Did an impact knock the Moon on its side?
We tend to think of the Moon as a static, dead world, with no atmosphere and no plate tectonics. But there are various signs the Moon has been active, volcanoes and indications of a magnetic field frozen in rocks. Impact craters that flooded with molten rock are also indications of more active periods in the Moon's history.
Now, some researchers are suggesting that the residual magnetic fields contain hints that the Moon was once flipped on its side by a violent event.
All evidence indicates that the Moon was formed when a Mars-sized body collided with the early Earth, leaving both in a molten state. This would have left the Moon with a sufficiently molten core that it should have generated a magnetic field for hundreds of millions of years.
Remnants of that field should remain trapped in rocks that solidified while it was still in place and remain trapped there to this day.
A team of Japanese researchers has now analyzed magnetic data from two lunar orbiters, the Lunar Prospector and Kaguya. Both orbited the Moon at low altitudes (under 40km) and tracked the local magnetic fields.
After eliminating a variety of areas with complex magnetic anomalies, the team looked at data from 57 different sites on the Moon and used the readings to calculate the orientation of the Moon's magnetic field at various points in its past.