NEEP533 Course Notes (Spring 1999)
Resources from Space

Lecture #10: What does the Moon tell us about the Earth?

Evolution of the Moon: The Apollo Model

Visuals

Slides illustrating the phases of lunar evolution and providing reconstructions of various periods.

Notes

Before Apollo 11, up through the completion of the Lunar Orbitor photographic missions and the Lunar Surveyor automated lander and surface analysis missions, we knew a lot about the relative sequence of major events in lunar history but not the span of time each represented.

Pre-Apollo Model for Lunar Evolution

After the analysis of Apollo 11-17 mission data we now had significant knowledge about the absolute ages of the stages of lunar evolution as well as details of events and processes about which we had never dreamed.

Apollo Model: Stages of Lunar Evolution

Cratering on the Moon: A dominating process!

Almost everything we think we know about the Moon must be viewed through the mask of cratering that has dominated lunar history from its origin to the present. The impact of comets, asteroids, meteors, micrometeors, dust, and energetic particles has modified the surface and near-surface expression of all the internally generated processes that have led to the physical nature of the Moon today.

Most comet, asteroid, meteor, and micrometeor impact velocities are between 15 and 20 km/sec, giving associated target pressures at the point of impact of several hundred GPa and extraordinary amounts of heat as conversion of kinetic energy into forward and rearward shock waves takes place almost instantaneously. About 98% of the projectile is melted, vaporized, or ionized and returned to space based on the amount of extralunar material that can be identified in the lunar regolith.
The general characteristics of craters as a function of diameter are as follows:
<10 m diameter craters (transition up to 100 m diameter)
o normally do not penetrate regolith
o depth to diameter ratio variable
o glass-lined pits in center of fresh craters
o deep pits in the center of some craters
o inner benches common if target material statified
o impact breccias present in and outside larger craters
>100m <10 km diameter craters (transition up to 30 km diameter)
o normally penetrate regolith
o hemispherical hole with raised rim
o depth to diameter ratio 1:3 to 1:4
o ejecta blankets extend to about one crater diameter
o target strata overturned but preserved in ejecta blanket
o inner benches and impact breccias associated with smaller craters
>30 km <100 km diameter craters (see Copernicus)
o flat floors with central mound or peak
o slumped (landslide) benches on walls
o indications of pools and flows of impact melt
o ejecta blankets extend to about one crater diameter
o target strata overturned but preserved in ejecta blanket
o secondary impact craters, crater clusters, and crater chains extent many crater diameters
>100 km diameter basins (see Orientale)
o generally flat, fractured floors with partial of complete inner rings of peaks
o slumped (landslide) benches on walls
o extensive pools and flows of impact melt inside and outside the primary impact crater rim
o ejecta blankets extend many crater diameters with evidence of significant lateral flow (do lithostatic pressures become a factor in diverting excavation from near vertical to near horizontal?)
o one to six rings of mountains outside primary impact crater rim (multiring basins generally >400 km in diameter, largest are South Pole-Aitken with a diameter of about 2500 km and Procellarum, 3200 km) .


Stage One: The Beginning - 4.56-4.57 eons. The Moon formed contemporaneously with the Earth.

  • Major Issue: Did the Earth and Moon and the other terrestrial planets form directly from the condensing solar nebula or was there an intermediate step of planetesimal formation?
  • Initial condensation as well as planetesimal growth would be the result of a slight tendency of particles to stick together after collisions.
  • According to modeling studies (D.N.C. Lin as reported by Peterson, 1993), this would be followed by "runaway" aggregation as gravitational forces of the more massive of the planetesimals began to play a larger and larger role?
  • Major Issue: Did the Moon form by impact induced fission of the Earth or by capture of a nearly co-orbiting small planet by the Earth (Hartmann, 1986 and Alfven and Arrhenius, 1972, Spudis, 1996, Taylor, 1999)?
  • Modeling simulations suggest that the collision of a Mars-sized asteroid with an early, differentiated Earth could have ejected debris that would have coalesced to form the early Moon and impart the existing high angular momentum to the Earth-Moon system.
  • Planetary capture of natural satellites has occurred frequently as shown by six satellites in the solar system that are in retrograde orbits. This origin is supported by the following arguments:
  • "Runaway" accretion of the Moon and other terrestrial planets from smaller planetesimals would result in a cool, initially undifferentiated interior with temperatures increasing outwardly as the energy of accretion increased.
  • An initially and still relatively cool interior of the Moon is supported by 1) primordial elemental and isotopic compositions associated with volcanic materials of deep-seated origin and 2) trace element systematics in lunar rocks that are not consistent with earlier differentiation as part of the Earth
  • Strong evidence that only about the outer 500 km of the Moon was molten and fully differentiated (see Magma Ocean discussion below).

