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Produced by the NASA Center for Aerospace Information (CASI)
. y 

• • 

Papers presented to the 

CONFERENCE 

ON THE 

ORIGIN OF 

THE MOON 

(NAS1-CB-17 .. 068) C<.IIUU:NCl:. CN IHe: OlllGU 

Of THE "CC~ (LUUdr dud P4au£tary lust.) 

04 p He l04/ft. A01 CSLL OJB 

~~

~ '''7 

CO·SPONSORED BY 

Division for Planetary Sciences 

of the American Astronomical Society 

Lunar and Planetary Institute 

National Aeronautics and Space Administration 

Kana, Hawaii 

October 13-16, 1984 

N8S- 137C9 

THBU 

Nl!!J-12757 

I' ,\ clas 

G3/') 1 211116 4 

I 
Abstracts and Program for the

Conference on the Origin of the Moon

Co-Sponsored by

Division for Planetar y Sr;enres

of the American astronomical Society

Lunar and Planetary Institute

Nationbl Aeronautics and Space Administration

October 14-16, 1984

Kona, Hawaii

LP' Contribution 546
Compiled in 1984

by

Lunar and Planetary Institute

Material in this volume may be copiad without restraint for library, abss-ract

service, educational, or research purposes; however, republication of any paper

or portion thereof requires the written permission of the authors as well aS

appropriate acknowledgment of this publication.
.a

PREFACF.

This volume contains abstracts that have been accepted by the Program Committee

for presentation at the Conference on the Origin of the Moon. This Committee

incl , ided the conveners and committee members listed below.

Conveners: William K. Hartmann (Planetary Science Institute), Roger J.

Phillip,, (Southern Methodist University), and G. Jeffrey

Tav)or (University of New Mexico).

.,,, i iani V. Boynton (University of Arizona), Alan Harris (Jet

Propulsion Laboratory), L. L. Hood (University of Arizona),

1 1 ;imela .lon g , (l,llnar and Planetary Institute), Gunter W.

Lugmair (University of California, San Diego), and Graham

Rydk•r (Lunar and Planetary Institute).

This voLume was prepared by the LPI Publications Office, and logistics and

administrative support for the conference was provided by the LPI Projects

Office. Tho Ltinar and Planetary Institute is operated by the Universities Space

Research Association under contract No. 14ASW-3389 with the National Aeronautics

and Space Administration.

.i i i
P R O G R A M

LPI TOPICAL CONFERENCE ON THE. ORIGIN OF THE MOON

Sunday, October 14, 1984

8:00 - 9:00 .,.r.;. Registration

8:45 a.m. Opening Remarks

INVITED REVIEWS

9:00 a.m. - 12:00 noon

Chairman: Michael B. Duke

O Minutes for presentation

(30) Wood J. A.

Review of theories of lunar origin.

(30) Larimer J.

How does Dinar bulk material relate to the solar nebula condensation

sequence?

(30) Wetherill G. W.

What were lunar accretion dynamics and early cratering history.

(30) Drake M. J.

Is lunar bulk material similar to Earth's mantle?

LUNCH

INVITED REVIEWS - CONTINUED

1:00 - 3:15 p.m.

Chairman: Robert 0. Pepin

(30) Taylor G. J.

What were the earliest lunar differentiation events?

(30) Hood L. L.

Is there a lunar iron core?

(30) Burns J.

What was the Moon's ancient orbital history?

COFFEE BREAK

V PRECEDING PAGE BLANK NOT 1'I(.\ir*^
Sunday, October 14, 1984

GEOPHYSICAL CONSTRAINTS

3:30 - 5:30 p.m.

Chairman: Roger . T Phillips

Page

(10) Turcotte D. L.

Geophysical and geochemical constraints favoring the capture

hypothesis. 3

(6) Matsui T. Abe Y.

Lunar magma ocean .ind its implication for origin of the moon. 4

(6) Binder A. B.

The initial thermal state of the mc,on. 5

(6) Yoder C. F.

The size of the lunar core. h

(6) Russell C. T.

On the Apollo subsateilite evidence for a lunar core. 7

(10) Cisowski S. M_ Fuller M.

Lunar magnetic history. 8

(10) Banerjee S. K.

Magnetic constraints on early lunar evolution revisited: Limits on

accuracy imposed by methods of paleointensity measurements. 9

(6) Runcorn S. K.

Implications of lunar palaeomagnetism for the origin or the moon. 10

6:00 - 7:30 p.m. SOCIAL

kl c 

=
Monday, October 15, 1984

CHEMICAL AND PETROLOGICAL CONSTRAINTS

e:00 - 11:30 a.m.

Chairman: John T. Wasson

Page

(10) Newsom H. E.

Constraints or, the origin of the moon from molybdenum and other

siderophile elements. 13

(10) Ringwood A. E. Seifert S.

Nickel-cobalt systematics and their bearing on lunar origin. ..4

(10) Delano J. W.

Abundances of Ni, Cr, Co, And major elements in the silicate portion

of Lhr. iuvuu: Cunstraintr, fr,)m pc Imary lunar magmas. 15

(10) Dickinson T. Newsom H.

Ge abundances in the lunar mantle and implications for the origin

of the moon. lh

(10) Goodrich C. A. Barnes S.

Is phosphorus predictably incompatible in igneous processes? 17

(14) WarrenP. H. Rasmussen K. L.

Megaregolith thickness, heat flow, and the bulk composition of the

moon. 19

Warren P. H.

The bulk-moon Mg0/Fe0 ratio: A highlands perspective. 19

(6) Goettel K. A.

BulF composition of the moon in the context of models for condensation

in the solar nebula. 20

COFFEE BREAK

(6) Shervais J. W. Taylor L. A.

Petrologic constraints on the origin of the moon: Evidence from

Apollo 14. 21

(10) Kreutzberger M. E. Drake M. J. Jones J. H.

Origin of the moon: Constraints from volatile elements. 22

(10) Koeberl C.

Volatile elements in and on lunar volcanic glasses: What do they

tell us about lunar genesis? 23

(10) Swindle T. D. Caffee M. W. Hohenberg C. M.

The I-Pu-Xe age of the moon. 24

(10) Taylor S. R.

Tests of the lunar fission hypothesis. 25

LUNCH

Vii
V.( 4

Monday, October 15, 1964

DYNAMICAL CONSTRAINTS

1:00 - 3:15 n.m.

Chairman: Alan Harris

Page

(10) Vanyo J. P.

Constraints on lunar origin: Evidence preserved in Precambrian

st^-rnaatolites. 29

(6) VanArsdale W. E.

Constraints on the origin of viscoelastic bodies. 30

(6) Yoder C. F. Williams J. G. Dickey J. 0. Newhall X. X.

Tidal dissipation in the earth and moon from lunar laser ranging. 31

(6) Hartung J. B.

Two lunar global asymmetries. 32

(6) Conway B. A.

The moon's orbit history and inferences on its origin. 33

(10) McKinnon W. B. Mueller S. W.

A reappraisal of Darwin's fission hypothesis and a possible limit to

the primordial angular momentum of the earth. 34

(6) Durisen R. H. Gingold R. A. Scott E. H.

Numerical simulations of fission. 35

(14)f Boss A. P. Mizuno H.

The dynamic fission instability and the origin of the moon. 11

Mizuno H. Boss A. P.

Tidal disruption and the origin of the moon. 37

(10) Cox L. P.

A numerical investigation of planetesimal collision trajectories with

a moon accumulating in earth orbit. 38

(6) Hartmann W. K.

Stochastic # ad hoc. 39

COFFEE BREAK
Monday, October 15, 1984

MY MODEL OF LUNAR ORIGIN I

3:30 - 5:15 p.m.

Chairman: G. Jeffrey Taylor

(6) e Malcuit R. J. Winters R. R. Mickelson M. E.

Page

A testable gravitational capture model for the origin of the

earth's moon. 43

Malcuit R. J. Winters R. R. Mickelson M. E.

Directional properties of "circular" mania: Interpretation in the

context of n tostable gravitational ca},ture model for lunar origin. 44

(10) Cassidy W. A.

The "problem" of iron partition between earth and moon during

simultaneous formation as a double planet system. 45

(10) Ringwood A. E.

Origin of the moon. 46

(14) Binder A. B.

On the origin of the moon by rotational fission. 47

(10) Wanke H. Dreibus G.

Geochemical evidence for the formation of the moon by impact inaaced

fission of the proto-earth. 48

Tuesday, October 16, 1984

MY MODEL OF LUNAR ORIGIN II

9:00 a.m. - 12:00 noon

Chairman: Torrence V. Johnson

(6) Greenberg R. Chapman C. R. Davis D. R. Drake M. J. Hartmann W. K.

Herbert F. L. Jones J. Weidenschilling S. J.

An integrated dynamical and geochemical approach to lunar origin

modelling. 51

(10) Hartmann W. K.

Lunar origin: The role of giant impacts. 52

(10) Herbert F. Davis D. R.

Models of angular momentum input to a circumterrestrial swarm from

encounters with heliocentric planetesimals. 53

4cx
7

Page

(10)Weidenschilling S. J.

Capture of planetesimals into a circumterrestrial swarm. 54

Weidenschilling S. J.

The Dinar angular v-;:entum problem. 55

(10) Chapman C. R. Greenberg R.

A circumterrestrial compositional filter. 56

COFFEE BREAK

(10) Wasson J. T. Warren P. H.

The origin of the moon. 57

(10) Cameron A. G. W.

Formatius of the prelunar accr-_tion disk. 58

(10) Kaula W. M. Beachey A. E.

Mechanical models of close approaches and collisions of large

protoplanets. 59

(10) Stevenson D. J.

Lunar origin from impact on the earth: Is it possible? 60

LUNCH

SUMMARY AND OPEN DISCUSSION

2:00 - 5:00 p.m.

Chairman: William K. Hartmann

(30) Sean C. Solomon

Summary: Geophysical perspective.

(30) John Longhi

Summary: Geochemical perspective.

(30) Stanton Peale

Summary: Dynamical perspective.

x 

—% — D 
(

GEOPHYSICAL CONSTRAINTS

S.inday, October 14, 1984

3:30 - 5:30 p.m.
r

N85 13710

GEOPHYSICAL AND GEOCHEMICAL C0NS'rRA1NTS FAVORING THE CAPTURE

HYPOTHESIS; D.L. Turcotte, Department ,)f Geological Sciences, Cor.lell

Untver ,► ily, Ithaca, NY 14853

There appears to be a near consensus Chat the early fractionation of

the moon war; the result of melting the outer layer of the moon. This

melting can he attributed to the accretional heating of moon. We have

carried out calculations of the early thermal evolution of the earth and

moon (Turcotte and Pflugrath, 1984) based on the assumption that these

bodies accreted independently. The results are consistent with many

observations.

Wv have aisw applied the concept of geochemical resevoirs to the

moon. The ruhldium-strontium isotope data for mare hasalts can he

explained by an early Eraclionatian of the source region. When the initial

isotope ratios Ei are plotted as a function of time we find very good

agreement with del/dt - -16. The fractionation factor f for the system

can be obtained from the relation f - (dri/dt) Q -1 where Q - 104 a

j ao/ iso where n is thL decay constant, jso is the initial concentration

of the parent, and Ao is the initial concentration of the daughter.

Taking the chondritic reference value Q - 18 we find f - -0.9. Thu g the

source region was almost completely fractionated. The good correlation of

the data and the reasonable results indicate that the reference isotope

ratio iAo /i go used in defining E and the ratio jso/iso used for the

earth are also valid for the moon. There is considerable scatter in the

data for the semarium-neodymium isotope system. This can be attributed to

either experimental difficulties or to variable degrees of fractionation in

the source region. These results are also consistent with the independent

accretion of the earth and moon from chrondri:ic type material.

A primary difficulty with the capture hypothesis is the difficulty of

dissipating sufficient energy to allow capture to occur. Howeve we have

shown that if the earth and moon were accreting sufti-lently close together

accretional capture could occur (Nordman and Turcotte, 1977). if a primary

body is increasing in mass the hyperbolic orbit of a secondary body with

respect to the primary body can be transformer+ Into an elliptic orbit.

Calculations of the capture cross section f , accretional capture indicate

that a significant window for capture exist6.

Nordmann, J.C., and D.L. Turcotte (1977), Proc. Lunar Sci. Conf. 8th, p.

57-65.

Turcotte, D.l.. and I.C. Pflugrath (1984), Proc. Lunar Sci. Conf. 15th,

submitted for publication.

PRECEDING PAGE BLANK NOT F1L 14 M
N$5 13'7" 1

LUNAR MAGh1A OCEAN AND ITS IMPLICATI01 1 FOR ORIGIN OF THE

MOON; Takafumi Matsui and Yutaka Abe, Geophysical Institute,

Faculty of Science, University of Tokyo.

It gives one of the most important thermal constraints on

origin of the moon whether a magma ocean covering the entire surface of a growing moon can be formed. Xcretional heating is

probably only one promising heat source for formation of such magma ocean M. However, so far have nol been proposed any accretional models which satisfy the magma-ocean constrdint (2). to

this paper, we will Study a plausible accreticnal model of the

moor, using as a constraint tha formation of a magma ocean.

Recently, we showed that the surface of a planet growing by

planetesimal impacts was heated over the melting temperature of

surface mater:_als due to the blanketing effect of an impact-induced atmosphcre (3). Using the same ca.lculational scheme for

the earth (3) early thermal histoiy of the moon growing by planetesimal impacts can be calculated for var 4 ous accretional models.

Figure 1 shows thermal evolution of moon accreting by planetesimal impacts. It is shown that a magma ocean covering the entire

surface was formed in both models. Most important parameters

related to surface temperature are safronov number and accretion

time. Our results show that very small safronov number is needed for formation of the magma ocean. Safronov number is usually

larger than 1 for accretion of planetesimals in heliocentric or--

bit (4). However, Safronov number decreases when the moon's

growth is dominated by the proximity of the earth. According to

Harris (5), safronov number falls in the range of 0.02-0.1 for

the binary accretion case. Therefore, we may suggest that the

moon was formed by accretion of planetesimals in geocentric orbits.

References

(1) Mizutani, H. et al. (1972) Moon, 4, 476-489.

(2) Kaula, W. M. (1979) J. Geophys. Res., 84, 999-1.008.

(3) Matsui, T. and Abe, Y. (1984) Lunar Planet. Sci. XV, 517-518.

(4) Safronov, V. S. (1972) NASA Tech. Trattsl. , TTF-677.

(5) Harris, A. W. (1978) Icarus, 34, 123-145.

O

co

O

T (K)

500 1000 1500 2000 2500

Safronov wimber 0.0:'

Acc• rc • t,ion t;itne "^.:'tnyc

-

T (K)

500 1000 1500 2000 2500

.. fronov number 0.02

Aceretinn tirre 10 .. V -

^b

0

O

N

O

O O

(n

O

co

O

6

0

F,N

^ O

\ .L

O

Figure 1.
' D3

N^ I 1 0x'71 2

5

THE INIfIAL THERMAL STATE OF THE MOON; A.B. Binder,

Senior NRC Fellow, Johnson Space Center, Houston, TX 77058

The initial thermal state of the moon may depend on its mode

of origin. For example, if the moon formed by accretion in geocentric orbit it ap^ears likely that the moon would have had a

relatively cool interior and an initially molten zone only a few

hundred km deep. If the moon formed by fission from the protoearth, it must have been totally molten initially. Results I have

obtained during the past few years all favor the concept that the

moon was initially totally molteg, this evidence includes:

1) The existence of young (<10 year old) thrust fault scarps in

the highlands and khar stress drop, shallow moonquakes both suggest that the thermoelastic stresses in the outer crust are currently in the kbar range. This is expected it the moon were ini -

tially totally molten, but not if the moon he.d a magma ocean only

a few hundred km deep.

2) Model studies, which successfully account for all the major

characteristics of the mare hasalts, indicate that the concentrations of the refractory incompatible elements (Sr, Ra, REE) in

the magma at the time the crust and mare hasalt magma source region began to form was 15 to 20 times CI values. This result can

be converted into bulk moon concentrations using the additional

result that the source region and crust formed as the outer 20%

of the mass of the moon crystallized. Thus the concentrations of

these refractory incompatibles in the bulk moon must be between

the limits of 3 to 4 x Cl if the moon were initially totally molten and 15 to 20 x CI i f onl y the outer 200 km of the moon wds

initially molten. A test of these end-member alternatives is provided by the limited heat flow data which suggest that the bulk U

and Th (also refratory incompatibles) contents of the moon are

also 3 to 4 x Cl values. Since it is most likely that the enrichment factor of all the refractory incompatible elements is about

the same, these results together suggest that the moon was initi -

ally totally molten.
N85 13713

'ME SIZE OF TIELUNAR CORE; C.F.Yoder, Jet Propulsion laboratory,

California Institute of Technology, Pasadena, California 91109.

A 0.2 arc second phase shift in the 18.6 year forced precession of the

lunar figure has been inferred from analysis of lunar laser ranging data. The

source of the phase shift is either viscous friction at a lunar core-mantle

boundary or solid friction caused by tidal flexing of the moon by the earth.

Core-mantle viscous coupling will explain the signature if the core radius R 

= 330 lam. On the other hand, solid friction can account for the signature

only if the lunar solid friction Q 1 30 is abnormally small compared with that

observed for, say, Mars ( + 100 - 200). Although the inferred core radius is

certainly within the limits imposed by the Apollo seismic experiment (R c <

500 km), it is significantly smaller than estimates of order 400-500

lam from electromagnetic sounding. How accurate is the estimate of the Re

derived from the phase shift? I shall discuss the effect of uncertainti6s in

the frictional. coupling mechanism, core density and ellipticity on the

inferred core radius. The excitation of lunar free librations by core

turbulence vis-a-vis other mechanisms (e.g. cometary or asteroidal impacts)

any the influence of changes in lunar precession in the past on lur.nr dynamo

generation will also be examined.

W 11W
3 i ^ Jl Ng, 1 14

ON THE APOLLO SUBSATELLIIF EVIDENCE FOR A LUNAR CORE: C. T. Russell,

Department of Earth and Space Sciences, and Institute of Geophysics and

Planetary Physics, University of California, Los Angeles, CA 90024

Electromagnetic sounding of the lunar interior is based on the fact that

electric currents are induced in a conductor in the presence of a changing

magnetic field in the direction as to oppose any change in the magnetic field

within the conductor. The length of time for the "new" magnetic field to

diffuse into the conductor is a function of both the conductivity of the

material and the dimensions of the conductor. Two independent techniques have

teen used to infer the electrical conductivity of the interior of the moon.

The first is the measurement of an incident spectrum of electromagnetic waves

in the solar wind near the moon and the separate measurement of the incident

plus reflected spectrum on the surface of the moon. The second is the measurement of the distortion of the ambient magnetic field of the geomagnetic

tail lubes by the conducting moon. The first technique is most sensitive at

high frequencies and short wavelengths where the scattering by the moon is

strung. The second technique is most sensitive to the deep conductivity of the

moon where tune scales are of the order of many hours or more. The first

^lchnique nas the limitation that two different sensors are used whose caliuration might differ slightly. A one percent difference in gain can strongly

affect the inferred conductivity profile at depth. The latter technique

depends on a single ,eirxr bu t. returns little information about lunar effects

on time scales much less than 90 minutes.

Apollo subsatellite measurements reveal a statistically very significant

signal corresponding to -4.23 x 10 22 Gauss-co13 per Gauss of applied field,

corresponding to a G-factor of -0.008 + 0.001. While these measurements do

not place any strong constraints on the conductivity of the lunar core as long

as it has a conductivity of greater than about 10 mho/m. However, they do

constrain the size of this conducting region to be greater than 400 km in

radius
`2j6l

S85 13

A

LUNAR MAGNETIC HISTORY; S. M. Cisowski, and M. Fuller, Department

of Geological Sciences, University of California, Santa Barbara, CA 93106

Intensity normalization results (1) have now been obtained from almost

all of the lunar samples for which some type of age determifnation is available. This amounts to about 90 samples. In the absence of intensity determinations of the Koenigsberger-Thellier-Thellier type, these results are

likely to remain the most complete data set upon which the history of the

intensity of ancient lunar fields must be based in the foreseeable future.

Magnetic contamination either during or after collection has been identified and minimized by a combination of AF demagnetization, thermal demagnetization over the range of the lunar diurnal cycle, along with investigation

of multiple samples from single rocks. The IRMs normalization method then

reveals that rocks whose radiometric ages lie between 3.6 and 3.9 B.Y. have

magnetizations indicative of being magnetized in far stronger magnetic fields

than those having older or younger radioactive ages. By comparison with both

terrestrial and lunar samples given thermoremanent magnetization in the

laboratorv, the strength of the ancient lunar field during this short period

of lunar history appears to leave been comparable with the present earth's

field, i.e. to order of magnitude tenths of an oersted, or tens of Am-'.

The only exceptions to this pattern of normalized magnetic intensity and

age are a few samples whose magnetization is very soft and appears to be dominated by a small number of large multidomain particles. These samples characteristically have non-repeatable AF demagnetization curves in both intensity

and direction. Although the number of samples with radiometric ages significantly greater than 3.9 B.Y. is limited, stratigraphic classification of the

lunar breccias for the Apollo 17 boulders and meteoritic trace metal content

correlations suggest that the oldest lunar samples were not magnetized in this

strong field. There is a significant absence within the high normalized

intensity group of sam p les exhibiting strong shock features.

The observation chat the Mare basaits older than 3.6 Q.Y. carry stronger

magnetizations than younger mare samples is in excellent agreement with the

surface field determinations by the electron reflectance method and with the

satellite magnetometer data. There is then some degree of agreement from different approaches that the moon may indeed have had a magnetic field comparable in intensity to the geomagnetic field for a period of about 300 M.Y.

between 3.9 and 3.6 B.Y.

The orig i n of this field remains a mystery. None of the various speculations are convincing. Suggestions of an early solar system field are not consistent with the low magnetization of the oldest samples. Models based upon a

close approach of the moon to the earth have to account for the 300 M.Y. approach. Lunar dynamo models must explain why this dynamo which must operate

in so small a core can operate so efficiently.

