Apollo 11 basalt petrogenesis and theories of lunar evolution comparing lunar and earth environment

Singer, S. F.

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iCO`,,MENTS ON RINGWOOD'S PAP1 K
"PETROGENESIS OF APOLLO 11 BASALTS t.ND IMPLICATIONS FOR
LUNAR OR:.GIN" *
Ringwood's (1970) paper consists of two parts: The first
summarizes arid organizes in an exceedingly commendable fashion
the geochemical and petrological data obtained from lunar
exploration. The second part is a discussion of various
t1,-ories of lunar origin, with particular emphasis on the so-
called "precipitation mechanism" for the origin of the Moon
(Ringwood 1966). In spite of my agreement with the major
summaries of the experimental features and direct interpreta-
tions, I cannot agree to the statem--nts put forward by Ringwood
with regard to the origin of the Moon.
Let us discuss first of all the composition of the lunar
basalts and show that many theories for the origin of the Moon
can give a satisfactory explanation. Ringwood makes much of
the apparent similarity between Earth basalts and mantle rocks
and lunar basalts to imply a genetic relation. Two things
might be said here. First of all, if we find that the Martian
basalts are similar, does this imply a genetic relation in
the sense that Mars should have been created from the Earth?
And, secondly, is the relationship really that close? Who is
to say that the similarities are stronger than the differences.
It really becomes a matter of taste. The major geochemical
* This paper consititues contribution No. 28 of The Lunar Science Institute.
Prepared at The Lunar Science Institute, Houston, Texas, under the joint
support of the Universities Space Research Association, Charlottesville,
Virginia, and the National Aeronautics and Space Administration Manned
Spacecraft Center under Contract No. NSR 09-051-001.
^^ _ ,.v	 N7Y
 
