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, 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 planet̵
Today, the Earth has a strong global magnetic field that protects the atmosphere and satellites with low orbits from harsh solar radiation. In contrast, the moon has neither a breathable atmosphere nor a global magnetic field.
Global magnetic fields are generated by the motion of molten iron in the nuclei of planets and moons. Keeping the fluid moving requires energy, such as heat trapped inside the core. When there is not enough energy, the field dies.
Without a global magnetic field, the charged particles from the solar wind (radiation from the sun) close to a planet generate electric fields that can accelerate charged atoms, known as ions, out of the atmosphere. This process takes place today on Mars, and it loses oxygen as a result – something that has been measured directly by the Martian atmosphere and the volatile evolution (Maven) mission. The solar wind can also collide with the atmosphere and knock molecules out into space.
The Maven team estimates that the amount of oxygen lost from the Martian atmosphere throughout its history is similar to that contained in a global water layer, 23 meters thick.
[Read: The Moon’s surface is rusting — and Earth may be to blame]
Probes old magnetic fields
The new research examines how the early fields of the Earth and the Moon may have interacted. But it is not easy to examine these old fields. Researchers rely on ancient rocks containing small grains that were magnetized as the rocks formed, 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 possibly as far back as 4.2 billion years with an average strength just over half its present value. We do not know much about how the field behaved earlier than that.
In contrast, the Moon’s field was perhaps even stronger than Earth’s about 4 billion years ago, before falling to a weak field state 3.2 billion years ago. At present, however, little is known about the structure or time variability of these ancient fields.
Another complexity is the interaction between the early lunar and geomagnetic fields. The new paper, which modeled the interaction of two magnetic fields with the North Poles either aligned or opposite, shows that the interaction expands the area of near-Earth space between our planet and the sun that is protected from the solar wind.
The new study is an interesting first step towards understanding how important such effects would be when they average over a lunar orbit or the hundreds of millions of years that are important in assessing the planet’s habitability. But to know for sure, we need further modeling and more measurements of the strength of the Earth and the 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 also matter, as do influences from space. For example, the sun’s brightness and activity have evolved over billions of years, and so has the sun’s ability to strip atmospheres.
How each of these factors contributes to the development of planetary habitability and thus life is still not fully understood. Their nature and how they interact with each other are likely to change over geological schedules as well. But thankfully, the latest study has added another piece to an already fascinating puzzle.
This article is republished from The Conversation by Christopher Davies, Associate Professor of Theoretical Geophysics, University of Leeds and Jon Mound, Associate Professor of Geophysics, University of Leeds under a Creative Commons license. Read the original article.
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