Surveying the South Pole-Aitken basin magnetic anomaly for remnant impactor metallic iron

Cahill, Joshua; Hagerty, J. J.; Lawrence, D. J.; Klima, R. L.; Blewett, D. T.

doi:10.1016/j.icarus.2014.08.035

Cited by 3 publications

(5 citation statements)

References 32 publications

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“…The distribution is centered downrange, and predicted to be thickest within the northwest quadrant of SPA ( Figure 1). This impact angle is consistent with magnetic anomalies in NW SPA ( Figure S1; Tsunakawa et al, 2010), which may result from downrange remnants of a Fe-rich impactor or mantle ejecta (Cahill et al, 2014;Wieczorek et al, 2012).…”

Section: Distribution and Evolution Of Mantle-derived Spa Ejectasupporting

confidence: 83%

“…As a brief conjecture, the magnitude of magnetic anomalies across SPA exhibits a loose correlation with the Th distribution, and, therefore, the region exhibiting a significant mismatch between M 3 band depth and LP Fe abundance (Figure 3, Figure, ). Magnetic anomalies in NW SPA have previously been attributed to subsurface dike swarms (Prurucker et al., 2012) or metallic impactor remnants (Cahill et al., 2014; Wieczorek et al., 2012). However, the results presented here suggest that ferroan uppermost mantle materials ejected by the SPA‐forming impact may contribute to the basin's magnetic signature.…”

Section: Origin Of the Spa Thorium Anomalymentioning

confidence: 98%

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Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

et al. 2021

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The evolution and compositional structure of the lunar mantle has been extensively modeled but insufficiently constrained by observations. Here, we identify and characterize mantle materials exposed by the Moon's largest impact basin to better understand the composition, stratigraphy, and evolution of the upper mantle. The vast South Pole-Aitken Basin (SPA) exhibits a broad, crescent-shaped thorium and potassium distribution. These incompatible elements are predicted to be concentrated in the dregs of the lunar magma ocean during end-stage crystallization. Through consideration of basin formation models convolved with subsequent geologic evolution, we demonstrate that the distribution and implied stratigraphy of Th-and K-bearing materials across SPA are consistent with an upper mantle ejecta origin. The most pristine exposures of these materials are confined to northwest SPA and also exhibit elevated Ti and Fe (relative to the farside highlands) in association with a gabbronoritic mineralogy. This is consistent with latestage magma ocean assemblages predicted by petrologic models. In contrast, SPA impact melt derived from greater depths is associated with a low-Ca pyroxene-dominated assemblage. Together, these compositional patterns are evidence for a stratified ancient upper mantle. Importantly, the incompatible-element-enriched, ilmenite-bearing ferroan gabbronoritic cumulates evidently had not participated in gravitational overturn at the time of SPA formation. Contrary to recent hypotheses invoking nearside sequestration of incompatible elements to explain hemispherical differences in crustal building and volcanic resurfacing, it follows that incompatible elements were globally distributed in the magma ocean at the time of SPA formation. Plain Language Summary Like the Earth, the Moon is layered into a crust and mantle. The Moon's layering was shaped by an early global melting event known as the "Lunar Magma Ocean." As the magma ocean solidified, dense minerals sank to form the mantle, while less-dense minerals floated to form the crust. Elements such as thorium are not easily incorporated into mineral structures, and remain in the liquid. Because of this, a thorium-rich dreg layer was sandwiched between the crust and mantle. These dregs are very dense and are expected to sink into the underlying mantle during or soon after crystallization. We demonstrate that the Moon's largest and oldest impact basin excavated material from this dense, thorium-rich layer before it sank. The exposed material was then diluted and obscured by four billion years of impact cratering and volcanic eruptions. However, we identify several pristine exposures created by recent craters. The impact basin also melted rocks from greater depths than the rocks it ejected. These melted rocks exhibit a much different composition. This indicates that the lunar upper mantle included two compositionally distinct layers that were exposed in different ways by this large impact event. These results have important implications for understanding the ...

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Section: Distribution and Evolution Of Mantle-derived Spa Ejectasupporting

confidence: 83%

Section: Origin Of the Spa Thorium Anomalymentioning

confidence: 98%

Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

et al. 2021

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“…Though this might provide a more realistic description of crustal magnetism, and fit the available data better, the downside would be of having to introduce an additional parameter in our inversions. Finally, it is possible that remotely sensed spectroscopic data could be used to investigate the abundance of macroscopic metallic iron in the crust and how it varies with depth (e.g., Cahill et al, 2014). Results from such studies could not only be compared with geophysical inversions of the lunar magnetic field but also be used as an independent constraint on the abundance of magnetic carriers in the crust.…”

Section: Discussionmentioning

confidence: 99%

Strength, Depth, and Geometry of Magnetic Sources in the Crust of the Moon From Localized Power Spectrum Analysis

Wieczorek

2018

JGR Planets

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Spacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean‐squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core‐generated dynamo field must have existed when the crust was cooling during the first 100 Myr of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat‐producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat‐producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.

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“…Petrographic examination of separated metal fractions will need to be conducted to distinguish unmodified metal-bearing impactors (Wood et al 1970;Jolliff et al 1993). It has been postulated that large amounts of unmodified iron-rich metallic meteorites may survive basin-forming events, accounting for some observed magnetic anomalies on the lunar surface (Wieczorek et al 2012), although this is still under debate (Cahill et al 2014). Identification of different iron oxidation states using Mössbauer spectroscopy of bulk soils could be employed to locate soils bearing oxidised phases such as hematite and magnetite (e.g.…”

Section: Iron-bearing Meteorites From Protoplanetary Cores and Primitmentioning

confidence: 99%

“…Meteoritic material originating from Venus is considered to be much more unlikely given the challenges of successfully surviving the ejection event through the dense venusian atmosphere (Gladman et al 1996;Armstrong et al 2002), however, it may be possible that in the past, if Venus's atmosphere was thinner, impact ejecta may have escaped to be thrown onto a Earth-Moon crossing orbit resulting in delivery to the lunar surface (see discussion in Armstrong et al 2002;Chapman 2002).…”

Section: Materials Derived From the Earth And Other Planetsmentioning

confidence: 99%

The Moon: An Archive of Small Body Migration in the Solar System

et al. 2016

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The Moon is an archive of impact cratering in the Solar System throughout the past 4.5 billion years. It preserves this record better than larger, more complex planets like the Earth, Mars and Venus, which have largely lost their ancient crusts through geological reprocessing and hydrospheric/atmospheric weathering. Identifying the parent bodies of impactors (i.e. asteroid bodies, comets from the Kuiper belt or the Oort Cloud) provides geochemical and chronological constraints for models of Solar System dynamics, helping to better inform our wider understanding of the evolution of the Solar System and the transfer of small bodies between planets. In this review article, we discuss the evidence for populations of impactors delivered to the Moon at different times in the past. We also propose approaches to the identification and characterisation of meteoritic material on the Moon in the context of future lunar exploration efforts.

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Surveying the South Pole-Aitken basin magnetic anomaly for remnant impactor metallic iron

Cited by 3 publications

References 32 publications

Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

Strength, Depth, and Geometry of Magnetic Sources in the Crust of the Moon From Localized Power Spectrum Analysis

The Moon: An Archive of Small Body Migration in the Solar System

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