Polar Wander on the Moon Inferred from its Shape and Magnetic Anomalies

Abstract:

The lunar shape can inform us about the Moon’s early spin pole location, or history of true polar wander. This history is important for understanding the stability of polar ice deposits [1], and possible relationships between large-scale lunar features and the lunar orbit. Recently, Garrick-Bethell et al. [2] showed that when the effects of large basins are ignored, the Moon’s early spin pole could be inferred from a tidal-rotational deformation that froze-in when the Moon was closer to the Earth. They also showed that the lunar shape is consistent with early tidal heating in the crust during the magma ocean epoch [3]. Here we will present some updates to this work, and discuss how the lunar spin pole may have evolved in time, as inferred from the progressive formation of large basins and components of the degree-2 gravity field that are not associated with basins.

Separately, magnetic anomalies can address the problem of lunar polar wander, assuming the ancient dynamo that magnetized them was dominantly dipolar and aligned with the spin axis. However, recent surveys of magnetic anomalies reveal paleopole distributions that are quite complicated and inconsistent across different studies [4, 5]. Some reported paleopoles are consistent with the early spin pole inferred from the lunar shape [2], while others are not. These paleopoles imply either very large amounts of polar wander, or that the dynamo evolved with a complex field geometry. Some possible resolutions to these problems will be discussed, including secular variation of the magnetic field and difficulties with inversions for magnetic sources.

References

1. Siegler, M. A. et al., 46th Lunar and Planetary Science Conference, LPI Contribution No. 1832, p. 2675 (2015).

2. Garrick-Bethell, I., et al., Nature 512, 181 (2014).

3. Garrick-Bethell, I., et al., Science 330, 949 (2010).

4. Arkani-Hamed, J. and Boutin, D., Icarus 237, 262 (2014).

5. Takahashi, F., et al., Nature Geoscience 7, 409 (2014).