We conclude that it is now hard to escape from the idea that there was a

relatively strong ancient lunar field between 3.9 and 3.6 B.Y. Moreover it is

curious that the existence of this field coincided with the termination of the

heavy bombardment of the moon and mare basalt extrusion.

(1) Cisowski, S. M., Collinson, D. W., Runcorn, S. K., Stephe-ison, A. and

Fuller, M. (1983) Proc. Lunar Planet. Sc'.. Conf. 13th, in Journal of

Geophysical Research, $B, p.A 691--VU—.
N85 13'716

9

MAGNETIC OONSTRAINTS ON EARLY LUNAR EVOLUTION REVISITED -- LIMITS ON

ACCURACY IM1, W ED BY METHODS OF PALE01NTENSITY MEASUREMENTS; Subir K.

BanerJee, University of Minnesota, Dept. Geology and Geophysics,

Minneapolis, MN 55455. Present address: University of California, Dept.

Geology and Geophysics, Berkeley, CA 94720.

Lunar internal magnetic field variation can provide a strong

constraint for early lunar evolution. However, the magnetic carriers in

lunar, samples are most unusual, ultrafine grains (100-10,000 A) of iron

either free or in intimate contact with troilite (FeS). thus it is

impossible to carry out conventional paleointensity experiments requiring

repeated heating and cooling to 7700C without chemical, physical or

microstructural changes. Therefore, alternative, non-thermal methods of

paleointensity determination have been sought, the two anhysteretic remanent

magnetization (ARM) methods of Baner,jee and Mellema (1) and Stephenson and

Collinson (2), and the saturation isothermal remanent magnetization (IRMs)

method of Cisowski et al. (3).

I have inve:jtigated the experimental errors inherent in these alternative

approaches to estimate the accuracy limits on the calculated paleointensities. In the light of these built-in errors, a review of the up-to-date

lunar paleointensity data yields the following conclusions: (1) It is most

likely that lunar surface field was between 1,000 and 5,000 nT during its

total history, (2) There is suggestiv4 ',ut not convincing evidence that the

field was as high as 100,000 nT ac 3.9 b.y. ago, (3) It is unlikely that the

3.9 b.y. high field epoch was preceded by a weak (( 1,000 nT) field lasting

between 4.0 and 3.9 b.y.

It should also be pointed out that recent discussions about the past

polar wander of lunar spin axis assume a strictly axial dipolar magnetic

field to have been present at the 4.0-3.9 b.y. period when it is simultaneously claimed that the lunar magnetic field was at its weakest strength

ever (( 1,000 nT). This may or may not be a contradiction in terms of dynamo

field generation.

Rararannaa

1) Baner,jee, S.K.

P . 177.

(2) Stephenson, A.

23, p. 220.

(3) Cisowski, S.M.

and Mellema, J.P. (1974) Earth Planet. Sci. Lett., 23,

and Collinson, D.W. (1974) Earth Planet. Sci. Lett.,

et P.I. (1975) Proc. Lunar Sci. Conf. 6th, P. 3123.
N 8 5 13717

it ►

IP, ITICA'I 'NS OF LIINAR

:.K.Nuncorn, Schoo ..I Physic s,

upon 'lyric NI I rRll, I n;land, IIK.

PALALCHAGNI:'I'1SM FOR '1'111: ORV IN OF '1'111; M(N)N.

Ifn i versi t y of Newcastle upon 'Tyne, New(,tst Ie

Lunar l alaeom ralSeti three IssUO 's relaLing to the origin of the

Moon: the (.art y f , rmiiL ion of it fluid iroU core • , the nature of primeval heat

sour( es in I he M0011 ;and the ex i5tence o a pr i meval sate I I it c system. The

remanent ma) IiCLiza, t , ,n of tlu Apol to :,,,mples wits interpreted as eviden( a for art

i lit er na I I y } er ► e rat('d I unar wagneL i L I I e 1 d (1 ). The three independent methods

of deteiminruK pal ioi)intensil.i.e5 (the Thellier,ARM (2) and IRM methods ('i)) are

now ill gene i a 1 igi ct meriL that Lite fiel d was about I C 3.9 h yr ago dec I i n i ng

exponc t ial lv to .0.1 6 3.2 t, yr ago. PalaeomagncrLic directions of crustal

st.ral a have been determined from Lite Apollo I) and 16 subsatel l ite magnetometer

obser vat ion., by Culp man, Russell and ilood (4). The question whether these are

randotn 1 f dire( led ,ut h as woo I d be exile( ted from local magnet i zaL i on pt oces,, es

or are proof (it Lite exi:,tence of an early core dynamo field is one of the key

iwoe's ul' lunar sc once. On the core dvnamu hypothesis Lite mean magnetic

t ield would have been a dipole along the axis of rotrtt ion: the north magnetic

pule por,il ions do c,,i.ulaLed from Lite observational daLa can he grouped into 3

bipolar gr oupi nf,s Ailing 3 axes different from the present. lunar rotation axis

(5). 1 ari,ue that those groupi ngs refer to different times: Pre-Nectarian

4.2 b yr, bower Ne(tarian 4.0 b yr and Upper Nectarian-bower Imbrian 3.9 h yr.

'fhe successive realtenlations of the Anon with respect to its axis of roLaLion

are the result of the creaLaun of Lhe mulLi•-ring impact: basinv. As these

b , tsfns 1 ie e lose I„ palaeoequator s ul corresponding age, it i i interred Lhat

the bodies which i.mp,tcted the Munn were in orbit about the Moon near its

equator rather Lh;in asteroids (5). The use of the Bingham statistical distribution Shows Lh;aL this (6) assocf;ition is highly signtticanL. Because each

group consists of between 6 and 10 multi-ring basins iL is concluded that there

were 3 larg( sate llites, each ui which broke up as they passed through the Ruche

limit, at 10" year tt ► feivals as a consequence OI Lidal friction. In fact it is

shower t.haL the suc( cr.r,ive muvements of the palaeormignet is pole through 90 0 are

required by EuIei's principle of rotat ion of solid bodies. The direction of

the i ncoming impac t i ng bod. ; r;in be deterani ned t rum the asymetry of Lite basins

and I he retiu I t s (Wf I ft( , ims( 7 )) ^;I,uw d i rec L f ons rough I y para I I e I t o the pa 1 aeomagnetic;alJv deLer ill illud equators of corresponding age. Some of this evidence can

he interpreted a:; implying that Uae sit LeIIites were in retrograde orbit but in

any case their exi.,tonce around the Moon until 3.H h yr ago sets a considerable

const rai nt nn Lhe evo I ut. i on of the ear t y ValLit-Moon system. nie creation of

the basins by bodies coming in at about 2 km/second in contrast Lo the former

view that. Lhe bodies were asteroids or comets, requires them to be of subsLanLi:al size so that :r strong component of solar system mualctial Should he present

in the 1 urrit regol it h. Alt hough t he preset (e of a lunar curd was long ago

suggested and I.ht re ore nuw var i ouc; di. f 1 erent, ;a l though individually not conc l us i ve it gument -,, t he I i L of ttie pa heomagnet i s dat a Lot he d i pot e h-1 pothesis

is st rong evidence for Lhe existence of it mo 1 ten I unar iron cord and implies it

powerful heat sour( c 1)resent in the ear I iesL history of the Moon.

( 1 ) Runcorn S h ( V) 70) Pt oc , Apo l l o I I Lunar Sc i . Con ,1).2369- 2'387. ( 2) St y phenson A, Runcor n S k, K Cal I inson 1) W ( 197`',) Proc.l,unar Sci.ConI .6th, p.3049-

3062. (;) Cisowski S M, Col I inson 1) 1•., Runcorn S K, St,vpl oi, son A h Vu Ier M.

( 1983) Proc.kunar I'Ianct .Sci.UonI . I it h, • it Journal of Gco rr rysicaI Researc h.88,

A691 - 004. ( 4 ) Hood I. I, (1980) Proc.Luni,r 1'Ia1le1..5ci.0,11:.1001, p. 1879-1896 K

Proc.Lunar PlarieL.:,ci.('onf.l'2li, 1).817-830 (1981). (5) Runcorn S K (1983) Nature,

iO4, 549-59h. ( 6 ) I;uncorn S K ( 1984) PhiI Trans. R.Suc, (7) Wi IheIms D I:.Prof.Pap.
CHEMICAL AND PETROLOGICAL CONSTRAINTS

Monday, October 15, 1984

8:00 - 11:30 a.m.

-,;* -,.ae - - 0 
N85 1318 ^

13

CONSTRAINTS ON THE ORIGIN OF THE MOON FROM MOLYBDENUM AND OTHER

SIDEROPHILE ELEMENTS; H.E.Newsom, Max-Planck-Institut fur Chemie,

Saarstrasse 23, D-65OU Mainz, F. R. Germany, and Institute of Meteoritics,

University of New Mexico, Albuquerque, NM 87131, USA (present address).

New data on the concentration of molybdenum in the mantle of the Earth

(1) and in lunar rocks confirm that most siderophile elements in lunar crustal

rocks and lunar basalts derived from the interior have significantly lower

concentrations than in the Earth's riantle and much lower concentrations

compared tc, chondritic meteorites (Fig. 1). Our data confirm the low Mo

concentrations reported by Taylor et al. (2). The Mo/Nd ratio in lunar samples

(0.0016) is a factor of 30 lower than the Mo/Nd ratio in terrestrial samples

(0.043, ref. 1), and a factor of 1,200 lower than in C1 chondrites (1.94).

Different but correlated depletions of Co and P have been found in different

lunar samples (3). The Mo data appear to he correlated with the P and Co

depletions. The variations in siderophile contents seem to be real (3), not

just due to experimental error, and may be explained by small variations in

the segregation of metal, possibly on a global scale.

As discussed by Newsom (4) the siderophile element data are consistent

with theories for either a terrestrial or an independent origin for the Moon.

Wanke et al. (3) suggest that the Moon formed from the Earth's mantl?

(depleted in siderophile elements, e.g. ref. 5), and segregation of metal

within the Moon further lowered the concentrations compared to the Earth's

mantle. From the terrestrial siderophile element concentrations, the concentrations in the lunar samples can be reached by segregation of only 0.15%

metal at 10% partial melting. For total melting of the silicates as much as 1%

metal is required to achieve the observed W, P, Mo and Re lunar depletions.

For an independent origin of the Moon, segregation of a small metal core

could explain the observed concentrations of siderophile elements in lunar

rocks (4). Starting with chondritic concentrations and assuming a metal content of 5.5% at 9% partial melting of the silicates, the calculated depletions

match the observed depletions. Metal contents less than 5.5.1, can explain the

depletions of W, Mo and Re if equilibrium occurred at smaller degrees of

partial melting (6). For a 2% core about 1% partial melting is needed.

The Mo data support evidence from other siderophile elements that the

Moon almost certainly contains a metal core or pools of segregated metal. A

terrestrial origin is indicated if the core is less than 2 wt% because the

1 W -Ma-—`R^-- siderophile element depletions could not be

+ + + + + + obtained by such a small core alone. An

^f 10' t t independent origin is likely, but not

z^

I^

required, if the lunar core is greater than

Z= I } 2 wt`t, because the siderophile element

10 ' ► depletions could then ')e explained by purely

W ; lunar processes. Determining the actual size

;W YJ , of the lunar core will be a k py step in

a^ answering the riddle of the lunar origin.

Ref.: (1) Newsom and Palme (1984)

oLL 10` Earth Planet. Sci. Lett. in press. (2)

W^ •Eartn Taylor et al. (1971) Proc. 2nd Lunar Sci.

b Mpon

a ,o D Moon. calculated Conf., 1083. (3) Wanke et al. (1983) in

for (ndeol 

melt

lt

mg f

e origin.

9%far bol ^ Lunar Planet. Sci. XIY 818. (4) Newsom

and 5.5% Molol I (1984) EOS 65, 369. (5) Ringwood and Kesson

lo° -- J (1.977) The Moon 16 425. (6) Newsom and

10 METAL SILICATE

/O

MELT PARTITION COEFF

,c Drake ( 1982) Nature 297, 210.

^i:ECrr)INC, PAC? 13LANK NOT rir. ` , 'r)

2 k 
N85 13719

14

NICKEL-C fiALT SYSTEMATICS AND THEIR BEARING ON LUNAR ORIGIN;

A.E. Ringwood and Seifert. Research School of Earth Sciences, Australian

National Universit; Canberra, A.C.T. 2601 Australia.

Tile earliest metal phases to crystallise from the least-fractionated lowTi mire basalts contain about 30% Ni and 2 to 7% of Co. Experimental investigations of partitioning of Ni and Co between primitive luu..: Lasalcic magmas and

metal phases are currently in progress (1). Preliminary results show that the

metal phase crystallizing near the liquidus of Green Glass (185ppm Ni, 70ppmCo)

would contain about. 40% Ni and 2.34 Co. Tt seems, therefore, that if a lunar

(-ore does exist, as suggested by the results of (2), it is relatively rich in

Ni (30-40%) and Co (2-7%). This composition presents a remarkable contrast to

that of the earth's core which is estimated to contain about 6% Ni and 0.3% Co.

One explanation for the differing densities of earth and moon is that both

bodies accreted independently in the same neighbourhood of the solar nebula

from a common mixture of metal and silicate particles. Enrichment of metallic

phase in the earth is attributed to a physical fractionation process. Ifyl:',the5es

in the category (eq. disintegrative capture,Fe silicate fractionation based on

magnetic properties or on differential ductility of metal and silicates) fail

to explain the very different chemical compositions of terrestrial and lunar

cores and can therefore be rejected.

Another class of hypotheses maintains that the moon accreted independently

in the solar nebula and was produced from a mixture of high temperature,intermediate temperature and low temperature condensates together with condensed

metal phase. These batches were fractionated by unspecified mechanisms and

assembled in desired proportions to form the moon. These hypotheses are subject

to certain cosmochemical cunstraints: (,k) metal phases condensing in the

nebula do not contain more than 13% Ni and 0.7% Co (3). (b) they imply a particular relationship between the Ni content of the metal and tae MqO/(MgO+FeO*)

ratios of coexisting silicates (manifested in chondrites as Prior's Second Law).

Thus a metal core containing 30-40% Ni implies a lunar mantle with M3O/(Mgo+Fe0*)

molar of about 0.5. This is far beyond the limits permitted by the moon's

physical properties; (c) known cosmochemical processes are ineffective in

fractionating cobalt from nickel; hence the bulk lunar Ni/Co ratio is probably

close to the primordial ratio of 21. The abundance of cobalt in Low-Ti basalt

is almost identical to that in terrestrial oceanic tholeiite and it seems

likely that this similarity extends to their respective source regions.

Acceptable lunar models consistent with observed Ni-Co metal, l silicate distriboit.ion coefficients and also possessing a bulk chondritic Ni/Co ratio appear

to require even higher concentrations of Ni in the lunar mantle (>1500 ppm)and

in the lunar core (>40% Ni) than are indicated by the Green Glass equilibria.

These conditions are very difficult, if not impossible to satisfy in terms

of existing models by which the moon was formed as an independent "planet" in

the solar nebula. They are readily explained if the moon was formed from

material possessiu(a similar Ni and Co abundances to the earth's mantle and

subse, luently differentiated a very small K0.59, by mass) Ni-rich core as

suggested inlependerrtly by Wanke, Ringwood and Newsom.

Refs (1) Sci.fert, S., O'Neill, li., and Ringwood, A., unpublished results.

(2) Newsom, If., and Palme, H. Earth Planet. Sci. Lett.., submitted 1984.

(3) Grossman, L., and Olson, J., Geochim Cosmochem Acta (1974) 38, 173.
N85 13720 3//

15

ABUNDXNCF.S OF Ni,Cr, Co, AND MAJOR EIJ : MENTS IN THE SILICATE PORTION

OF 'lliE MOON: CONSTRAINTS FROM PRIMARY LUNAR MAGMAS. J.W. Delano, Dept. of

Geological Sciences, State University of New York, Albany, N.Y. 12222

The lunar volcanic glasses are samples of primary magmas derived by partial melting of the Moon's mantle. The twenty-seven varieties of glass define

three linear arrays having nearly constant Mg/Si ratios. 'The low-Ti magmas in

each array approach the chondritic Ti/A1 ratio. In addition, the Ca/A1 ratio

in these low-Ti magmas trends toward the chondritic value as one proceeds from

array 1 through array III. Using the chemistries of these primary magmas, as

well as the assumption that the Moon possesses chondritic Ca/Al/'1'i/Mg ratios,

the silicate composition of the Moon has been estimated ('fable). The results

have the following implications:

(A) In agreement with 1e.g. 1-41, the silicate portion of the Moon is

enriched in FeO by a factor of _. 2 compared to the Earth's upper

mantle at present.

(B) In agreement with [e.g. 2,3,5,61, the Moon is not strongly enriched

in the refractory lithophile elements relative to either chondrites

or the E'arth's upper mantle.

(C) The Mg/Si ratio of the Moon lies outside the range exhibited by chondritic meteorites but is similar to that within the Earth's upper

mantle.

(D) According to the Co vs. (FeO + MgO) systematics [21 in lunar volcanic

glasses, the Co/Mg ratio in the silicate portion of the Moon is identical to that in the Earth's upper mantle at present (" 8x less than

chondritic value).

(E) The abundance of Ni (as NiO) in the Moon's silicate-portion is - 250

ppm. This is a factor of 8 lass than in the Earth's upper mantle at

present and 60x less than in li-group chondrites. In agreement with

17,81, this abundance of Ni requires the presence of a metallic core

within the Moon. The distinctly non-chondritic Fe/Ni ratio (i.e. 650

vs. chondritic value of 18) suggests that the Moon's metallic core is

Ni-rich. This fact, in combination with the high Fe0-abundance in the

lunar silicate (Table), may present formidable obstacles for the in

situ depletions of P and IV required by the 'binary planet' hypothesis

re.g. 91. For a 'modified Earth-fission' h ypothesis [2] to be consistent, the mantles of the Larth and Moon must have segregated Fe0-rich

[191 and Ni-rich cores, respectively, subsequent to the fission event_

SILICATE PORTION OF 111F MOON

Si02 41.3 - 42.7 wt.,, FeO 16.3 - 16.7 Ni 250 ppm

TiO 2

0.18- 0.19 MnO 0.23 Co 90 ppm

Al 2 0 3

3.5O- 3.65 MgO 33.6 - 35.0

Cr 2 01 0.33- 0.43 CaO 2.80 2.91

REFERENCES: [1] Morgan et al. (1978). Moon and Planets, 18, p. 465-478; [21

Wanke et al. (1977) Proc. Lunar Planet. Sci. Con 8th, p. 2191-2213; [31

Ringwood. Origin of the Barth and Moon, -1 95 pp., Springer-Verlag, New York,

1979; [4] Taylor. Planetary Science: A Lunar 1 1 erspuctive, 481 pp., lunar and

Planetary Institute, Houston, 1982; 151 Warren and Wasson (1979) Proc. Lunar

Planet Sci.. Conf. loth, p. 2051-2083; 1o1 Binder (1983) LP1 Tech. Rep. 83-02,

p. 17-19; [71 Wanke et al. (1983) Lunar Planet Sci.-XIV, p. 818-819; [8]Ncwsom

(1984) EOS, 69, p. 369-370; 191 Newsom and Drake (1982) Nature, 297, p. 210-

212; [10] McCammon et al. (1983) Proc. Lunar Planet. Sci. Conf. 13th, p. ASO1-

A506.

__ _
131721

16

(IF Al-UNPAN( IN THE LUNAR MANTLE; AN[) IMPLICATIONS FOR THE ORIGIN Of THE

MOON Tammy U-kinson r ' Ho(Lon Newsom, DepartmereL of geology and Institute of Meteoritics, UniversiLy

of New Mexico , tibuquergw Nnw Mexico 81131

Regardless of the orq n cif the moon, metal segregation must have occurred within Lhe moon in order to

account for its low siderophde element abundances relative to the earth or chondriLes (1) Evidence of variability

of sider ophile elements in the moon may provide additional clues r. the origin of the moon Germanium is a

strongly sideropinle element whose bulk distribution coefficient indicates that it is not fractionated during igneous

processes on the moon Thus , variability in absolute (;e abundances in more basalt s and pristine highland rocks,

rather than elemental ratio-. , can be used to infer lunar mantle abundances and processes We have compiled

literature data for Ge abundances in more basalts and pristine highland rocks (Fig 1) For some landing sites,

samples with ) 12 ppb Ge were considered to be extreme outliers and are not included The Apollo 15 samples

are enriched in Ge by a factor of 25 over the Apollo 17, 16, and 17 samples Other siderophde element

variations have been found in the moon For example, variation-, in the (P/Nd) and (Co/MgO-FeO) ratios from

landing site to landing site have been found ( 2) Based on the data on Fig 1, our best estimate of the average

Cie abundance in the silicate portion of the moon is 352 ppb The moon is depleted relative to chondritic

abundances, by a factor of 38,000 normalised to Si Two possible explanations for the observed variations in Ge

abundance in the moon are (a) more metal may have segregated from some regions of the moon than from

others, or (b) Ge -bearing material may have been added later in the evolution of the moon The Ge concentrations in Apollo 1^ 16, and 17 materials could be produced from the Ge concentrations m Apollo 15 materials by

009% metal segregation, assuming D(Ge) of 1283 (3) Palme ( 4) suggested that the target material at the

Apollo 16 and 17 landing sites had different indigenous Ni and Co abundances, and concluded that the Apollo 17

material could be produced by segregation of 003% metal from the Apollo 16 material The segregation of this

metal should cause the Apollo 17 material to be depleted in Ge by a factor of 3 compared to the Apollo 16

material However, the Ge abundances of the Apollo 16 and 17 samples are roughly the same The material

added later in the evolution of the moon could be Willing meteoritic material, remelting of cumulate layers, or

pieces of a cold primordial core which has been ditiplaced into the mantle For example, plagioclase grains in

norite 18235 are enriched in both incompatible elements (Be and Yb) and siderophile elements (Co and Cr) over

plagioclase in anorthosite 15415 (4) Palme et al (S) explain these conflicting data by assuming a mixture of

a highly evolved KREEP liquid with a component enriched in Mg , Cr and s,derophde elements such as Ni and Co

Herbert (6) has shown that relaxation of an inverted density distribution in the mantle, caused by fractional

crystallisation of olivine and pyroxene from the magma ocean, could displace upward a cold primordial core Also

segregation of metal during core formation would displace such a core Ba yed on Ge abundances, the Apollo 15

material could be produced from Lhe Apollo 12, 16, and It materials by addition of 031% of material of

terrestrial mantle composition, or 001% of material of C1 composition Since terrestrial mantle and chondriLic

material have different siderophile. element signature-,, it may be possible to distinguish among these alternatives

for the material out of whim the moon formed The available Ge data confirm some of the previously suggested

element variations in lunar samples The siderophde element signature of a primordial silicate core could provide

an additional clue to the origin of the moon

References (;) Newsom (1984) EOS, Vol 65 No 22 ---- Apollo 17 pr^sirne

(2) Wanke et al (1984) (,PS-XV, 818 (3) Schmitt z ^rpMana locks

et al (1984) 41th Meteoritical `.,ocieLy Meeting , H -9 ^J^IL.- ,— T--r O^• Boson.

