1) Lunar Orbiter tracking data revealed large mass concentrations (mascons) beneath the lunar maria. 2) The largest mascons were found beneath Mare Imbrium, Mare Serenitatis, Mare Crisium, Mare Humorum, and Mare Nectaris. 3) The mascons suggest a relationship between the formation of the maria and large impact events on the lunar surface.

1. craft have been estimated, become the
data which are further processed. These
data contain the local gravity effects,
Reports since the other variables such as sta-
tion motion and the primary seleno-
centric orbit motion have been removed
by the least-squares fit.
Mascons: Lunar Mass Concentrations The next step in the reduction was
to extract accelerations from the resid-
Abstract. Lunar Orbiter tracking data have been processed to supply a qualita- uals. As shown in Fig. 2, they were fit
tively consistent gravimetric map of the lunar nearside. While a simplified model with patched cubic polynomials (5)
was employed, the results indicate that there are large mass concentrations under to smooth the data and to permit deter-
the lunar ringed maria. These mass concentrations may have important implica- mination of an accurate derivative.
tions for the various theories regarding lunar history. Since the spacecraft altitude above the
lunar surface is not constant, the raw
The Lunar Orbiter missions have pro- noise on these data is between 0.1 accelerations represent the gravity
vided both high quality photographs of and 1 mm/sec except at times of pic- effects at the height actually flown.
the moon, and supplementary scientific ture readout where the degradation fac- What we really desired was a measure
information concerning the gravitational tor is approxintately 3. These data per- of the relative acceleration changes for
field of the moon. Previous investigators mit examination of the observed sys- a constant spacecraft altitude because
have concluded that the moon was grav- tematic effects which are in the neigh- such measurements would correlate with
itationally rougher than anticipated in borhood of 10 to 200 mm/sec (65 gravitational changes at or near the
the sense that comparatively high de- mm/sec = 1 hz). lunar surface. For that reason we
gree terms in the spherical harmonic Our sample of doppler data spans normalized the accelerations so that the
expansion would be required for effec- a 10-day period with 80 consecutive plotted values represent the accelera-
tive representation of the gravity field orbits of the Lunar Orbiter V space- tion a spacecraft would have experi- f
(1). This roughness of the moon has craft, during which it was continuously enced if it had been at an altitude of
been- of interest to the Apollo Project tracked. The-spacecraft had the follow- 100 km, under the assumption that the
because of the resulting perturbations on ing orbital characteristics: semimajor typical mascons were at a depth of 50
the trajectory of the Apollo orbiting axis, 2636 km; eccentricity, 0.27; in- km below the surface, that is,
spacecraft. For these reasons, a new clination to the lunar equator, 850; and Anorm = Acomp X (H + 50)2/(150)2
analysis has been done with the use of orbital period, 3hl im. The closest ap-
the accurate tracking data received here proach to the lunar surface was 100 km where Anorm is the normalized accelera-
by the NASA Deep Space Network at 2°N latitude. tion, Acomp is the computed acceleration,
operated by Jet Propulsion Laboratory. The central 90-minute data span, cen- and H is the altitude in kilometers.
We now report that this new proc- tered on perilune, was taken from each While this arbitrary choice was im-
essing (2) of the Lunar Orbiter data has individual orbit for the data processing. posed upon us by our desire to make
produced unexpected results. A study The combined coverage of the 80 orbits, simplifying assumptions, we had some
of local accelerations on the spacecraft south pole to north, east limb to west, evidence as a basis for judgment. The
resulted in a gravipotential map (Fig. was adequate to cover the lunar nearside analysis of distinct spacecraft (with
1) of the lunar nearside which has hemisphere between + 700 latitude and different altitudes) flying over the same
revealed very large mass concentrations longitude. features had yielded 25 to 125 km for
beneath the center of all five nearside Whereas the processing of the 9000 the depth of perturbing points below
ringed maria (Imbrium, Serenitatis, individual data points was a substantial the surface. We therefore chose 50 km
Crisium, Nectaris, and Humorum). In undertaking, involving hand working of as an average depth expectation and
addition, they were observed in the area the punched cards, this was only a small used spacecraft accelerations normalized
between Sinus Aestuum and Sinus Medii fraction of the data available from five to the perilune distance of 100 km.
