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. 2017 Aug 9;3(8):e1700207.
doi: 10.1126/sciadv.1700207. eCollection 2017 Aug.

A two-billion-year history for the lunar dynamo

Sonia M Tikoo  1   2   3   4 , Benjamin P Weiss  1   2 , David L Shuster  2   3 , Clément Suavet  1 , Huapei Wang  1 , Timothy L Grove  1
Affiliations

A two-billion-year history for the lunar dynamo

Sonia M Tikoo et al. Sci Adv. .

Abstract

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.

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Figures

Fig. 1
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.
Fig. 2
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.
Fig. 3
Fig. 3. 40Ar/39Ar thermochronometry constraints on the formation age of breccia 15498.
(A) Multi-phase, multi-domain diffusion (MP-MDD) model predictions for diffusion of radiogenic 40Ar* experienced by a 1-cm-diameter basalt clast within 15498 resulting from breccia formation between 650 and 3300 Ma (that is, from heating to temperatures ranging between 450° and 675°C), followed by daytime heating to effective mean temperatures ranging between 25° and 56°C after 600 Ma. Observed step heating ages ±1 SD (dark gray boxes) are plotted against the cumulative release fraction of 39Ar released. The 3310 ± 24–Ma age inferred from the HT release steps represents the minimum crystallization age of the basalt clast. The colored steps are model release spectra calculated using MP-MDD model parameters corresponding to breccia formation at varying times (different formation ages are indicated with different colors). The inset displays the number of model degassing steps that are within error of the sample degassing path (individual steps connected by dashed red line) in the LT release fraction (heating steps 2 to 6) for different breccia formation ages (a value of n = 5 indicates all steps fit within error of the model). (B) Reduced χ2 misfit values for model release spectra shown in (A). Misfits are shown both including (black circles) and excluding (gray circles) the first degassing step of the heating experiments. Red shaded box indicates formation ages precluded by the cosmogenic exposure age (≤600 Ma).
Fig. 4
Fig. 4. Vector endpoint diagrams showing demagnetization of 15498 subsamples.
(A) AF demagnetization of subsample 282c. (B) Thermal demagnetization of subsample 282t. Open and closed circles represent projections of the NRM vector onto the vertical (Up-E) and horizontal planes (N-E), respectively. Blue, red, and green arrows denote LC/LT, MC/MT, and HC/HT components, respectively. Subsample masses as well as selected AF levels and temperature steps are labeled.
Fig. 5
Fig. 5. Equal-area stereographic projections of LC/LT and MC/MT magnetization components observed for interior subsamples of 15498.
(A) LC (circles) and LT (squares) component directions. (B) MC (circles) and MT (squares) component directions. Lines encircling component directions represent the maximum angular deviations associated with each direction. Open symbols (dashed lines) represent directions in the upper hemisphere, whereas filled symbols (solid lines) represent directions in the lower hemisphere. Subsamples from parent chips 274, 282, 287, 313, and 314 are denoted with light blue, medium blue, dark blue, dark green, and light green symbols, respectively.
Fig. 6
Fig. 6. Equal-area stereographic projection of HC and HT magnetization component directions.
Shown directions are observed for mutually oriented matrix glass subsamples from the interior of 15498. Symbols and surrounding ellipses represent directions and associated maximum angular deviation values obtained from principal component analysis. AF and thermally demagnetized subsamples are displayed using circles and squares, respectively. Subsamples from parent chips 274, 282, 287, 313, and 314 are shown by light blue, medium blue, dark blue, dark green, and light green symbols, respectively. Open symbols (dashed lines) represent directions in the upper hemisphere, and filled symbols (solid lines) represent directions in the lower hemisphere. The Fisher mean direction and α95 confidence interval (star and surrounding ellipse, respectively) are shown.
Fig. 7
Fig. 7. Thellier-Thellier paleointensity experiment for subsample 15498,313e.
(A) Arai plot displaying NRM lost during progressive thermal demagnetization (ordinate) versus laboratory pTRM gained (abscissa). Peak temperatures for selected steps are shown. pTRM checks for alteration are shown as triangles. Paleointensities for unblocking temperature ranges of 250° to 540°C and 560° to 680°C are denoted with dark gray and green symbols, respectively. Gray segments link consecutive thermal steps. (B) Vector endpoint diagram showing zero-field thermal demagnetization steps for subsample 313e. LT and HT components are denoted using blue and green symbols, respectively. Paleointensity experiments were conducted following the IZZI protocol (alternating zero-field and in-field measurements).
Fig. 8
Fig. 8. Magnetization versus distance from the peripheral impact glass spatter.
Shown are residual magnetization values for thermally demagnetized 15498 matrix glass subsamples after heating to 300°C. Individual subsample names are labeled. The gray shaded box denotes the zone likely to have been remagnetized by emplacement of the impact glass spatter (approximately three half-widths of the local glass spatter thickness).
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