    •
  • SUMMARY OF THE ORIGIN OF THE MOON
  • CONSTRAINTS RELATED TO THE MOON
  • OLDEST LUNAR ROCKS CRYSTALLIZED FROM SILICATE MELTS BETWEEN 4.5 AND 4.6 BILLION YEARS AGO.
  • ELEMENTS OF THE ATOMIC NUMBER OF SODIUM (22) OR LESS ARE DEPLETED IN SAMPLES OF THE MOON'S CRUST AND MANTLE
  • THE MOON BEGAN WITH A GLOBAL SILICATE MAGMA OCEAN AT LEAST 480 KM DEEP
  • THE MOON'S DENSITY REVERSES FROM 3.6 TO 3.3 BELOW ABOUT 500 KM.
  • BELOW ABOUT 500 KM, THE MOON IS NOT GEOCHEMICALLY DIFFERENTIATED WITH RESPECT TO LEAD AND MANY VOLATILE AND SIDEROPHILE ELEMENTS
  • Hf/W SYSTEMATICS INDICATE THAT FeS SEPARATION FROM THE MAGMA OCEAN WAS COMPLETE ABOUT 51 MILLION YEARS AFTER THE SOLAR SYSTEM FIRST FORMED
  • AT LEAST 45 LARGE IMPACT BASINS FORMED ON THE MOON IN ITS FIRST 600 MILLION YEARS AND SOME WOULD HAVE AFFECTED ANGULAR MOMENTUM AND ROTATIONAL AXIS ORIENTATION ORIGIN OF THE MOON
  • CONSTRAINTS RELATED TO THE MOON AND THE EARTH
  • RATIOS OF OXYGEN ISOTOPES IN THE EARTH AND THE MOON ARE THE SAME
  • THE DENSITY OF THE EARTH IS 5.5 G/CM3 AND OF THE MOON IS 3.3 G/CM3
  • THE MANTLES OF THE EARTH AND THE MOON HAVE DISTINCTLY DIFFERENT SIDEROPHILE ELEMENT SIGNATURES
  • REFRACTORY ELEMENT CONCENTRATIONS ARE HIGHER IN THE MOON THAN IN THE EARTH
  • THE MOON AND THE EARTH HAVE DISTINCT DIFFERENCES IN VARIOUS ISOTOPIC RATIOS
  • ANGULAR MOMENTUM OF THE EARTH-MOON SYSTEM IS HIGHER THAN ANY PLANET-SATELLITE SYSTEMS AROUND THE SUN
  • THE MOON IS SEPARATING SLOWLY FROM THE EARTH DUE TO TIDAL INTERACTIONS AND IS ESTIMATED TO HAVE BEEN UNSTABLE ABOUT ---- BILLION YEARS AGO.
  • OLD HYPOTHESES
  • SIMULTANEOUS FORMATION AS A DOUBLE PLANET SYSTEM
  • PROBLEM WITH ANGULAR MOMENTUM AND DIFFERENCES IN DENSITIES AND COMPOSITIONS
  • BREAK-UP OR FISSION FROM A RAPIDLY SPINNING EARTH
  • PROBLEM WITH TOO MUCH ANGULAR MOMENTUM AND DISTINCT COMPOSITIONAL DIFFERENCES
  • DISINTEGRATION OF NEAR-EARTH CROSSING PLANETESIMALS
  • PROBLEM IN MODELING THIS HYPOTHESIS
  • CAPTURE OF AN INDEPENDENTLY EVOLVED PLANET
  • STILL UNDER CONSIDERATION
  • RECENT ADDITION
  • EARTH IMPACT OF A MARS-SIZED PLANETESIMAL
  • CURRENT HYPOTHESES
  • EARTH IMPACTOR HYPOTHESIS
  • Mars-sized planetesimal impacted a young Earth after separation of their iron-rich cores
  • EVIDENCE
  • COMPUTER MODELS APPEAR TO PRODUCE A MOON
  • CAN ACCOUNT FOR THE HIGH ANGULAR MOMENTUM OF EARTH-MOON SYSTEM
  • CAN ACCOUNT FOR THE MOON'S OUT-OF-ECLIPTIC ORBIT AND NON-PARALLEL ROTATIONAL AXIS
  • CAN ACCOUNT FOR THE LOSSES IN ELEMENTS BELOW THE ATOMIC NUMBER OF 23 IN THE MOON'S CRUST AND UPPER MANTLE
  • CAN ACCOUNT FOR THE MOON'S TOTAL IRON DEPLETION OVER CHONDRITES AND THE EARTH
  • CAN PRODUCE A MAGMA OCEAN
  • CAN ACCOUNT FOR COMPOSITIONAL DIFFERENCES BETWEEN THE EARTH AND THE MOON BY SELECTING SPECIFIC PORTIONS OF THE EARTH AND THE IMPACTOR TO FORM THE MOON
  • PROBLEMS
  • REQUIRES THAT THE ENTIRE MOON BE INITIALLY MOLTEN AND ACCRETED FROM DEVOLATILIZED MATERIAL, I.E., DOES NOT ACCOUNT FOR MOON'S LOWER MANTLE APPARENTLY LARGELY UNDIFFERENTIATED COMPOSITION
  • REQUIRES THAT THE IMPACTOR BE ACCRETED FROM THE SAME OXYGEN RESERVOIR AS THE EARTH.
