The Moon: Internal Structure & Magma Ocean 1

Lunar Magma Ocean & Lunar Interior 2

Two possible views of the Moon’s interior:

The Moon: Internal Structure 3

Like Earth, the Moon is a differentiated body. However, the exact nature of the interior structure of the Moon is still debated.

The exact nature of the core (specific composition, size, liquid or solid) is still debated.

The Moon is believed to possess a rather small Fe-rich core that is surrounded by a thick, solid mantle.

Planetary Dynamo 4

Thermal & compositional gradients in liquid outer core result in convection (Fe-rich outer core), which in turn produces a magnetic field.

The Lunar Dynamo 5

to be able to power a dynamo this late in history(Figs. 4 and 6).These results confirm some Apollo-era conclu-

sions and refute others. Modern paleomagneticmeasurements have now demonstrated that acore dynamo likely existed on the Moon at leastas far back in time as 4.25 Ga, in contradiction tothe hypothesis of a late origin (i.e., after 4.0 Ga).On the other hand, modern measurements haveconfirmed the existence of a high-field (mean of~77 mT from six samples) epoch lasting from atleast 3.85 to 3.56 Ga, as originally recognizedduring the Apollo era. However, we now see thatthe dynamo subsequently declined precipitouslyto <∼4 mT by 3.2 Ga. In particular, modern analy-ses of samples 3.3 Ga or younger suggest thatmost or all of the Apollo-era paleointensity analy-ses from this late epoch, which ranged up to ap-parent values of 20 mT, are only upper limits onthe lunar field and therefore do not require adynamo at this time. Finally, a recent analysisof a potentially very young lunar sample (66)hints that the dynamo may have nevertheless

persisted in a weakened state (surface field of~2 mT) until sometime after 3.3 Ga.

The global context of crustal magnetism

Coincident with these new paleointensity ex-periments, there have also been important re-cent investigations of the lunar crustal magneticanomalies. As discussed above, a key findingfrom the Apollo-era measurements was the dis-covery of several intense magnetic anomalies,including those on the southern farside, thatapparently require crustal materials with NRMlarger than that of nearly all known Apollosamples (35). Global magnetic field maps fromthe Lunar Prospector spacecraft, which only be-came available 6 years ago (8, 29, 68), have nowconfirmed the unique size and intensity of thesouthern farside anomalies and their spatialcorrelation with antipodes of four, or perhapseven five (69), of the eight youngest basins. How-ever, the acquisition of the first high-resolutionglobal topography data in 1993 by the Clemen-tine laser altimeter (70) led to the identification

of another geologic association with the south-ern farside anomalies: They lie on the northernedge of the vast South Pole-Aitken (SPA) basin,the largest known impact crater on the Moon.This discovery has led to two explanations

for these anomalies as alternatives to the impact-plasma fields hypothesis. Both propose that rath-er than reflecting locally high Bpaleo, they are amanifestation ofmaterials with locally high cTRMthatweremagnetized in a dynamo field. The firstof these proposals (71) identifies the anomalieswith the inferred locations of <4.0 Ga basalticdikes locatedwithin the SPA (Fig. 3). The assumedgreat vertical thickness (30 km) of these dikesenables them to produce the strong SPA anom-alies despite theweakNRMof typicalmare basalts.The second proposal (72) emphasizes that

the anomalies are in the expected locations ofejecta and impact melt derived from the SPAimpactor (Fig. 3). The key advantage of thisproposal is that typical chondritic impactors,which are far more Fe-rich, have cTRM ~100and 104 times that of mare basalts and pristinefeldspathic rocks, respectively. Therefore, dep-osition of relatively thin layers (perhaps just tensto thousands of meters thick) of such materialshould have fundamentally enhanced the mag-netic properties on the surface. SPA ejecta, as wellas that from more recent impacts, could also ac-count for most of the other isolated crustal anom-alies. However, thus far no evidence has yet beenidentified indicating that at least the surface layer(top ~30 cm) of the southern farside anomalyregion is anomalously iron metal-rich (73).Both of these new proposals require only dy-