(4) Palme (1980) PLPSC 11th, 481 (5) Panne et al (1984) U z Apollo 16

L,PS-XV, 625 (6) Herbert (1980) Pt.SC 110, 2015

Apollo 15

A , 14 1) 1 41 .n_ -- _11-.1A

Apollo 12

r s a u ip

Ge ( ppb)

J
N85 13'72 -J,/3

r 2

17

Is 11 hotphorus Predictably Incompatible in Igneous Processes? C.A. Goodrtch o and S.

Barnes •Institute of Meteoritics, Dept, of Geology, University of New Mexico,

Albuquerque, Nb1 87108, f NASAIJohnson Space Center, Houston, TX 77058

Siderophile element abundances have been central to recent models for core formation to

the Earth and Moon and the origin of the Moon (e.g. 1,2,3). It Is important to identify

siderophile elements whose behavior in igneous processes is predictable, so that primary

mantle abundances can be deduced by subtracting out the effects of igneous processes (2).

W, Mo, and P appear to be well correlated with La or Nd in terrestriAl and lunar samples,

and therefore to have behaved as lithophile incompatibles during igneous processes (1,2,3).

The model of (4) for core formation in the Moon is consistent with initially chondritic

abundances of N and Mo, but requires subchondrftic P, and suggests that P was depleted

due to volatililty. Data compiled by (1) suggests that P and La are correlated for lunar

samples. However, it is possible that the correlation they observe is partly due to mixing of

a primitive and a KREL•P component, rather than incompatibility.

Rare occurrences of P-rich silicates suggest that P is not always incompatible.

Phosphoran olivine (with up to 46 P2O ) occurs in pallasites (5,6), where it might have

formed by reaction between P-bearing in and olivine (7). Olivine grains containing up to

0.7% P220 have recentl y been found in fine-grained mm-sized objects in type 3 ordinary

chondrftel (8), which suggests that P-bearing silicates might have been present in early

solar system materials. Phosphoran olivine and pyroxene occur in silicate inclusions within

iron-carbon alloy from Disko Island (9). These minerals contain up to 2.7 wtA 1 1

2 05 and

show P silicate crystal/liquid distribution coefficients (D ) on the order of 0.5-I.U. Low

fU 2

, rapid cooling, or high bulk P 0 5 coupled with low PSiO2

 content might have led to F'

corhputibility in the Disko rocks, And2 low fO 2 might also have been an important factor in the

case of pallasites. It' low f'O,Z leads to P compatibility, then P-rich silicates might have formed

during core formation, and -hidden reservoirs of P-rich silicates might exist. The apparent

depletion of P in the Moon observed by (4) could be due to the existence of such hidden

reservoirs.

We have begun an experimental study to determine P olivine/liquid distribution

coefficients. Preliminary experiments were conducted at fO =FMQ, with the following starting

composition: SiO2

2

28.43 , TiO 6.83 , Al 2 O 6.62 , FeO 332 53 , MnO 0.39. MgO 10.26 , CeO

9.14, Na O 0.27, K 2O 0.02, ' O5 4.45. ?n isothermal runs (held for 24 arid hours at

1250 arid2 1206°C) average D P w6s slightly high (0.03-0.07). However, in a run held 49

hours at 1081°C the average 1) P was less than 0.003, which Indicates that the equilibrium

tendency is toward very low D 

'

A series of experiments at cooling rates ranging from 20 -

400°C/hr gave the following refults. (1, DP increases with cooling rate. D P as high as 0.43

was observed for crystals formed during quenching from 1198°C to room temperature. (2)

Isothermal annealing at temperatures ranging from 1038-1180°C led to loss of P from olivine.

P gradients (P 2O contents ranging from 0.2-1.3 wtA over a distance of 0.1 mm) were

observed in crystals homogeneous in Fo. (3) The effect of annealing is temperature

dependent, reflecting the dependence of diffusion rate on temperature. Experiments with

lower P2205 contents and at lower fO2 are planned. We have also measured P 2 O 5

contents of

late pyroxenes in several Apollo 11 basalts, which crystallized at fO 2 =1W of lower. Some

pyroxenes that grew in interstitial immiscible liquids rich in FeO and P22O5 show high DP.

However, FeO -rich outer zones of large pyroxene crystals have very iow P 'contents. O 5

This indicates that low fO2 is not sufficient to cause high D P . The r_rystals that grew in

the immiscible liquids mighC have grown very rapidly in closed systems.

Our preliminary results indicate that P can be compatible with olivine during rapid

cooling, but ib not during isothermal crystallization with long growth times, and tends to be

expelled during annealing. It is therefore not likely that P is compatible under any

-widespread igneous conditions, and the incompatible behavior of P in lunar crustal rocks can

be safely assumed. In addition, low fO 2 is insufficient *.- cause P compatibility, so it is

unlikely that P-rich silicates formed during the early evolution of the Earth or Moon. These

results indicate that P is depleted in the Moon as suggested by (4). Our conclusions may not

apply to phosphoran olivine in pallasites or ordinary chondrites, because these minerals

might not have formed by igneous processes.

References: (1) Newsom H.E. and Drake M.J. (1983) GCA 47, 93-100. (2) Drake M.J. (1983)

GCA 47, 1759-1767. (3) Newsom N.E. and Palme H. (1984) EPSL, submitted. (4) Newsom

H.E. (1984) LPS 15, 605-606. (5) Buseck P,R. (1977) GCA 41, 711-.40 (6) Buseck P.R. and

Clark J. (1914) Min. titag., in press. (7) Kracher A. (1983) LPS 14, 405-406. (8) S.I.

Recca, personal communication. (9) Goodrich C.A. (1984) GCA 48, 1115-1126.
VW S85 13723

18

MEGAREGOLITH THICKNESS, HEAT FLOW, AND THE BULK COMPOSITION OF THE MOON

Paul H. Warren l 6 Kaare L. Rasmussen l , 2 , Institute of Geophysics, I UCLA, Los

Angeles, CA 90024, USA, 2 Univ. of Copenhagen, DK-2200, Copenhagen N, Denmark

The Moon's bulk composition is a major constraint on its origin. Bulk

density alone shows that the Moon is depleted in metallic FeNi relative to the

Earth or to chondrites of comparable MgO/FeO (e.g., H chondrites). Depletions

of minor-trace volatile elements are also obvious from geochemical data. The

simplest assumption would be that in terms of nonsiderophile, nonvolatile

elements the Moon is not much different from the silicate portions of the

Earth or chondrites. But heat flow data [1] have been interpreted to imply

that the Moon's U content is abouL 46 ng/g, i.e., about 2x that inferred to be

in the Earth's mantle, and 3x that of H chondrite silicates. It is difficult

to envisage a nebular process that would fractionate refractory lithophiles

like U from major lithophiles like Si. Thus, a disparity in U/Si implies a

disparity in provenance of parental materials, i.e., origin. However,

previous studies have suggested that (a) the two sites where lunar heat flow

was measured are unrepresentative, because their megaregolith 13yers are

unusually thin [2); and (b) heat flow is about 1.4x greater than steady-state

models imply, mainly because roughly half of the Moon's mass is convectionfree (conductively cooled) lithosphere !3; also implicit in 21.

Our models further assess effects of megaregolith on lunar thermal evolution. We confirm that the two measured sites are probably unrepresentative,

with heat flows about 25% higher than regional averages, due to focussing of

heat flow towards regions with thin megaregolith. These effects are much

smaller than originally suggested [2], because we make more realistic assumptions about lateral differences in megaregolith thickness, and about the

difference in thermal conductivity of megaregolith vs. bedrock. Numerous

lines of evidence indicate that the megaregolith is generally 2-3 km thick

under highlands (which cover about 83% of the total lunar surface), and <1 km

thick under maria. In most of our models, megaregolith thickness is assumed

to be roughly 6x greater over highlands than over maria. In previous models

[2] this ratio ranged from 10 to 1000. Based on sparse data for porosity

among lun:,r rock types, and the correlation between thermal conductivity and

porosity [1], we assume that megaregolith conductivity is roughly 20 kiloerg

8- 1 cm -1 K -1 , and bedrock conductivity is roughly 7x greater. In previous

models [2) the ratio of these parameters ranged from 10 to 1000. We also find

that insulation by megaregolith exacerbates the problem 141 of reconciling

modest temperatures inferred for the (present) mantle with a high rate of heat

production; an upper limit of 30 ng/g for the bulk-Moon U content can be

derived from this constraint alone.

One of the two sites (Ap 15) is probably further unrepresentative because

it is in an exceptionally U-rich region of the Moon. It is probably not mere

coincidence that measured heat flow is 501 greater for the Ap 15 site than for

the Ap 17 site [1J. Correcting the Ap 17 measurement for the effect (*25%) of

locally thin megaregolith, our estimate for present global mean heat flow (Q)

is 11 erg s

-1 cm-2 , which is only 0.6x as great as previously estimated [1].

If present heat production is 0.7x present heat flow [2, 31, a Q of 11 erg s-1

cm-2 implies a bulk-Moon II content of 20 ag/g, only 0.43x that previously

suggested [1]. This estimate is equally consistent with derivation of the Moon

by depletions of FeNi aid volatiles from the Earth, or from chondritic

silicates. The Moon's bulk composition is less exotic than generally assumed.

References: [1] Langseth et al. (1976) PLPSC 7, 3143. [2J Conel b Morton

(1975) The Moon 14, 263. [31 Schubert et al. (1980) JGR 85, 2531. [4] Keihm b

Langseth (1977) PLSC 8, 499.

_
N85 13724 J^^

1 1

111F; BULK-MOON "X;O/FEO RATIO: A HIGHLANDS PERSPECTIVE

Paul II. Wirren, 1116t1tUte of Geophysics, UCLA, Los Ange)Qs, CA 90024, USA

The main: observational coustraints on the origin of the Moon involve its

orbital parameters, its thermal history, and its bulk composition. The f irst

two of these are now known to excellent and fair precision, respectively. The

bulk composition is still a tuatter for enlightened speculation, but some

aspects can be constrained well enough to have genetic significance. The

ratio MgO/FeO, mo.e commonly expressed as njK (- molar MgO/(MgO+FeO)), evolves

curing igneous fractionation in ways that are well understood. The simple fact

of the Moon's proximity to the Earth suggests similar provenance of parental

materialb. However, bulk density alone shows that the Moon is depleted in

metallic FeNi relative to the Earth. Depletions of minor-trace volatile

elements are also obvious from geochemical data. The simplest assumption

would be that in teruis of nonsiderophile, nonvolatile species such 36 MgO and

Fro the Moon is compositionally similar to the silicate portion of the Earth.

The V.& ratio of the Earth's upper mantle is about 0.896, and clearly >0.86.

But most estimates of the mmj^ ratio of the bulk Moon are <0.82, implying that

the Moon and the Earth come from fundamentally different batches of material.

Compositional data for nonmare (highlands) samples suggest that the

Moon's m. & ratio is higher than generally estimated. Geochemically

representative highlands soils have n^ ratios of 0.66 (Apollo 16), 0.69 (Luna

20) and 0.73 (ALIIA81OU5). These soils are mixtures of unrelated pristine

nonmare rocks, of which there are at least three groups: Mg-rich rocks,

ferroan anorthusites, and KREEP. Other than Mg-rich rocks, virtually all

pristine rocks have r4K <0.65. 1hus, assuming the mixing process that sampled

Mg-rich materials was random, the average BL& of Mg-rich parent magmas wab

probsbly at least 0.70. More direct evidence can be derived from the Mg-rich

rocks themselves. Nine of them (about 1/4 of the total) have bulk-rock

>0.87. Two ( 15445 "A" and 67435 "PST") contain Fo 92 olivine. Production

melts that crystallized F092 olivine implied shat the pix ratios of source 

regions in lunar mantle were cumn,ensurably high.

Assuming that the parent melts formed by equilibrium ("batch") partial

melting, this constraint can be quantified as:

mg(systam) = (f•Ml+(1-f)•m •A)/(f+(1-f)•A)

where the subscripts are 1 = liquid and x = crystals; f - the degree of

partial melting; and A = XMg+Fe/XLlg+Fe. For any given value of K (defined as 

(Fe/Mg) x / (Fe/Mg) l ), m1;1 and My,, are interdependent. Literature data indicate

that K for olivine is in the range 0.26-0.36, and K for low-Ca pyroxene is

close to 0.30. A is probably never «2; an extreme lower limit would be 1.

During partial melting, as f increases beyond a certain threshold, tt.e

crystalline framework of the rock breaks down, and the buoyant melt tends to

rapidly separate from the residual crystals. This critical f i_ probably

<0.4, and almost certainly <0.5; 0.6 would be an extreme upper limit.

Plugging these sorts of values into the above equation, it can be shown that

' source regions of melts parental to F092 of iv ines must have !gK >0.84, and much

more realistically >0.86. For example, assume K = 0.3, f = 0.5, and A = 2.

Then 2y, (the olivine Fo content) = 0.92 implies E&I

= 0.78, and E&(system)

0.875. If K = 0.3, f = 0.5, but A - 1, m(system) still = 0.851; if K = 0.3, A

=2, but f = 0.6, (system) still = 0.861. The constraint cannot be eased

much by invoking a different K, either.

Assume for the sake of argument that the Fo 9 2 olivines are flukes.

Widespread MK-rich melts evidently had awl >0.70. If during their genesis K =

0.3, f = 0.4, one' A = 2, tl-t system ^. must have been at least 0.839. The

Moon's bulk composition is less exotic than generally assumed.
N8 13725

20

BULK COMPOSI110N OF THE MOON IN THE CONTEXT OF MODELS FOR

CONDEN:iATION IN THE SOLAR NEBULA; Kenneth A. Goettel, Dept. of Geological

Sciences, Brown University, Providence, Rhode Island 02912

The compositions of the terrestrial planets may be governed in part by

variation with heliocentric distance of the composition of condensates in the

solar nebula, with due account taken of the effects of mixing and possible

addition of volatile-rich material. Equilibrium condensation in the nebula

produces material at two temperatures with a den31ty equal to that of the

moon: high-temperature, refractory-rich material or low-temperature,

fully-oxidized material. Either composition is grossly discordant with lunar

geochemical data, althougn extensive devolatilization of the latter

composition could perhaps result in an viable composition. Furthermore, to

account for the composition differences between Earth and moon, nebular

condensation models for the moon presumably require formation of the moon

remote from the Earth, with subsequent capture. Capture models for the

origin of the moon appear implausible because of dynamic difficulties and

particularly because of Lhe oxygen isotope systematics that suggest formation

of the moon was intimately associated with formation of the Earth.

Therefore, double-planet (formation in geocentric orbit) or fission models

for, lunar origin appear more plausible. Recent siderophile trace-element

data (1) suggest that either of these two models may be viablL if segregation

of a small metal core occured within the moon. The focus of the present

paper is not to argue for either of these models, but rather to address an

implication that is common to both.

The FeO content of the moon, about 13%, is substantially higher, than the

present FeO content of the Earth's mantle, about 8%. If' the moon formed by

fission from the Earth's mantlr., teen the conclusion that the Earth's mantle

must have been much richer in FeO at the time of fission appears firm; there

de not appear to be viable mechanisms to fractionate FeO so markedly during

fission or to add FeO to the moon after fission and not add FeO to the Earth

as well. If the moon formed independently in geocentric orbit, then the FeO

contents of the two bodies should be similar, because both would be accreting

from the same 2ource of silicate material and it is very difficult to

envision a process that would so markedly fractionate FeO between Earth and

Moon. Therefore, Earth's mantle at the time of lunar formation probably had

an FeO content quite similar to the present FeO content of the moon. This

conclusion, if valid, has profound implications in two areas: 1) the

differentiation history of the Earth's mantle and core, and 2) the processes

responsible for governing the bulk compositions of the terrestrial planets.

Reduction of the FeO content of the mantle requires growth of the core either

by addition of FeG or by addition of Fe from disproportionation of FeO. If

the Earth initially contained more FeO then Earth should also have higher

abundancfs of other volatiles; this conclusion applies whether the FeO was

produced by reaction of H Z O with Fe in the nebula or produced in the Earth by

addition of volatile-rich material to metal-bearing material. If Earth had

more FeO than previously thought, then the composition differences between

Earth and Mars are less than previously believed. This suggests that

condensation temperature and heliocentric distance may have been less

important in governing planetary compositions and other mechanisms, including

iron/silicate fractionation may have been more important. The implications

of this model for the compositions cf the moon and the other terrestrial

r)lanets will 1.,e discussed.

(1) Newson, H.E. (1984), EOS 65, 369-370.
N85 137;6 ^_Dl 7

21

PFTROlOG1C CONSTRAINTS ON THE ORIGIN Of THE MOON: EVIDENCE FROM APOLLO 14.

J. M. SHERVAIS and L. A. TAYLOR, Department of Geological Sciences, University of

T rnnessEe, Knoxville, TN 37996.

All the thecries for the origin of the Moon ultimately must be able to explain

the Cive,rsity of lunar rock types and their distribution across the lunar surface.

The diversity and distribution of these rocks that are fundamental characteristics

reflecting the Frocosses involved in lunar origin, including accretionary

mechanisms, primordial differentiation, and subsequent melting events.

The Fra Mauro breccias at Apollo 14 contain distinctive suites of mare basalts

and highland crustal rocks that contrast significantly with equivalent rocks from

other Apollo sites. These contrasts, which are outlined below, imply lateral

heterogeneity of tho lunar crust and mantle.on a regional scale. This

heterogeneity may date back to the earliest stages of lunar accretion and

differentiation.

Highlands Highland crust at the Apollo 14 site is represented by clasts of

magnesia q troctolite and anorthosite (01 >Fo75; F'lg >An92; ! En, Di, Sp), alkali

anorthosites (Plg <An87; ± Pig. Aug), ilmenite gabbro/norite (Plg . An84; ± Aug,

Pig, Ilm) and granite (Qtz, Kspar, P1g, ternary Feldspar). These rocks are

unusually rich in alkalis and the incompatible elements. Similar rock-types and

compositions occur at the nearby Apollo 12 site, but not at the other, more distant

Apollo locations. Ferroan anorthosite and magnesian norite are both rare in the

Apollo 14 highland crust, in contrast to Apollo 16, the archtypical highland site.

Mare Basal! i Ten (10) compositionally distinct varieties of mare basalt have been

docurnented at the Apollo 14 site. Seven of these are low-T1, high-Al mare basalts;

others include tow-Al basalts similar to thosf, from Apollo 12 *

and high-A1, very

low-Ti (VLT) glass spheres. High-Ti basalts are not found. The low-Ti basalts a-e

all enriched to alkalis relative to other low-Ti basalts and have compatible trace

element (Co. Ni, Sc, V, Cr) abundances intermediate to normal low-Ti basalts tnd

high-Ti basalts. Incompatible elements vary widely. e.g., La varies from 8x to

100x chondrite. These and other trace element abundances and ratios (e.g., La/Lu,

Lu/Hf, Ti/Srr, Sc/5m) span the range observed between VLT, low-Ti, and high-Ti mare

basalts from other Apollo and Luna sites. These data suggest an interfingerirg of

low-Ti and high-Ti basalt source regions, or alternatively, that these distinct

source regions never developed in the lunar mantle beneath Apollo 14. They alsc

suggest, like the Crustal data, that the mantle under this part of the moon is

enricheC in alkalis.

Discussion Current theories requiring a moon-wide crust of Ferroan Anorthosite are

based largely on samples from Apollo Its where all but a few samples represent the

FAr, suite. However, at the nearside sites, FAt, is either scarce (A-15) or

virtually absent (A-12, A-14, A-17). These sites are dominated by troctolites anc

norites of the Mg-suite and, to lesser extent. alkali . , northosites (A-12, 14 only).

Two possibilities exist: 1) a thick crust of FAN never formed on the western lunar

nearside. Instead. a thin protocrust may have been succeeded directly by later

intrusive suites; or 2) large basin-formin g, impacts such as Procellarum could have

removed much of whatever FAi; crust did exist.

The younger intrusive suites demand a variety of origins. lrcluding 'he

assimilation of KREEP and the diapiric rise and remelting of early, Mg-rich

cumulates. However, none of the processes discussed above accounts for the

LIL-enriched nature of the western lunar nearside. This enrichme'rt is evidenced

by; 1) orbital gamma-ray data showing high K, U, and Th in this region; 2) the

common occurrence of enriched rock types such as alkali anorthosite and granite at

Apollo 12 and 14; 3) the 'enriched' character of "west r'I rocks noted by

Warren et al. (1983); 4) the abundance of KREEP in soils and breccias; and 5) the

'enriched' character of Apollo 14 aluminous mare basalts. Wasson and Warren (1980)

have addressed asymetrical fractionation in the magma ocean as a possible

explanation.

We suggest another possibility: the accretion of a large mass of material

(e.g., 0.1-0.2 moon masses) late In the crystallization history of the magma ocean.

