The habitability of a planet depends on many factors. One is the existence of a strong and long-lasting magnetic field. These fields are generated thousands of miles 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 finds it difficult to cling to a breathing 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 formed around 4 billion years ago.
Today, the Earth has a powerful global magnetic field that shields the atmosphere and low-orbiting satellites 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 movement of molten iron in the cores of planets and moons. Keeping the fluid moving requires energy, such as heat trapped in the core. When there is not enough energy, the field dies.
Without a global magnetic field, the charged particles of the solar wind (solar radiation) passing near a planet generate electric fields that can accelerate charged atoms, called ions, out of the atmosphere. This process is happening on Mars today and it loses oxygen as a result – something that has been directly measured by Mars’ Atmosphere Mission and Volatile Evolution (Maven). The solar wind can also collide with the atmosphere and project molecules into space.
The Maven team estimates that the amount of oxygen lost from the Martian atmosphere during 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]
Probe ancient magnetic fields
The new research examines how the first fields of the Earth and the Moon may have interacted. But probing these ancient fields is not easy. Scientists rely on ancient rocks that contain small grains that have been magnetized as the rocks formed, saving the direction and strength of the magnetic field at that time and place. These rocks are rare and extracting their magnetic signal requires careful and delicate laboratory measurements.
Such studies, however, revealed that the Earth had generated a magnetic field for at least 3.5 billion years, and possibly as far back as 4.2 billion years, with an average force of just over half of the current value. We don’t know much about how the field behaved before this.
In contrast, the Moon’s field was perhaps even stronger than Earth’s around 4 billion years ago, before precipitously falling to a weak field state 3.2 billion years ago. . At present, little is known about the structure or temporal variability of these ancient fields.
Another complexity is the interaction between the first lunar and geomagnetic fields. The new paper, which modeled the interaction of two magnetic fields with the north poles aligned or inverted, shows that the interaction extends the region of near-Earth space between our planet and the Sun which is sheltered solar wind.
The new study is an interesting first step towards understanding the significance of these effects when averaged over a lunar orbit or over the hundreds of millions of years that are important in assessing planetary habitability. But to be sure, we need more in-depth modeling and more measurements of the strengths of the early magnetic fields of the Earth and the Moon.
Moreover, a strong magnetic field does not guarantee the continuous habitability of a planet’s atmosphere – its surface and deep interior environments also matter, as do influences from space. For example, the brightness and activity of the Sun have evolved over billions of years, as has the ability of the solar wind to strip atmospheres.
How each of these factors contributes to the evolution of planetary habitability, and therefore of life, is not yet fully understood. Their nature and the way they interact with each other are also likely to change over geological time scales. But thankfully, the latest study 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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