namo fields rather than plasma fields to explainthe magnetic anomalies. Yet more evidence fora dynamo comes from the recent identificationof many anomalies within the interiors of mostNectarian (i.e., ~3.85 to 3.92 Ga) basins and somepre-Nectarian (i.e., more than ~3.92 Ga) basins(Fig. 3) (74–76). These impacts likely demagne-tized any existing NRM, producing impact meltsheets that should have cooled from the kama-cite Curie point to ambient lunar surface tem-peratures far more slowly (given their >1 kmthicknesses, over a period of >104 years) thanany impact-generated field. Therefore, thesecentral basin anomalies likely require a steadyfield like that expected from a core dynamoduring the Nectarian and pre-Nectarian periods.However, once again, the inferred TRM inten-sities (~1 A m−1) exceed what is achievable withknown endogenous lunar materials and may re-flect the addition of Fe-rich impactor materials.It has been suggested (29) that the apparent

lack of anomalies in the centers of Imbrian ba-sins (<1 nT at the surface, suggesting that thecentral impact melt layer has an NRM less than~10−3 A m−1) may indicate that the dynamo wasno longer active by the time of their formation(i.e., after ~3.85 Ga). However, this proposal isnot consistent with the strong evidence for acore dynamo from paleomagnetic analyses of3.56 to 3.85 Ga (Imbrian-aged) Apollo 11 and 17basalts (see above). In fact, the sample and or-bital data sets are consistent if the composition

1246753-8 5 DECEMBER 2014 • VOL 346 ISSUE 6214 sciencemag.org SCIENCE

Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods and esti-mated lifetimes of various lunar dynamos. Each point represents measurements of a single Apollosample. Circles represent actual paleointensities, downward arrows represent upper limits on paleo-intensity, and right arrows represent upper limits on age. Green and blue points were measured using theIRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARMmethod.Note the datum at <7Ma at the extreme right of the figure.The shaded green box encompasses themeanpaleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty(upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) iscurrently not well constrained due to spurious demagnetization effects. Vertical dashed lines showestimatedmaximum lifetimes of various proposed lunar dynamomechanisms: purely thermal convectionin a dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantlerotational changes, purely thermal convection in a wet mantle; precession; and thermochemical coreconvection driven by core crystallization. The horizontal line shows the maximum lunar surface field asestimated from Eq. 2.The lifetimes of the precession and thermochemical dynamos are highly uncertain.Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6.

RESEARCH | REVIEW

to be able to power a dynamo this late in history(Figs. 4 and 6).These results confirm some Apollo-era conclu-

sions and refute others. Modern paleomagneticmeasurements have now demonstrated that acore dynamo likely existed on the Moon at leastas far back in time as 4.25 Ga, in contradiction tothe hypothesis of a late origin (i.e., after 4.0 Ga).On the other hand, modern measurements haveconfirmed the existence of a high-field (mean of~77 mT from six samples) epoch lasting from atleast 3.85 to 3.56 Ga, as originally recognizedduring the Apollo era. However, we now see thatthe dynamo subsequently declined precipitouslyto <∼4 mT by 3.2 Ga. In particular, modern analy-ses of samples 3.3 Ga or younger suggest thatmost or all of the Apollo-era paleointensity analy-ses from this late epoch, which ranged up to ap-parent values of 20 mT, are only upper limits onthe lunar field and therefore do not require adynamo at this time. Finally, a recent analysisof a potentially very young lunar sample (66)hints that the dynamo may have nevertheless

persisted in a weakened state (surface field of~2 mT) until sometime after 3.3 Ga.

