NASA sent astronauts to the moon during the Apollo missions to resolve the composition of the moon’s interior. Unfortunately, scientists still aren’t sure if the moon’s interior is molten with a metal core, or if it’s mostly unmelted like an asteroid. There is evidence that the moon once had a very powerful magnetic field, but it isn’t known if it was caused by dynamos generated from a molten core, or through the less likely option that it came from external sources. Benjamin Weiss of Massachusetts Institute of Technology is lead author of the paper, which was published in Science.

“Our data suggest that, despite the moon’s tiny size — only 1 percent of the Earth’s mass — its dynamo was surprisingly intense (stronger than Earth’s field today) and long-lived, persisting from at least 4.2 billion years ago until at least 3.56 billion years ago,” Weiss told MIT’s Jennifer Chu. "This period, which overlaps the early epoch of intense solar system-wide meteoroid bombardment and coincides with the oldest known records of life on Earth, comes just before our earliest evidence of the Earth’s dynamo.”

Earth’s magnetic field is believed to be generated from a dynamo, a process in which mechanical energy is converted into electric energy. Differences in temperature, pressure, and composition of materials in the core create movement that generates electric currents. Weiss and co-author Sonia Tikoo believe that a similar process may have occurred on the moon in a similar matter. However, a possible lunar dynamo would likely have been much shorter and more intense than the one surrounding Earth. 

“Both the strong intensity and long duration of lunar fields are surprising because of the moon’s small size,” Weiss continued. “Convection, which is thought to power all known dynamos in the solar system today, is predicted to produce surface magnetic fields on the moon at least 10 times weaker than what we observe recorded in ancient lunar rocks.”

While a convection-based dynamo should have been able to sustain the magnetic field around the moon for longer than it did, there may have been some unexpected factors at play. If the mantle were solid, motion from the moon’s spin about its axis could have generated turbulence in the core. Because the moon moved faster then than it does now, that might explain the curiously strong field that had been generated.

“We know that the moon’s field declined precipitously between 3.56 billion years ago and 3.3 billion years ago, but we still do not know when the dynamo actually ceased. Establishing this will be a key goal of the next phase of lunar magnetic studies,” Weiss concluded.

In order to better understand the direction of the moon’s magnetic field, scientists like Weiss would need to know how samples from the moon were oriented. The samples Weiss studied had all been broken due to impacts with the lunar surface, and were not found where they formed.

Understanding the former magnetic field of the moon would not only give critical insight to the moon’s internal structure and geological history, but it would also provide clues about how our largest satellite formed and allow scientists to explore the principal mechanisms of how magnetic fields form on planets.