Our solar system was formed from a cloud of dust and gas. Most of that cloud coalesced to form the sun, the rest formed the solar disk. The dust and gas in this disc gradually, over many millions of years formed larger and larger clumps. These clumps crashed into one another, forming larger and larger bodies. These bodies eventually formed the planets, but in the early years the planets continued to be bombarded by asteroids and comets.

It was during this period that the Earth’s moon was formed. About 4.5 billion years ago a Mars sized object collided with the early proto-Earth. These two objects formed the planet we know today and the material ejected into space by the collision formed the Moon.

Currently the Moon does not have a magnetic field, but remnants of magnetization suggest that it once had a very strong one. A new study of lunar samples brought back by Apollo astronauts suggests that the field may have once been stronger than the one currently generated by the Earth.

“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. 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,” said Ben Weiss in an interview with MIT News.

Weiss is a professor of planetary sciences in MIT’s Department of Earth, Atmospheric and Planetary Sciences. He and former student Sonia Tikoo are authors of a paper on the subject which was published today in the journal Science.

The Earth’s magnetic field is caused by the rotation and convection of liquids in the planet’s outer core. Because the Moon would have been incredibly hot for a time following the collision that formed it, it is not surprising that it generated a magnetic field. The strength and duration of the field demonstrated by Weiss and Tikoo, however, has left researchers puzzled.

“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 ten times weaker than what we observe recorded in ancient lunar rocks,” said Tikoo according to Science Reporter.

The Moon’s mass is only about one percent that of Earth, which should minimize magnetic activity. However, the Moon was much closer to the Earth at that time which would have caused it to rotate faster on its axis.

“The findings suggests that our understanding of how planets form magnetic fields is incomplete. It’s possible that the Moon generates a magnetic field in a number of different exotic ways. One speculative idea was that the wobble of the Moon’s spin axis dumps a lot of energy into the core and that could have produced a much stronger magnetic field,” said Weiss, according to ABC Science.

Follow up studies on the subject will focus on what caused the magnetic field to subside and end, which may provide further clues about its unexplained strength and duration.