The habitability of the planet depends on many factors. One is the existence of a strong and long-lasting magnetic field. These fields are generated thousands of kilometers below the planet’s surface in its liquid core and extend far into space – protecting the atmosphere from harmful solar radiation.
Without a strong magnetic field, the planet struggles to stay in 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 already extinct magnetic field may have helped protect our planet̵
Today, the Earth has a strong global magnetic field that protects the atmosphere and low-orbit satellites from harsh solar radiation. In contrast, the Moon has neither a breathing atmosphere nor a global magnetic field.
Global magnetic fields are generated by the movement of molten iron in the nuclei of planets and moons. Maintaining fluid movement requires energy, such as heat trapped in the core. When there is not enough energy, the field dies.
Without a global magnetic field, charged particles of the solar wind (radiation from the Sun) passing near a planet generate electric fields that can accelerate charged atoms, known as ions, outside the atmosphere. This process is happening on Mars today and as a result it is losing oxygen – something that is directly measured by the atmosphere of Mars and the mission of volatile evolution (Maven). The solar wind can also collide with the atmosphere and kill molecules in space.
The Maven team estimates that the amount of oxygen lost from Mars’ atmosphere throughout its history is equivalent to the contents of a 23-meter-thick global water layer.
[Read: The Moon’s surface is rusting — and Earth may be to blame]
Probing of ancient magnetic fields
The new study examines how the early fields of the Earth and the Moon may have interacted. But exploring these ancient fields is not easy. Scientists rely on ancient rocks that contain small grains that have magnetized during rock formation, saving the direction and strength of the magnetic field at that time and place. Such rocks are rare and the extraction of their magnetic signal requires careful and delicate laboratory measurement.
However, such studies have revealed that the Earth has generated a magnetic field for at least the last 3.5 billion years, and perhaps up to 4.2 billion years, with an average strength of just over half of today’s value. We don’t know much about how the field behaved before that.
In contrast, the Moon’s field was perhaps even stronger than Earth’s about 4 billion years ago, before falling sharply to a weak state in the field 3.2 billion years ago. Little is currently known about the structure or time variability of these ancient fields.
Another complication is the interaction between the early lunar and geomagnetic fields. The new document, which models the interaction of two magnetic fields with north poles, aligned or opposite, shows that the interaction expands the region of near-Earth space between our planet and the Sun, which is protected from the solar wind.
The new study is an interesting first step toward understanding how important these effects would be when averaged over a lunar orbit or the hundreds of millions of years that are important for estimating planetary habitability. But to know for sure, we need additional modeling and more measurements of the forces of the early magnetic fields of the Earth and the Moon.
Moreover, the strong magnetic field does not guarantee the continued habitability of the planet’s atmosphere – its surface and deep inner environment are also important, as are the influences from space. For example, the brightness and activity of the Sun has evolved over billions of years, as has the ability of the solar wind to take away atmospheres.
How each of these factors contributes to the evolution of planetary habitability, and hence life, is not yet fully understood. Their nature and the way they interact with each other are also likely to change in geological terms. But fortunately, the latest study added another piece to the already compelling puzzle.
This article was republished by Christopher Davis, Associate Professor of Theoretical Geophysics, University of Leeds, and John Mound, Associate Professor of Geophysics, University of Leeds, under a Creative Commons license. Read the original article.
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