(presumably a newly discovered ancient orbiters, in several mission phases, with Therefore the accelerations are based
ringed mare), and Mare Orientale. The distinct orbit characteristics. upon these assumptions.
Urey-Gilbert theory of lunar history These data were processed by least- The accelerations are of necessity
(3) has predicted such large-scale high- squares fitting, in which the theoretical measured along the spacecraft-earth di-
density mass concentrations below these model (4) included a triaxial moon rection. Since our method of data proc-
maria, which, for convenience, we shall and gravitation perturbations due to essing is a point-by-point scalar system,
call mascons. the earth, the sun, Venus, Mars, Jupiter, we are left with the unfortunate fact
The Deep Space Network tracks un- and Saturn. It is necessary to limit the that a single data point cannot tell us
manned deep space probes launched by data arcs to 90 minutes in order to ob- from what direction the acceleration
the United States. We use earth-based tain the consistency required for the acted. What we see is the projection of
radio transmissions at 2300 Mhz in a analysis. The method is described in the true acceleration on the spacecraft-
coherent loop between the spacecraft reference (2). where the definite corre- earth vector. Only a modeled system
and receiver. The doppler cycle count, lation between the residuals and varia- fitting substantial quantities of continu-
which is the difference between the tions in the lunar gravitational field is ous data can determine the nature of
transmitted and the received signal, is demonstrated. these geometrical effects.
continuously accumulated and sampled The residuals from the raw doppler Since the true direction of the forces
at regular 1-minute, 30-second or 10- data, obtained after the best selenocent- is unknown, a simplifying- assumption
second intervals. The high frequency ric position and velocity of the space- was made in order to tie the observed
680 SCIENCE, VOL. 161

2. accelerations to the lunar surface for of the angular separation of the space- sumptions that permitted rapid conver-
comparison with a map. Because the craft from the lunar zero latitude and sion of the raw residuals into gravi-
acceleration decreases with the distance longitude point, resulted in increased metric data. As already noted, the
squared, we assumed that the force was uncertainty in the acceleration ampli- qualitative aspects of the new observa-
directly below the trajectory. tudes near the limb. Such a correction tions are rather well preserved at the
The decision to omit a correction for seemed inadvisable because the true expense of some of the quantitative
the projection on the spacecraft-earth direction was unknown. information.
vector, approximately a cosine function These constitute the simplifying as- From the analysis so far we have ob-
Fig. 1. Gravimetric and acceleration map of the lunar nearside. Ranges are indicated below.
Range * Symbol Range* Symbol Range* Symbol
Beyond ±20. +X ± 7.5 to ± 8.5 ±8 + 2.5 to ± 3.5 ±3
±15. to +20. +C + 6.5 to +- 7.5 ±7 + 1.5 to + 2.5 ±2
+11.5 to -+-15. +B + 5.5 to + 6.5 ±6 + 0.5 to ± 1.5 +1
± 9.5 to +11.5 ±A + 4.5 to ± 5.5 +5 .o to + 0.5 +0.
± 8.to±9.5 ±+ 9 +9
+3.5 to 4.5 +4 -0.5 to 0.0 -0
* Above X 0.1 = mm/sec2; above X 10 = milligals; these scaling factors also apply to the cover.
16 AUGUSI 1968
681

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I SCIENCE, VOL. 161

4. tained 74 sets (of the 80 orbits, 6 had Table 1. Large positive mascon locations; occur directly above it. Since these
insufficient data) of normalized accel- P.E., probable error.