  • DOES NOT ACCOUNT FOR A NECESSARY DENSITY REVERSAL BELOW UPPER MANTLE
  • REQUIRES THAT DIFFERENTIATION OF THE EARTH AND THE IMPACTOR, AND THEIR IMPACT, OCCUR WITHIN THE HF/W 55 MILLION YEAR MODEL AGE FOR THE LUNAR MAGMA OCEAN
  • DOES NOT ACCOUNT FOR THE CUMULATIVE EFFECT OF MANY LARGE IMPACTORS ON THE MOON'S NON-PARALLEL ROTATIONAL AXIS
  • DOES NOT ACCOUNT FOR THE NECESSARY CHRONOLOGY OF TIDAL SEPARATION OF THE EARTH AND MOON ORIGIN OF THE MOON
  • EARTH CAPTURE HYPOTHESIS ­ 1
  • The Moon and the Earth formed as independent planets in the same part of the solar system with the Moon being captured by the Earth prior to at least 1 billion years ago.
  • EVIDENCE
  • CAN ACCOUNT FOR THE SIMILARITY IN AGES OF ORIGIN OF THE MOON AND THE EARTH
  • CAN ACCOUNT FOR THE MOON'S LOWER MANTLE BEING APPARENTLY LARGELY UNDIFFERENTIATED
  • CAN ACCOUNT FOR THE LOSSES IN ELEMENTS BELOW THE ATOMIC NUMBER OF 23 IN THE MOON'S CRUST AND UPPER MANTLE
  • CAN ACCOUNT FOR THE MOON ACCRETION FROM THE SAME OXYGEN RESERVOIR AS THE EARTH.
  • CAN ACCOUNT FOR COMPOSITIONAL DIFFERENCES BETWEEN THE EARTH AND THE MOON BY DIFFERENT EFFECTS DURING PRIMARY ACCRETION
  • CAN ACCOUNT FOR A HF/W MODEL AGE FOR THE LUNAR MAGMA OCEAN OF 55 MILLION YEARS
  • CAN ACCOUNT FOR A NECESSARY DENSITY REVERSAL BELOW THE MOON'S UPPER MANTLE
  • CAN ACCOUNT FOR THE CUMULATIVE EFFECT OF MANY LARGE IMPACTORS ON THE MOON'S ROTATIONAL AXIS
  • CAN ACCOUNT FOR THE NECESSARY CHRONOLOGY OF TIDAL SEPARATION OF THE EARTH AND MOON
  • PROBLEMS
  • ACCOUNTS FOR THE HIGH ANGULAR MOMENTUM OF EARTH-MOON SYSTEM ONLY BY THE ASSIMILATION SEVERAL LARGE IMPACTORS AFTER CAPTURE
  • ACCOUNTS FOR THE MOON'S OUT-OF-ECLIPTIC ORBIT NON-PARALLEL ROTATIONAL AXIS ONLY BY THE CUMULATIVE EFFECT OF MANY LARGE IMPACTORS
  • ACCOUNTS FOR THE MOON'S TOTAL IRON DEPLETION OVER CHONDRITES AND THE EARTH ONLY BY BEING NON-COMPETITIVE WITH THE ACCRETION OF A MORE MASSIVE, CO-ORBITING EARTH
  • Major Issue: Once formed, to what extent did the Earth, Moon, and other terrestrial planets follow similar courses of geologic evolution?
  • Initial differentiation of sufficiently heated outer portions of the terrestrial planets and the Moon probably followed similar courses, including formation of a calcium-rich crust over a magnesium and iron-rich mantle with the protocore formed by the gradual migration of immiscible iron-sulfur liquid to the center of the planet (see Stage Two, below).
  • Planets as large as the Earth and Venus continued to differentiate as radioisotopic heat, in excess of that that could be transferred to the surface by conduction, induced internal convection which recycled the early crust and mantle (plate tectonics on Earth, overplating on Venus,) and continually modified the chemical composition of each and of the core .
  • Geochemical evidence from the oldest crustal rocks on Earth (4.0 billion year old and younger Archean rocks) indicates that such recycling began very early (Bowing and Housh, 1995).
  • Recycling of the crusts and mantles of the Moon and Mercury have not occurred due to the absence of significant thermal convection and plate tectonics.



