A magnetic planet
Ken Tapping, 4th November, 2015
Imagine yourself in a canoe somewhere on a northern lake. It is a moonless night, the stars are brilliant and a curtain of aurora shimmers overhead. That picture is about as Canadian as it gets. Displays of the aurora borealis are quite familiar to us, but that is not the case for many other worlds. It all comes down to magnetic fields.
The core of our planet is a great lump of quasi-molten iron and nickel. As it circulates it acts like a dynamo, generating electric currents which in turn give rise to a magnetic field. This emerges in the Polar Regions and extends outwards into space, forming a bubble around the Earth. The solar wind pushes this bubble into a long teardrop shape, with its tail pointing away from the Sun. Most of the time the solar wind flows smoothly around the bubble, but when the Sun is active, solar wind particles penetrate the bubble and are guided by the magnetic field down toward the poles, where they hit nitrogen and oxygen atoms in the atmosphere. This makes them glow, giving us an auroral display. The biggest benefits from our magnetic field are invisible, but without them we would probably not be here.
The solar wind is a blast of particles and magnetic fields coming out from the Sun at hundreds of kilometres a second. Some of those particles have high energies capable of frying spacecraft electronics and endangering astronauts. Fortunately the space station and most orbiting hardware lie within the bubble and are largely protected. Our magnetic field and dense atmosphere together protect us from the solar wind and dangerous radiation. To better appreciate what our magnetic field does for us, we can look at two planets: Mars, the fourth planet out from the Sun, and Venus, the second. We live on the third. We think that Mars once had a magnetic field like ours. However, it is a smaller planet than Earth, and cooled faster. When its core finally solidified the dynamo stopped and the magnetic field disappeared. When that happened, the solar wind started to flow over the top of Mars’ atmosphere, slowly scrubbing it away, so that today there is little left. As the atmosphere thinned, harmful ultraviolet and other radiations reached the surface. This affected the soils, forming highly reactive and toxic chemicals such as perchlorates. Any future colonist will have to get rid of these chemicals before planting anything in the local soil.
Venus is about the same size as the Earth, but has a very weak magnetic field. This is odd. We would expect that just like the Earth, its core would be still liquid and the dynamo still working. Why isn’t it? One possibility is that our Moon plays a role. The Earth is unusual in the Solar System in having a moon that is more than a quarter of the diameter of the planet it orbits. It is believed that the Earth and Moon are the result of the collision of two bodies, some 4.5 billion years ago. Did this give the Earth a different inside from Venus? Does the continual tidal kneading of the Earth by the Moon’s gravity make a difference?
With no magnetic bubble, the solar wind flows over the top of Venus’s atmosphere, scrubbing it away. However Venus is more massive than Mars and has a tighter gravitational grip on its atmosphere. The lighter gases like hydrogen find their way to the top and are removed while heavy gases like carbon dioxide stay near the surface and are retained. Carbon dioxide is a greenhouse gas and contributes to Venus having a surface temperature high enough to melt lead.
We know that for life as we know it to exist, we need liquid water, oxygen and stable, mild temperatures. Now we can add one more thing: a fairly strong and stable magnetic field.
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