New models may solve the mystery of the moon's weird magnetic field

For years researchers have puzzled over why the moon has odd areas of magnetism. New research suggests that it came from an asteroid impact.
Written by Rose Eveleth, Contributing Editor

For centuries, humans have looked at the moon in wonder. While we have come a long way from thinking the moon was a god, or perhaps made of cheese, there are still some mysteries about our glowing companion. Why, for example, does the moon have a weird magnetic field? New research suggests that perhaps these unexpectedly magnetic zones are the remnants of an asteroid impact.

When the Apollo missions went to the moon, one of the things they discovered was that the little rocky body had magnetic hot spots. Some parts of the crust were strongly magnetized, while others were not.

"The conundrum has always been that the magnetism we see on the Moon is not correlated with any surface geology," researcher Sarah Stewart-Mukhopadhyay said in the press release. These magnetic bits did not correspond with mountains, or craters or plains.

Except for one area – the South Pole-Aitken basin. This is a huge crater on the far side of the moon, about 1,370 miles wide. It is also the oldest, and biggest crater that science knows about in the solar system. The biggest group of magnetic anomalies falls on the northern rim of the South Pole-Aitken basin.

But no one thought that the basin was involved because the magnetic fields near it only seemed to show up in the last five to ten years – while the crater was certainly much older than that.

Now, researchers are rethinking that assumption, and think that perhaps the fields were indeed caused by an asteroid impact. One of the reasons is that the magnetism observed in these hot spots is far greater than found on any lunar rock. “We know the magnetic properties of asteroidal material are much higher than that of the Moon. It is possible that metallic iron from an asteroid could have been magnetized by the impact, and deposited on the Moon,” Stewart-Mukhopadhyay said.

The real dilemma here was how to test their idea. To do so, Stewart-Mukhopadhyay created complex models that took into account the moon’s surface, core and mantle. Then, the team modeled all kinds of different impacts, from strong to weak at all different angles. "Each time, the model produced similar results to what we see on the Moon," she said.

Solving this puzzle could also push scientists to reevaluate how they view magnetic fields on other planets. Often researchers use the magnetic fields of distant planets to make inferences about their geologic history. But, Stewart-Mukhopadhyay’s work suggests that a body’s magnetic field might not be solely the result of its own geology. It could be formed by outside factors as well. "It may also be true that extra-terrestrial materials play a larger role in the magnetic fields of other planets than anyone has appreciated," she said

Photo credit: Wikimedia Commons, Mark Weiczorek

This post was originally published on Smartplanet.com

Editorial standards