Magnetic studies of lunar rocks indicate that the Moon generated a core dynamo with surface field intensities of ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga). The field subsequently declined to <~4 μT by 3.19 Ga, but it has been unclear whether the dynamo had terminated by this time or just greatly weakened in intensity. We present analyses that demonstrate that the melt glass matrix of a young regolith breccia was magnetized in a ~5 ± 2 μT dynamo field at ~1 to ~2.5 Ga. These data extend the known lifetime of the lunar dynamo by at least 1 billion years. Such a protracted history requires an extraordinarily long-lived power source like core crystallization or precession. No single dynamo mechanism proposed thus far can explain the strong fields inferred for the period before 3.56 Ga while also allowing the dynamo to persist in such a weakened state beyond ~2.5 Ga. Therefore, our results suggest that the dynamo was powered by at least two distinct mechanisms operating during early and late lunar history.

INTRODUCTION

The Moon is a unique venue for exploring the longevity of dynamos generated by planetary bodies intermediate in size between planets and asteroids. A central conundrum is that the lunar dynamo was apparently intense and long-lived, with surface fields reaching ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga) (1–6). The field intensity then precipitously declined by at least an order of magnitude (possibly even to zero) by ~3.19 Ga (7–9). It is unknown whether this decrease reflects total cessation of the dynamo or whether the dynamo persisted beyond 3.56 Ga in a markedly weakened state.

The mechanisms that generated such a long-lived dynamo are uncertain but may include thermal convection (10–13) and mechanical stirring produced by differential rotation between the lunar core and mantle driven by impacts (14) or mantle precession (15, 16). Geophysical evidence for a ~200- to ~280-km-radius solid inner core (17, 18) within a larger ~220- to ~450-km-radius liquid outer core (17–23) also suggests that thermochemical convection resulting from core crystallization—the driving force behind the Earth’s dynamo—may have helped sustain the dynamo (24–26). Without invoking special conditions, such as an early thermal blanket enveloping the lunar core, a hydrous lunar mantle, or a low core adiabatic heat flux, purely thermal convection dynamos are unlikely to persist beyond ~4 Ga (6). Impact-driven dynamos are transient [lasting up to a few thousand years after each large basin-forming impact (14)] and cannot have occurred after the last basin-forming impact at ~3.7 Ga (5). On the other hand, mantle precession or thermochemical convection may be capable of powering a dynamo well beyond 3.5 Ga (6).

Key to distinguishing between these lunar dynamo mechanisms is establishing the lifetime of the dynamo. However, the poor magnetic recording properties (7, 8, 27) and complex thermal and deformational histories of most lunar samples (6, 7) as well as the rarity of young (<3.2 Ga) Apollo igneous samples have thus far hindered efforts to establish when the dynamo ultimately ceased. Although some Apollo-era studies have suggested that lunar samples as young as ~200 million years old (Ma) formed in lunar paleofields of ~1 to ~10 μT (28), most of these values are likely upper limits given the samples’ magnetic recording fidelities (8, 27, 29). Furthermore, it has been proposed that impact-generated plasmas could generate transient magnetic fields [lasting up to ~1 day for large basin-forming impacts (30) or <~1 s for the small impacts after 3.7 Ga (8)] that could magnetize shocked and quickly cooled rocks throughout lunar history (31, 32). Given these complexities, determining when the lunar dynamo actually ceased requires a young rock with exceptionally high-fidelity magnetic recording properties and a well-constrained thermal and shock history. To address these deficiencies, we conducted a new analysis of such a young lunar sample, glassy regolith breccia 15498.