The global context of crustal magnetism

Coincident with these new paleointensity ex-periments, there have also been important re-cent investigations of the lunar crustal magneticanomalies. As discussed above, a key findingfrom the Apollo-era measurements was the dis-covery of several intense magnetic anomalies,including those on the southern farside, thatapparently require crustal materials with NRMlarger than that of nearly all known Apollosamples (35). Global magnetic field maps fromthe Lunar Prospector spacecraft, which only be-came available 6 years ago (8, 29, 68), have nowconfirmed the unique size and intensity of thesouthern farside anomalies and their spatialcorrelation with antipodes of four, or perhapseven five (69), of the eight youngest basins. How-ever, the acquisition of the first high-resolutionglobal topography data in 1993 by the Clemen-tine laser altimeter (70) led to the identification

of another geologic association with the south-ern farside anomalies: They lie on the northernedge of the vast South Pole-Aitken (SPA) basin,the largest known impact crater on the Moon.This discovery has led to two explanations

for these anomalies as alternatives to the impact-plasma fields hypothesis. Both propose that rath-er than reflecting locally high Bpaleo, they are amanifestation ofmaterials with locally high cTRMthatweremagnetized in a dynamo field. The firstof these proposals (71) identifies the anomalieswith the inferred locations of <4.0 Ga basalticdikes locatedwithin the SPA (Fig. 3). The assumedgreat vertical thickness (30 km) of these dikesenables them to produce the strong SPA anom-alies despite theweakNRMof typicalmare basalts.The second proposal (72) emphasizes that

the anomalies are in the expected locations ofejecta and impact melt derived from the SPAimpactor (Fig. 3). The key advantage of thisproposal is that typical chondritic impactors,which are far more Fe-rich, have cTRM ~100and 104 times that of mare basalts and pristinefeldspathic rocks, respectively. Therefore, dep-osition of relatively thin layers (perhaps just tensto thousands of meters thick) of such materialshould have fundamentally enhanced the mag-netic properties on the surface. SPA ejecta, as wellas that from more recent impacts, could also ac-count for most of the other isolated crustal anom-alies. However, thus far no evidence has yet beenidentified indicating that at least the surface layer(top ~30 cm) of the southern farside anomalyregion is anomalously iron metal-rich (73).Both of these new proposals require only dy-

namo fields rather than plasma fields to explainthe magnetic anomalies. Yet more evidence fora dynamo comes from the recent identificationof many anomalies within the interiors of mostNectarian (i.e., ~3.85 to 3.92 Ga) basins and somepre-Nectarian (i.e., more than ~3.92 Ga) basins(Fig. 3) (74–76). These impacts likely demagne-tized any existing NRM, producing impact meltsheets that should have cooled from the kama-cite Curie point to ambient lunar surface tem-peratures far more slowly (given their >1 kmthicknesses, over a period of >104 years) thanany impact-generated field. Therefore, thesecentral basin anomalies likely require a steadyfield like that expected from a core dynamoduring the Nectarian and pre-Nectarian periods.However, once again, the inferred TRM inten-sities (~1 A m−1) exceed what is achievable withknown endogenous lunar materials and may re-flect the addition of Fe-rich impactor materials.It has been suggested (29) that the apparent

lack of anomalies in the centers of Imbrian ba-sins (<1 nT at the surface, suggesting that thecentral impact melt layer has an NRM less than~10−3 A m−1) may indicate that the dynamo wasno longer active by the time of their formation(i.e., after ~3.85 Ga). However, this proposal isnot consistent with the strong evidence for acore dynamo from paleomagnetic analyses of3.56 to 3.85 Ga (Imbrian-aged) Apollo 11 and 17basalts (see above). In fact, the sample and or-bital data sets are consistent if the composition

1246753-8 5 DECEMBER 2014 • VOL 346 ISSUE 6214 sciencemag.org SCIENCE

Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods and esti-mated lifetimes of various lunar dynamos. Each point represents measurements of a single Apollosample. Circles represent actual paleointensities, downward arrows represent upper limits on paleo-intensity, and right arrows represent upper limits on age. Green and blue points were measured using theIRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARMmethod.Note the datum at <7Ma at the extreme right of the figure.The shaded green box encompasses themeanpaleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty(upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) iscurrently not well constrained due to spurious demagnetization effects. Vertical dashed lines showestimatedmaximum lifetimes of various proposed lunar dynamomechanisms: purely thermal convectionin a dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantlerotational changes, purely thermal convection in a wet mantle; precession; and thermochemical coreconvection driven by core crystallization. The horizontal line shows the maximum lunar surface field asestimated from Eq. 2.The lifetimes of the precession and thermochemical dynamos are highly uncertain.Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6.

