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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.