Besides adding fresh, primordial material, this would remelt a large pocket of

crust and mantle, thereby allowing a second distillation to occur in the resulting

"magma sea". This late accretion of large bodies of primary composition would have

contributed significantly to large-scale heterogeneity in the lunar crust and

mantle. As emphasized by Palme et al. (1984), the chemistry of the highlands

crust virtually requires a large meteorite component. Therefore, it would appea.

that meteorites may have played a much bigger part in crustal evolution than simply

metamorphosing the rocks and creating the lunar landscape. The large variety of

rocks with no apparent common parentage demands an unusual set of circumstances -

i.e., different processes. not ,Just variations on a theme.

-- J
^ 8 3 '7 2 '7

vak N85 1

22

ORIGIN OF THE MOON: CONSTRAINTS FROM VOLATILE ELEMENTS*. Melanie

E. Kreutzherger**, Michael J. Drake, John Vi. Jones, Lunar and Planetary Laboratory, University of Arizona, Tucson, AL 85721. **Present address: University of Michigan.

Fission of the Moon from the Earth following core formation is a hypothesis for the origin of the Moon (Ringwood, 1979). Supporting arguments

include the low density of the Moon corresponding to the density of the

Earth's mantle and the low volatile content of the lunar rocks vs. those of

terrestrial origin. Vapor pressures of the alkali elements and their oxides

increase in the following order: Na, K, Rb and Cs. The Moon should, therefore, be more depleted in Cs relative to Rb, Rb relative to K, and K relative

to Na than the Earth if the fission model is correct. Analyses of lunar mare

basalts and terrestrial mid-ocean ridge and other young basalts indicate that

this behavior is not observed; for example, the Moon shows a higher Cs/Rb

ratio than the Earth--i.e., it lies between chondrites and the Earth.

It i.s possible that monovalent alkali elements might be lost from silicate materi.als in a differei.t order than that inferred from elemental and

oxide vapor pressures, as a result of differences in the way they are bound in

silicate materials. To test this hypothesis we conducted a ser!es of experiments to investigate alkali loss at high temperatures. A synthetic basalt of

composition D1 75An 25 containing approximately one wt.% each of Na, K, Rb, and

Cs was prepared. The mixture was ground under acetone, fused twice at 14000C,

and quenched to form a homogeneous glass. The glass was used as our starting

material. Experiments were conducted using the wire loop technique at 1

atmosphere and 1400 0C and 10500C in air, allowing 1 , nlatile loss as a function

of time to be determined. The resulting glass beads were then analyzed using

the scanning electrcn microprobe.

Analyses indicate that the behavior of volatiles dissolved in a silicate

malt is similar to that inferred from elemental and oxide vapor pressures. At

14000C (superltquidus), Cs is most readily volattIt zed, followed by Rb, K, end

Na (see figure). The mass of the glass beads is not a major factor in the

rate of volatile loss. At 1050 0C, no significant loss of volatiles was detected after 1 week.

These results are consistent with previous experiments by Gibson and

Hubbard (1972) on 12022. We conclude that alkali element rattcs in the Earth

and Moon are not readily interpreted i.n terms of the Fission hypothesis.

*Research conducted to conjunction with Lhe Tucson Lunar

Origin Consortium.

REFERENCES: GIBSON E.K. AND

HUBBARD N.J. ( 1972) Proc. Lunar Sci Conf. 3rd, 2003-2014.

RINGWOOD A.E. (1979) Ortgi.n of o

d

the Earth and loon. Springer- Cr

Verlag.

1400° C

00

o }

a

{

10 Symbols

• No 0

• K •

Rb

Cs

I

0.1 1 10 100

Time (hours)
N85 13 28

VOLATILE ELLIAENTS IN AND ON LUNAR VOLCANIC GLASSES:

'THAT DO T7 1EY TELL US ABOUT LUNAR GENESIS ? C. Koeberl,

Institute for- Analytical Chemistry, University of Vs-enna, P.O.

Bux 73, A-100 44 `Jien%a, Austria.

Lunar volcanic Masses exist in many different varieties(1,2)

(at the moment about '3 have been identified), and there are good

reasons to believe thHt they have originated from a deep lunar

interior source, giving the opportunity to probe the composition

of these regions. One of the most important observations, namely

the presence of surface correlated elements (e.g. 3-7), has caused

some interest because of the implications on the chemistry of the

deep lunar interior.?ost of these elements (like FZNa,S,C1,Cu,7n,

lia,ve,Br,Cd,Tn,Sb,I,'.L1,Pb) are clearly volatile. Since the lunar

interior has been thought to be depleted in volatile elements

when ^^Y:.ieii to the earth (8), the detection of a possible source *I volatiles is of importance. Models of the origin of the

(noon have been develo_,ed,hased on the assumption that the moon is

depleted in volatiles. Although th, - s Ftrems to pose some problems

to the fission theory (deriving the moon from the earth's mantle).

a fission model involving devolatilization has been discussed (9).

Recently the surface correlated elements have been interpreted

to indicate that the moon has some reservoirs that are enriched

in volatil--^s, and that the deep ll.0^.ar interior is not depleted in

volatile elements.Delaro and Livi (1) suggest three different

scenarios for the origin of these elements:early differentiates

from a magma ocean, a deer undifferentiated source (heterogenous

accretion of the moor_?), or cosmic xenoliths with random distribution in the lunar interior. Of course, a lunar interior which

is not depleted in volatiles bas important iml:licattons on the

origin of the moon.3o it woul ,! be cf interest to know something

about the amount of volatiles present in the 'Lunar interior.

Since most of these elements are abundant only in the form of

very thin surface layers on the glasses, and since the ;lasses

themselves do not :nor±r a similar enrichmf-nt, the source should be

of r.atner limited extent. Also the driving mechanism of lunar

volcanoes must have been quite different from the terrestrial

mechanism, making the lunar eru p tions rather'bubbling' than really 'explosive fountaining'. This is implied alFo by the absence

of large burbles including volatiles (and the driving gas, like

on earth). The total absence of water or the moon (in all samples,

including volcanic glasses) precludes the presence of water also

very probably in the lunar interior. However, water is a very

vigorous 1riving gas for volcanoes on earth.None of the other volatile compounds which may have formed (10),is equally capable of

launching melts with great en.ermies.The rather limited amounts

of these compounds (halides etc.) have condensed fast on the

quenched glasses. From the condensation behaviour of the volatile

compounds (10),which leads to heterogenous condensation,we may

conclude that comr,aring element ratios of surface correlated elements gives little sense.Thus it seems as if the "volatile reservoirs" are of rather limited extent,and that they do not enlarge

the volatile content of the bulk moon significantly.

References: (1) J.M. Delano and K. Livi, GCA 45 (1981) 2137-7149. (2) J.W. Delano, LPS XV (1984) 216-217. (3) G. Heiken, D.S. McKay,

anFR.W. Trown, GCA 38 ( 1974) 1703-1718. ( 4) C. Meyer, D . S. McKay, D.H. Anderson, and P. Butler, PLSC 6 ( 1975) 1673 - 1699. (5) E.N.

cirlin and I.M. HousTey, PLPSC 10 (1979) 341-354. (6) S. Jovanovic and G.W. Reed, PLSC 5 (1974) 1685-1701. (7) R.N. Goldberg, D.S.

Burnett, and T.A. Tombrello, PLSC 6 (1975) 2189-2200. (8) R. Ganapathy and E. Anders, PLSC 5 (1974) 1181-1206. (9) A.B. Binder,

EPSL 41 (1978) 381-385. (10) C. Koeberl, M. Klesl, F. Kluger, and H H. Melnke, J. Mon-cryt Solids (1984) in press.

^y

23

-0;* .,Ar -- _
^,6 w N 8 5 13'7 2 9

24

THE I-PU-XE A;E OF THE MOON. T.D. Swindle, M.W. Caffee and C.M. llohenberg, McDonnell Centet for Space Sciences, Washington U., St. Louis, MO 63130.

Rb-Sr analyses of some lunar samples indicate that the Moon is close to the

age of primitive meteorites (11, but are only reliable to within about 1A0

m.y. A potentially more precise chronometer is the I-Pu-Xe system. 1111 has

a 17 m.y. halflife and decays to t3 'Xe; 144 Pu, with an 82 m.y. halflife, produces 111 Xe- 116 Xe in fission. The 110 I/ 144 Pu ratio has a halflife of 21 m.y.

The egjSrimenl&I obsee y p n (+as-rich lunar highland breccias contain xenon that is surface-correlated and has an isotopic composition matching that

of 1 " Pu fission, with variable contributions from 110I [2, 31. The exact ^-

method of incorporation of this surficial component is not known, but the gas

probably comes from degassing of the bulk Moon (rather than some smaller

reservoir), since the presence of this component is a global lunar phenomenon

[2]. The "Ole/116Xe ratio of the gus incorporated into a given sample seems

to bz time-ordered. This is proportional to the relative rates of decay of

119I and 144 Pu if production, release and re-incorporation are closely coupled

[3]. From this, we cau calculate the 129 I/ 244 Pu ratio at the time of incorporation, which, in principle, allows us to calculate the time of incorporation onto the host surfaces. The ratios 1191/1171 and 1 " Pu/ 1 / s U at the time

of formation of primitive meteorites are fairly well known [4]. The I/li ratio

of the Moon, however, is poorly constrained, since iodine is difficult to

measure, making this the most uncertain part of the procedure. S. R. Taylor

estimates the present-day I/1) of the Moon to be .01 or less (personal communication). Using this valve, we get a xenon incorporation time of about 45 m.y.

after xenon closure in the most primitive meteorites. If we assume an uncertainty of a factor of two in the lunar I/U ratio, the corresponding uncertainty in the age of the host grain surfaces is about 20 m.y.

The imp licatins: Various authors have calculated the xenon retention time

for the Earth to be 50-100 m.y. (after rrimitive meteorites) on the basis of

the isotopic composition of the Earth's atmosphere [5] or of certain mantle

samples [6]. The formation time we have computed for the Moon is near the

older edge of this range. These calculations are consistent with xenon retention for the Moon beginning anywhere from several times 10 7 years before it

began on Earth to about 10 7 years after.

How do these conclusions constrain models for the Moon's origin? The model

that has the most rigorous chronological constraints is the earth-fission

model (and its variants). Xenon retention for the Earth could have begun as

late as the event that gave birth to the Moon. For the Moon, it is hard to

imagine that xenon retention could have begun before re-accretion of the fissioned (and initially dispersed?) material, particularly if that material got

hot enough to account for the depletion of the volatile elements [7]. Thus,

if fission models are correct, xenon retention in the Earth certainly began no

later than in the Moon, and possibly began earlier. Therefore, the I-Pu-Xe

system is only marginally consistent with a fission origin. If further study

confirms that the I/U ratio of the Moon is .01 or less, or if gas-rich lunar

highland breccias with higher ratios of 120 1 to 3 "Pu are found, it would be

difficult to explain the results in an earth-fission model of lunar origin.

References: [1] Albee et al. (1974) LSC V, 3; Papanastassiou and Wasserburg

(1976) Pr. LSC 7th, 2035. [2] Bernatowicz et al. (1978) Pr. LPSC 9th, 1571.

[3] Swindle et al. (1984) Pr. LPSC 15th, submitted. [4] Hohenberg and Kennedy

(1981) GCA 45, 251; Hudson et al. (1982) LPSC XIII. , 346. [5] Pepin and Phinney

(1976) LSC VII, 682; Wetber.ill (1975) An.Rev.Nuc.Sci. 25, 283. [6] Staudacher

and Allegre (1982) EPSL 60, 399. [7] Ringwood and Kesson (1977) Moon 16, 425.
-^D?

. N85 13'730

25

TESTS OF THE LUNAR FISSION HYPOTHESIS; S.R. Taylor, R.S.E.S.

Australian National University, Canberra.

The progress of science upends on the construction .f testable nypotneses. Following the Apollo

sam p le return, the concept that the moon was r1331oned .'r^m the Earth after .ore separation has become

the most readily testable hypotheses of lunar origin, since direct compar130n3 of lunar ana

terrestrial compositions can be made. The following differences, in order )f decreasing certainty,

nave been established:

'a) The moon is depleted in volatile elements relative to the Earth,whi:h In turn is depleted relative

to primitive solar nebula abundances. Depletion factors range from 2-4 for RbiSr and K/U to

over 5G for the most volatile elements (e.g. B1,T1)(1).

(b) The FeO content[2; of the bull( moon 1.13%) is nigher than that of the terrestrial mantle (8f1

may ;Ing the hulk lunar 4g/Mg-Fe atomic ratio • 0.31. When •2S Fe 13 removed by a lunar core the

Mg/M%-Fe value of the lunar mantle (0.84) IS still lower than that of the terrestrial primitive

mantle plus crust value of 0.89.

(e) The silicate portion of the moon is depleted in sideropnile elements relative both to the Earth

and to solar nebula abundancea[3). The presence of a small core ( . 2t of lunar volume) accounts

for some of this depletion if the anon was fully molten following accretion. Removal of Ni and Co

by olivine and ort-iopyroxene crystallisation In the loop interior can alto lepiete the source

region of the mare ba3alt3 in these elements.

(d) Two observations indicate that the moon is enriched in refractory elements relative to the Earth

and solar nebula abundances. Heat flow values indicate 1 ; abundances of at least 33 ppb[4). The

Al content of the 60-100 em tnicK highland crust together with the additional budget fcr the more

basalt source regions requires bulk lunar A1,0, contents of about 6t consistent with the eo3ric

Al/U ratio for those refractory elements[5]. These compositional differences are collectively

sufficient to rule out simple versions the fission hypothesis. More complex versions must

incorporate the following processes:

(i) Depletion of volatile elements. This process 13 limited by the absence of such effects from

the volatile REE elements, Eu and Yb. The alkalies are lepleted leas than the very volatile

elements, although volatility effects should produce constant depletion factors[1;,

(11) Fission *.ust occur wnen the Mg/Mg-Fe ratio 0f the terrestrial mantle 13 0.80, 109 lower than

the Dresent value[6;.

111) Enrichment or selact:ve condensation of refractory elements.

These problems introduce so :many ad hoc adjustments to the fission hypothesis that It becomes

unto3table.

Further constraints may bo obtained from attempting to late the volatile-refractory element

fractionation. Meteorites recorl retal-suifide-silicate and volatlle-refractory element fractionation

close to 4.56 Ae. Mechanisms for large scale separations, required )n a planetary scale, of volatile

elements occur via str-ng solar .Inds In the T Tauri pnase during the very early stages of solar

°.ebula evolution, witnln aoout 1 0' years of the arrival ^f the sun on the main sequence;";. Meteorite

nvilence records early 1033 of 935[3; so that a population of fractionated planetesimals :s left In

the inner solar syetem. Sweep-up of these Into planetary oodles takes of the order of 1 0 • year3[9].

Two methods for :sting the :unar volatile depletion may to lerived from the volatile-refractory

pairs Rb-Sr and :-P-j. Lunar initial ratios are clone to or lower than 3AEI, implying very early

separation )f Rb 'r pm .:r= 1 C. 1 1 1 . If the terrestrial initial ;r ratio 13 also :lose to 3ABI, tnen

fission :oust occur :Lose to -A .56 As ratner than 1 0' years later :terrestrial PbiSr mar:o • 0.1-31).

'he moon has acquired excess Xe from 1 7 m.y. " • I and 82 m.y. "Pu[12). if the moon fl33l0ned from

the Earth following Accretion over 10' years, then 'ne volatllizatlon required to remove K, 3b, Pb,

31,71- 9 ,C', etc., will .remove Xe, and so Xe ler:Ved fran ,r P'1 should be rare in the moon. If the

moor. aocreted from a separate group of fractionated planetesimals, which underwent metal and volatile

element, :ractionatlon at To, then 1 °'I and "`Pu can ne ;resent in the precursor planetesimals. The

^minatlon or chemical and isotopic problems suggests tnat the fission hypothesis Is no Ionger viable,

and separate ^errestrlal and lunar accretion from a populati^n of fractionated precursor planetesimals

provl•les a more reasonable explanation.

References

Waif, R. and Anders, E. (19e0) GCA, 44, 211'.

:23 Tavlor, S.R. 0982) Planetary Science A Lunar Perspective LP'..

33 p rase, M.J. (1983) JCA, 4, 1 "51-

+] Ke1nm, S.J. and Lang3eth, M.G. 11977) PLC 9, ,99.

[5] Taylor, S.R. and Bence, A.E. (1975) PLC 6, 1121.

[6] WAnK-, H. it al, (1983) LPS XIV, 818.

[7) Imhoff, C.L. (1979) in ProtO3tars and Planets (Ed. T. Genre13, 'Jniv.Ariz.Press) 699.

L 8] Wetherill, G.W. ('980) Ann.Rev.A3t.A3tr ,)phy3. 13, 77.

[9] ':0swami, J.1, and :.al, ). (1979) Icarus, 4 0, 510.

[10] Papana3ta33iou, D.A. and 'easserburg, G.J. (1976) LPS VII, 665; ly.ui3t, L.E. (1977) PCE, 10, 103.

[1'; Taylor, S.R. (1984) Meteoritics, 1 9, 405.

0123 Swindle, T.D. et a:. (1984) Proc.15th LPSC. JGR (in press).

^e "•rI+ J
DYNAMICAL CONSTRAINTS

Monday, October 15, 1984

1:00 - 3:15 p.m.

"P IT.F.DT,"'G I Xri±, I3t;. NX IMIT

_,,
t. N85 13731

29

CONSTRAINTS ON LUNAR ORIGIN: EVIDENCE PRESERVED IN PRECAMBRIAN

STROMATOLITES: J.P. Vanyo, Department of Mechanical and Environmental

Engineering, University of California, Santa Barbara, CA 93106

In that growth rate and structure of many living organisms are directly

influenced by solar radiation and often by conditions in an aquatic environment, these organisms should be expected to respond to various earth-moonsun relationships. Tile bio-geological period of major interest for theories

of lunar origin is prior to the beginning of the Cambrian some 0.57 billion

years ago, but in the Precambrian, life forms were restricted to subaqueous

unicellular algae and bacteria. Fossils of such organisms are abundant in

the geological record. Their existence is also evidenced by abundant fossilized structures (stromatolites) consisting of many thin layers of usually

darker algae-bacterial growth alternating with layers of usually lighter

sediment-precipitate. The earliest of these have been dated to 3.5 billion

years ago, spanning most of the earth's history and predating many postulated

lunar origin times. Although stromatolites are forming in isolated locations

today, their preservation dropped off sharply with the advent of burrowing

and grazing worms in the Cambrian. In shape the laminated structures vary

from nominally horizontal mats, e):tending over many square kilometers, to

large and small bumps, and to a variety of columnar shapes.

A form identified as Anabaria juvensis and characterized by packed

1 to 2 cm. dia. columns many centimeters tall „ith convex upward laminae

(dark plus light layer) about 0.2 mm thick were analyzed by Vanyo and Awramikl

A sinusoidal columnar growth pattern was interpreted to be a response of

3tromatolite forming microbes to the changing inclination of the sun over the

Geasons,with microbe growth rate N

positively related to solar intensity. 45' ,I

A best estimate of laminae per Q

ern sinusoidal wavelength agrees with North

• UMMer

an estimate of days per year (435) at {},

the same age (0.85 billion y ors) by

{Y,

Munk,hased on theoretical extrapo- vertical at

lstions from present data. Maximum o' io+nuae

tilt of the average column after correction for index of refraction yields."

an obliquity of the ecliptic of 26.5°. Northern I

Both estimates are based on one small "1n 1•r

sample and require further refinement. Model necessities that the sinusoidal

plane be aligned with the paleo north-south direction and for growth at a

near equatorial location have been confirmed with paleomagnetic analyses.

Vanyo and Awramik using additional specimens and with NSF support are

developing a systematic methodology for extracting data evidencing earthmoon-sun dynamics at time of stromatolite formation. In particular stromatol.tes span the time from 1 to 2 billion years age, critical for several

theories of lunar formation and/or an earth-moon near encounter. Such cataclysmic events would most certainly influence stromatolite formation.

1. Vanyo J.P. and Awramik S.M. (1982) Geophys. Res. Lett. 9:1125-1128

PT17-ri"DINTC, r,k(:r rrr::A;^'K NOT FTT

2 ZJ
N85 1 C j7JrG

W

CONSTRAINTS ON THE ORIGIN OF VISCOELASTIC BODIES: W. E. VanArsdale,

Department of Mechanical Engineering, University of Houston-University Park,

Houston, TX 77004

A possible explanation for the time scale of lunar orbital evolution has

eluded investigators until recently. Lambeck (1980, p. 288) asserts that

"...only a variable energy sink can solve the time-scale problem and the only

energy sink that can vary significantly with time is the ocean." He suggests

that the oceans are a significant energy sink accounting for more than 85% of

all dissipation (Lambeck, 1975). While the oceans undoubtly contribute to

energy dissipation, it seems implausible that such a small amount (0.02%) of

the Earth's mass could play such a dominant role in this process. An alternate hypothesis assumes that most of the dissipation is associated with solidbody tides. This mechanism is also capable of varying significantly over time

for viscoelastic bodies.

In this analvsis of orbital. evolution, the bodies are modeled as incompressible, Kelvin-Voight solids. While this material is simplistic, it does

provide an analytical characterization of solid-body dissipation and the resulting tidal moment. This moment is calculated assuming an elliptic orbit

and then used in approximate equations of motion. A solution to these equations is obtained for slowly varying orbital parameters. These results suggest that the orbital parameters are currently changing at a comparitively

large rate. This rate was smaller in the past if the Earth's viscosity n

changed significantly with time (VanArsdale, 1984).

The orbital history of a two body system is traced back in time to develop constraints on the satellite's origin. These ,alculations were performed

using parameters for the Earth-Moon system and neglecting the tidal moment

exerted by the Earth on the Moon. This analysis predicts a decrease in the

lunar orbit to a minimum separation distance of ten Earth radii in a period of

time controlled by rl . The Earth's obliquity drops to r/2 during this close

approach. These constraints suggest the Moon appeared in a significantly inclined orbit at a distance greater than ten Earth radii. This distance is

Outside of thc• Roche 1?mit and would appear to alleviate some difficulties

associated with capture hypotheses discussed by Kaula and Harris (1975). However, the small eccentricity at close approach is not favored by this hypothesis. As suggested by Goldreic• h (1966), this constraint suggests an origin by

accretion.