    • Stage Two: The Magma Ocean - 4.56-4.4(?) eons. Accretionary melting, due to conversion of kinetic energy to heat, FexNiySz liquid migration towards the Moon's center, and the decay of short lived radioisotopes (e.g. 26Al, 182Hf, etc.) created an approximately 500 km deep magma ocean.

    Loss of some elements and redistribution of others in this magma ocean (differentiation) occurred because of volatile depletion due to high temperatures and low gravity, and crystal settling and floating as the magma ocean cooled.

    Reaction of migrating iron liquid with any accreted water in the unmelted lower mantle to produce hydrogen and FeO probably would be sufficient to eliminate any remaining internal indigenous water from the entire Moon as no evidence of such water has be found in Apollo samples.

    One of the more illuminating differentiation processes during the Magma Ocean stage was that involving Hf and W (Lee, et al., 1997). The systematices of this pair is made even more interesting by the fact that 182Hf is radioactive, decaying to 182W with a half life of 9 million years. The following figure summarizes the Hf-W data obtained by Lee,et al (1997, Table 1) in the context of the model of lunar evolution that assumes both a magma ocean and a largely undifferentiated lower mantle as contained in the Capture Hypothesis summarized above:
     

    Schematic representation of the interior of the Moon showing Hf-W systematics (data from Lee, et al., 1997, Table 1) relative to the period 4.44 to 4.0 billion years.

    • SUMMARY OF Hf-W SYSTEMATICS FOR THE MOON - 4.55-4.0 BY
      • SIGNIFICANT LOSS OF FROM THE MAGMA OCEAN WILL OCCUR WITH THE EARLY SEPARATION AND DOWNWARD MIGRATION OF FexNiySz LIQUID.
      • MOST 182Hf WILL DECAY TO 182W (HALF-LIFE 9M YEARS) BEFORE CRYSTALLIZATION OF THE BULK OF THE MAGMA OCEAN.
        • THE CLOSING OF THIS SYSTEM IN THE MAGMA OCEAN MAY BE REFLECTED IN THE ~45 MY MODEL AGE IT GIVES FOR THE APOLLO 17 CLEAN ORANGE GLASS (APPARENTLY DERIVED FROM THE DEEP UPPER MANTLE) AND THE APOLLO 15 OLIVINE BASALTS (APPARENTLY DERIVED FROM THE LOWER PORTIONS OF THE UPPER MANTLE) BOTH OF WHICH APPEAR TO HAVE BEEN PRODUCED BY THE PARTIAL MELTING OF EARLY CUMMULATES IN THE MAGMA OCEAN THE MODEL AGE OF WHICH IN TURN MAY REFLECT THE END OF 184W AND 182W REMOVAL BY FexNiySz LIQUID.
      • THE COMPOSITION AND MODEL AGES (~67 MY) OF APOLLO 17 NORITES AND AN APOLLO 16 Mg-GRANULITE (APPARENTLY DERIVED FROM SPLASH INTRUSIONS FROM THE MAGMA OCEAN INTO THE LOWER CRUST) MAY INDICATE THE CLOSING OF THE Hf-W SYSTEM IN THE THERMALLY AND DYNAMICALLY MORE AGITATED UPPER MAGMA OCEAN.
      • THE ISOTOPIC PROPORTIONS IN APOLLO 16 FERROAN ANORTHOSITES (APPARENTLY DERIVED FROM THE UPPER CRUST) MAY INDICATE THE OVER-ALL PROPORTIONS IN THE UPPER CRUST.
        • THE HIGHER PROPORTIONS OF 182W IN THESE ROCKS MAY INDICATE THAT THE CRUST HAD BEGUN TO FORM BEFORE ALL 182Hf HAD DECAYED. THEIR MODEL AGE OF ~53 MY MAY REFLECT THE TIME OF ISOLATION OF AN INCREASINGLY COHERENT UPPER CRUST FROM THE MAGMA OCEAN.
      • THE VERY HIGH Hf AND W IN THE APOLLO 15 KREEP BASALTS INDICATE THE CONCENTRATION OF THESE REFRACTORY ELEMENTS IN THE RESIDUAL LIQUIDS OF THE MAGMA OCEAN.
        • THEIR MODEL AGE (~53 MY) MAY INDICATE WHEN THESE LIQUIDS BECAME A CLOSE SYSTEM, I.E., ABOUT THE SAME TIME AS THE UPPER CRUST. THIS SUGGESTS THAT THE MAGMA OCEAN MAY HAVE BEEN LARGELY CRYSTALLIZED AT THIS TIME EXCEPT FOR THESE RESIDUAL LIQUIDS.