2,43 09
features to explain are certainly the low iron content in the
Moon and the depletion of volatiles. Let us see how various
hypotheses of lunar origin deal with this explanation.
Ringwood himself has a rather special mechanism for
achieving these geochemical separations. fie assumes that n
the process of accretion the Earth heat: up to precisely the
temperature at which the silicate materials will vaporize
but the iron will not. This temperature is supposed to be
maintained while a very l arge fraction of the mass of the
Earth accretes. Witho- any kind of thermostat, this is very
difficult to believe. In the first place, the surface tempera-
ture must surely depend on the rate of accretion as well as on
the gravitational potential. (In fact, if the accretion rate is
low enough, then the Earth may well accrete in a cold state
as has been previously supposed by Harrison Brown, Urey, and
others.) There are good reasons to believe that the accretion
rate goes through a maximum when the planet is of intermediate
sizq and certainly diminishes after most of the accreting
material has been swept up. Incidentally, it is not permissible
simply to equate the average kinetic energy of the accreting
matter to an a.eraged heating of the surface by assuming a heat
loss proportional to (ave. temp.) 4 . On the contrary, since
each accretion constitutes a high-speed impact (with the
attendant production of hot gases and intense radiation), the
2
leffective heat loss w X11 be ,nuch larger, and the retained 'heat
will only be a fraction of the energy input. In addition,
much of the iron will vapori7_^ in each impact.
To return to Ringwood's discussion, he has the silicates
evaporating into the atmosphere. For some r.--al, on the atmos-
phere is also at a high temperature whose causa is not explained.
In general, atmospheres would be much cooler. If so, then the
silicate material ought to precipitate out at some altitude and
then simply fall-out back to the Earth's surface. (There it
may reevaporate so that, in essence, we have here a very
effective additional cooling mechanism for the Earth's surface.)
After the accretion is completed, Ringwood '..,rings in a
special event, a T-Tauri stage of the Sun, to remove the
atmosphere except for the silicate materials. FIow they
happened to be left behind is not explained. Three other
mechanisms for dissipating the primitive atmosphere are
mentioned, but they are all rather vague. It is not explained
how the "rapidly spinning high-molecular-weight terrestrial
atmosphere" can mix with the low-molecular-weight solar
nebula in view of the existence of a magnetosphere, and why
this mixing, in any case, should remove the terrestrial atmos-
phere. If "magnetohydrodynamic coupling" transfers angular
..momentum from the condensed Earth to this primitive atmosphere,
this would certainly not dissipate it but, on the contrary,
3
F
implies that the magnetic lines of force would hold on tightly
to the atmosphere. The fourth mechanism which depends on the
formation of a core (why did it not form during accretion?)
would not produce an instability of the atmosphere. It would
simply prevent the accretion of more material which would be
thrown off as soon as it hit the Earth.
Therefore, I am not convinced that an atmosphere could
be so conveniently dissipated. I am further not convinced
that this dissipation would carry away just the volatile
materials so as to explain their absence on the Moon. If the
removal mechanism is based on atomic physics or plasma physics
phenomena, then there should be vast differences exhibited
between lead, on the one hand, and sodium on the other. Yet
no systematic variations among the volatiles have been shown
except that volatility is the primary determinant.
The fission theory (O'Keefe 1969, 1970) can explain some
of these features more directly. The absence of iron on the
Moon is, of course, explained simply, since the iron has
condensed to form the core of the Earth. The depletion of
volatile on the Moon is explained by O'Keefe on the basis of
intense heating of the Moon which must have occurred during
the fission process, and a resulting evaporation and dissipation
of the volatiles.
4
5i
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The capture theory has no built-in explanation for the
low abundance of iron on the moon. If Urey'.- old arguments
( 1962, 1966) concerning the iron abundance on the Sun or
concerning the origin of diamonds in meteorites are invalid,
• then this does not invalidate the capture theory as such. It
simply removes Urey's justification for the capture theory.
(It is impo: ,^ cant to keep the argument logical.) The capture
theory must explain the absence of iron in other ways. It
is possible, for example, that when the solar nebula condensed
and planets formed, there was formed essentially one nucleus
for iron at the orbit of the Earth,and that at a later time
when the ne:1ula had cooled, several nuclei formed for the
silicates. In view of the fact that there are other examples
in the solar system of bodies that are genetically related;
e.g., meteorites, but have large differences in iron content,
we don't necessarily have to burden a theory of lunar origin
with explaining this fact.
On the other hand, the depletion of volatil.es on the
Moon can be explained by the capture theory in a very natural
way. If the low abundance of volatiles is due to the fact
that they were accreted at the very end of the accretion
process from a cooling solar nebula and added as a veneer
(Ganapathy et al 1970; Anders 1970), then the observed depletion
ratio of 10 to 100 between Moon and Earth can be accounted for
i1
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very satisfactorily if the Moon is formed independently, but
not if the Moon is formed in orbit around the Earth (Singer
and Bandermann 1970).
Let us now look at the dynamical considerations. It
• should be stipulated right away that capture of the Moon by
the Earth is an event of low intrinsic probability. If one
wishes to introduce atmospheres or bodies around the Earth
that can absorb some of the kinetic energy of an incoming
Moon, then the process becomes easier but morb ad hoc. The
important thing to note is that the capture pro;:ess does not
do violence to any of the laws of physics, that angular
momentum is strictly conserved, that the energies involved can
be accounted for, and th6 the energy dissipation may in front
be responsible for initiating melting and core formation of the
Earth (Singer 1968).
Ringwood misinterprets the basis for the capture
hypothesis. The earlier calculations by Gerstenkorn (1955)
and MacDonald (1964) which showed that the Moon was captured
from a retrograde orbit can actually be used as an argument
against capture since they would produce excessive dissipation
in the Earth's interior, as well as other physical difficulties.
I also should point out that these Cidal calculations
do not define a time scale of capture. They simply give the
orbit at sequential time intervals. In fact, I rejected the
2 billion year time scale as far back as 1966 simply because
6
I did not believe in the constancy of the tidal dissipation
parameters of the Earth, and I wanted to capture th- Moon
immediately after its formation so as to produce a physically
more reasonable capture theory (Singer 1966, 1968). "ub-
• sequent data from the Moon have borne out the fact -`hat capture,
if it occurred, must have happened about 4.5 billion years
ago. This -is gratifying and shows that the capture theory was
not contrived to fit the experimental data.
i
There is another aspect of the tidal calculations that
is misunderstood by Ringwood; namely, the fact that the Moon
comes as close as 2.6 earth radii. (This is the semi-major
t	 axis; the perigee distance is much closer.) This distance
has nothing to do with the Roche limit and depends entirely
e
on the total angular momentum of the Earth-Moon system. This
is well established in a series of computational runs in
I
which I have chosen different initial angular momenta.
,► 	 Let me say, however, that the tidal calculations cannot
'	 be used to p
 