Goldretch, P. (1966) Rev. Geophys. 4, 411-439.

Kaula, W. M. and Harris, A. W. (1975) Rev. Geophys. Space Phys., 13, 363-371.

Lambeck, K. (1975) .1GR 80 (20), 2917-2925.

Larnbeck, K. (1980) The Earth's Variable Rotation: Geophysical Causes and Consequences, Cambridge University Press, New York_.

VanArsdale, W. E. (1984) "Orbital Dvnamics of a Viscoelastic Body" (submitted

to JGR).
^ 85 1333 3"^

31

'iIr =.L t!IS:'.IE'AT'IGN IN THE F.AbTH AND MOON FROM LUNAR LA:-"Ek

4ANGING; E. Yoci-, r, J. 6. Williams, J. U. LickPv, r IJewhall.,

liL Eropulsi;---n Lab-ratorv, California Institute of Technology,

Fabadena , CA '411(.' '

The evolution of the moon s orbit is mainl y g overned b y tidal

dissipation in the earth while the evolution of its spin i.s

controlled b y its own internal dissipation. The analysis of

lunar lases r,ng inq data from Aug ust 1969 through Ma y 1562 yield=

QLe present values cif both of these parameters The lunar secular acceleratign is determined to be n = -25.3 + 1.2 ar

=econds;centurv_`, wtiich is Equivalent to a secular change in the

mean distance of +3.7 + 0.2 cm/yr. If the moon has orbited the

e-irth since it: f , .:rmation, this must be an anomalousl y hi g h value

r-resumabl y due t_ changes in dissipation in the oceans glue to

c."ritinent.al drift. The rotational dissipation of the moon is

measured by k.,T = u.0047 + 0.00! da y s. With a lunar k,- _ 0.032

0.007, this - dissipation would imply Q = 30, but we piefer the

explanation that the dissi pation c,ccurs at the interface between

the mantle and a liquid sere or shell.
-^^ N 8 ^- rl 

3 4

32

TWO LUNAR GLOBAL ASYMMETRIES; J. B. Hartung, Solar System

Associates, 9108 Potomac Ridge Road, Great Falls, Virginia 22066

The Moon's center of mass is displaced from its center of figure about

2 km in a roughly earthward direction(1). Most maria are on the side of the

Moon which faces the Earth(2).

It has been argued that because the center of mass is so displaced and the

crust of the Moon has a lower density than underlying material, the lunar crust

must be thinner on the Moon's earth-facing side(1). The observed distribution

of lunar maria is said to support this conclusion because it may be expected

that mare basalt magma would reach the surface through a thinner crust more

easily. A problem for this model is to explain how the lunar crust thickness

asymmetry developed in the first. place.

We suggest an alternative view and assume the Moon was initially

spherically symmetric, or nearly so. The emplacement of mare basalts repre,ents

a transfer of mass which produces most of the observed center-of-mass displacement toward the Earth. As a worst case, if all of the center-of-mass displacement is due to basalt emplacement, then a mass equal to all of the mare basalts

on the Moon(3) would have had to have been transferred a distance equal to the

diameter of the Moon. Such a requirement might be satisfied, given the

uncertainties involved, if mare basalt magmas were in "communication" over

distances of thousands of km. The remaining question in this case is what

causes the asymmetric distribution of lunar maria.

Because the Moon is in a spin-orbit-coupled relationship with the Earth,

the effect of the Earth's gravity on the Moon is 3symmetric(4). The earthfacing side of the Moon is a gravitationally favored location for the extrusion

of mare basalt magma in the same way tha n. the topographically lower floor of a

large impact basin is a gavitationallj favored location. This asymmetric effect

increases inversely with the fourth power of the Earth-Moon distance. If the

iMoon were one-tenth its present distance from the Earth, the front-back asymmetric effect of the Earth's gravity would be equivalent to an elevation

difference on the Moon of about 3 km(4).

A scenario for history of the Earth-Moon system consistent with the above

discussion includes: formation of the Moon by accretion processes in a heliocentric orbit near that of the Earth; a gravitational encounter with the Earth

about 4 billion years ago resulting in captire of the Moon into a geocentric

orbit and heating of the Moon through dissipation of energy related to tides

raised during close approaches to the Earth(5) to produce mare basalt magma;

and evolution of the Moon's orbit to its present position, slowly at first to

accomodate more than 500 million years during which magmas were extruded.

References

1. Kaula W.M., Schubert G., Lingenfelter R.E., Sjogren W.L., and Wollenhaupt

W.R. (1972) Proc. Lunar Sci. Conf. 3rd, p. 2189-2204.

2. Lipskiy Y.N. -TFT6T Tn- Tf—ie74 one eeorites and Comets (B.M. Middlehurst

and G.P. Kuiper, eds.), P . Univ. icago ress, Chicago.

3. Head J.W. (1975) In Papers presented to the Conference on the Origins of

Mare B asa lts and Their p ica ion Tm for unarvo uT-ion, p. 6'1-69.

TFieTunar ^cience institute, -Flousfon.

4. Hartung J.B. (!976) Proc. Lunar Sci. Conf, 7th, p. 3097-3112.

5. Singer S.F. ( 1968) Geopys. , T5, ^^-^'f^.
N $ 5 13 '7 3 5

33

' f111•: MOON'S ORBIT HISTORY AND INI. h:Rl-:Nt :ES ON ITS ORIGIN.

II.A.Conway, Dept. of Aeronautical and Astronautical Engineering, Unive[SiLy of

Illinois, Urbana, it 61801.

A frequency-dependent model of tidal fri(Lion is used in the determinaLion

of the evolution of the Earth-Moon system. The anal / sis considers the lunar

orbit eccentricity and inclination, the solar tide or. the Earth, Earth oblateness, and higher-order terms in the tidal potential. A Solution of the

equations governing Lhe precession of the E'arth'.. rutaLiunal angular mouent.um

and the lunar ascending node is found. TW s allows the analyr_ ical averaging

of the variational equations over the period of relative precession which,

though large, is necessarily small in comparison to the time step of the

numerical integrator which yields the system evolution over geologic Lima.

The resulting history is consistent with a capture origin for the Moonl.

It appears to rule out, in agreement with a previous result , origin of the

Moon by fission. Results are shown for a range of assumed values for the

lunar tidal dissipation (that is, for various k of the Moon) alLhough they

remain qualitaLively the same. ' tidal dissipation within the Moon, during what

would be the immediate-post-capture period, is shown to bo capable of signif'icantly heating the Moon. This result is consistent with large-scale malting

of the lunar surface dated at about 4.45 x 109 yr3.

periapsis within the Earth's Roche

rth's gravitational field as this

a mechanism to prevent so close,

show that the probability of such

otter alternative hypotheses for

The immediate-post-capture orbit has a

l in) iL. Capture into resonance with the Ea

orbit tidally evolves has been suggetLed as

and perhaps destructive, an approach . We

capture is negligibly small, P .002, and

the survival of the Roche I ► mr^ passage.

I. Conway, B.A., Icarus 51, 610-6:.'_' (19132).

2. Goldreich, P., Rev.Geuphy. 4, 411-439 (1966).

3. Taylor, S.R., Nature _310, 98-99 (1984).

4. Ativen, H. and Arrheni us, G., Science li;5, 11-17 (1969).

J
a^

"*N85 137 366

i.,

A REAPPRAISAL OF DARWIN'S FISSION HYPOTHESIS AND A POSSIBLE LIMIT TO THE

PRIMORDIAL ANGULAR MOMENTUM OF THE EARTH; William B. McKinnon and S.W. Mueller,

Dept. Earth and Planetary Sci, and McDonnell Center for the Space Sciences,

Washington University, Saint Louis, MO 63130.

A century ago G.H. Darwin proposed that the primordial earth may have rotated

fast enough that the solar tidal period was nearly resonant with the fundamental free

oscillation period of a fluid earth. A large and unstable tidal oscillation would then

have split off to become the moon. Jeffreys developed an argument that dissipation

during resonance would be sufficient to prevent such an unstable oscillation (1), however, and the hypothesis was laid to rest.

If the earth and moon are recombined so as to conserve angular momentum, and the

resulting abject is modeled as a Maclaurin spheroid with a density equal to the earth's

present density o, then its spin frequency w will be 0.303 3rGp (period = 5.36 hr.).

So l ar tidal forcing will be at twice_ this frequency. Lord Kelvin showed that the fundamental (;=2) free oscillation -Tor a fluid sphere occurs at (16/15)• 3nGp. This oscillation is composed of 22+1 - 5 modes that are degenerate when W = 0 only. The sectoral ("football") modes (m=!2), relevant to tidal resonance, are split such that the frequency of the "backward" propanating tide (m=-2) is significantly decreased (as seen in

the inertial frame). This frequency, based on the calculations of Chandrasekhar (2),

is 0.654 3nGp (period = 2.48 hr.) for the rapidly rotating proto-earth, or only 8%

greater than the tidal frequency (period = 2.68 hr.). Considering that solar tides

have extracted angular momentum from the earth-moon system over 4.5 b.y. (a few percent

of the present total '3)), the correspondence of the primordial tidal and resonant

frequencies is nearly exact. (The effect of central condensation of the proto-earth

is to increase both frequencies by a similar, though for the resonance not precisely

known, amount ( 1 0 - 201,J). This result, unknown to Darwin or Jeffreys, is curious.

to say the least.

To evaluate the effects of resonance, consider the problem as a driven, damped

oscillator, governed by

x + .(A) x + , , 0 '(w)x = F cos(w [tjt)

where Y, w, and F have their usual meanings, and A is the amplitude of the tidal

dis p lacement x. Classical results apply, notably that, for large Q (=u, /Y) and equilibrium conditions, A _ = QA . .. ,. . If A today 1 m, and Ares needs to exceed 106 m

for resonant f i ssion, then Q must excee d 106 . This seems dubious even for an accretionary earth with very low viscosity (10' - 10 11 P) due to supersolidus temperatures

(4). Q may be estimated from GM/2w,,Rv, where M and R are the earth's mass and radius,

and v is the kinematic viscosity derived from mixing length theory (5,6). For very

large A, , might approach R-,,,, so the resonance is likely to be too damped for fission.

This argument is more general than Jeffreys' (1), who considered friction between the

oscillating mantle and a rigid core. An even more robust argument follows from the

fact that Q must be so great for fission that equilibrium is not maintained; the

fluid proto-earth passes so quickly through resonance that maximum amplitude is not

reached (tit for resonance at Q''; the system r a Q-1).

If resonant "ission is not possible, what is the significance of the commensurate

periods above? Numerical coincidence? We suggest not; perhaps solar resonant tides

acted as a brake on the spin of the primordial partially molten earth. Certain proposed origins for the moon (binary accretion, capture) do not necessarily involve

addition of substantial amounts of angular momentum to the earth-moon system. The primordial earth-moon system may have had nearly the same angular momentum as it has today.

References: (1) Jeffreys, H. (1930) Mon. Not. R. Astron. Sot. 91, 169-173; (2) Chandrasekhar, S. (1969) Ellipsoidal Figures , Yale Unix.. Press (New Haven),

Ch. 5; (3) Kaula, W.M. and Harris, A.W. Rev_. Geo^^ys._Space Ph s. 13 (1975) 363-371;

(4) Cooperman, S.A. (1983) Geophys. Res. Lett. 10, 925-928; ( Hubbard, W.B. (1974)

Icarus 23, 42-50; (6) Goldreich, P. and Nicholson, P.D. (1977) Icarus 30, 301-304.
N 8 5 13 '7 3 '7

35

NUMERICAL SIMULATIONS OF FISSION; R.H. Dt,risen, Indiana Un;^.,

R.A. O'ngold, Mt. Stromlo and Sid i ng Spring Obs., and E.H. Scott, National

Sprc.: science Data Ctr.

Up until recently, the fission theory of lunar origin had to rely on

classic results for the equilibrium and stability of self-gravitating,

incompressible fluids (1). Because this work was mostly anal y tic, conditions

for the onset of dynamic instability could be determined but not the outcome. Over the past seven years, several research groups have applied

modern hydrodynamic simulation techniques to the fission problem. The most

comprehensive study to date (2) indicates that, for fluids with the compressibility of stars, dynamic fission instabilities lead to spiral-arm ejection

of mass ana angular momentum in the form of a ring or disk of debris, not as

a single body. Some quantitative aspects of these results seem favorable

to lunar origin by fission (3). We have expanded on this work by considering

fission instabilities in fluid objects with a smaller degree of comp ressibility, more closely approximating terrestrial material. Although :he

qualitative features are similar, there are significant quantitative

differences for the stiffer equation of state. We will discuss she implications of our new results for the fission hypothesis of lunar origin.

We also hope to present evolutions illustrating possible a p proaches to

instability.

1. Chandrasekhar, S. (1969) Elli soidal Figures of E uilibrium (`ilia)

University Press, New Haven).

2. Durisen, R.H., Gingold, R.A., Tohline, J.E., and Boss, A.P. 1984,

A_s_t_

r

_^op

hys J., in preparation.

3. Durisen, R.H. an dyScott, E.H. 1984, Icarus 58, in press.
r -^r 13'7 3 8 N8^

36

THE DYNAMiC FISSION INSTABILITY AND THE ORIGIN OF THE MOON;

Alan Paul Boss and Hiroshi Mizuno, DTM, Carnegie Institution of Washington

One theory for the formation of the Moon involves the dynamic fission of

a rapidly -rotating protoplanet, which might then result in the formation of

the Earth and the Moon. The fission hypothesis was originally based on

analytic, linearized models of the growth of asymmetry in homogenous

(incompressible) bodies. Recently the fully nonlinear evolution of the

dynamic instability in inviscid, compressible bodies has been calculated

using numerical techniques. The dynamic instability was found to degenerate

into the ejection of a rind of matter with a substantial fraction of the

mass, leaving behind a central body with most of the mass, which is still an

attractive means for forming the Moon ( 1). All theories of fission require a

similar catastrophic, dynamic phase in order to obtain two bodies from the

initial single body, even those theories which involve a prior phase of

secular evolution (2). We have used both the linearized analytical approach

and the more recent, numerical approach to show that dynamic fission probably

does not oc^.ur in rocky protoplanets.

The numerical calculations are performed with a fully three dimensional

hydrodynamical code, which allows the nonlinear, time evolution of the

instability to be followed. The protoplanet i 9 represented by a fluid with a

Murnaghan eq^ation of state ( p = (K /Y)((p / p ) - 1)) in the solid regions

(p > 3 g cm = P o

 ) and zero pressure otherw?se. The numerical method

ensures global coservation of both the total mass and the total angular

momentum of the sy3tem. The protoplanet is constrained to rotate essentially

+: a rigid body. The kinetic energy in the solid regions (i.e., motion other

than rigid body rotation) is dissipated throughout the evolution in order to

simulate the effects of viscous dissipation in rocky bodies. The basic

numerical method is the same as that which originally helped discover the

outcome of the dynamic instability in compressible, inviscid bodies. The

above alterations to the numerical methods have been successfully tested on a

series of test problems involving the equilibrium of a body subjected to

tidal forces (the Roche problem and lower dimension analogues). Sequences of

uniformly-rotating equilibria (= Maclaurin spheroids) have been constructed

and are used as the initial models for the fission calculations. The results

can he scaled to protoplanets of arbitrary size, with the same K and Y.

We have studied the dynamic instability its a numerical mode? consisting

of a Y = 5 protoplanet with a mass of 6.2 x 10 `7 g, rotating well above the

limit for dynamic instability (rotational energy/Igravitational energy) =

S = 0.30 > 8 c - 0.2.74). An initially imposed asymmetry consisting of a 10%

binary perturbation in the density was found to disappear on the rotational

period time scale. No dynamic instability occurred.

This . sult has been verified analytically by including our velocity

dissipation terms in the linearized analysis of the stability of a Maclaurin

spheroid (using the tensor virial equation approach): the dynamic

instability disappears when our simulated viscous dissipation terms are

included (the secular instability remains). We intend to extend this

analysis to ellipsoidal equilibria.

These results apgly_>t,o bodies with large viscosities (kinematic

viscosity v > 10 cm s ). Any rocky body, even with considerable partial

melt or a molten core, should be stable to dynamic fission; an y rotational

instability that occurs can only result in equatorial mass loss.

(1) Durisen R. H. and Scott E. H. (1984) Icarus 58, p. 153-158.

(2) O'Keefe J. A. and Sullivan E. C. (1978) Icarus 35, p. 272-283.

c
X85 13'739

TIDA1, DISRUPTION AND THE ORIGIN OF THE MOON; Hiroshi Mizuno

and Alan Paul Boss, DTM, Carnegie Institution of Washington

The dynamic problem of the tidal disruption of a rocky planetesimal has

been solved by a direct integration of the fully three dimensional, nonlinear

equations of motion. Tidal disruption has long been thought to be a crucial

process in the accumulation of the terrestrial planets. Previous work has

been limited to static, equilibrium models in circular orbit about a primary,

and the result that stable equilibria do not exist inside a critical radius

(= Roche limit) has been hypothesized to mean that any object that passes

within the Roche limit is disrupted. We have disprcven this hypothesis.

Our time-dependent solution is performed numerically, treating the

planetesimal as a ^luid with a Murnaghan equation of state_3

(p - (K°/Y)((p1p ) - 1)) in the solid regions (p > 3 g cm = p o ) and

zero pressure otRerwise. The kinetic energy in the solid regions is

dissipated throughout the evolution in order to simulate the effects of

viscous dissipation in rocky bodies (tcnsile strength is negligible). The

boundary between the solid regions and the "atmosphere" is accurately

:

follc:4ed. The numerical methods have been extensively tested on a series of

t one, two (Jeans), and three (Roche) dimensional test problems involving the

equilibrium of a body subjected to tidal forces; the test results predict

critical radii for disruption of the equilibrium which agree very well with

R the analytic solutions. Sequences of uniformly-rotating equilibria

(= Maclaurin spheroids) have been constructed for use as initial nodels for

the calculations. The results may be scaled to planetesimals of arbitrary

i size, providing that the same equation of state applies, (same K and Y).

Tne calculations show that a rocky body (radius - 1000 km,°Y - 1) which

passes by the Earth on a parabolic orbit with a perigee within the Roche

limit (- 3 R E rh ) is not tidally disrupted, even for grazing incidence

(= 1.2 R arth^. At most, a few percent of the mass is lost from the surface

of the planetesimal. Objects on hyperbolic orbits would experience even less

tidal disruption. There are two coupled reasons for why this result differs

from previous hypotheses: (1) in a dynamic encounter, there is insufficient

time to disrupt the planetesimal (compared to the infinite time available in

the Roche problem), and (2) the velocities in the solid region must remain

small because of viscous dissipation, and hence it takes many orbital periods

to completely disrupt a rocky planetesimal (in which case the disruption

consists of gradual mass loss from the surfaro, riot a catastrophic failure).

Our results do not ap^;y t2 bodies with very low viscosity: if the

kinematic viscosity v > 10 cm s , then our results apply. Nakazawa and

Hayashi's study (private communication) of tidal disruption indicates that

tnviscid bodies can tidally disrupt, which is consistent with our results (as

we decrease the amount of dissipation, moce mass is lost). Thus only

completely molten bodies are tnviscid enough to undergo tidal disruption; any

rocky body, even with considerable partial melt, or with a molten core,

should be able to resist disruption.

In the accumulation theory of the formation of the terrestrial planets

(including the Moon), one of the important factors for determining the

relative velocity in the swarm of planetesimals (and hence the growth time)

is the mass spectrum. We have shown tha t-, tidal disruption can be ruled out

as a mechanism for reducing planetesimal masses. Furthermore, mechanisms for

forming the Moon whit'. rely upon tidal disruption (e.g., tidal disruption by

Earth in order to capture a circumplanetary ring of matter which can later

accumulate into the Moon), are unlikely to be correct.

37

is .r+w
► N85 13740

38

A N11MP;RIGAL, INVF.'M ;AT10N OF PLANETESIMAL COLLISION

TRA,IECTt)?TES WITH A MnnN ACCUMULATING I N EARTH ORBIT: L. P. COX

J. S. 1•ewl:, A.-t ncriat.es, In-.: `Iijoonn, Arizonfj and Concord, Mass.

Tn the actin-Trio of lunar origin in which the moon is assumed

to :rave acorPtr-d most. of i t„ mass while in orbit about the Earth.

a knnwledgo of the relative imprint rateG of heliocentric planetnnim rlls on the icc:reting Frirt.h and moon is essential for any

attempt, to eo-tablish dynamical constraints on lunar origin. All

previous model:; of lunar accumulation in Earth orbit have treated

gravitational onenuntPrs, including* collisions, between planetoolmnls and the necreting moon as two-body interactions. However,

this ideal i ro i two-hotly t.rPat.ment, of encounters has been found to

brerik ]own whPr, the ratio of the unperturbed relative encounter

vi-locit.y of tho two hodiPS to their mutual surface escape velocity. V/V is less than -0.3(1,2).

Conseluently, numerical integrations of the regularized

Equations of motion fnr four bodies (Sun, Earth, moon, planetesimal) are in progress. A planetesimal impact trajectory is

calculated by assuming that, the planetesimal has hit the surface

of the moon nt. an as., umed location, traveling in an assumed

direction. and with ,in assumed impact speed. Next, the equations

of motion nrn l i:ii-ri efi l ly int«grfited backward in time in order to

determine whore the pinnotonima ► hn., come from as given by its

osr•irinting nrbitril olnmentc; nt large enough separations from the

Earth and m(-)on that the o.l p monts are sufficiently close to their

ho 1 i ocentri c of 1 ur s . Tn thi w^3 ,y those volumes in heli ocentric

orbital element ;;pace whiol contribute trajectories that directly

impact the moon, anri I ogous to t.hc, "hand.," of tra'ectories impacti n,-7 a planetoid Farth found by Giuli (3) and Dole.(O. can be

mapped out..

Tn numeri o.-i I 1nt,P,-rat.i(-)ns of the threo-hody Pquat.ions of

mnt•ion, many were. ob:,orvad at, lnw velocities, V/V e < 0.1,

to result, from nncn , rnt.or trr;,joot,orios having unperturbed separation distances far be,yord the two -body gravitational radius(1,2).