      • THE VERY HIGH 182W AND 180Hf APOLLO 11 OLD HIGH Ti BASALT MAY INDICATE THAT BOTH Hf ISOTOPES WERE CONCENTRATED IN THE LATER LIQUIDS THAT CRYSTALLIZED INTO THE UPPER PORTIONS OF THE UPPER MANTLE.
      • THE DISTINCTIVE ISOTOPIC PROPORTIONS IN THE APOLLO 17 BULK ORANGE GLASS (APPARENTLY RELATED IN PART TO THE VOLATILES ON THE SURFACES OF THE GLASS SPHERES) SUGGEST A SOURCE REGION DISTINCT FROM OTHER BASALTS. THIS IS CONSTANT WITH OTHER INDICATIONS THAT THESE VOLATILES WERE DERIVED FROM BELOW THE UPPER MANTLE.

        The above analysis and interpretations rest in part on the assumption that radioactive 182Hf was created shortly before the accretion of the terrestrial planet, including the Moon. The apparent consistency of Hf/W systematics and the derived model ages with a plausible detailed model for the evolution of the Moon lends support for the origin of 182Hf, 26Al (daughter 26Mg found in C1 chondrites) having been in a near-by supernova that was coincident in timing with and possibly responsible for the collapse of the interstellar cloud that became the solar nebula (see Cassen and Woolum, 1999, p 56).

      • Major Issue: Although probably created by the accumulation of planetismals of varying composition, how homogeneous did the Magma Ocean become as it was stirred by continuing and increasingly energetic accretion and convection?
    • Spectroscopic analysis of Clementine and Galileo images of deep impact basins on the lunar far-side from which significant depths of crustal material have been removed suggest that the most of the present crust is more chemically heterogeneous than that exposed at the surface.
    • Similar heterogeneity has been mapped by Pieters in much smaller craters (see Nozette, et al., 1994, p1844-1848) using Clementine and telescopically gathered spectra.
    • Rather than suggesting that the deep impacts penetrated 50-70 km of crust to excavate lunar mantle material (P. G. Lucey et al., 1994 in Nozette, et al., 1994 and, 1996, as reported in Science News, 149, 204), these heterogeneities may reflect the mixing of Magma Ocean melt at various stages of differentiation with the still forming crust, an extremely dynamic process that was occurring as the crust formed on the still partially molten mantle.
    • Stage Three: The Cratered Highlands - 4.4(?)-4.2(?) eons. Impacts of comets, asteroids, and other debris, capable of forming craters at least 50 km in diameter, saturated the surface of the lunar crust once it was capable of supporting itself over the residual liquids of the magma ocean. (Note: Oldest identified Earth crust is at least 4.0 eons old.)

    This breaking, fusing, and mixing of the upper 4-5 km of the upper crust, and the extensive fracturing of the the next 5 or so km below, created a thermally insulating layer over the entire Moon without which remelting of the upper mantle in Stage Six could not have occurred.

      Once crystallized from the Magma Ocean, and without the addition of new energy or a reduction in lithostatic pressure, the the mantle could not remelt. This need for new energy or reduced pressure was exacerbated by the concentration of radioisotopic heat sources in the residual liquid and the gradual migration of that liquid upwards toward the base of the crust.

