The Moon used to have its own shield against the Sun — it died a billion years ago

• A new study by the Massachusetts Institute of Technology (MIT) has been able to determine that not only did the Moon have its own magnetic field, but it was twice as strong as the Earth's.
  
• Benjamin Weiss and his team were able to determine that the shield was at its most powerful 4 billion years ago when the Moon was closer to the planet.
  
• As the Moon moved away, its core began to harden and the magnetic field began to weaken before it phased away entirely.
  

The Earth has its own magnetic field. It's what protects solar winds from ripping the planet's ozone layer apart. The Moon used to have its own shield against the Sun and it might even have been stronger than Earth's. But, a new study explains that it disappeared a billion years ago.

"We've shown that the dynamo that produced the moon's magnetic field died somewhere between 1.5 and 1 billion years ago, and seems to have been powered in an Earth-like way," said Benjamin Weiss, a professor at MIT.


How the Moon lost its shield

Scientists believe that the Moon was a lot closer to the Earth 4 billion years ago. At that time, the Earth helped maintain the Moon's magnetic field. The planet's gravity was exerted on the Moon causing it to wobble. The wobble stirred up liquid in the lunar core — including the iron within.


A magnetic field is powerful by what is called a ‘dynamo'. Just like on Earth, the Moon's dynamo was generated by electric currents. These electric currents are a result of the movement of iron. It's the same movement that also causes volcanoes to erupt and what caused the Moon's magnetic field to strengthen.

However, this was short-lived. As the Moon moved further from the Earth, it was on its own.

Without the Earth's gravity to shake up the iron inside, the lunar core started to crystallise around 2.5 billion years ago. The process of liquid turning into solid was enough to maintain the magnetic field for another billion years.


Although this was not a permanent solution either. Once the lunar core had completed hardened, there was nothing left to generate a magnetic field. The Moon was left defenceless against the onslaught of the Sun.

The Moon's magnetic field was twice as strong as the Earth's

Weiss and his team studied Moon rock samples brought back the National Aeronautics and Space Agency's (NASA) Apollo mission. They measured microteslas in order to determine changes in the magnetic field.

On Earth, the magnetic field today is around 50 microteslas. On the Moon, rocks from 4 billion years measured around 100 microteslas. That means the lunar magnetic field was twice as strong as the Earth's.


By the time the Moon had moved away from the effects of Earth's gravity, rocks from 2.5 billion years ago only record 10 microteslas - indicating that the magnetic field was significantly weaker.

The Earth, thankfully, still has its magnetic field but is undergoing some changes. The planet's magnetic North seems to be moving faster than ever before — heading to Siberia.

See also:
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