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. 2020 Jan 1;6(1):eaax0883.
doi: 10.1126/sciadv.aax0883. eCollection 2020 Jan.

The end of the lunar dynamo

Saied Mighani  1 , Huapei Wang  1   2 , David L Shuster  3   4 , Cauȇ S Borlina  1 , Claire I O Nichols  1 , Benjamin P Weiss  1
Affiliations

The end of the lunar dynamo

Saied Mighani et al. Sci Adv. .

Abstract

Magnetic measurements of the lunar crust and Apollo samples indicate that the Moon generated a dynamo magnetic field lasting from at least 4.2 until <2.5 billion years (Ga) ago. However, it has been unclear when the dynamo ceased. Here, we report paleomagnetic and 40Ar/39Ar studies showing that two lunar breccias cooled in a near-zero magnetic field (<0.1 μT) at 0.44 ± 0.01 and 0.91 ± 0.11 Ga ago, respectively. Combined with previous paleointensity estimates, this indicates that the lunar dynamo likely ceased sometime between ~1.92 and ~0.80 Ga ago. The protracted lifetime of the lunar magnetic field indicates that the late dynamo was likely powered by crystallization of the lunar core.

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Figures

Fig. 1
Fig. 1. NRM demagnetization of matrix glass subsamples from breccias 15465 and 15015.
Shown are endpoints of the NRM vectors during progressive alternating field (AF) and thermal demagnetization. Closed and open symbols represent projections of the NRM vectors onto the horizontal (N-E) and vertical (U-E) planes, respectively. (A) 15465 subsample 4-2. (B) 15465 subsample 5-3. (C) 15015 subsample 229a1m. (D) 15015 subsample 229b8. Inset: Magnified view of HT demagnetization steps for 229b8. The legend in (A) shows the sample holder magnetic moment (denoted by the size of the large black box) and the MIT SRM moment resolution (denoted by the size of the small black box) (section S3). The initial NRM, AF levels, and temperatures for selected demagnetization steps are labeled. For both breccias, after removal of low coercivity (LC) and low temperature (LT) components (blue arrows), there is no discernible origin-trending magnetization in the high coercivity (HC)/high temperature (HT) range (as indicated by scattered vector endpoints).
Fig. 2
Fig. 2. Paleointensity estimates for subsamples of breccias 15465 and 15015.
(A and C) ARM paleointensity experiments on 15465 subsample 6-3 and 15015 subsample 229a1m, respectively. Paleointensities are estimated from NRM lost during AF demagnetization as a function of ARM gained in a 50-μT DC bias field and a 260-mT AF. AF steps used to calculate the LC and HC paleointensities are colored blue and red, respectively. Paleointensities and their uncertainties (95% confidence intervals) are shown for the HC range. Insets in (A) and (C) show the decay of NRM and ARM during progressive AF demagnetization. (B and D) ARM paleointensity fidelity tests on 15465 subsample 6-3 and 15015 subsample 229a1l, respectively. Legends list TRM-equivalent fields for ARMs acquired in a range of DC bias fields in an AF of 260 mT and assuming ARM/TRM = 1.34 (section S4) (49). Horizontal dashed lines indicate the noise level due to acquisition of spurious ARM during AF demagnetization. Inset in (D) shows a magnified view of the moment decay.
Fig. 3
Fig. 3. Thermal paleointensity estimate for 15015 matrix glass.
Shown is the NRM lost during progressive thermal demagnetization versus pTRM gained by heating in a laboratory field of 3 μT for subsample 229b8. Inset: Magnified view of 300° to 680°C temperature steps. NRM lost and pTRM gained steps are denoted with squares, with blue and red symbols denoting data in the LT and HT ranges, respectively. pTRM checks for alteration are denoted with triangles. The HT range has a paleointensity value of 0.24 ± 0.24 μT.
Fig. 4
Fig. 4. Paleointensities for breccias 15465 and 15015 and those of other modern measurements of lunar rocks.
Points labeled 15015 and 15465 are new paleointensity estimates reported by this study, while the remaining points are previously measured values (6, 9). Red points denote upper limits on the field (i.e., values indistinguishable from zero), while blue points denote nonzero values (i.e., detections of the paleofield). Vertical and horizontal arrows and error bars indicate the paleointensity and age limits and 1-SD uncertainties, respectively. The blue and red shaded regions indicate the epochs when the dynamo is inferred to be active and have ceased, respectively. Vertical and horizontal arrows and error bars indicate the paleointensity and age limits and uncertainties, respectively. The vertical dashed lines are the lifetime for proposed dynamo mechanisms: thermal convection in a dry mantle (11), impact-driven changes in mantle rotation (50), thermal convection in a dry mantle covered by a thermal blanket (12), thermal convection in a wet mantle (13), lunar precession (16), and core crystallization (15). The upper and lower horizontal dashed lines denote the maximum field predicted by energy flux scaling (36) and the predicted field exceeded by all published dynamo models for >90% of the dynamo lifetime, respectively. Data are tabulated in table S22.
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