Answer to why Mercury has a magnetic field and not Mars. (Off-Topic)

by Funkmon , Tuesday, May 12, 2015, 01:17 (3280 days ago) @ dogcow
edited by Funkmon, Tuesday, May 12, 2015, 01:28

Disclaimer: I missed taking a planetary geology class, so I'm not 100% about this stuff. There's a TLDR at the end.

Mercury we know has a liquid core that is 80 percent of its radius. This means it's big. More stuff means it'll take a bit longer to cool. We also know that Mercury has lighter elements in its core, particularly sulfur. This supports a concept of radial mixing, which is the idea that in the proto-sun's accretion disk, things from the outer edge managed to still mix with things in the inner edge. We don't, AFAIK, have a good idea about why this happens. Our ideas about solar system formation are completely wonky...but we do know radial mixing happens. We have evidence from comets, meteors, and a few other solar system bodies, and the finding of sulfur, a lighter element, that far in supports this idea.

So, Mercury started with some sulfur in its core, which means the melting point of the substance making up the core is lowered, making it easy to keep spinning. We know its magnetic field is made by a dynamo due to higher than expected libration.

So that's a bit on Mercury.

Mars may not have a magnetic field now due to lack of convection in the core, which is liquid, or at least has a portion that is liquid. If there is no convection, there is no dynamo effect that produces the magnetic field. The question is why. A popular idea that has come about lately which is supported by computer modeling is the idea that during a period called the Late Heavy Bombardment (LHB, a time in the early solar system where there were lots of asteroid-like objects flying around smashing into things), Mars was hit very very hard, heating its mantle to a temperature similar to the core. The core doesn't need to be much hotter than the mantle, on the order of 200 K, but it does need a differential for convection currents to continue. As a result of the lack of temperature difference, convection currents stopped, the dynamo process failed, and the magnetic field was lost.

This hypothesis fits with observations of the surface of Mars, which has old craters (dating to within 500 million years of the planet's formation, the time of the LHB) that show remnants of the old magnetic field. Craters dating to after the LHB however, show no magnetic anomalies like this. At the very least, this is a good indicator the LHB had something to do with it, and there are many hypothesis based upon this, like one that states that Mars HAD no dynamo until the LHB, which some (read: me) find implausible.

TLDR
So, it's hypothetical, but it's possible that, pending more evidence or better hypotheses, Mercury still has a magnetic field due to an accident of its composition (with more sulfur than expected, resulting in a lower melting point), and Mars does not due to an accident of the LHB heating up the mantle and removing convection.

There you are.