      With the insulating layer represented by the Cratered Highlands, and the consequent reduction in heat conduction to the surface, heat could begin to accumulate below the crust , keeping the accumulating residual liquid molten and gradually increasing the temperature at progressively lower zones of the upper mantle.

      After few hundred million year long period of magma ocean/upper mantle cooling and contraction, a comparably long period of upper mantle heating and expansion began. This heating and expansion began to be counteracted by the movement of basalt magma to the surface during Stage Six.

      It is likely that the regional release of lithostatic pressure in the upper mantle by basin forming events in Stages Four and Five generally accelerated the initial partial melting of underlying portions of the upper mantle.

     

     

     

     Changes to Lunar Features: End of the Cratered Highlands Stage

     Mapping Camera 1566: Typical farside cratered highlands

     Typical farside cratered highlands

     

  • Major Issue: What process or processes produced the light plains materials that partially fill many Cratered Highland craters, pre-Mare Basalt (Stage Six)basins, and presumably underlay many portions of the mare basalt piles generally?
    • Hot and gas mobilized flows of fine impact debris radiating from Large Basin forming impact events (Stages Four and Five)?
    • Pre-Mare Basalt (Stage Six), volatile driven eruptions of the very earliest initial partial melting components of the mantle with entrained crustal debris?
      • These volatiles may have been dominated by hydrogen, migrating upward after being produced by the breakdown of water as iron rich liquids moved through the lower mantle (see Stage Two discussion).


    • Stage Four: The Old Large Basins/Crustal Strengthening - 4.2(?)-3.9 eons. The Old Large Basins/Crustal Strengthening - 4.2(?)-3.9 eons. More than 30 large impact basins formed, but rapid crustal and mantle adjustment (isostatic compensation) occurred so that no mass concentrations remain (see Stage Five, below).

    Figure: Map of Farside Young Large Basins (Nectarian System) (Wilhelms, 1987)

    The strengthening was probably due to potassium, rare earth elements, and phosphorus-rich residual liquids (urKREEP), left over from the crystallization of the magma ocean, moving in response to fracturing beneath the large basins from the mantle-crust boundary, a strengthening process in and of itself, and into the crust where it crystallized into interlocking dikes within the myriad of crustal fractures.

  •  

     

     

    Changes to Lunar Features: End of The Old Large Basins/Crustal Strengthening Stage 

    Full Moon showing eastern limb basins

    Topographic map of the South Pole-Aitken Basin area based on Clementine data. The basin is about 2500 km in diameter and the largest of the Old Large Basins

    Major Issue: When did the event take place that created the largest of the old large basins, the now vague but none the less huge (more than 3200 km in diameter) Procellarum Basin on the near side of the Moon?

      Wilhelms (1987) and Wieczorek and Phillips (1998) provide lunar mapping and lunar gravity interpretations, respectively, that strongly indicate that the roughly circular region of the Moon, the Procellarum Basin, that underlies Mare Procellarum, Sinus Roris, Mare Imbrium, and a number of smaller maria reflects a very large basin forming event that pre-dates all other identifiable old large basins.

     

    		 Outline of the Procellarum Basin based on lunar geologic mapping (Wilhelms, 1987)
         

    		   Gravity data from the Clementine Mission indicates a thin crust beneath the Procellarum Basin, lying between -75o and +30o.
         

    		   Global topography from the analysis of Clementine laser altimetry data, showing the extensive regional topographic low contained within the Procellarum Basin.

       

      Ejecta from the Procellarum event, combined with that from the later, nearly as large South Pole-Aiken event (see discussion below), created the thickest lunar crust we see today.

        The event, however, may have removed most of the young Cratered Highland crust beneath the region, requiring that a new insulating zone of brecciated crust be developed before upper mantle temperatures below the region could reach the point of partial melting.

        This in turn may explain why the mare basalts of the region are apparently younger (3.2-3.5 by with some crater counting suggesting even younger flows in the western portion of the basin) than most of those to the east.

      There is no evidence in the Lunar Prospector global gamma ray spectrometer analysis (Lawrence, et al., 1998) that any KREEP materials were excavated by the Procellarum event even though the later Imbrium event, occurring within the Procellarum Basin, was responsible for the wide spread distribution of KREEP material on the near side of the Moon.

        This suggests that the Procellarum event occurred before urKREEP had moved upward into the crust and may have been responsible for triggering that movement in this region of the Moon.

