Unlike the Earth the Moon lacks a global magnetic field (though it does still have a field of sorts) but evidence from rocks brought back by the Apollo missions shows that this was not always the case. We haven’t seen an article on planetary magnetism from the ICR in some time now, but they used to be quite common. Brian Thomas’ latest, The Moon’s Latest Magnetic Mysteries, breaks the silence.
The topic is a recent paper in PNAS, “Persistence and origin of the lunar core dynamo” – the paper is not open access, but a conference abstract on the same subject is available as is a Phys.org article. The methodology is quite interesting:
First they took two lunar rocks returned by Apollo and calculated how long it would have taken them to cool. The minimum period for one of the rocks was about 100 days, while for the other it was 1000. According to the paper it is possible for impacts to produce magnetic fields, but these would last no longer than a day and so they concluded that these rocks were unlikely to have been affected by a temporary field of that nature and instead any ancient record of a magnetic field from when the rocks were created would come from something more long lived.
The next important piece of information is, therefore, what strength of field is recorded in the rocks. According to the Phys.org article:
Using newer technology to analyze the rocks, they found that they had, on average, fields of 13–70 microtesla—the higher readings are on a par with that of Earth’s magnetic field.
This is a fairly significant field, though they cannot determine it with great accuracy. The next piece of information is the age of the rocks: 3.56 billion years old, meaning that there must have been a planetary field of around that strength at the time. This adds 160 million years to the known lifetime of the dynamo that caused it, but this is too long for the simple fluid dynamo model. What else is there?
While small impacts can only create fields that persist for less than a day, really big impacts could give the core of the Moon a kick and create a dynamo that would persist for around 10,000 years. However, the last known impact large enough to have caused such an event was around 3.72 billion years ago, which is too long ago to have affected the rocks measured.
This leaves a third option: precession. Interaction between the Moon and the Earth “appears to be capable of powering a dynamo until as late as ∼1.8–2.7” billion years ago, which is plenty of time for the rocks we have (this result was actually the conclusion of the paper being discussed last time the Moon’s magnetic field came up in 2011).
There is one slight wrinkle, though: we have as yet no mechanism that can produce a field strength greater than 15 microtesla, while what we’re aiming for is nearer 60. After casting aspersions on the very concept of a dynamo, this is what Thomas seizes upon:
In technical terms, “Nevertheless, the high paleointensities of [three moon rocks] still present a major challenge…” according to the PNAS study authors.
This doesn’t seem to be a quote mine. The full paragraph from which that quote comes from (the last in the conclusion) reads:
The Late Imbrian 3.56-Ga crystallization age of the high-K basalts means that they are very likely too young to have been magnetized by an impact-driven dynamo. Furthermore, attributing the paleomagnetic records of 76535 at 4.2 Ga, 10020 at 3.7 Ga, and 10017 and 10049 at 3.6 Ga to an impact-driven dynamo would require a series of transient impact-driven dynamos. The fact that the 10017 and 10049 paleointensities are so similar to one another, as well to those of the 3.72-Ga basalt 10020, argues strongly in favor of a stable lunar dynamo at least between 3.72 and 3.56 Ga. This lifetime is inconsistent with existing models of core convection, which have been unable to power a dynamo unambiguously after 4.1 Ga by thermal convection alone. Rather, these results support the possibility of a longer-lived power source for the lunar dynamo, such as precession or thermochemical convection due to core crystallization, although impact-induced core dynamos could have operated earlier in lunar history. Nevertheless, the high paleointensities of 10017 and 10049 [and 10020] still present a major challenge, given that all current lunar dynamo models are only thought to be capable of producing surface fields <15 μT. It currently remains unclear when the dynamo finally decayed.
This doesn’t seem to be totally insurmountable – while we don’t have one now that doesn’t mean we wont ever have such a model in the future. Thomas begs to differ:
This “major challenge” understates the problem, which might better be termed an impossible challenge. Neither a 2,000 nor a 3,000 foot long rope can span the mile-wide gap across the top of Grand Canyon.
Well that analogy came out of left field. Some Grand Canyon facts for you: the average width of the canyon is actually 10 miles (16 kilometres), and the maximum is a whopping 18 miles (28.8 km). But if you took your rope upstream to the Navajo bridge in the Marble Canyon portion you’d find that you needed less than 1,000 feet, and in fact the minimum width of the canyon is only 600 feet (180 metres).
I wonder if a similar solution can be found for the Moon? If his choice of analogy is any indication, Mr Thomas’ certainty that they will not is quite unfounded.