The present investigation sKnuld quantify the extent to which

the moon benefit,. , in the late : to Jos of its accrete on from this

"anomalous gravitational fn(:using" of low - velocity heliocentric

p1nnetesimrils by the Fa rt.h. assumed at, this stage to be closely

approaching its, present, m-ass.

(1) Wetherill G. W. and Cox I,. P. (1984) The range of validity

of the two-body r,pproximati.on in models of terrestrialplanet

accumulat,inn: T. irnvittattinnnl perturbations. (in press)

(2) WethPrill G. W. and Cox L. P. (1984) ...: II. Gravitational

cross-sections and runawn ,y n(-oretion. (in press)

(3) Giuli R. T. (1968) in Tc,aru.:, 8, P. 301-323.

(4) Dole S. H. (1962) in Planr,tary Cpr3en Sr,i . , 9, p. 541-553.
1 X85 13741

39

STOCHASTIC # 9Q UQQ; William K. Hartmann, Planetary Science

Institute, Tucson, Arizona 8571Q.

Many lunar origin theorists have felt constrained by Occam's razor to

avoid postulating a role for stochastic events, such as large impacts. This

attitude is a misreadinq of Occam ► s razor, which asks that we not posit " hQ

events. Some Q1daa_Q2 of influential events in solar system history are

class-predictable but not event -eredictablQ: we believe the class of events

occurred, but we cannot predict times and magnitudes of individual events. The

events are thus stochastic, but not " hDQ.

An example is the probable K-T boundary astQroi^ impact: As-ieroid

statistics long ago implied such events every few 10'-10 yr, but we could not

convincingly tie specific geologic effects to specific impacts. As a result,

large impacts tended to be ignored; we should have given more thought to

geologic and climatic consequences of these class-predictable events.

Analogously, we shou l d be careful about adopted constraints on lunar

origin and not ignore class-predictable stochastic events. As long ago as 1958,

0. Yu. Schmidt emphasized the statistical efforts of innumerable small

planetesimals, explaining the tendencies toward regularity: proqrade rotations,

small obliquities, is and e's, etc. Superimposed Yere the effects of

large-scale stochastic events, emphasized by Safronov as early as 1965. The

magnitude and timing of the largest impact in Iach planet's history may have

been crucial to that planet's development. Theoretical and observational

results support the im portance of large-scale stochastic events Jimpacts or

encounters?) in explaining differences among planets: (1) Safronov attributed

obliquities to impacts of masses up to a few gercRnt of planetary mass (5% for

Uranus; 0.1% for Earth); (2) O.R. Davis and I obtained similar results for e's

and i's. Items (1) and (2) imply planetesimals as large as O > 1000 km striking

planets during the planet-forming period; (3) Lunar and planetary basins dating

ba q an estimated 4.0 Oy. -live direct evidence of impactors as large as D=150

km; (4) Several satellites show impact scars nearly big enough to disrupt

them, 34 - 401 of the ^ ;ameters for P hobos, Mimas, and Tethys. Voyager analysts

concluded that Mimas and other moons had been completely disrupted and

reassembled -- accretion involved catastrophe; (5) Some asteroids have been

fragmented, but other_ have survived with orimitive surfaces; (6) Differences

in ring systems and satellite systems also suggest stochastic processes.

Im p acts and cl•._ • se encounters with large objects during planet formation

are class-predictabii:. These stochastic events, such as large impacts that

triggered ejection of Earth-mant!e material into a circum-Earth cloud, should

not be rejected ar. sU bgY. A way to deal with such events scientifically 1s to

investigate their consequences; if it can be shown that they might produce the

moon, they becomf- iable concepts in theories of lunar origin.

Reference

(1) Schmidt, C (1959) A Theory of the Origin of th e Earth, Moscow.

(2) Safronov, (1966) . ^strQSr. AJ. Q, p. 987.

(3) Hartmann 'J.K. (1977) in Comets. Asteroids. Meteorite-5, A. nelsemme, Fd.
MY MODEL OF LUNAR ORIGIN I

Monday, October 15, 1984

3:30 - 5:15 p.m.

If

PRECEDEN , PArI; I'L- K NOT
N85 13742

43

A TESTABLE GRAVITATIONAL. CAPTURE. MODEL FOR THE ORIGIN OF THE

EARTH'S MOON; Robert J. Malcult, Dept. of Geology, Ronald R. Winters,

Michael E. Mickelson, Dept. of Physics and Astronomy, Denison University,

Granville, Ohio 43023

A great-circle pattern of large circular maria can be interpreted as the

signature of a tidal disruption event associated with a gravitational capture

origin for the Earth-Moon system. The features that delineate this lunar

surface rock pattern are Maria Orientale, Imbrium, Serenitatis, Crisium,

SmyLliii, and the mare-filled crater Tsiolkovsky. Our proposed gravitational

capture scenario '.s divided into five eras.

ORBITAL ERA 1. .Jupiter-Perturbed Heliocentric Orbital Era (-4.6 to -4.3

b.y.). The model begins with formation of the planets from nebular condensates about 4.6 b.y. ago. The Earth accumulates in the vicinity of its present orbit but Luna (- pre-capture Moon) forms as a small planetary unit on

the inner edge of the Asteroid Zone at about 1.7 - 2.0 A.U. Gravitational

perturbations by .Jupiter cause an increase in eccentricity of Luna's orbit

so that it becomes Mars-crossing and eventually Earth-crossing. During this

orbital era, the lunar magma ocean would lose heat and eventually become

mainly crystalline by 4.3 b.y.

ORBITAL ERA II. L•'arth-Crossing Heliocentric Orbital Era (-4.3 to ~3.9

b.y.). This orbital era commences when Luna's orbit becomes Earth-crossing.

AL this point, occasional energy-dissipating gravitational encounters of the

non-capture type can occur between Luna and Earth. A large quantity of orbital energy (-4 X 10 38 ergs) must be dissipated within the bodies of Luna

and Earth during this orbital era. Although much of this energy can be

absorbed by the Earth's upper mantle over this time interval, periodic tidal

activity would be effective in reheating Luna's magma ocean zone.

ORBITAL ERA III. Luna's Capture (N3.9 b.y.). As Luna undergoes periodic

gravitational encounters with Earth while in a near L•'arth-like orbit, the

lunar body becomes warmer, more deformable, and thus more capturable. However, gravitational capture of a lunar-sLzed body during a close encounter

would entail dissipation of about 2 X 10 35 ergs if Luna had a voo of about

0.2 km/sec. liven if we assume the Earth to be about twice as deformable as

it is today, the lunar body must have Love numbers around h=0.5 and k-0.25

to facilitate capture in one pass. Since energy dissipation is related to

r -6 (r= distance of separation between the two bodies), only encounters

within 1.4 Re could be expected to result in Luna's capture. Encounters

within 1.4 Re are well within the Weightlessness Limit for the Earth-Moon

system and such a close encounter of a sufficiently "warm" Luna could result

in tidal disruption during the time of the encounter.

ORBITAL ERA IV. Early Post-Capture Geocentric Orbital Era ( N3.9 to - 3.6

b.y.). After the lunar body is inserted into a geocentric orbit within the

stability limit of the Earth-Moon system, a geocentric orbital evolution

would occur which results in Circularization of an initiall y highly elliptical.

orbit. Computer simulation of this early geocentric orbital evolution

suggests that ci l

lariz.1tion of the orbit occurs rapidly, i.e., within a

few 10 5 years af^. capture. About 5 `: 10 35 more ergs must be absorbed by

the Earth and Moon during this orbital era.

ORBITAL. ERA V. Subsequent Geocentric Orbital Evolution (-3.6 b.y. to

Present). Following orbital circularization at about 35 to 40 R e (prograde),

the lunar orbit gradually expands to its present dimensions by way of

of tidal friction processes.

PRECEDING PACE l3LA-,iK NOT FILMM

ft _;., . ..Mow .- - J_

_^ 
N85 '7 4 

44

DIRECTIUNAL PROPERTIES OF "CIRCULAR" MARIA: INTERPRETATION IN THE

CONTEXT OF A TESI'ABLE t;RAVITATIONA[. CAPTURE MODEL FOR LUNAR ORIGIN; Robert J.

Malcuit, Dept. of g eology, Ronald R. Winters, Michael E. Mickelson, Dept. of

Physics and Astronomy, Denison University, Granville, Ohio 43023

Photogeologic study of "circular" maria and mare-filled craters reveals

that several of these features are either cl . iptical and/or asymmetrical.

The ellipticity/asymmetry is interpreted as a directional property which may

have genetic significance. The features with the most apparent directional

properties are Maria Orientale, Crisium, Humorum, Moscoviense, and the marefilled crater Tsiolkovsky. Other features with weaker vectorial signatures

are Maria Imbrium, Serenitatis, and Smythii. The asymmetric pattern associated with Mare Orientale is somewhat different from that of the other maria.

There are extensional features to the southwest of Orientale and compressional features to the northeast. 'thus movement appears to have been from

southwest to northeast. All other maria, except Moscoviense, indicate movement: in eastcrl • directions and these vectors are within 300 of the trend of

a great-circle pattern of large circular maria described by [1). The vector

for Mare Moscoviense is in a north-easterly direction and is oriented nearly

perpendicular to the great-circle trend. The directional properties of these

features are consistent with at '.east three models of mare basin formation:

(1) that the mare basins were excavated by a swarm of fra:ments from a tidally

disrupted planetary body impacting from a westerly direction [2]; (2) that the

mare basins were excavated by lunar satellites whose orbits decayed sufficiently to cause the satellites to impact onto the lunar surface from a westerly

direction 131; and (3) that the basins were formed by the impact of large

spheroids of lunar basalt onto the lunar surface from a westerly direction

fl]. According to [1], such basaltic spheroids may be necked off a lava

column during the tidal disruption phase of a very close gravitational encounter between Earth and Moon early in Solar System history. In this tidal

disruption :model, each major basaltic spheroid impact zone should be characterized by a basaltic lava "lake" surrounded by anorthositic crust "flooded"

by the overflow of basalt resulting from the collapse of the basaltic spheroid

onto the .lunar surface. A raised rim could then result from the rebound of

the area in response to excess loading during impact.

General tests for the model are the following: (1) there should be no

anorthositic crust under the postulated tidal disruption zones -` Mare Orientale and a restricted region of Oceanus Procellarum; (2) there should be a

normal thickness of crust under the postulated fall-back zones; and (3) the

crust under the mare basalt in the mare basins in the fall-back zones should

not be greatly disturbed by the impact of the magmatic spheroids. Specific

tests for the Mare Crisium region (a typical impact zone in this model) are:

(1) the morphology of the anorthositic crust under the mare basalt should

be spoon-shaped (asymmetrical) with the deep part on the western end; (2) the

shape of the mascon associated with the basin should mimic the shape of the

mare basalt fill; (3) the bulk of the rim-mantling material should be composed of a black-matrix breccia consisting of anorthositic crustal fragments

within a matrix of lunar basalt; and (4) a negative gravity anomaly should

be associated with the raised rim. Other "circular" maria in the fall-back

zones should have similar characteristics.

REFERENCE'S: [1[ Malcuit, R.J., et al. (1975) The Moon, V. 12, p. 55-62.

[21 Hartmann, W.K., (1977) Sci. American, v. 236, p. 84-99.

[3] Runcorn, S.K., (1983) Nature, v. 304, p. 589-596.
N85 13744

45

T1iE'VROBLEM"OF IRON PARTITION BETWEEN EARTH AND MOON DURING

SIMULTANEOUS FORMATION AS A DOUBLE PLANET SYSTEM: W. A. Cassidy, Department

of Geology and Planetary Science, University of °ittsburgh.

Given certain conditions, there should be no problem in forming the

moon as a sister planet of the earth, from the same reservoir of material,

within the same time frame. Tne model described here requires fractional

vapor/liquid condensation, planet accumulation during condensation, a late

start for accumulation of the moon, and volatile accretion to the surfaces

of each planet only near the end of the accumulation process. While most

workers resist the suggestion that liquid phases could be stable in the

primitive solar nebula, transient conditions as distant from the suit

1.6 A.U. produced during nebular flares or a T Tauri phase of the early sun

could be characterized by pressures and temperatures within the stability

fields of iron and silicate liquids and vapors (1). That glassy chondrules

formed, possibly as far from the sun as 3 A.U., is not inconsistent with the

suggestion. At 1 A.U., therefore, cooling vapor may have condensed to

liquids, and may have fractionated in the process. In the model, initial

accumulation of small objects is helped if the agglameratiug particles are

somewhat sticky, as liquids might be, but still there would be a critical

size range within which rotating bodies are unstable. Assuming that growth

proceeds through this range, agglomeration continues. If the reservoir of

vapor is being preferentially depleted in iron by fractional condensation,

an iron-rich planetar-1 core forms. As the temperature decreases, ccndensing

material becomes progressively richer in silicates and poorer in iron,

forming the silicate-rich mantle of an already differentiated earth

(cf. 2,3). Continuing to develop the model, a second center of agglomeration

successfully forms near the growing earth after most of the iron in the

reservoir around 1 A.U. has been used up. The bull, composition of the moon

then is similar to the outer mantle of the accumulating earth, since from

some instant in time they drew on the same iron-depleted reservoir. As

each body increased in cross-sectional area and mass it became a more

efficient collector of additional material, but the earth had started sooner

and became more efficient more rapidly. Volatil.es ac_umulated late and were

incorporated into surface materials of both planets, but disproportionately

more were gathered by the r.rrth because by thaL time it was competing much

more successfully for the remaining available material. (1) Cameron. A.G.W.

(1984) Lunar and Planetary Science XV, p. 118-119. (2) Cassidy, W. A. (1964)

Ann. N.Y. Acad. of Sciences 119, n. 17-40. (3) Turekian, K. K. and Clark,

S. P., .Jr. (1969) Earth ane Planetary Sci. Lett. 6, p. 346-348.

J
N85 1374_

46

ORIGIN OF THE MOON; A.E. Ringwood, Research School of Earth

Scit•nces, Austr..alian National University, Canberra, A.C.T. 2601 Australia

The earth's mantle and the moon possess similar (%x2) abundances of the

siderophile elements (Co,Ni,Fe,W,P,S,Se and Te. (1) The abundances of these

elements in the earth's mantle have been determined by the interaction of

several complex processes unique to the earth, which relate to core formation

and the distribution of siderophile elements between netal and silicates.

These processes could not have operated similarly within the ,noon, in which the

maximum size of a core is smaller than 10 percent of the proportional size of

the earth's core and which would have formed under drastically different

P,T,f0 2 conditions tv the earth's core. The similarities in siderophile abunda,ices strongly ,uggest that the moon was derived from the earth's mantle after

the earth's core had formed (1). This conclusion is not weakened by the existence of some important geochemical differences between the moon and the earth's

mantle, which can readily be explained by selective loss of volatile elements

(1) and segregation of a small amount (<0.5%) of metal within the moon (eg,2).

The energy required to remove material from the earth's mantle and place

it into geocentric orbit can be supplied most readily by impact processes

during accretion of the earth (3,4,5,6). 'Theoretical studies of tektites,

together with experimental impact mechanics investigations (6) show that

impacts from aceret.inq planetesimals can accelerate material from the mantle

to velocities exceeding lOkm/sec and can also evaporate large amounts of target

material. Currently most of t'is material would be recaptured by the earth or

it would escape. However, at ..r late stage of its accretion, after segregation

of the core, the earth probably possessed a strong magnetic field and a

rotation period of 4-5 hours (3). Impacts of late-accreting, high-velocity

planetesimals (possibly comet.,ay bodies from the outer planet reqion) would

evaporate many times their masses of mantle material (6). These gases would be

accompanied by a massive spray of shoe'--melted silicate droplets. Evaporated

gases would be transiently ionized and thereby coupled hydromagnetically to the

earth's rotation. It is suggested that the (lases produced from such near-equatorial impacts were rapidly shun out into equatorial geocentric orbit. Viscous

coupling between the gases and shock-melted liquid spray also cau3ed the latter

to achieve similar. orbits. The evaporated material was selectively recondensed,

and, accompanied by the shock melted, devolatilised silicate droplets, accreted

to form a sediment-ring of earth-orbiting planetesimals. The volatile components condensed further away at lower temperatures as sub-micron smoke particles

and were blown away by an enhanced uarl.y solar wind. This sediment-rinq also

captured a significant proportion of earth-bound planetesimals a advocated by

(7), thereby increasing the total iron content of the sediment-ring to a level

exceedinq that in the earth's mantle. The moon was formed by accretion from

planetesimals comprising the sediment ring.

References (1) Ringwood, A.E. (1979) Origin of the Earth and Moon, Springer,

N.Y.; (2) Newsom, H., and Palme, 11. I.arth Planet. Sci. Lett., submitted 1984,

(3) Ringwood, A.E. (1972) Phys. Earth Plan. Int. E, 366-376; (4) E:c.rtmann, R.

and Davis, D. Icarus 24 504-515; (5) Cameron, A. and Ward, W. Lunar

Science 7 120-122. (6) Boslough, M. and Ahrens, T. (1983) Lunar Planei:. Sci.

14, 63-69 '7) Ruskol, E. Soc. Astron. AJ. 15, 646-654.
N85 1 3'746

47

IN THE ORIGIN OF THE MOON BY ROTATIONAL FISSION; A.R.

Binder, Senior NRC Fellow, Johnson Space Center, Houston, TX

77058.

There is an increasing body of evidence which suggests that

the moon originated by fission. The observations, etc. which support this concept are as follows.

1) Essentially all stars are members of close or contact binary

systems (1). These observations suggest that such systems formed

by fission of the proto-star at the end of its formation. Also,

like the earth-moon system, the Pluto-Charon system has a large

secondary to primary mass ratio and orbital characteristics which

are expected for a fission pair. These empir-^cal observations

suggest that the formation of hinary stars and plane t s by fission

is a prevalent process in the formation of astronomical bodies.

2) Recent advances in the dynamical study of fission show that

the " .. results are consistent with some of the requirements of

the fission hypothesis for the origin of the moon." (2) and in -

dicated that stellar bodies also u.idergo fission. Also these studies co,firm that fission leads to a ,-econdary body having a mass

of 10 to 20% of that of its primary.

3) As shown earlier (3), the new1y formed proto-moon, whose

temperature was 2000 to 3000°C and which therefore had an extensive, volatile enriched atmosphere of vaporized silicates, would

have lost a large fraction of its original mass via mass transfer

through the L point. While most of this mass would have been recaptured by Jhe earth, some of it would have escaped the earthmoon system, thereby removing angular momentum from it. In addi -

tion, the proto-moon itself was rotationally unstably and this

would have lead to the loss of mass from the proto-moon and the

system. Calculation show that if only 0.7 to 1.5 lunar masses of

material , i .e, only 5 to 10% of the original mass of the protomoon, escaped the earth-moon system, the current angular momentum

deficiency of the system can be accounted for (3).

4) The processes described above would result in a moon of terrestrial man tI a material which was depeted in both metallic iron

and vol atiles. The moon would have been totally molten and would

have differentiation in to a dunitic lower mantle, a peridotite

upper mantle, and a feldspathic crust (4). As small core should

have been formed mainly from Fe produced by the reduction of FeO

as the moon lost its volatiles. The formation of a small lunar

core would account for the observed lunar depleation pattern for

the sideroohile elements (4,5).

The above characteristics are consistent with our current

understanding of the moon's bulk compoition, interr3l structure,

seismic, and tectonic characteristics.

1 ) Abt 1­1 A. and Levy S.G. ( 1976 ) Astrophys. J . , Suppl. Ser. 30,

p. 213-306. 2) Durisen R.H. and Scott E.H. (1984) Icarus 58, p.

153-158. 3) Binder A.R. ( 1980 ) P roc . Lunar Planet. Sci. Conf.

11th, p. 1931-1939. 4) Binder A.B. (1974) Moon 11, p. 53-76. 5)

Newson H.E. (1984) EOS 65, p. 369-370.
N85 13747

48

r,FOCHFMICAL FVIOFNCF FOR THE FORMATION OF THE MOON BY IMPACT INnuan FISSION OF THE PROTO-EARTH. H. Wanke and G. Dreibus, Max-Planck-Institut

fur ChPmie, Saarstr.23, 0-6500 Mainz, F.R.Germany.

The Earth's mantle exhibits a number of geochemical peculiarities shich

make our planet to an almost unique object in the solar system. This becomes

evident if one compares terrestrial hasalts (TR) with those from the eucrite

parent body (FPB), and the shergotty parent body (SPB), which to all p robability is Mars. EPS and SPB basalts differ in their oxygen isotope systematics

and in their higher contents of Mn from TR. Under "normal" planetary conditions (ol+opx+cpx, beinq the major Fe- and Mn-bearing phases), the liquidsolid partition coefficients of FeO and MnO are only slightly above 1. Hence,

comparing hasalts from different planetary objects the depletion of Mn for

Earth and Moon is immediately evident.

Aside its lower abundance of volatiles and hi g hly siderophiles, the Moon

comes compositionally in all details very close to the Farth's mantle except

the lower Fen content of the latter which can he explained by a gradual transfer of some FeO into the core durinq geological times (1). Com p ared to their

metal/silicate p artition coefficients (f),m/s), all siderophile elements are

hiahly overabundant in the Farth's mantle assumin q eq uilihration with a pure

metal phase. Lunar hasalts contain much less Tr, Au, P3tr.. (nm/s , 10 5 ) than

T1, while these differences diminish for Mi (')m/s , 10 ) to abou^ a factor 3

(2), and to less than a factor of ? for W, Co and p (0m/s b In ). In terms

of a huge lunar magma ocPan formation or addition and segregation of small

amounts of metallic iron (^ 0.5 %), cotild account for the depletion of siderophiles according to their Dm/s.