        On the other hand, Lunar Prospector thermal neutron data (Feldman, et al, 1998) indicates a probable iron + titanium-rich annulus just outside the rim of the Procellarum Basin. This material, representing the deepest excavated, may be from the potentially abundant splash intrusions (Fe and possible Ti-rich liquids from the Magma Ocean during Stage Two) present in the lower portions of the crust.

        Thermal neutron data indicating an annulus of iron+titanium (Feldman, et al., 1998) around the Procellarum Basin. Potassium is not significant in this annulus by comparison with the gamma ray data reported by Lawrence, et al (1998).


      Major issue: When did the event occur that formed the South Pole Aiken Basin, a 2500 km diameter basin over 12 km deep at its deepest, and what were its consequences?

      First detected in bits and pieces of out of place mountain arcs in pre-Galileo photographs by Earth-based lunar mappers (Wilhelms, 1987), conformatory data and images of South Pole-Aiken have been gathered by Galileo, Clementine and Lunar Prospector. This information has confirmed that mare basalts are largely absent from the floor of the basin (Feldman, et al., 1998) and KREEP containing materials are large absent from the region (Lawrence, et al., 1998).

        A possible explanation for the absence of both significant mare basalt and KREEP in or around the basin may lie in the South Pole-Aiken event occurring after most of the Cratered Highlands had formed, thus removing most or all of the insulating layer which in turn would prevent the accumulation of the heat necessary to partially melt the upper mantle beneath the region to create basalt magmas. The non-mare dark albedo surfaces within the basin (some iron, little titanium, small amounts of potassium and thorium based on Clementine and Lunar Prospector sensing) may be related to impact melts, related to both South Pole-Aiken and younger large basins within its borders, of mixtures of anothositic crust and splash intrusions. The absence of a significant KREEP signal makes it seem unlikely that these surfaces could be related to magmas from either the incompletely solidified Magma Ocean or its residual liquids

        As with Procellarum, the absence of significant KREEP around the basin is explained in this scenario by the South Pole-Aiken event occurring before the movement of the urKREEP residual liquids into the lower crust. As with Procellarum, Lunar Prospector thermal neutron data (Feldman, et al., 1998) indicates a probable iron + titanium-rich annulus just outside the rim of the Procellarum Basin. This material, representing the deepest excavated, may be from the potentially abundant splash intrusions (Fe and possible Ti-rich liquids from the Magma Ocean during Stage Two) present in the lower portions of the crust.


    Stage Five: The Young Large Basins - 3.9-3.8 eons. An additional 10-12 large impact basins, more crisp and sharply circular that the more numerous earlier basins, formed when the crust was strong enough to support central mass concentrations and rim deficiencies indefinitely (see Wilhelms, 1987).

    Figure: Map of Nearside Young Large Basins (Lower Imbrian System) (Wilhelms, 1987)
    Figure: Map of Farside Young Large Basins (Lower Imbrian System) (Wilhelms, 1987)
    Figure: Map of Nearside Young Large Basins (Upper Imbrian System) (Wilhelms, 1987)
    Figure: Map of Farside Young Large Basins (Upper Imbrian System) (Wilhelms, 1987)


    		Changes to Major Lunar Features: End of the Young Large Basin Stage

    • The best fit of gravity data to a masscon model results from taking the mare basalt fill as a flat plate of given thickness and density with some upward central displacement of an upper mantle mass (as much as 25 km above non-mascon areas) and downward circumferential displacement of the mantle around that mass.

     
    Mass concentrations in and mass deficiencies around Young Large Basins as shown by Bouguer gravity anomalies derived from global topography and free-air gravity models based on Clementine data (Nozette, et al., 1994, p.1841)

     
    Gravity anomalies measured by Lunar Prospector (Konopliv, et al., 1998). A and B show near and far side accelerations, respectively, in milligals. B and C show near and far side relative crustal thickness models, respectively, in kilometers and should be compared with the middle figure under the Procellarum discussion, above. Although major similarities exist, the results are clearly model dependent. The Wieczorek and Phillips analysis appears most consistent with other data and inferences about crustal thickness.

    • The ejecta of young large basins was studied and sampled by Apollo 17 in the Valley of Taraus-Littrow, a radial valley that cuts the rim of the Serenitatis Basin. It as found that both melted and unmelted ejecta and intrusions are created by these events. The melted ejectal and intrusions are viscular, probably the result of the incorporation of hydrogen in the crust.
    • The young large basins also may have accelerated partial melting in the upper mantel by temporarily lowering lithostatic pressure belows the basin. If the mantle were near its partial melting point at a point along the column beneath the basin, magma could form and, indeed, volatiles could be released deep within the Moon (see Stage Six discussion in lecture 11).