RESEARCH | REVIEW

Magnetic measurements of Apollo samples indicate that the Moon had a very intense magnetic field from ~4.5 - 3.5 Ga, and it then declined substantially by 3.3 Ga.

The relatively ‘long-lived’ nature of this

dynamo suggests

crystallization of a core.

[from Weiss & Tikoo, Science, 2014]

The Moon: Internal Structure 6

Recent re-interpretation of seismic data from the Apollo missions suggests the presence of a solid inner core, a liquid outer core, and a partially molten zone at the base of the mantle. This structure is a bit more similar to Earth than previous ideas, but it remains to be seen if it is accurate.

Data from the NASA GRAIL (gravity) mission will help to answer these questions.

We only have rocks from the Moon’s crust, not the mantle, but some volcanic glasses (the green glasses) are believed to be very ‘primitive’ (chemically speaking) and inform us about the mantle composition.

Magma Cooling & Crystallization 7

Different minerals have different crystallization (or melting) temperatures; as a magma cools, minerals will crystallize out in a certain order. Different minerals have different densities, and some mineral crystals can settle to the bottom of a magma chamber because they are more dense than the magma. When a mineral crystallizes, the elements that go into that mineral will be removed from the magma, thus the magma will become depleted in those elements and enriched in others.

The small amount of melt remaining was very enriched in elements that didn’t want to go into olivine, pyroxene, or plagioclase.

This included potassium (K), the Rare Earth Elements (Th, U, Ba, Zr, etc.), and phosphorous (P).

Mg & Fe go so Ca & Al go (in a relative sense)

Olivine and Pyroxene (orthopyroxene, to be precise) are the first minerals to crystallize.

Lunar Magma Ocean 8

The KREEP rocks imply significant amounts of initial magma in order to get this kind of concentration, and they are ~4.36 billion years old, so melting was large-scale, and crystallization was early & fast.

The increase in Ca and Al causes plagioclase minerals to crystallize.This plagioclase is less dense than the magma and floats to the top.

These minerals are rich in Fe and Mg, so the residual melt/magma is depleted in these elements.

These minerals are dense, sink to the bottom, & accumulate in a thick, dense zone: these are the cumulate rocks

The plagioclase accumulates at the top to form the anorthositic crust, about 4.46 billion years ago.

Lunar Magma Ocean 9

The magma ocean hypothesis is able to explain:the thick anorthositic crust, the different types of mare basalts, the KREEP rocks, and it is consistent with the ages of the lunar samples.

Hf-W measurements show that the Moon’s core formed about ~30 million years after solar system formation. Other isotopes show that most of the magma ocean crystallized by 4.4 Ga and the final KREEP melt solidified by 4.36 Ga: it all happened early in the solar system!

Lunar Magma Ocean 10

Based on the chemistry of lunar rocks, it appears that at least 50% of the entire Moon was once in a molten state. This has been referred to as the “Lunar Magma Ocean” hypothesis.

Incompatible elements (ones that tend to remain in the magma) are highly concentrated at the lunar surface, and these rocks are very old, so the concentration process happened fast.

The voluminous anorthositic crust (rich in plagioclase feldspar) requires a mechanism to concentrate signficant amounts of Al in the crust.

Lunar Magma Ocean 11

A consequence of the crystallization sequence of minerals and the cooling of the lunar magma ocean is that very dense minerals tend to form last, especially from the later melts that have a lot of titanium.

Therefore, the upper part of the mantle has a layer that is denser than the material below.

It has been suggested that this was not stable, and that the dense, Ti-rich material at the top sank back down to the bottom of the mantle (‘magma ocean overturn’).

Depending on various assumptions about the Moon’s composition and internal structure/temperature, simulations show that this can in fact happen under certain conditions, but it’s not yet clear that it actually did happen.

This is an important idea because these Ti-rich melts also contained a lot of the heat-producing elements that produced long-lived volcanism.

