Magnetic fidelity of lunar samples and implications for an ancient core dynamo

A long-lived lunar dynamo powered by core crystallization

“…Nevertheless, the current view suggests that a magnetic field of several tens of μT was present at the surface of the Moon between 4.2 and 3.56 Ga ago (Garrick-Bethell et al, 2009;Shea et al, 2012;Tikoo et al, 2012a;Suavet et al, 2013). The lack of data before 4.2 Ga ago implies that the dynamo could be older, and there is also no definitive proof that the dynamo shut down 3.56 billion years ago, only that the surface magnetic field beyond that time was weaker (Tikoo et al, 2012b).…”

“…Lunar rocks are poor magnetic recorders and their thermal histories are often uncertain, therefore many of the Apollo era estimates should be used with caution (Lawrence et al, 2008;Tikoo et al, 2012a). Nevertheless, the current view suggests that a magnetic field of several tens of μT was present at the surface of the Moon between 4.2 and 3.56 Ga ago (Garrick-Bethell et al, 2009;Shea et al, 2012;Tikoo et al, 2012a;Suavet et al, 2013).…”

“…These low estimates are consistent with paleomagnetic studies of the sample younger than about 3.3 Ga (Tikoo et al, 2014). Nevertheless, the much larger magnetic fields (several tens of microteslas) that are inferred between 4.2 and 3.56 billion years ago (Garrick-Bethell et al, 2009;Shea et al, 2012;Tikoo et al, 2012a;Suavet et al, 2013) can not be accounted for by any current model.…”

Dynamics and Thermal History of the Terrestrial Planets, the Moon, and Io

“…In the case of the Moon, the lunar magnetic field has been studied indirectly via the natural remanent magnetization of the returned lunar samples and directly with magnetometers carried to the surface and placed in orbit at low altitude above the surface on the Apollo 15 and Apollo 16 subsatellites (see reviews in Dyal et al, 1974;Fuller and Cisowski, 1987). Estimates have been made for the Moon (e.g., Garrick-Bethell et al, 2009;Shea et al, 2012;Tikoo et al, 2012) and Mars (e.g., Arkani-Hamed and Boutin, 2012;Lillis et al, 2008, Mitchell et al, 2007. In the case of Mars, Mars Global Surveyor (MGS) measured the magnetic field of the entire planet (Acuñ a et al, 1998).…”

“…The new data, however, suggest a core dynamo may have existed on the Moon from at least 4.2 to 3.2 Ga with a peak in intensity of more than 10 mT at around Figure 10 The 3-D thermochemical structures and 2-D cross sections for a model with a ilmenite-bearing cumulate (IBC) layer at the CMB that is enriched in radioactive elements and an activation energy of the mantle material of 100 kJ mol À1 (a and b) after 301 Ma and (c and d) at the present day is assumed. Reproduced from Zhang N, Parmentier EM, and Liang Y (2013) (Garrick-Bethell et al, 2009;Shea et al, 2012;Suavet et al, 2013;Tikoo et al, 2012). Most of the 3-D thermal structures are plotted as isosurfaces of residual temperature with contour levels of À0.07 (blue) and 0.07 (orange) except for (a) that uses the contour level of À0.15 (blue) and 0.15 (orange).…”