Recently, impact induced fission became a most plausiblto physical process

for the formation of the Moon (3,4). According to Wetherill (5), the largest

objects impacting on the Farth durinq its accretion was in the order of several lunar masses. Wanke et al. (6) have presented evidence for the presence

of unf°actionat.ed primary matter in the lunar hi g hland breccias, which in composition should be almost identical to that of the bulk Moon. This observation

also a-oues for an impact induced fission scenario. In such an impact process

large fractions of vaporized target material will be placed in orbit around

the Earth and after recondensation - loss of volatiles - forms the Moon in a

fast time scale. It is plausible that in addition to a massive Froto-Moon several smaller satellites formed which were swept y in by the Proto-Moon on a larg er time scale (7) (basin forming objects). These smaller satellites would of

course have a cor.iposition identical to that of the Proto-Moon.

It is also plausible that impact indt:^ ,?d fission occurred at the moment

at krhich accretion of the Farth was not yet totally completed. In terms of an

inhomogeneous accretion of the Earth (8), most of the oxidized and volatiles

containin g component i5 added towards the end of accretion. Hence at time of

fission the mantle mould have had somewhat lower concentrations of (oxidized)

siderophiles and volatiles as toda y . Formation of the Moon from material of

the Earth's mantle has been advocated since lonq by Ringwood (9,110).

(1) Jaanutz F. and Wanke H. (19R2) Lunar o lanet.Sci. XIII, 358. (2) Wolf R.

and Anders F. (1980) Geochim. Cosm p chim. ^^cta^l^, ^^II^3^ Hartmann W.K. and

Davis x).(1975) Icarus 24, 504. (4) Poslou q h M .R. and Ahrens T.J. (1983) Lunar

PlanetSci.-XIV, 63. (7 Wetherill G.W. (1976) Proc. Lunar Sci. Conf -7FF,_

3245. 6 Wanke H . et al. (1975) Proc. Lunar Sci. Conf. 6th, 1313. (T Runcorn

S.K. (19,94) Lunar Planet. Sci.-XP;_

.

^ >?an '"^e T^— Tfgf^l) Phil. Trans. R.

Soc. Lond. A 303, 2R% 9 Rinnawnod A.F. (1960; Geochim. Cosmochim. Acta 20,

241. (in) Ringwood A.F. (1978) Lunar Planet. Sci.IX, 961.
0

MY MODEL OF LUNAR ORIGIN II

Tuesday, October 16, 1984

9:00 a.m. - 12:00 noon

0
N 8 5 13 '7 4

r...^

8

AN INTFGRATFD DYNAMIrAI_ AN rj GEOCHEMICAL APP ROACH TO LUNAR ORIGIN

MODCLLINn; P. Greenberg*, C.R. Chapman*,D.R. Davis*, M.J. Drake**, W.K.

Hartmann*, F.l. Herbert**, J. Jones**, and S.J. WHidenschilling*, constituting

the Tucson Lunar Oriqin ronsortium, *Planetary Science Institute, Tucson,

Arizona, **Lunar & Planetary Laboratory, Tucson, Arizona.

None of the three major categories of models of lunar origin readily explains the Moon's pro p erties: The fission model suffers from dynamical uncertainties and from compositional inconsistencies with the mantle of the

Earth; l the model of growth in circum-Terrestrial orbit suffers from the gross

bulk compositional differences between the Earth and Moon, ;.,^., the latter's

lack of metallic iron; the ca p ture hypothesis requires some unknown capture

mechanism to slow a full-sized Moon into a bound orbit, and also fails to

address the problem of low iron content.

Our consortium considers each of those models to represent an end member

of a more general scenario: As the E arth grew by planetesimal bombardment, a

circum-terrestrial cloud of particles was created from a combination of impactejected mantle material and planetesimals captured directly into orbit around

the Earth. Such a swarm cont ; nued to capture planetesimals and to receive

ejecta until the bombarding population thinned, the Earth stopped growing, and

the Moon accreted in orbit. If Farth-mantle material dominates the swarm, the

model resembles the fission hypothesis; if small planetesimals dominate, the

model re p resent ,, the "growth in Earth orbit" end-member; if the swarm were

dominated by a single large planetesimal, we would essentially have a capture

model. A model intermediate between these extremes appears most promising.

In this context, we see two wave to exp!ain compositional properties.

First is the stochastic mute: A few big late planetesimals of diverse composition are captured in orbit and/or hit Farth. Such rare events control the

final composition of the Farth and " goon. This approach may be reasonable

because some late planetesimals werFa prohahly biq, and they had large orbital

eccentricities and thus may sample a range o' compositional zones. However, a

stochasti^ explanation is difficult to analyze or test. Some might call it ad

hoc, yet such events are quite plausible.

We consider a second route which may provide a systematic explanation of

composition: ?'he circum-terrestrial swarm acts as a f ilter, preferentially

capturing small weak silicate bodies, while passing large iron planetesimals

(cores of broken p%ronts).,Jhe dynamics of this filtering process are described in a subsequent paper.' There are a numher- of issues related to this

hypothesis that arrr also considered ; in quhsequent napers: What other geochemical priip , ^ties need to be expieined? How was the swarm produced in the first

place? And finally, how was the swarm maintained in orbit? Or more

specifically, can captured material coy ribute enough angular momentum? The

answer to the latte r seems to he Q, a snemin q stumbling block to any model

involving a circum-terres +r ial swarm. Unless some systematic source of angular

• momentum4c^n be identified, we may be forced to rely on stochastic dynamical

processes 4, for an explanation.

r References

(1) Drake, M.J., and J. Jones (1984). Invited review (no abstract) this conf.

(2) Cha pman, C.R., and R. f;reenberq (1984). This conference.

(3) Kreutzbe-ger, M .F. et 21.. (1984). This conference.

(4) Weidenschillinq, S.J. (1984). This conference.

I. (5) Hartrrenn, W. K . (19 RAI). "Role of Giant impacts", this conference.

(5) Davis, D.P. and F. Hcrbert (19134). This conference.

i:liCi IMNG PAGE I^I.:% Nl ^ NOT I^ lUNI''1)

- — 
r 

N8^ 13 '7 4 9

LUNAR ORIGIN: ROLF OF GIANT IMPACTS*; William K. Hartmann, PIaratary

Science Institutio, Tucson, Arizona 85719.

The impact flux required to acc5ete Ear T^ I s mass during accretir time T =

0.03 to 150 m.y. ran 9 es from some 10 to 10 x the present flux. In 1975,

Hartmann and 0avis suggested that impacts of a large planetesimals?) during

this period ejected "iron-deficient crust and upper mantle material, forming a

cloud of refractory, volatile-poor dust that could form the moon."

The high primordial 14 u im9Y es 1 impact/week of bodies ranging about

-50 km across and about 10 to 10 kq. A small fraction of the resulting

ejercta reaches near-Earth space, with sub-orbital stay-times of the order of a

week.. (In fast-accretion models, the "weekly impactors" have dimensions larger

than the present atmospheric thickness; they are espociAlly efficient in

eje(tinq debris into near-Earth space.) Therefore, the "weekly impactors"

maintain a time-varying circum-Earth swarm or disk; its density needs further

evaluation. The swarm may have interacted with incoming material, or accreted

onto a small proto-moonlet already (captured?) in orbit.

Less frequent 100+ km impactors ejected transient surges of mass into the

circum-Earth swarm. The largest impactors probably approached or exceeded lunar

size. One or more giant impacts may have added enough heated, volatile-depleted

upper-mantle material (from a magma ocean?) to contribute much of the moon's

mass. Lunar formation#n such a swarm is supported by Thompson and Stevenson,

possibly within 100 yr. If Earth spun rapidly, as in fission models, the

largest impact may have introduced enough angular momentum and energy to

trigger the ejoction/fission event. Impact-induced Fission thus overcomes

certain problems of classical fission models.2 4,5 The suggestion by Hartmann And Davis and later by Cameron and Ward '

that lunar origin involved giant impacts remains attractive. Large

planetesimals are consistent with current accretion models, and may have been

widely scattered in the early solar system: their existence is a reasonable, if

not necessary, assumption in moon-origin models. Furthermore, isotopic data

require the moon's tormation primarily from local material resembling Earth's

upper mantle, not material from elsewhere in the solar system. Giant impacts

are stochastic, class-predictable events that would provide the required type

of ejected Earth-mantle material without requiring large moons to form near

other planets (a problem with less stochastic processes). Such material may

have mixed with incoming meteorites during lunar formation, affecting lunar

chemistry. Further work on this hypothesis should include dynamical studies of

whether: (1) Some elected material in each large imnact would remain in orbit,

causing growth of the swarm; (2) a small moonlet captured or accreted in the

swarm could serve as a nuc l eus to catalyze lunar accretion; (3) spontaneous

accretion could yield a moon without a catalyst-moonlet; (4) tidal action could

help the primordial moon move outward instead of spiraling in due to drag; (5)

Earth's rotation rate or an oblique impact is critical in launching the swarm;

and (6) the swarm helps differontiate incoming matar;nI , preferentially

capturing low-density silicate dust.

*Research conducted in conjunction with the Tucson Lunar Origin Consortium.

References

(1) Hartmann, W.K. (1980) Pyoc• Cunf. Lunar Highland Crust, p. 155-171.

(2) Hartmann, W.K. and Davis, D.R. (1975) Ic arus 2A, p. 504-515.

(3) Thompson, A. and b;cvPnson, 0. (1983) Lunar Planet, Sci. XIV, p. 787-788.

(4) Cameron, A. and Ward, W. (1976) Lunar Sci. VII, p. 120-122.

(5) Ward, W. and Cameron, A. (1978) 1_ nor P ane t. Sci. IX, p. 1205-1207.
ORKAWN'- ^ - ---

OF POOR QUALITY N85 13750

e 0.1 =

4+

e

e

n

-f - --+

0.9 l.n 1 11,9 i.n 1. 1

Fig. I a Fit,. 2 )

r' 1

MODELS OF ANGULAR MOMENTUM INPUT TO A CIRCUMTEHRESTRIAL SMARM FROM ENCOUNTER - MIN

riLLIOCENTRIC PLANETESIMALS + ; Floyd Herbert, Lunar and Planetary Laboratory, University of Arizona,

Tucson Al 85721, and Donald H. Davis, Planetary Science Institute, Tucson AZ 85719.

Modals of lunar Origin in which the Moon accretes In orbit about trio Earth frun, material

approaching the Earth frorr. hellocentric orbits must overcome a fundamental problem: the approach

orbits of such material would be, In the simplest approximation, equally likely to be prograde or

retrog r ade about the Earth, with the result that acc r etion of such material adds floss but not angular

murnerltum to eircumterrestrial satellites. Satellite orbits wculd then decay due to the resulting

drag, ultimately Impacting onto the Earth.

In the present work we investigate one possibility for adding both material and angular momentum

to Earth orbit: Imbalance In the delivered angular momentum between pru- and retrograde

Earth-passing orbits which arises from the three-body dynamics of plarretesimals approaching the Earth

from heliocentric space. The existence of this Imbalance for planetesimals directly striking the

Earth and its tendency to be of prograde sense was first demonstrated by Glul1 (1) In numerical

computations and later In analytic wurk by Harris(2).

In order to study angular momentum delivery to eireemterrustrial satellites we have numerically

computed near-Earth velocities as a function of distance frorn the Earth for d large array of orbits

systematically spanning heliocentric phase space. The resulting distribution f(v,r,a,e) of

Earth-passing velocities v at each distance r resulting from orbits at each holioeentrlc a and e

together with one of the various plausible assumed heliocentric planetesimal number densities n(a,e)

determines trio me.1n velocity near the Earth of all plarletesimalr from all parts of the feeding zone by

<v> = fv f( v ,r,a,e) n(a,e) d 2 v do de'ft( v ,r,a,e) n(a,e) d l v dade.

Th© results of this experiment for most assumed planetesimal distributions produced mean

Tangential Earth f lyby velocities that wore positive (prograde) both near the Earth's surface and at

distances up to 40 R E . A constant n(a,e) yielded mean flyhy tangential v,Ioeities of only a few

percent of local circular Earth orbit speed vc(r), whereas d Quasi-thermal distribution around

heliocentric eccentricity of 0.03 gave flyby tangential velocities varying from a few to 10% of v (at

40 RE ). Raising the mean thermal eccentricity to 0.05 and assuming that planetuslTuls crossing the

Earth's orbit without terrestrial pertu-bdtions are reduced in frequency by a factor of 10 produced a

mean flyby tangential velocity 20% of >.

c

 at 1 RL and 30% of vc at 40 nl (but about zero at 2.5 RE).

The slgnificance of heliocentric feeding zone weighting is sho..n In Figs. I and 2, which are

histograms of the sign of the zonal statistic < v (i,e)> at 4 RE , computed by evaluating the above

expression over -^mdll a and a bins wittl n(a,e) constant. Fig. 2 values are relative to v c (r), while

Fig. 1 refers to a nun-rotating frame. Thuso diagrams indica t e trio hellocentric phase space source

regions for significant angular momentum input to the near-Earth region. Therefore, while other

weightings of Thu heliocentric population might incrredse angular momentum input, Fig. 2 suggests that

there is no Simple weighting that would spin up the eircumterrestrial swarm.

Thesu preliminary experiments show tr;at l iulioeentric planetesimals passing throu5h the Earth

environment possess significant an^ulrr momentum. ' f , #ever it also appears that these same

planetesimals impacting a .ircula ;zed ci cui r.

;e5tri.. planetesimal swarm would likely remove

angular momentum (though E slbly increasin g mean kinetic energy), presumably promoting both swarm

,nfall upon the Earth or; ra%cape to huliocer.tric space. Only a distribution of highly eccentric

satellite orbits with medl tangential velocities of d few tens of percent of local circular velocity

would be Immune against angular momentum loss to passing heliocentric planetesimals.

+Res,3areh conducted in conjunction with the Tucson Lunar Origin Consortium

ACxNOw.EGEMENTO: The authors thank C. P . Sonett

for ma..ing computer time available. This work was

funded in pa r t by NASA under grant number

NASw-3516.

RE F ERENCES: (1) A.T. Glull, Icarus 5, 301 (196b)

(2) A.w. Harris, Icarus 31, 165 (1977)

FILIAL "APTIONS:

f ly. 1 A 2-D histogram of the sign of the mean

tanyvn • ial ve;ocif, of ha l iocantrrc planoles reis

passing through the Earth neighborhood at a

distance of a AE , axpresse° as a function of a and

e for the hellocentric source orbits. Symbols only

appear at >Ositions co r respanding to orbits capable

of reaching trio 40 R E c;rcum1e r res t rlal disc.

rig. 2 A 2-D his t ogram the same as Fig. 1 except

thatfine zero of ve loelty is taken to be (prograde)

cl r c,la' orbit velocity a' < RE.

J
) 

,. N 8 J 1 J ^ ^1

54

CAPTURE GF PL ANFTE S I fAAL S INTO A C I RCIIMTFRRESTR I AL SWARM*; Stuart J.

Weidenschilling, Planetary Science Institute, Tucson, Arizona 85719

The lurar origin model considered by the Tucson Consortium 1'? involves

processing of proto-lunar material through a circumterrestrial swarm of

particles. Once such a3 swarm has formed, it can gain mass by capturing

iiifalIing planetesimals and ejecta from giant impacts on the Earth, although

the angular momentum supply from these sources remains a problem. Here we

examine the first stage of formation of a geocentric swarm by capture of

planetesimals from initially heliocentric orbits.

The only pla-isible capture mechanism that is not depFndent on very low

approach velocities is the mutual collision of plan e tesimals passing within

Earth's sphere or influence. The dissipe.tion of energy in inelastic collisions

or accretion events changes the valuR of the Jacobi parameter, allowing capture

into bound geocentric orbits. Ruskol noted that this F.-ocoss is most effective

for small bodies with larger area/mass, possiyl^ allowing compositional sorting

(e.g., if mean Fe content varies with size). She assumed that planetesimals

captured in this way would bring net prograde angular momentum sufficient to

ensur ri stab'e orbits about the proto-Earth.

We have tested this capture scenario directly by many-body numerical

integration of planetesimal orbits in near-Earth space. Initial orbits are

chosen randomly from a uniform distribution in a outside the sphere of

influence. Th9 program can integrare up to 200 trajectories simultaneously. For

a reasonable chance of collision with this liSted number, only the planar case

is considered, with approaches within 4 x 10 cm scored as collisions. All

collisions result in coagulation. A p article's status, free or bound, is

determined by the value of the Jacobi parameter. Collisions within the sphere

of influence are scored as free-free, free-bound, or bound-bound. The number of

events is severely limited by computer time, but allows some useful

conclusions.

With no initial random velocities (e = 0), 79 free-free collisions yielded

?7 ca p tures (351), 9 prograde and 18 retrograde. A uniform distribution of

random velocities up to 0.05 times the heliocentric circular velocity yielded

58 free-free collisions with 14 captures (21`(), evenly divided

prograde-retrograde. In both cases, free-bound collisions tend to cause

captured bodies to spiral inward until accreted by Earth. These results agree

with those of Davis and Herbert that the systematic contribution of angular

momentum is insufficient to maintain an orbiting swarm under heavy bombardment.

Thus, a circumterrestrial swarm can be formed rather easily, but is hard to

sustain because the mean net angular momentum of a many-body swarm is small.

The requisite angular momentum can be supplied by a single collision (or a few,

at mosr) of large bodies within Earth's sphere of influence. A swarm formed it

this wav could have subsequently accreted up to several times its original mass

without collapsing on D the planet, possibly allowing enough processing of

proto-lunar material to produce iron-silicate fractionation.

*Research conducted in conjunction with the Tucson Lunar Origin Consortium.

References

I. Chapman, C. and R. nreenherq (1984). This conference.

2. Greenberg, P. t al. (1984). This conference.

3. Davis, D.R. and F. Herbert (1984). This conference.

4. Hartmann, W.K. (1984). This conference.

5. Ruskol, E. (1972). Izv. Earth E y5. 7, 99.
X85 13'752

55

THE DINAR ANGULAR MOMENTUM PROBLEM*; Stuart J. Weidensch;llinq,

Planetary Science Institute, Tucson, Arizona 85710.

Formation of the Moon by classical Darwin-type fission of a rapidly

sninninq proto - Earth requires =3 112 times the present angular momentum of the

Earth-Moon system. Proponents of fission have proposed mechanisms for the

escape of the excess after fission, but have generally assumed that the

proto-Earth could.asquire the requisite angular momentum during its accretion.

'loth numerical ''' and analytic studies yield the result that the systematic

angular momen+urr delive-ed by impacting planetesimals is small, and should

produce a slowly rotating planet. Even the most favorable case of Davis and

Herbert, 2 with !a and 2 distributions maximizing prograde angular ni^mentum of

impacting orbits, yields bare!y the present angular momentum of the system.

More plausible distributions yield much lower values. These results appear to

rule out acquisition of enough angular momentum for fission by the accretion of

small bodies directly impacting the planet.

More angular momentum could be supplied if Earth was surrounded by a

prograde accretion disk. Orbital decay of disk particles due to viscous

spread ; q or "accretion drag" dice +o impact of low angular momentum material

onto th • disk would add mass to Earth with specific angular momentum of

circular orbital motion at ;ts surface. For fission instability, Earth would

have to acquire 3/4 of its total mass via a disk. The low mear angular momentum

delivered by infallinq planetesimals makes it hard to maintain such a disk, but

viscous spreading might prevent it from collapsing onto the planet, while

allowing most of the mass to flow inward. To maintain a steady-state disk, the

mean specific angular momentum of material impacting the disk must equal that

reaching the Fnrth at the surface orbital velocity. The most favorable case of

Davis and Herbert requires a disk radius ()reater than 10R for a steady state;

the processing of so much mass through a region well Atside the Roche limit

would favor a co-accretion scenario rather than fission.

Even if the entirA planetary mass were processed through an accretion

disk, it might not reach rota*tonal instability. At-'80% of the critical angular

momentum, the planet would evolve from an axially symmetric Maclau,-in spheroid

to a triaxial Jacobi ellipsoid. The non-axisymmetric gravitational field would

transfer angular momentum back to the disk beyond the synchronous altitude(=

1.5 R 1 via resonances and density waves. Thus, fission insiab;lity could not

be reeached by disk accretion, either. The requisite angular momentum could be

delivered only by a large (stochastic) impact of a Mars-sized body, but this

scenario does not resemble classical fission.

There is a lesser but similar p roblem with co-ac:retion models. 5 To avoid

orbital collapse by accretion drag, the Moon must gain angular momentum from

the rapidly rotating Earth via tides. Accumulation of the planet by direct

impacts of planetesimals does not supply enough angular momentum. It would

require processing--fl.2 M through an accretion disk, or else a giant impact, to

spin up the proto-EarA before the lunar embryo formed. Thus, co-accretion

requires an earlier event that might in itself have sufficed to form the Moon.

*Research conducted in conjunction with the Tucson Lunar Origin Consortium.

References

1. riuli, R. (1968). Icarus 8, 301.

2. Davis, fl. and F. Herbert (10841. This conference.

3. Weidenschilling, S. (1984). This conference.

4. Harris, A. (1977). Icarus 31, 168.

5. Harris, A. and W. Kaula (1975). Icarus 24, 516.
4/4/13753 5 N8

56

A CIRCU NITF.RRFSTRIAL COMPOSITION'L FILTER*; Clark R. Chapman and

Richard Greenherq, Planetary Science Institute, Tucson, Arizona 85719.

A ma , ior q , restion about the moon is its under-abundance of iron. We seek

(as others have) to understand whether a metal-silicate fractionation of

heliocentric bodies can be achieved through collisional interactions with a

circumterrestrial swarm. Large, dense metallic cores of disrupted,

differentiated planetesimals could pass throuqh such a swarm relatively

unimpeded, while silicate fra(,ments would be filtered out and captured by the

cloud, which might later accrete into a metal-depleted moon. We envisi?n

planetesimals left over during the very late stages of accretion of the Earth

with heliocentric orbits extending a few tenths of an AU beyond the Earth's

orbit. They may have been heated and geochemically differentiated into silicate

budies with iron cores, just as has apparently happene6 to a major fraction of

the asteroid pop ulation. Such bodies would diffuse toward Earth's orbit. We

consider the g ates of such diffusion and of the mutual collisional destruction

within the population. We then consider the interaction of the differentiated

planetesimals and their collisional products (both silicate mantle-fragments

and iron cores) with a swarm of Earth-or-bitinq lunesimals (perhaps ejecta from

the Earth) of km-scale, totaling 0.1 lunar mass, extending out 10 or 20 Earth

radii. We find that such a small near-Earth population of lunesimals can filter

out silicate-rich material, while passing iron cores, and form a moon composed

partly of terrestrial material, but more substantially of the captured

silicate-rich 0ortions of the planetesimals. This silicate-separation process

and accretion of 1^unesimals into the moon must qo to completion in a time short

compared with 10 yr, which is the timescale fnr the planetesimals to be swept

up by, or scattered away by, the Earth.

