At a time when we’re developing ways to watch single-neuron firings inside a living human skull, or using cosmic rays to peer inside the super-shielded reactor core at Fukushima Daiichi, it might seem like astronomers ought to have the tools to peer deep inside planets, as well. The truth is though, we don’t have a particularly detailed idea of what is going on inside out own planet, and even direct confirmation of the existence of tectonic plates is only a few decades old.
As such, it’s useful to find more easily studied proxies for planets, objects that undergo many of the same processes during formation but which are smaller or in some other way amenable to study. As has been the case for so many astronomical studies these days, the best candidate may be a space-rock; using “paleomagnetic” measurements, scientists have peer inside a meteorite to gain insight into our own planet’s future death.
The findings come by way of Germany’s BESSYII synchrotron, which used high-powered X-ray beams to peer inside a particular meteorite and look at the geological history of its parent asteroid. They used micro-scale deposits of a magnetic material called tetrataenite, which is made of an iron-nickel compound and is only found on meteorites, reading them almost like the magnetic units of a hard drive. This allowed them to look back into the magnetic history of the parent asteroid.
The sun’s magnetic field also comes from a dynamo, though obviously with different substances and no hard outer core.
The results show that the asteroid’s magnetic field must have persisted much longer than anticipated, despite the fact that the smaller bodies cool much more quickly than moons or planets. The (relatively) rapid cooling of the metorite allowed the scientists to observe the meteorite’s final moments of magnetic life.
Both the Earth’s magnetic field and that of an asteroid are caused by something called the Dynamo Effect, which you can think of as the very rough inverse of most forms of power generation. In a power station, you generally have steam powering a magnet’s movement around a coil of a conductive material (say, copper wire). The moving magnetic field causes electrons to flow through the copper, which we capture as electricity. In a magnetic dynamo, we have the opposite chain of events: the kinetic energy of moving molten substances on the interior of a planet, moon, or asteroid is naturally transformed into electrical current, and just as a moving magnetic field causes current, current also causes a moving magnetic field. When the molten samples stay molten and moving for aeons, the magnetic fields hang around for aeons, too.
The type of meteorite studied in this case is important, since they come from asteroids formed in the early days of the solar system. As a result, they are useful microcosms of pleasantry formation, since they came into being in a hot enough time to have formed a hard mantle around a molten core; not all asteroids form in this helpful way, as they are usually too small to create much internal heat through simple pressure. This study uses the meteorites as little magnetic time capsules carrying evidence about a particular asteroid, which in turn acts as a capsule full of evidence about the solar system’s formation. The researchers were even able to measure the intensity of the meteorites’ magnetic fields as their dynamos slowly cooled and died on the interior — the process played out exactly as expected.
This implies that when the Earth’s molten core eventually cools and hardens to the point that there is little or no slip-sliding of different substances, that its magnetic field will die out as well. The findings in this paper support this, but they also imply that the kinetics might not be what we expect them to be. That’s good, since while a magnetic flip is largely meaningless, magnetic death certainly would not be. With the Earth now thought to have begun cooling sometime in the last billion years, we certainly have a lot of time left; in all likelihood, the Sun will swallow the Earth long before then, as it convulses and expands as a part of its natural death throes.