largest accelerations would be normal
erations (mm/ sec2) as shown in Fig. 1. Latitude Longitude to the probe-earth line at this longitude,
Ringed (deg) (deg)
While this approach was intended pri- maria one would expect essentially zero effect
marily to supply a priori information Value P.E. Value P.E. from the overflight orbit. However,
for a more quantitatively precise fu- Imbrium 32 1 -17 1 from a family of orbits bracketing this
ture analysis, the results proved to be Serenitatis 25 1 19 1 time, we would expect those preceding
very enlightening and of scientific im- Crisium 18 1 56 2 the overflight to show a modest positive
portance. We emphasize that approxi- Nectaris -15 1 33 1 acceleration (at about -220 latitude),
mations were made, and those intend- Humorum -23 1 -38 1 and those following to show a negative
ing to analyze these data should use Orientale -20 3 -95 3 acceleration. The residuals from these
caution as dictated by the limitations Aestuum-Medii 6* 5 -6* 5 several orbits were plotted and accelera-
on the method of data reduction. * Missing data and possible skewed fits make It tion estimates were computed for lati-
A contour plot made from the data impossible to ascertain whether there are one or
two mascons in this location. More sophisticated tude -22° with eight consecutive orbits
of Fig. 1 is shown on the cover. The processing and additional data from other orbiter (Table 2) resulting in precisely the
missions will permit a determination.
large, distinct positive accelerations effect predicted.
show clearly, along with a smaller Examination of the detailed computer
number of distinct negative accelera- output permitted consistent estimates of
tions. Large positives lie in the centers The contour map shows two nearby
mascons of intermediate magnitude be- the mascon locations. Results of these
of all five major ringed maria, (Tm- computations are presented in Table 1
brium, Serenitatis, Crisium, Humorum, tween Sinus Aestuum and Sinus Medii.
A crucial orbit is missing which might with the estimated probable error.
and Nectaris). The large rate of change Even though we have computed both
in the accelerations over these mascons further clarify the situation, and it is
masses and depths for the largest mas-
reveals their relatively small physical not possible with these data to specify
whether they are really distinct. How- cons, nevertheless our present quanti-
extent (50 to 200 km). In fact, anal- tative data require further refinements.
ysis of the detailed computer results ever, this location certainly contains at One may easily compute approximate
indicates a nonspherical troughlike least one definite mascon. It is consis-
masses from an assumed depth, such
mass distribution (approximately 50 by tent to assume that this represents an
as 50 km. The Mare Imbrium mascon
200 km, running generally east-west) ancient feature similar to one of the
modest ringed seas, obliterated by the yields numbers on the order of 20 X
in Imbrium and Serenitatis. 100 lunar masses. A spherical nickel-
Apart from occasional short intervals debris from Serenitatis and Imbrium
and only now revealed by the new data. iron object about 100 km in diameter
of poor fit, the data are consistent. We would be a rough equivalent. This type
have plotted the individual polar orbits There are other, lesser mascons which
may or may not be significant. There of calculation gives a qualitative esti-
(south to north) on Fig. 1. The con- mate for the large size of these objects.
sistency of the results can easily be seen may be correlations with the Russian
magnetometer observations from their The presence of large mascons under
by reading across the (Fig. 1) map, every ringed maria, excepting Iridum,
that is, from west to east, perpendicular Luna probes if data over the specific in-
dicated areas is analyzed. Their pre- and their relative absence elsewhere,
to the individual polar orbits. The varia- suggests a relation between the two
tions tend to be smooth and consistent liminary statement of results, however,
does not indicate any unusual effects. phenomena which may be similar to
across the adjacent orbit sets. Further- that suggested by Urey and Gilbert (3).
more, computed comparisons between A side investigation of residuals not
displayed in Fig. 2 was the examination Among questions that arise are these:
residual accelerations from an equatorial Does each of these mascons represent
orbiter (Lunar Orbiter III) agree well of data in the area of Mare Orientale
an asteroidal-sized body which caused
with the plotted data. These results (-20° latitude, -950 longitude), a
ringed mare discovered by Lunar Or- its associated mare by impact? If not
constitute the data consistency check
and initial estimate of gravity varia- biter photography. If Orientale has the simply the original impactor itself, by
tions which was sought from the sim- same characteristics of the other ringed what processes were they formed in
plified approach. maria, then high accelerations should the lunar interior? Is the presence of
these objects consistent with a molten
The more diffuse positive and nega- lunar interior?
tive variations shown on the plots near Table 2. Accelerations in the area of Orien- Urey has proposed (6) that the
the limbs must be considered doubtful tale, 850 to 99°W longitude at a fixed 220S Mare Imbrium collision object entered
because of the normalizations intro- latitude.
