Life on earth could be because of the Moon's magnetic field.

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. A new study suggests that the moon's magnetic field may have protected our planet’s atmosphere as life was forming around 4 billion years ago.

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

Molten iron in the cores of planets and moons are in motion which is the key resource for the generation of the global magnetic field. Energy is required to keep the fluid moving, such as heat trapped inside the cores.

The field dies when there is sufficient energy. 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 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. The new study is an interesting first step towards understanding how important such effects would be when averaged over a lunar orbit or the hundreds of millions of years that are important for assessing planetary habitability. But to know for sure we need further modeling and more measurements of the strengths of the Earth and Moon’s early magnetic fields.

What's more, a strong magnetic field does not guarantee the continued habitability of a planet’s atmosphere – its surface and deep interior environments matter too, as do influences from space.

How each of these factors contributes to the evolution of planetary habitability, and hence life, is still not fully understood. Their nature and how they interact with each other are also likely to change over geological timescales. But thankfully, the latest study has added another piece to this fascinating puzzle.