Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Geophys. J. R. astr. SUE.(1967) 13, 545-546. Research Note The Earth-Mars-Moon System T. F. Gaskell (Received 1967 June 6) Many different opinions still exist concerning the origin of the Moon (see for example Marsden & Cameron 1966), but very few writers discuss the possibility of a tri-partite system consisting of the Moon and the Earth and Mars (Gaskell 1967). One of the main objections to the old Darwinian theory that the Moon was thrown off the Earth when the Earth was molten has been that a body will either return to its parent or will be lost completely, but it will not settle down in orbit without some extra application of force. Lyttleton (1960) showed that, if Mars was ejected from the Earth and captured by the Sun, the Moon could be left as piece of the Earth which remained to orbit the Earth. Having once accepted the dynamical feasibility of Mars and the Moon both being originally part of the Earth, the whole picture of the evolution of the inner planets becomes clear. It is generally accepted that Urey's theory of cold accretion of the planets is correct, otherwise it is difficult to account for the low atomic mass elements (Urey 1958). However, a molten Earth is desirable in order to provide a ready explanation of the separation of the crust and the core from the original material. Recently work on the structure of iron meteorites has suggested that all the cold accreted bodies heated up at an early stage, possibly due to the presence of the short-lived radioactive isotope of aluminium A1 26. This material could do its work and be lost without trace in the 4500 million years of the Earth's life. The differentiation of original planetary material consequent on melting would cause a decrease in the moment of inertia of the Earth as the heavy iron settled to form the core (Wise 1963) and the period of rotation of the Earth would decrease. The fast spinning molten Earth would then break up as Darwin (1879) proposed, except that Mars and the Moon and many other smaller bits of debris would be thrown off the Earth. The Earth can lose Mars and the Moon without changing its size very greatly, since the mass of Mars is one-tenth and of the Moon one-eightieth that of the Earth. The densities (Valley 1965) of Mars 4.1 and the Moon 3-3 indicate that considerable differentiation had proceeded before the ' spin-off' occurred. Mars probably contains a small core formed by completion of the differentiation process, but the Moon is probably crust and mantle. The removal of Mars and the Moon provides the simplest explanation of the observed fact that the continental crustal material covers only one-third of the Earth's surface. A crust formed by separation of light material and subsequent cooling should be uniformly spread over the Earth's surface. The Moon, having commenced its orbit of the Earth, would recede slowly due to tidal friction (Jeffreys 1928). In doing so it would traverse the small bits of debris associated with the disruption of the Earth, and therefore many of the large craters on the Moon would be old ones. As the Moon recedes so the concentration of bits of the Earth will decrease. 545 546 T. F. Gaskell Calculations of the tidal energy dissipation at the present time (MacDonald 1966) indicate that the Moon was very cloee to the Earth 1700 million years ago. However, it is unlikely that the tidal friction has remained constant during the life of the Earth. If the theory of continental drift is accepted, the Atlantic Ocean is only about 120 million years old, and such large contributors to tidal friction as the Bay of Fundy and the English Channel did not exist. The Behring Strait, which Jeffreys (1928) calculates accounts for two-thirds of the tidal friction, only came into being when America moved away from Europe. Lyttleton (1960) proposes that Mercury was once part of Venus. The mass of Mercury is only about one-fortieth that of Venus, so the material could have been spared. The densities, according to Valley (1965) are about the same, indicating that separation took place before any appreciable differentiation of material in Venus had taken place. This is not unexpected, since, being much closer to the Sun than the Earth, it was only necessary for Venus to melt before being pulled apart by the attraction of the Sun. BP Britannic House, Moor Lane, London, E.C.2 1967 June. References Darwin, G., 1879. Nature, Loiid., 19, 292. Gaskell, T. F., 1967. The Earth’s Mantle, ed. by T. F. Gaskell, Academic Press, New York. Jeffreys, Sir Harold, 1928. The Earth, Cambridge University Press. Lyttleton, R. A., 1960. Mon. Not. R. astr. SOC.,121, 551. MacDonald, G. J. F., 1966. The Earth-Moon System, p. 165, Plenum Press, New York. Marsden, B. G. & Cameron, A. G. W., 1966. The Earth-Moon System, Plenum Press, New York. Urey, H. C., 1958. Proc. chem. Soc., 67. Valley, S. L., 1965. Handbook of Geophysics and Space Environments, Air Force Cambridge Research Laboratories, McGraw-Hill, New York. Wise, D. V., 1963. J. geophys. Res., 68, 1547.