Apollo 15 sample 15498 was collected on 1 August 1971 as unoriented float on the southern rim of Dune Crater within eastern Mare Imbrium. The rock consists of a cohesive impact melt glass matrix containing <1-mm- to ~2-cm-diameter mare basalt clasts (Fig. 1) (33, 34). The clasts are petrogenetically related to the Apollo 15 quartz- and olivine-normative mare basalt suites, suggesting that the breccia was melted and assembled in close proximity to the Apollo 15 landing site (35). The rock is partially coated by a variably ~1- to 6-mm-thick spatter of impact melt glass (textural relationships indicate that this rind is younger than the interior breccia, but its precise age is unknown). A network of fissures lined with vesicular glass crosscuts the interior of the rock (see section S1). Although the basalt clasts contain abundant shock deformation features, including maskelynite (33, 34), the lack of microfractures within the glassy matrix (see section S1) indicates that the rock has not been significantly shocked [peak pressures likely <~3 GPa (36)] since lithification.

Fig. 1 Mutually oriented 15498 parent chips. (A) Chips 15498,274, 15498,282, and 15498,287. (B) Chip 15498,313. (C) Chip 15498,314. The sample contains abundant mare basalt fragments (blue arrows and labels) within a glassy matrix (purple arrows and labels). Surficial melt glass spatter locations are denoted with red arrows and outlines. The scale cubes have widths of 1 cm. The subsamples and scale cube are oriented following the Johnson Space Center (JSC) system for 15498.

The petrography and degree of crystallinity of the glassy matrix of 15498 suggest that it formed by viscous sintering of a clast-laden melt (37). During this primary cooling, ferromagnetic metal grains crystallized from the melt portion of the breccia. Consistent with a previous study of metal compositions in 15498 (38), our electron microprobe analyses found that the major ferromagnetic minerals within the resulting glass matrix are kamacite (α-Fe 1−x Ni x for x < ~0.05) and martensite (α 2 -Fe 1−x Ni x for ~0.05 < x < ~0.19). If an ambient magnetic field was present at the time 15498 formed, kamacite and martensite grains with the observed compositions would have acquired mostly thermoremanent magnetization (TRM), with some possible contribution of thermochemical remanent magnetization (TCRM) during primary cooling on the Moon.

Conductive cooling calculations indicate that the glass matrix cooled from above the Curie temperature of kamacite (780°C) to ambient lunar surface temperatures (<100°C) over a period of at least a few hours (see section S1). However, this cooling time scale is long relative to the expected <1-s duration of impact fields when the rock formed. Therefore, impact fields are extremely unlikely to be the source of any TRM or TCRM acquired by the breccia’s glass matrix during primary cooling.

Hysteresis data (see section S4) (39) and our electron microscopy imaging indicate that, in stark contrast to the multidomain grain size of metal in virtually all lunar crystalline rocks, metal within the glass matrix of 15498 is a mixture of predominantly superparamagnetic to pseudosingle domain grains, with only a relatively small contribution from multidomain grains in lithic fragments (Fig. 2). The presence of these fine-grained magnetic carriers indicates that the melt glass portion of 15498 should provide unusually high-fidelity paleomagnetic records. The relatively low Ni content of kamacite and martensite grains present within the rock suggests that the rock should retain any primary TRM and TCRM through laboratory thermal demagnetization experiments up to maximum temperatures between ~600° and 780°C [which correspond to the austenite-start solid-state phase transformation temperature for the observed martensite compositions and the kamacite Curie temperature, respectively (see section S1)]. In combination, these rock magnetic properties indicate that 15498 is an excellent target for lunar paleomagnetic studies.

Fig. 2 Hysteresis curves for 15498. The red curve shows the measured data. The blue curve shows the data after application of a paramagnetic slope correction.

A previous analysis of 15498 found that the glass matrix of the sample contained a stable natural remanent magnetization (NRM) component that persisted during alternating field (AF) demagnetization to at least 40 mT and during thermal demagnetization to at least 650°C (39, 40). A paleointensity value of ~2.1 μT was obtained from one subsample (40) using a modified Thellier-Thellier (41) technique. However, this study was not able to conclusively demonstrate a robust record of the lunar dynamo due to lack of measurements of mutually oriented subsamples, checks for sample alteration during the paleointensity experiment, a detailed characterization of its postformational shock and metamorphic history, and, most importantly, a radiometric age.