Clementine: Crustal Thickness 12

GRAIL: Gravity & Crustal Thickness 13

Average crust thinner than previously recognized!Al content likely similar to bulk silicate Earth.

14

3

rsta.royalsocietypublishing.orgPhil.Trans.R.Soc.A372:20130242

.........................................................

30

40

50

60

70

1970 1975 1980 1985 1990 1995 2000 2005 2010publication date

30

40

50

60

70

80cr

usta

l thi

ckne

ss (k

m)

average crustal thickness

seismic crustal thickness

A12/14Apollo zone

A16

Apollo zone

Apollo zone

Apollo zone

A14A12

A16A15

Figure 1. Seismically determined crustal thickness estimates (bottom) and average crustal thickness estimates as a function ofpublication date. As analyses of the Apollo seismic data have improved with time, and as orbital missions have improved ourknowledge of the lunar gravitational field and topography, estimates of the Moon’s crustal thickness have decreased by almosta factor of two over a 40 year time period.

average crustal thickness of the Moon would be even greater than that obtained from the seismicanalyses [17]. After accounting for lateral variations in crustal density, as estimated from orbitalγ-ray spectrometer data, Haines & Metzger [18] showed that the farside crust should be onaverage 22 km thicker than the nearside crust. In using the seismic constraint of Goins et al. [16],they reported an average crustal thickness of 73 km.

The Clementine and Lunar Prospector missions, launched respectively in 1994 and 1998,obtained the first near-global topographic maps of the Moon [19] and allowed for the constructionof a vastly improved gravitation field, especially over the nearside hemisphere of the Moon [20].With these data, it was possible to create the first global crustal thickness models by assumingdensities for the crust and mantle, combined with knowledge of the crustal thickness at one locale.Neumann et al. [21] obtained an average crustal thickness of 61 km by requiring their modelsto match the seismically determined 55 km thickness at the Apollo 12 and 14 sites. Using animproved inversion approach, Wieczorek & Phillips [22] obtained an average thickness of 66 kmwith a 60 km constraint at the Apollo 12 and 14 sites. However, if the crust of the Moon wereisostatically compensated by crustal thickness variations, Wieczorek & Phillips [23] showed thatthe relation between the lunar geoid and topography implied an average crustal thickness of43 ± 20 km (revised to 49 ± 16 km [24]). At the low elevations of the Apollo 12 and 14 sites, thecrustal thickness should have been about 28 km. Rather than call into question the accuracy ofthe Apollo seismic constraints, these authors instead interpreted this finding to mean that thelunar crust was stratified in density, with a low-density anorthositic layer overlying a densernoritic layer.

Two independent re-analyses of the Apollo seismic data were performed following theLunar Prospector mission, with both suggesting that the lunar crust was considerably thinnerthan previously thought. Khan et al. [25] suggested that the crustal thickness was 45 ± 5 kmin the Apollo zone (which is most representative of the Apollo 12 and 14 landing sites giventhe distribution of seismic events used in their analysis). In a subsequent analysis that tested thehypothesis of having a crust thinner than the Apollo-era analyses, they showed that models with

on March 1, 2016http://rsta.royalsocietypublishing.org/Downloaded from

GRAIL: Gravity & Crustal Thickness

As we learn more about the Moon (new missions!), our estimates of crustal thickness have decreased over time.

[Taylor & Wieczorek, 2014]

GRAIL: Oceanus Procellarum 15

What is the origin of “Oceanus Procellarum”?

Why are maria concentrated on the near side?

In addition to being an area of thinner crust, there is also a high Th anomaly here (heat producing elements).

16GRAIL: Oceanus Procellarum

The gradient in the GRAIL gravity data indicate large fracture (dike) systems.

These are due to uneven cooling of the crust and act as conduits for magma.

Thermal stress, not a giant impact!

[Andrews-Hanna et al., 2014]

17

There is a similar feature in the South Polar Terrain of Enceladus (moon of Saturn).

Can this cooling/thermal stress process also explain some of the features observed on that icy body?

GRAIL: Oceanus Procellarum

[Andrews-Hanna et al., 2014]