rove -that capture has occurred. They merely
give an evolutionary path for the orbit,but do not say at
which point the Moon has been inserted. It rather works the
other way around; namely, tidal calculations can be used to
eliminate certain possibilities for lunar origin, those that
give an initial orbit which would not have evolved into the
present orbit. Speci^i.cally, as wasp earlier pointed out by
7
oaf
MacDonald (1964) and proven more definitively by Goldreich
(1966), any hypothesij of lunar origin which starts the Moen
in the equatorial plane of the Earth will not fit the
dynamical picture and give the presently-obnerved orbit.
This is a firm statement and covers all theories of fission,
accumulation, precipitation, etc., and should be taken very
seriously. If one wants to overcome this very fundamental
and basic objection by means of ad hoc arguments, these ad
hoc arguments have to be well-founded and well-proven (see
below).
The principal problems with the precipitation hypothesis
are dynamical. How, for example, do the silicates precipi-
tate in the primitive terrestrial atmosphere and how do these
precipitating particles end up in Earth orbit? If the atmos-
phere is corutating with the Earth, the particles which
1
condense below the synchronous altitude will simply fall down
i
on the Earth,and particles which condense above the synchronous
altitude will be ejected. (Can we really suppose that a
thick atmosphere extended well beyond the synchronous altitude?)
But let us assume that all of these rather crucial steps
can be explained and handled quantitatively, and that we end
up with large chunks of silicates in Earth orbit which slowly
accrete to form a Moon orbiting well within 5 or 6 earth radii,
perhaps just beyond the Roche limit. This Moon would be in
8
ri
an equatorial orbit. We now have the difficulty of
explaining how this Moon evolved into its present orbit.
Ringwood suggests a way out that had been originally proposed
by Wise (1969); namely, that the Earth's rotational axis was
tilted by about 10° to the plane of the ecliptic before the
Moon was formed (italics supplied by Ringwood). But this
won't solve the problem,. The Moon will still assemble in the
equaLOrial orbit, although at a different inclination to the
ecliptic; but the total angular momentum will not have the
correct value.
A more sophisticated "out" has been Proposed by Cameron
(1970). But it won't work either. In fact, Cameron gives
two possibilities: 1) that the Eart,, accreted further material
afterthe Moon was formed, but that this material was accreted
asymmetrically so that the Earth's axis was Lilted by 1000
But as can be shown ;Goldreich 1965), suer a% adiabatic change
i.n the Earth's obliquity will maintain the Moon in the
equatorial orbit. 2) The other possibility suggested by.
Cameron is that the Earth was suddenly hit by a large body
which tilted its axis by 10 0 . The Moon could certainly not
follow this kind of a non-adiabatic change in obliquity.
However, I have calculated what it would take to produce this
change and it turns out that, even under favorable conditions,
the impacting body would have to have a mass about twice that
of our Moon. In other words, we would have to assume the
existence of moon-like objects in the vicinity of the Earth's
9
orbit in the solar system. but then why not assume that such
an objoct was captured in ;he first place?
It seems to me that Ringwood has to contend with two
further points. Why is the Earth-Moon system unique in the
solar system? Why hasn't the precipitation process opera-ted
for Venus? And, finally, why does Venus have no appreciable
angular momentum? Was it removed by capture of a moon from
a retrograde orbit (Singer 1970)? A piavalence of lunar-
sized objects in the early solar syst(-m, coupled with a low
but finite probability of capture can acrunt in a very
satisfactory way fox- these features.
S. Fred Singer
Office of the Secretary
Department of the Interior
January 1970
Prepared at the Lunar Science Institute under the joint
support of the Universities Space Research Association
and the National Aeronautics and Space Administration
Manned Spacecraft Center under Contract No. NSR 09-051-001.
r
1G
...ti
REFFREINCES
Anders, E., "Water on the Moon," Scier ►ce X69, 1309, 1970.
Cameron, A.G.W., "Formation of the Earth-Moon System," E®S,
Trans. Amer. Geophys. Union 51, 628-633, 1970.
• Ganapii thy, , R . , et al, "Truce Ea err,ents in Apollo 11 Lunar Rocks,"
in Proceed ngs of the Apollo 11 Lunar Conference, Houston,
Texas (vol. 2, A.A. Levinson, ed ), pp. 1117-1142,
Pergamon Press, New York, 1970. 
Gerstenkorn, M., "Uber Gezeitenreibung beim Zweikorperproblem,"
Z. Astrophys. 36, 245-279, 1955.
Goldreich, P., "Inclinations of Satellite Orbits About an
Oblate Precessing Planet," Astron. J. 70, 5-9, 1965.
Goldreich, P., "History of the Lunar Orbit," Rev. Geophys. 4,
411 -539 5 1966.
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O'Keefe, J.A., "Origin of the Moon," J. Geophys. Res. 75,
6555-6574, 1970.
MacDonald, G.J.F., "Tidal Friction," Rev. Geophys. 2, 467-
' 541 5 1964.
Ringwood, A.E.,,"Chemical Evolution of the Terrestrial Planets,"
Geochim. Cosmochim. Acta. 30, 41-•104, 1966a.
'	 Ringwood, A.E., "Petrogenesis of Apollo 11 Basalts and
Implications for Lunar Origin," J. Geophys. Res. 75,
6453--6 1{79, 1970.
Singer-, S.F. and L.W. Bandermann, "Where Was the Moon Formed?",
Science 170, 438-439 5 1970.
Singer, S. F., "Origin and Dynamical Evolution of the Moon,"
in Physics of the Moon, pp. 207-237, Proceedings of
AAS/AAAS Symposium, Washington, D. C., 1966, published
by Amer. Astron. Soc., Tarzana, Calif.
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Geophys. J. Roy. Astron. Soc. 15, 205-226, 1968.
Singer, S.F., "How Did Venus Lose Its Angular Momentum?",
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A-,trnr,nr:iv of ti,n "nnr,_ (l._ Yn , ) ;ll., ed.), pp. 481-523,
it-ive to the Solar System,"
Q., 199-223, 1966.
)in the Earth," J. Geophys.


Comments on Ringwood's paper 'Petrogenesis of Apollo 11 basalts and implications for lunar origin'

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