Sometimes, enough heat is dumped into the CMB to cause a large blob of hot mantle rock to migrate upwards, causing other forms of excitement, of which, more soon. Because the pattern of Earth’s geomagnetic reversals are imprinted in rocks as they are formed, they are incredibly valuable for dating rocks. Once the polar parts of the core mantle boundary (not to be confused with the other CMB) are sufficiently heat saturated, the mechanism shuts down and restarts nearer to the equator, sometimes with the opposite polarity, which causes a geomagnetic reversal. Over time, the transported heat gets dumped into the lower mantle, which has a much longer dynamical timescale (~100 million years) and the storm currents migrate away from the equator towards the poles, just like hurricanes/cyclones, which are also heat engines but mercifully do not involve magnetic fields. These rotating cells provide the necessary geometrical characteristics to generate magnetic fields, which feed back on themselves to create yet more magnetic fields. Combined with coriolis force we end up with self-intensifying, multiscale, turbulent rotating “storms” of molten metal, with a characteristic timescale of a few thousand years. Convective efficiency is impeded by magnetic forces on the molten metal currents, both fluid and amperic. This heat is transported through the molten iron/nickel outer core through convection. For a necessarily imprecise heuristic explanation of what is going on, the Earth’s core generates heat from nuclear decay and iron crystalization. I haven’t read deeply on this topic since about 2013, but when I last checked there weren’t any published follow up studies (despite advances in computation) that added substantial new insights (updates welcome!). We have a bunch of theorems that exclude all the simple possibilities, but magnetohydrodynamics in the general case is really complicated! Still, in 1995 Glatzmeier and Roberts published a simulation of Earth’s geodynamo (or a numerically tractable approximation with much higher viscous dissipation) performing a reversal. I studied it during undergrad when, in ~2008 it still was not really understood, and even today we don’t get it. Second, the way in which planets make magnetic fields is still poorly understood, and was in fact identified by Einstein as one of the big mysteries more than a century ago. While we have no reason to suspect that Mars’ dynamo couldn’t have reversed before it ultimately stopped, there doesn’t seem to be evidence for the continual formation of new crust that this sort of imprinting would require, so chalk it up in the “solved one mystery, found six more” column. Even weirder, the crustal magnetic field is striped similarly to Earth’s oceanic basalt, which was only understood in the context of plate tectonics (invented in its modern form in 1957!) and geomagnetic reversal. While its magnetic field isn’t nearly strong enough to prevent gradual loss of its atmosphere due to stripping by the solar wind, it is strong enough in some places to form aurorae. First, Mars Global Surveyor (launched in 1996) mapped Mars’ magnetic field and found surprisingly strong remnant crustal magnetism, indicating that Mars almost certainly had a dynamo once. We now know this isn’t the entire story, which is super cool. Next up, we read that Mars’ small size led to premature heat loss, core solidification, loss of differential rotation, and as a result no magnetic field. The problem is that electrostatic forces can be repulsive as well as attractive. They are too small to have sufficient gravitational interactions, at least at close range, so broadly speaking there must be something happening with electrostatic forces. For example, it’s not completely understood how primordial dust grains, which are ~10 microns in size, first stick together. Planetary accretion is still an area of active research with several unsolved problems. In the text, KSR describes planet formation as “rocks banging together in space,” which is not wrong. Still, what we have is not nothing, and it is very cool. One of the main reasons I am writing this commentary is to reflect on the knowledge we have gained in the ~30 years since the Mars Trilogy was written, but in reviewing this section I am reminded that despite our incredible gains, many of the mysteries raised are still mysteries and will likely remain thus until many years after humans walk there. The opening prolog to this section describes the formation of the planet Mars, so we have many opportunities to put our nerd hats on. Contains spoilers for this chapter and earlier chapters. Part of the Mars Trilogy Technical Commentary Series.
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