at Sinus Iridum as a low elevation im-
duced in the data processing. An orbit Orbit Accelerations pactor. Others have held that the two
determination program, operating un- No. (mm/sec) 2 features are independent formations.
der a least-squares fitting, should mini- 88 0.22 Sinus Iridum is the smallest ringed mare
mize the sum of residuals squared and 89 0.14 and, Urey (6) and others (7) have
tend to compensate for large variations 90 0.22 pointed out, is a unique feature. It is
by "splitting the difference." Precisely 91 0.13 larger than the other ringed and filled
how this effect has appeared in the re- 92 0.08 craters such at Ptolemaeus, and it has
sults is difficult to determine with cer- 93 -0.17* a similar appearance to the larger ringed
tainty, but we suspect that the large 94 -0.17
95 -0.09
maria discussed. If it is a separate im-
accelerations will give rise to diffuse, pact mare, we expect that its physical
lower amplitude, opposite and com- 96 Occultation
pensating signatures in the processed * Taking geometry into account, and noting the properties would resemble the others,
zero-crossing in the table, permits determination and hence contain a mascon for the
data. of the Orientale mascon's longitude at -95°. same reasons.
16 AUGUST 1968
683

5. ^Examination of the data reveals in- local gravity effects," Tech. Rep. 32-1072 (Jet
Propulsion Laboratory, Pasadena, Calif.). filling of silicates. The silicates on the
stead a sharply defined, negative ac- 3. H. C. Urey, in Vistas in Astronomy, A. Beer, exterior can very easily be overlooked,
celeration. While this appears to support Ed. (Pergamon Press, London, 1936), vol. 2,
p. 1676. since they often are rust colored.
the Urey hypothesis, it is not necessarily 4. M. R. Warner and M. W. Nead, "SPODP- The present mass of Colomera is
inconsistent with other selelenological Single precision orbit determination program,"
Tech. Mem. 33-204 (Jet Propulsion Laboratory, 129.3 kg including all known pieces
theories. Pasadena, Calif., 1965). which are in the Spanish and U.S. Na-
5. T. M. Lang, "LEASTQ-A program for least-
P. M. MULLER squares fitting segmented cubic polynomials tional museums. Considering the thick-
W. L. SJOGREN having a continuous first derivative," Space ness of the cuts made on this meteorite,
Programs Summary (Jet Propulsion Labora-
Jet Propulsion Laboratory, tory, Pasadena, Calif., 1968). and the loss of weight during removal
Pasadena, California 6. H. C. Urey, in Physics and Astronomy of the of the rust, we can account for almost
Moon, Z. Kopal, Ed. (Academic Press, New
York, 1962), pp. 484-486. all the originally reported mass of
7. R. B. Baldwin, The Face of the Moon (Univ. 134 kg.
References and Notes of Chicago Press, Chicago, 1958), p. 200.
8. We thank R. W. Davies, W. L. Kaula, J. The surface of the meteorite shows
Lorell, P. Gottlieb, and D. W. Trask for many
1. W. H. Michael, Jr., R. H. Tolson, J. P. helpful contributions; D. K. Davies (and clear evidence of subjection to some
Gapcynski, Science 153, 1102 (1966); J. Lorell,
in Measure of the Moon, Z. Kopal, Ed. (Gor- staff), R. Hanson, P. A. Laing, C. L. Law-
son, and R. N. Wimberly for programming
form of forging; hammer and chisel
don and Breach, New York, 1967); R. H.
Tolson and J. P. Gapcynski, Proc. Int. Space and computer assistance. This paper presents marks are obvious at several points. The
Sci. Symp. 8th (COSPAR assembly, London, the results of one phase of research carried extent to which the marks reflect its
July 1967); J. Lorell and W. L. Sjogren, out at the Jet Propulsion Laboratory, Cali-
Science 159, 625 (1968). fornia Institute of Technology, under NASA history before acquisition by the Span-
2. P. M. Muller and W. L. Sjogren, "Consistency contract NAS 7-100. ish Museum is not obvious. Fractures
of Lunar Orbiter residual with trajectory and 1 August 1968 in the meteorite and certain peculiar
surfaces suggest that a fragment of iron
was ripped away over a single area of
about 100 cm2. We cannot prove that
the present surface is primary and was
Potassium-Feldspar Phenocrysts in the Surface of not significantly altered by ablation.
