Why the moon’s early magnetic field might be responsible for life on earth

The habitability of a planet depends on many factors. One is the existence of a strong and long-lived magnetic field. These fields are generated thousands of kilometers below the planet’s surface in its liquid core and extend far into space – shielding the atmosphere from harmful solar radiation.

Without a strong magnetic field, a planet struggles to hang on to a breathable atmosphere – which is bad news for life as we know it. A new study, published in Science Advances, suggests that the Moon’s now extinct magnetic field may have helped protect our planet’s atmosphere as life was forming around 4 billion years ago.

Today, Earth has a strong global magnetic field that protects the atmosphere and low-orbiting satellites from harsh solar radiation. In contrast, the Moon does not possess either a breathable atmosphere or a global magnetic field.

Global magnetic fields are generated by the motion of molten iron in the cores of planets and moons. Keeping the fluid moving requires energy, such as heat trapped within the core. When there is insufficient energy, the field dies.

Without a global magnetic field, the charged particles of the solar wind (radiation from the Sun) passing close to a planet generate electric fields that can accelerate charged atoms, known as ions, out of the atmosphere. This process is happening today on Mars and it is losing oxygen as a result – something that has been directly measured by the Mars atmosphere and volatile evolution (Maven) mission. The solar wind can also collide with the atmosphere and knock molecules into space.

The Maven team estimates that the amount of oxygen lost from the Martian atmosphere throughout its history is equivalent to that contained in a global layer of water, 23 meters thick.

[Read: The Moon’s surface is rusting — and Earth may be to blame]

Probing ancient magnetic fields

The new research investigates how the Earth’s and Moon’s early fields may have interacted. But probing these ancient fields isn’t easy. Scientists rely on ancient rocks that contain small grains that got magnetized as the rocks formed, saving the direction and strength of the magnetic field at that time and place. Such rocks are rare and extracting their magnetic signal requires careful and delicate laboratory measurement.

A picture of the ancient moon with magnetic field lines.