Our quantitative conclusions are.: (1) Silicate fragments will be trapped,

while iron cores cap pass throuqh the circumterrestrial swarm dozens of times,

taking at least 10 yr before being trapped. (2) Until the supply of

planetesimals is depleted. it may prevent the circumterrestrial swarm from

accretinq: planetesimals larger than 100m in size c)uld f • agment rocky-strength

lunesimals and would impact them on a timescale shorter than tho timescale for

accretion of lunesimals (1 year). (3) The iron cores are large and strong

enough to withstand collisions with the lunesimal population. (4) If collision

velocities among the planetesimals are <300 m/sec, as expected if they are

governed chiefly by gravitational interactions with each other (rather than

with larger planets), then the planetesimals and iron cores will survive long

enough for the segregation process to go to completion. (5) The planetesimals

apparently can diffuse toward the Earths orbit on a timescale of 10 yr by

their own collisional dissipation; they interact with the circumterrestr'al

swarm at velocities of severri1 km/sec, as required for the segregation proces:^.

(6) During the course of 10 yr, the iron cores will be swept up by the Earth

and Venus for gravitationally scattered away), so little metal will remain for

accretion by the moon once the moon forms; the stirring-up of the

circumterrestrial swarm will slow as the cores are depleted, and the moon will

accrete. (7) The whole scenario works only if there is a way to maintain the

hypo-^hesized circuni-rrestrial^ swarm, which otherwise would collisionally

diff.ise on a timescale of 10 yr (much of it collapsing on the 2Eirth); The

source of angular momentum to maintain the swarm remains a mystery.

*Research conducted in conjunction with the Tucson Lunar Origin Consortium.

References: (1) Greenberg gt i:. W)79) Icarus 35, 1. (2) Oasis, D.R. and F.

Herbert (1984). This conf. (3) Weidenschillinq, S.J. (1984). This conf.
N85 85 13'754

F 57

1111' ORIGIN OF THE MOON. John 1'. Wasson, and Paul li.

Warren, In titute of Geophysics and Planetary Physics, University

of Ualil'or ►► ia, Los Angelcs, CA 911024, USA,

Bulk density alone shows that the Moon is depleted in

metallic FVNi relative to the Earth or to chondritic meteorites

having similar• FeO/MgO ratios (e.g., II chondrites). This

depletion implies that the Moon formed not from chondrites but I- rom diflcrentiated material. Origin of the Moon by fission frorn

the Earth offers a simple explanation for its depletion in I:e`i,

but this mechanism seems ,III likeIy because of associated dynamical

difficulties. Furthermore, the currently accepted moment of

inertia, 0.3905+00-13, impIies that the Moon possesses a cord

rou ti hly 300 km in radius, inconsistent with for►natioil I'rom the

maritIC. Lunar volatile clement depletions have been invoked in

support of Fission, but volatile contents of eucritic meteorites

are siiniIarl

y low and the eucrites did not form b

y Earth fission.

Much of the High volatile content of the Earth may reflect

efficient kite capture of volatile-rich cometary material due to

the Earth's possession of an atmosphere.

A more plausible origin of the Moon is that developed by

Rrrskol ( 19

- 7), accretion from the circrunterrestrial swarm. She

suggests that the low FeNi content of the Moon can be understood

it' the mean sire of" interplanetar

y Silicate p:a

► • ticles was much

smaller than that i'or metal particles, since this would have led to

preferential capture of silicates into liarth orbit, but we

question whether the mean particle si-e of the metallic particles

would h;rve been great enough to prevent their capture into the

swa I'M.

Although igneously formed meteorites are much less common

than chondrites, the statistics are hi;rsed beCau5e most

meteorites come from parent bodies that are near or beyond the

Asteroid Belt; it is plausible that most of the asteroidal Sic

holies 1 All from the Sun melted within 100 Ma of the birth of

the solar system, ;rnd that most of the protoplanet.iT'

y matter was

in these holies. The eucritic pare body experienced

metal/si 1 irate I'ractionation --4.4 Gsa ago. We (Wasson ;rrld Warren,

1979) sug ,r;csted that near I AU asteroid di fferent iation occurred

before the hulk of Lhe protolrrnar material had Ihcen captured into

a circurrTterrestrial Swarnr. These drffercntiated bodies broke up

as a result ol" mutual impacts. Fragmentation tended to reduce

the more brittle silicate crusts ;rnd mantles to relatively small

(perhaps -"1 in) si:e, where;rs because of their much greater

strength mane of the metallic: cores Ina 'v have Survived intact.

Collision of small silicate fragments t%ith debris alread

y in

liarth orbit led to the orbital capture, but the debris cloud was

essentially transparent to the metallic cores, and these objects

continued in heliocentric orbit until removed by a dose

planetary encounter of the first or second kind.

Ruskol lc.l,. (1977) In Soviet-Amer. Conf. Cosmochem. Moon Plan.

Wasson J.T. ;rnd Warren 1

1 .11. ( 1979) Lunar 1 1 1_inet. Sci 10, 1310. „ -- J
-11

*tQ ^-

5 ti 1^1 V a^ 13 "5

0

F I MMATION OF THE PRELUNAR ACCRETI ON DISK; A.(,.W. Cameron, HarvardSmithsonian Center for Astrophysics.

Bill Ward and 1 (1,2) were led to the suggestion of a collisional origin of the Moon through the

following consideration. The angular momenturn of the Earth-Moon system is less than sufficient to

spin the Earth to rotational instability, we were nevertheless interested to determine the mass of

the body which, striking a tangential blow to the protoearth, could impart the angular momentum of

the Earth-Moon system to the protoearth. The required projectile turned out to be about the mass

,)f Mars. That defined the basic scenario of our lunar formation process.

in such a collisk- we assume that both target and projectile, having formed fairly rapidly, are

molten and have de,.eloped iron cores. A collision at the escape velocity of 11 km/sec (or slightly

inure) is sufficient to vaporize the rocky mantle of the projectile and a like amount of material in

the mantle of the target. The result is a major explosion centered slightly below the surface of the

protoearth. Material ejected from the site of this explosion would normally, under gravitational

control of the protoearth, be on trajectories which would return it to collide with the protoearth.

We were interested in the fact that, as long as the material was in the form of vapor, it would also

he accelerated by prossure gradients. This would assure that a minor parr of the material would be

the )wn into orbit about the Earth, forming a prelunar accretion disk.

The subsequent process of lunar formation consists of Oe viscous dissipatior of this accretion

disk, accelerated by the fact that the self-gravity of the disk would attempt to form local

condensations in the disk, only to have them sheared apart inside the Roche lobe with

accompanying dissipation (2). The material transported beyond the Roche lobe can accumulate to

form the Moon; the efficiency of the process is 5o high that the disk is likely to be largely

vaporized in the process (3). An accompanying process with observable consequences is the loss of

the Earth's primordial atmosphere by flow along the surface of the accretion disk (4).

At the site of the impact the vaporizat;on of the rock is largely a result of the compression of

the material, and, with an expected efficient nucleation, recondensation of the vapor should occur

before the exploding material has expanded very far. Assuming th.:t the condensed material is still

fairly finely divided (particles of the order of a centimeter), collisions would ordinarily be plentiful

and the material could still act as a compressible fluid. However, relative internal motions are

probably largely eliminated in the vapor phase, and the expansion is thus likely to take place with

little farther transverse acceleration (the. 'gas' is 'cold'). This situation will form a prelunar disk

of small dimensions and the yield of material at the Roche lobe is likely to be too small.

This situation would be improved if relative motions can be induced among the particles when

they have been carried to larger distances from the protoearth following the explosion (so that

they will act as a 'hot gas'). Transverse accelerations under these conditions impart much more

angular rnornentum and make a larger disk. Two processes are under study that can contribute to

such accelerations. one is the resi.fual valor from the more volatile constituents of the original

rocks that is unlikely to be able to recondense under the expected temperature conditions. The

other is the effect of gravitational accelerations acting within the expanding material due to its

own self-gravity. Some simple numerical illustrations of these processes will be presented.

The precise line of impact of the projectile onto the target is not known. However, there is a

substantial probability that the iron core of the projectile will not be absorbed by the protoearth at

the time of this impact, but will be collected on subsequent impact. If so, it is likely to be

substantially fragmented and sheared following the collision, and significant mutual gravitational

effects of these fragments near the top of their trajectories can be quite effective in stirring the

condensed particles. These fragments carry a great deal of angular morp entum, and transverse

accelerations of the condensed particles at these large distances from the Irotoearth can lead to

the formation of a much larger prelunar disk.

REFERENCES: (1) A.G.W. Cameron and W.R.Ward (1976) Lunar Sci. VII, 120-122, LSI,

Houston. (2) W.R. Ward and A.G.W. Cameron (1978) Lunar Planetary Sci. IX, 1205-1207, LPI,

Houston. (3) A.C.Thompson and D.J. Stevenson (1983) Loinar Planetary Sci. XI`J, 787-788, LPI,

Houston. (4) A.G.W. Cameron (1983) Icarus, 56, 195-201.

J
uwic-MALr. N8 5 13756 ^yJ

OF POUR QUALIT't;

r) y

iiI P p 'O 1i i S 1:41)

i..L^l a r, t l'r(11~ t.^, "ei. A.N. h, IU1,1 1, ^. . 1 , 'dcli p y, ), t. :)f : t! t II

t c': )ci-. t;r !::, Jniv«^L.:Lty t)^ (,iltfol'.1'.1, Lu:; R!I ,,:lea t1(i)24.

In mf0tL; of lur,lr origin by great iirl',act., attention it

t,:^uatly pail f,a tLo hyIrodynsuic exl• inc;ion resalt:ny from tlie,

-it ?at imour,t ,)'- thcrala' ,'II • tL j,, . 'luw,;vt!r, th-, -i,oiirce of thi.;

n•!ryy i 'Ifter all, ]rdvitational. Givi, rl th•tt tit'l1

iic;LUFtic)r. i:; inevital:ly si,1111fi 1:ant ill a CIUSE: ai:, roach tut wepa

large t^o,lies (1,1,3, 1 1), it. !-.i tc be Expectod that ,:i,lnific.tt.t

,Iui,uirturc,u fc-)a) or],its auull occur even i;eforc

im;«(ict. '^hesu 4t L1 lirtures coU13 arise from p er ltdni.a1 ef:ort.

,in l hence purely muc:lanica1 m•)4e1 s aze w'crth I ursuing.

For «:l o:; ^ .ip,)ro,ichcs 1,(''u p on pro f. ) Planets tLe --:i •tc• of I.Irs

or 1 1L JEr, we re i• ruLant o,ic1, 1)11nel t d^ an arru,mulatiov of

holi «:s,

tyi)icct11y 125. T!, ­ c:ticulatictl is starte.] Lei,.ind ti..,,,

'sect!) ,? lilvit, !1)It clo;;e eiio-viii that tidal etrf_sses %i11 b? L) iy

enough that o a ,lloi)sl :;c itl the i.ltn-t is vs.;^, ntiallj flui-,.

For economy, th-'! mol ')1 i.; :,.il..itarll.1y .iyrt -jet Lic about th,^ plan

•ief.ne'.] by the initial po.;atiun an" velocity vectors, d11,]

1j1 ,2nt-t Si- iris aze. i1:i.;JClt 1 orf-ho oll-I1 to t.hi:; i:lan r.. Ihp initi_11

configuration is the tidall.y 1ijtorte•1 P)lilsoi I - g out the lint

:;et.ween pl.inet ;. ^he tra ject.;ry of each of tl„a eletrent.11 ludic.,

is c>ilcul-itcd NUnmirl j too yrivitatiunal attractions

of ,111 bniir:, il, ')ot1, p1 1rtr'. T !I'-, I'rin(-iF711 diftic; ilty is

tah in j int.) acru ,lnt r•:s.;-lre f.f.).:c;t., 1)4!tti+«a!11 )-, o1io s which are

in contact, a:; i;; t c-%:;F ho f or,i ti 31. disrurtion and after arty

sui),tr^guetit ccll This; is %un ' y rec]uirirlcl to.:ics withi:l

i rescril)(•)1 distauc-4 of each other to h,Ava itutual accelrration:i

leu(l4111 to zorc. :.Pl ative velJ,:it1, CO;ISELVi%1; 1i1lEaL tri-nent!im

for any gLoup of :;uch 1)o , lies, :)ut. 111 , )winy tl.e yro .;I: to rot•ite.

1hi_ inte,jritiort is uontro' I.-ii m,tiriy by not 111owin3

this simulittion tO CMIS—! an IL'tifi(:ial pulsi ticn.

The to.st Lntt:r,^st.ir.j mtr.^ c`ataineI f c t tpi.roich

offsets which are a :,X,111 al'lltiClo cf the! pl..inet radius and

approach v-:locitie:. ')f i E.M. i second. In tha ca so

:)f an int.er,i^.ti0L t:etuc:,)n 'tar.; aul ::art!I S I z o d lrotovlal1Ct;;,

:Host of tlla! m,it?Lial on ,ls in ­ cllision, Lut a few percert and in

c: 11 lif tic orF i:s .in(l a F•-^w ,)Pr :.:r.t Psca1)e. .hc 1)l a n c t i.;

ty,)ically spun ur. to rilicut. :)n•^ rota t ior. i FivN hour..

,lnothcar tro«iel is to :on.;i'1FL to ')ffset r.ollisi.cn , such a

-iri.,es f. rot I wi l^ r+lnlc )^: i1).,r.t.1ch velocities anti offL;cts.

The f 1ui,1 rt.at ut ti► a 1 1 1-inA ; ii ^.n impief lea el . to es3.!nti.111 f

t t,r-c 1:adie thF -)vial q)i) i ; ,)rti m, 1 l ua the two poLt jock

whirll -,r,! .; h.d:" r-. 3 U:. f, ,i:i Pzp v ilu^sly Sil jC ?it;: I ('),U) . H owo v°.r,

t` k' state of th4- p:.anats at th i:; . tage is tlrcarly juit.e

:istcrtatl, su,l it is not r,:dlisi`ic to tre,lt t h ( m as s1)hPrc:;

nr)til the instant c)F impdct.

neferelic--i: (1) npik 1`+72 i .:;'., 4str.J. 1 1 ): 130; (2)

1it1 g r 1975 7,_.iLiv; ?4: :'Sr,; (3) ,,etnPri11 1970 7.un.FI.Sci.cunf.

7: 3245; (4) Kaula 1'?77 Lu1 ► .^1. :i.con`_. 

n: 321; (5) Jlartman , l F.

'avis 1975 : c ,ivu:, 24, 501; C ixeron r, hard 1076 Iun.Sci. VII:

1'20.

,.......
60 N8 5 137 5"7

TZAR ORIGIN FROM IMPACT ON TIE EAR71h: IS IT POSSIELP David J. Stevenson, Div.

moologi<al and Planetary Sciences, Galtech, Pdsadena, CA 91125.

All themies of lunar origin involve events or processes which seemirgly have low efficiencies or low probabilities or both. The model I am pursuing is an impact-triggered

"fission" lunar origin. Prelimitary results indicate that the process is not necessarily

inefficient ncr unusually improbable. The model is, of course, not new (see refs 1-4,

especially refs. 3,4) but this effort outlined here is the first attempt to replace handwaving with physics. It is : ped to present 5ume detailed modeling at the mee . t

abstract only eutliness the issue_ thet the models seek to address. Part of the work relating ating

to disk evolution has been presented previously; emphasis here , is on the emplacement of earthdwived material into earth orbit immediately after a large impact. lire "Seoorg Burn"

If impact ejecta (a mixture of target and projectile) leave the impact site ballistically and

are subs aquently acted upon only by the gravity field of a spherical Firth, then the e jecta

either reimpacts the Earth or escapes on a hyperbolic trajectory. Hence the need for a

"second burn" ,,^gyve oortsidered three possible resolutions: pressure gradient acceleration, rorr-central gravity and visecus spreading. (i) Pressu,e Gradient Acceleration: The

expanding vap,T-liquid-solid a jecta clod may have highly rr,^Xeplerian motion for distances

at least as great as a few projectile radii away from impact. Suppose that Keplerian motion

occurs after material has reached a height h above ttv surface (measured in units for which

earth's radius = 1) and has initial radial and tangential velocity oomponents at this height

of 6 and 1 - e, respectively (measured in units for which escape velocity _ 1). Assuming h,

6, a all «1, one finds that subsequent periapse occ ri " qw earth's surface IT/2bourd ,

(negative total energy) trajectories provided), h/2 , e ^ 2/3 h , 161 J (h - 9/4 e ) . It

follows that very a e oblique impacts Q10' km bodies) are high desirable since they would

lead to relatively large h with some of the material having low 6 and potentially acceptable

e, leading to orbital inljection, (ii) Non-centrdl grvity can have a somewhat similar effect:

near the impact site and time of impact, the actual gravitational field can have a markedly

rur-central character and a di stance h - few projectile i u::: ; must be exceeded before central

Keplerian motion occurs. Numerical results indicate comparable (but additive) effects to

^ es.%u,c:-gr-ddient aooeleration. (iii) wwider a steady, 2-D visok= jet ? emanating

fYom the earth's surface. it is possible to approximate the equation of motion in the form

dX/dt = Q + cN(' - JU /D where V is velocity at come position in the jet, a is the

acceleration due earth's gravity (assumed central), u is a nmmerical constant, y is the

viscosity ir. the jet (assumed turbulent; due to qwr instabilities), is velocity of the

oentr-,,U streamline (i.e. equidistant from the inner and outer edges of ,jet) which can to

strewn to be approximately Keplerian, and D(t) is the jet width. I find that material in the

outer (ire. further ftom Farth center) part of the jet gains both emV and angular momentum

from viucaus stresses and that orbital injection is possible (and, less commonly, escape) at

the expense of dumping over half of the jet back on the -th.

Spin-out of a Superrotatirlg Atmosphere. Although "second burn" is possible, the

a^ of m^:Lerial emplaced in orc.:

ci

r tends to be small if the impact velocity -escape veloty. This difficulty is reduced if very large impacts occur since they cause an eart]-

ennirciing superrotatirg atmosphere (T - 10 4 K) of Mg0-Si0-0 plus liquid which can bleed out

through the new equatorial plane to V m a disk. A similar model has been proposed for the

formation of the Uranian satellites. The earth is then enveloped in a highly oblate and

opaque silicate atmosphere which has a photaspheric temperature of ^2000 K and a cooling time

-104 yr. In this time, the disk lore evolve and emplace at the Roche limit sufficient material

to form one or more proto-floors. It is concluded that orbital emplacement of material i_Q

Dmsible and that the process is much more efficient fo r very large impacts than small (radius

j0' km) impacts. Major quantitative unoeertainties remain.

Referenoes

(1) Urey, H.C. (1952) "The Planets, their Origin and Development." (Oxford Univ. Press).

(2) Ringwood, A.E. (1979) 'Origin of the Earth and Moon" (Springer-Verlag). (3) Hartmann,

W.K. and Davis, D.R. (1975) Icarus 24, 504-515. (4) Ward, W. and Cameron. A. (1978) L. ,nar

Plane Sci• Abs. IXIV p. 1205-1207. (5) Thompson, A. and Stevenson, D. (1983) Lunar Planet.

Sci, Abs. XIV, p. 787-788. (6) Zeldovich, Ya. and Raizer, Yu.P. (1966) "Physics of Shock

Waves and High-temperature Hydrodynamic P.`jenomena" (Academic Press), p. 103. (7) Birkhoff,

G. and Zarantonello, E.H. (1957) "Jets, Wakes and Cavities" (Academic Press), Ch. XV. (8)

O'Keefe, J.D. and Ahrens, T.J. (1982) GSA Special Paper 190, p. 103-120. (9) Stevenson, D.J.

(1984) Uranus and Neptune: Prot. NASA Workshop, Part IV, p. 13.
AC'IHOR INDEX

Abe Y. i Malcuit R. J. 43 44

Matsui T 4

i Banerjee S. K. 9 MCKlnno" W. B. 34

' Barnes S. 17 Mickelson M. E. 43, 44

IBeachey A. E. 59 Mizuno H. 36, 37

Binder A. 5, 47 Mueller S. W. 34

Boss A. P. 36, 37

N,2whall X. X. 31

i Caffee M. W. 24 P;ewsom H. E. 13, 16

Cameron A. G. W. 58

Cassidy W. A. 45 Rasmussen K. L. 18

Chapman C. R. 51, 56 Ringwood A. E. 14, 46

Cisowski S. M. 8 Runcorn S. K. 10

Conway B. A. 33 Russell C. T. 7

Cox L. P. 38

Scott E. ll. 35

Davis D. R. 51, 53 Seifert S. 14

Delano J. W. 15 Shervais J. W. 21

Dickey J. 0. 31 Stevenson D. J. 60

Dickinson T. 16 C.-A ndle T. D. 24

Drake M. J. 22, 51

DreibuE G. 48 Taylor L. A. 21

Durisen R. H. 35 Taylor S. R. 25

Turcotte D. L. 3

' Fuller M. 8

VanArsdale W. E. 30

Gingold R. A. 35 Vanyo J. P. 29

Goettel K. A. 20

Goodrich C. A. 17 W^nke H. 48

Greenberg R. 51, 56 Warren P. H. 18, 19, 57

Wasson J. T. 57

Hartmann W. K. 39, 51, 52 Weidenschilling S. J. 51, 54, 55

Hartung J. B. 32 Williams J. G. 31

Herbert F. L. 51, 53 Winters R. R. 43, 44

Hohenberg C. M. 24

Yoder C. F. 6, 31

Jones J. H. 22, 51

Kaula W. M. 59

Koeberl C. 23 

Kreutzberger M. E. 22

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