The mean density of the meteorite,
Colomera, and Iron Meteorite determined on the main mass with a
dynamometer in a Joly balance experi-
Abstract. Silicate aggregates, including large single crystals of potassium feldspar ment, is 7.613 + 0.048 g/cm3. Estimat-
as long as 11 centimeters and sodium feldspar, are embeded in the surface of the ing the densities of the metallic iron-
medium octahedrite Colomera. Silicate nodules in the interior appear to be much nickel and silicate at 7.88 to 7.90 (4)
smaller (about 0.3 centimeter). Glass nodules are abundant both on the external and 3.3, respectively, and correcting for
surface and in the interior. These observations are evidence that some iron the presence of remnant rust, we
meteorites formed as segregations within a silicate matrix and did not originate obtain a range of 4.3 to 5.8 percent
in a metallic planetary core. (by volume) of silicates in the whole
meteorite. Clearly, large amounts of sili-
The presence of silicates in iron drid, the main mass was shipped to our cate inclusions are incompatible with
meteorites has been well known for laboratory. the density data, the suggestion being
more than a century. Because of the ob- Preliminary observations showed that that the abundance of large silicate
vious importance of the existence of the large silicate inclusions, partly obscured masses on the exterior is not typical
metallic iron, interest in the silicate frac- by rust, occur on the surface. In order of the interior. This idea is supported
tion has been rather small until recently. to remove the rust, we cleaned the main by the observation that a large sur-
The presence of a wide variety of sili- mass by "sand blasting" with TiO2 face inclusion, exposed on the cut sur-
cates in iron meteorites (1) in much spherules. During this process it was face and in the adjacent slices, extends
lower abundance than in pallasites and discovered that the external surface to a depth of only about 1 cm.
mesosiderites has permitted use of the contained at least four large silicate Several samples were taken from
Rb87-Sr87 and K40-Ar40 methods for the inclusions. One of these exhibited the each of the large external-surface in-
dating of these objects (2). Previous characteristic reflectance of a single clusions and from several of the small
work in our laboratory on silicate in- cleavage fragment; it was 11 cm long, spheroidal inclusions exposed on both
clusions from several iron meteorites 2.5 cm wide, and deeper than 1 cm. In the external surface and the cut surface;
showed very regular Rb87-Sr87 and K40- the center of this inclusion were large they were studied optically, by x-ray
Ar40 ages of about 4.6 x 109 years. Sili- green pyroxene aggregates (- 1 cm). diffraction and electron-microprobe
cate material taken from slices of Several of the largest of these inclusions techniques. Quantitative chemical anal-
Colomera yielded results that did not were found in depressions or "holes" yses were obtained from the microprobe
define an isochron. These anomalous in the surface. In addition to these large analyses using the procedures described
results suggested that this meteorite silicate masses, small (- 0.3 cm) by Bence and Albee (5). The average
merited further investigation. droplike masses were present on the chemical analysis for each phase is
Colomera, first recognized as an iron external surface, resembling in size reported in Table 1.
meteorite in 1934 (3), was found and form the silicate inclusions Macroscropic examination of the
buried in a patio in the village of Colo- present in the interior and ex- largest of the surface inclusions revealed
mera near Granada (Spain) in 1912. posed on the cut surface. These small two distinct phases: the large crystal
The original mass was reported as 134 inclusions also occur in slices of Colo- (11 by 2.5 cm) already mentioned for
kg; it measured roughly 50 by 40 by mera. Some of the surface pockets are which a cleavage face was observed,
16 cm. By courtesy of the Museo Na- almost covered with- silicates that sug- and aggregates or single crystals of
cional de Ciencias Naturales de Ma- gest that they are relicts of a continuous green pyroxenes (- 1 by 1 cm). Sam-
684 SCIENCE, VOL. 161