     

     

    Mapping Camera 1578: View of Tsiolkovskiy

    Mapping Camera 2797: View of Tsiolkovskiy

    Boulder made up of impact generated breccia.

    Contact between two impact breccia units in boulder.

    Older, blue-gray impact breccia unit.

     

    Sample of blue-gray impact breccia
    Sample of anothrosite fragment from blue-gray unit
    Younger, vesicular impact breccia unit
    Sample of vesicular impact breccia
    Injection veins of glassy impact breccia in breccia

    •  The above figures relate to a contact between an older, blue gray breccia unit (rock made up of pieces of other rock) and a younger brown-gray, visicular unit.

    • One of the apparent findings of the Lunar Prospector Mission (Lin, et. al, 1998). has been an apparent correlation between the large areas of residual magnetism on the far side of the Moon, discovered by subsatellite magnetometers deployed by Apollos 15 and 16, with the antipodes (opposite farside points) of the Imbrian and Serenitatis basins.


    Far side areas of remenent magnetism and their correlation with the antipodes of Imbrium and Serenitatis Basins (dotted circles)

    • If this correlation with Imbrian (3.87 by) and Serenitatis (<3.92 but >3.87 by) is valid and not coincidence, it suggests that circulation in the lunar core to produce a global magnetic dipole field began and ended in a period of less than 100 my around 3.9 by ago. This might represent the point at which downwardly migrating FexNiySz liquid (see Stage Two discussion, above) finally created a core of the critical size with a sufficiently high temperature to briefly sustain a dynamo circulation.
    • Major Issue: Lunar-wide cataclysm at 3.9 eons or not (see Spudis, 1996, 141-147).
      • Most, but not all, Apollo highland breccia samples point to a maximum age of brecciation at about 3.9 billion years - is this evidence of a lunar-wide cataclysm at that time or of a bias in the sampling to areas affected by Young Large Basin forming events?
      • The sense of continuum in the for-going discussion would suggest that ejecta from the Imbrium event (3.9 by) has biased the sample from the six Apollo landings. This is particularly true in light of a few highland breccia samples that are significantly older than 3.9 by.
    • Major Issue: What insight does the evolution of the Moon give us about the conditions on Earth, and possibly on Mars, as complex organic molecules appeared, evolved to self-replication and life began?
      • Carbon isotopic evidence (Mojzsis, et al., 1996) indicates that biological processes were modifying isotopic ratios on Earth at least by 3.85 billion years ago and fossil evidence indicates that complex one cell life forms were abundant by 3.5 billion years ago (Schopf, 1993), suggesting that the earliest biological evolution occurred within the space environment of lunar Stage Five or earlier.
      • The implications are very strong that the surface of the Earth was exposed to the same extraordinary level of violence, transient energy release, rock pulverization, glass formation, and space contamination as was occurring on the Moon during this period.
      • The surface environment of the Earth differed from that of the Moon primarily in the six time greater force of gravity and thus the presence of a water-rich fluid sphere.
      • Sheet structure minerals (clays) were probably the most abundant mineral species to which organic and/or biological molecules were exposed in this initial evolutionary environment.
      • Disruptions, if not restarts, of the evolutionary processes may have occurred frequently until after 3.8 billion years ago, i.e., the end of large basin formation on the Moon and thus probably on the Earth.
    Text

    Evolution of the Moon: The Apollo Model

    Originally published by the author in American Mineralogist, v 76, 773-784, 1991.



    Questions

    1. What is the best evidence that the Moon was not pulled from the Earth (leaving the Pacific Ocean Basin in its wake) since life evolved on the Earth? Explain.

    2. What explanation(s) can you give for the concentration of ages of samples of lunar impact breccias around 3.9 eons other than a cataclysm at that time?

    3. Give a geophysical explanation for the mass concentrations (mascons) and mass deficiencies in and around young large basins on the Moon and what conditions are necessary for them to persist for almost 4.0 billion years. Contrast with the Earth.



    References

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    Cassen, P., and Wooum, D.S., 1999 The origin of the Solar System, in P.R. Weissman, McFadden, L., and T.V. Johnson, editors, Encyclopedia of the Solar System, Academic Press, San Diego, 35-64.

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    Taylor, S.R., 1999,

    Wilhelms, D. E., 1987, The Geologic History of the Moon, U.S. Geological Survey Professional Paper 1348, U.S. Government Printing Office, Washington, 302p.

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