Moon's magnetic field lasted much longer than previously thought

The reason for that difference is because our planet's core is a dense mix of iron and nickel surrounded by flowing rock. That balance keeps charged particles moving in what is known as a dynamo.

Andrew McDonald | Aug 15, 2017

Researchers analyzing lunar rocks brought back to Earth in 1971 have found evidence that the moon's magnetic field may have lasted 1 billion years longer than previous estimates guessed, a new study published in Science Advances reports.

The team -- comprised of scientists from Rutgers University, the University of California, Berkeley, and the Massachusetts Institute of Technology -- looked at the rock sample and found it held evidence of a magnetic field that measured 5 microteslas. That is roughly equal to a microwave oven and fifteen times weaker than Earth's field.

The reason for that difference is because our planet's core is a dense mix of iron and nickel surrounded by flowing rock. That balance keeps charged particles moving in what is known as a dynamo.

In contrast, the moon is smaller and less dense than our planet. That has caused it to lose its field over time.

Past studies have shown that the moon's magnetic field started to fade roughly 400 million years after it first formed. That is odd because the molten core died out half a billion years before that point. While something kept the field going, nobody is surewhy the field lasted as long as it did.

"The concept of a planetary magnetic field produced by moving liquid metal is an idea that is really only a few decades old," said study co-author Benjamin Weiss, a researcher at the Massachusetts Institute of Technology, in a statement."What powers this motion on Earth and other bodies, particularly on the moon, is not well-understood. We can figure this out by knowing the lifetime of the lunar dynamo."

Some scientists believe constant meteorite impacts may have helped generate heat energy on the satellite, while others postulate the moon "wobbled" more when it was closer to Earth. Heat coming from crystallization of minerals may have also moved charged particles around.

To shed light on this mystery, the team in the study looked at a series of Apollo 15 mission samples that date back over 3 billion years. The rocks revealed that, while weak, a non-zero field did exist long after the core went cold. That suggests the moon must have had two mechanisms powering its dynamo: one when it was young and another force that lasted a bit longer.

While researchers have their own theories, more research will be needed before any solid conclusions can be reached. Even so, this study could help astronomers better understand why planets like Mercury still have a magnetic field and others do not. It could also shed light on the mechanisms on exoplanets and give astronomers better insight into Earth-like worlds.

"Whenever we look at exoplanets or the moons of exoplanets that could be in the habitable zone, we can consider the magnetic field as an important player in habitability," said lead author Sonia M. Tikoo, an assistant professor at Rutgers University, according to Science Alert.