giant impact hypothesis, is required to account for it. The concept of the magma ocean has proven robust. Several decisive pieces of evidence require that the Moon was mostly melted at or shortly following accretion....
Hypsometry
One manifestation of the terrestrial continental-oceanic crust dichotomy is bimodal hypsometry that corresponds to the crustal type i.e., oceanic vs. continental . Martian hypsometry is also bimodal, broadly similar to Earth. The martian bimodal distribution corresponds largely to the hemispheric dichotomy separating ancient southern highlands from younger low-standing smooth plains to the north. On Earth, the bimodal distribution in crustal elevation reflects isostatic response to differing...
Europium as a universal tracer
In many respects, the rare earth element REE europium, an element whose abundance rarely exceeds a few parts per million in crustal rocks of any planet, turns out to be one of the most useful elements in geochemistry and cosmochem-istry. By its enrichment or depletion relative to the other REE, one can trace much of the history of processes in the Solar System and the Earth. But the element is also useful to astrophysicists. Europium is formed in stars almost entirely by the r-process of...
A primary crust the highland crust of the Moon
Comparisons with the Earth's geologic style, though inevitable, have proved to be treacherous guides to the Moon. Every school child is aware that the Moon is not a planet. So why begin this discussion on planetary crusts with examples from a planetary satellite The reason is that the two types of crusts on the Moon, that form the lighter highlands and the darker maria, are among our best examples of primary and secondary crusts. Their origin and evolution are better understood than those of...
Atmosphere
The point about including this section in a book on planetary crusts is that the presence of sodium, potassium and calcium ions in the atmosphere of Mercury enables some inferences to be drawn about the nature of the crust. Mercury indeed has a tenuous atmosphere, H, He, O, Ca, Na and K ions having been detected 46 . This is similar to the observations on the Moon of Na and K ions at elevations up to 1200 km above the lunar surface. As these ions are derived by sputtering from the lunar...
Area thickness and density of the present continental crust
The continental crust presently occupies 41.2 of the surface area of the Earth or 2.10 x 108 km2 71.3 or 1.50 x 108 km2 lies above sea level. There are at least ten major continental blocks and four submerged microcontinents. The mean elevation of the continental crust above present sea level is only 125 m, but the elevation of the area above the shelf slope break the 200 m isobath is 690 m. Crustal thickness varies between 10 and 80km from an average of 41km 36 . The thickness correlates with...
Meteorites and planetary composition
The most reasonable internal structures for the terrestrial planets involve metallic iron cores overlain by silicate mantles, and it was Victor Goldschmidt who pointed out that the metallic, sulfide and silicate phases in meteorites were analogues 27 . Although this generalization still holds, it has not proven possible to correlate specific classes of meteorites, either alone or in combination, with the composition of the bulk Earth. Neither K U ratios, volatile element compositions, nor...
Notes and references Thu
1. Variously attributed to Yogi Berra, Niels Bohr, Winston Churchill, Albert Einstein, Enrico Fermi, Eugene lonesco, Dan Quayle, Bernard Shaw and Mark Twain amongst others. 2. Taylor, S. R. 1982 Lunar and terrestrial crusts A contrast in origin and evolution. PEPI 29, 233-41. 3. Cameron, A. G. W. and Ward, W. R. 1976 The origin of the Moon. LPSC VII, 120 Stevenson, D. J. 1987 Origin of the Moon - the collision hypothesis. Ann. Rev. Earth Planet Sci. 15, 271-315 Canup, R. M. 2004 Dynamics of...
Crustal composition
Although it might appear difficult to establish an overall composition for the crust of Venus, there are many clues. Much information is provided by the geomorphology of the surface as revealed by the Magellan radar. The smooth plains that dominate the surface appear to be volcanic. This is reinforced by the presence of a million basaltic-looking volcanoes. But the lessons of comparative planetology must engender caution and portraits may be deceptive. Fortunately the extraordinary USSR Venera...
The solar nebula and the giant planets
The solar nebula from which the Sun and planets formed had three basic constituents loosely gases, ices and rock. The dominant component was gas 98 hydrogen and helium . The heavier elements metals to the astronomers that amounted to about 2 by mass, had accumulated in the interstellar medium from 10 billion years of nucleosynthesis in previous generations of stars. Abundant elements such as carbon, oxygen and nitrogen were present in the nebula as ices e.g. as water, methane, carbon monoxide,...
The differentiation of Venus
How much differentiation has occurred on Venus and what fraction of the incompatible elements have been concentrated in the crust This is an inherently difficult question, on a planet where the crustal thickness is uncertain and where events occurring earlier than about one billion years ago, such as the possible formation of an early crust, are not represented by surface exposures. Melting in the mantle of Venus, even if producing mostly basaltic magmas, is likely to incorporate the...
Notes and references Imq
1. Wilhelms, D. E. 1993 To a Rocky Moon, University of Arizona Press, p. 75. 2. Konopliv, A. S. et al. 1998 Improved gravity field of the Moon from Lunar Prospector. Science 281, 1476-80. The density of the Moon is also intermediate between that of Europa d 3.014 g cm3 and Io, the innermost of the Galilean satellites of Jupiter, with a density of 3.529 g cm3. The other satellites in the Solar System are mostly ice-rock mixtures and so are much less dense. But such comparisons of objects formed...
Interaction with seawater
There is considerable interaction between the ocean and the upper portions of the underlying crust 18 . Studies of ophiolite complexes on land have revealed that Fig. 8.2 The wide variation of TiO2 versus Mg in lunar mare basalts compared to MORB. Data for MORB from 23 and for lunar basalts from Shearer, C. K. et al. 2006 Thermal and magmatic evolution of the Moon, in New Views of the Moon eds. B. L. Jolliff et al. , Reviews in Mineralogy and Geochemistry 60, pp. 365-518. most of the process...
The depletion of the volatile elements in the inner nebula
One might have expected that the composition of the Earth and the other inner planets would mirror that of the original rock component represented by the CI chondrites of the solar nebula. However the moderately volatile elements that have condensation temperatures in the range 400-1100 K are strongly depleted both in the Earth, Venus and Mars as well as in many classes of meteorites Fig. 1.2 . 1 The planets their formation and differentiation 100000 TTTT1 I I I I I I I I I I I I I I I I Fig....
Mare basalt ages
The mare basalts are very old by terrestrial standards. The oldest ages for returned lunar mare basalts come from Apollo 14 breccias. These aluminous low-Ti basaltic clasts found in the breccias range in age from 3.9 to 4.3 Gyr. The oldest basalt sample returned from a maria is Apollo sample number 10003, one of the very first samples collected by the Apollo 11 crew. It is a low-K basalt from Mare Tranquilitatis with an age of 3.86 0.03 Gyr. This gives a younger limit for the age of the Imbrium...