Volcanism, A Molten Core And Geomagnetism Options

[dvandorn]

This question has been bothering me for some time:

A lot of data suggests that Mars lost its magnetic field a *long* time ago -- like, in Noachian times. Something like three and a half billion years ago. More than anything else, the pattern of atmospheric depletion suggests this strongly.

There is also significant evidence that Mars has undergone volcanism for almost its entire history -- some lava flows have been dated via crater counts at only 10 million years or so.

The "accepted view" is that Mars lost its magnetic field because its core solidifed. Now, how does it logically make sense that Mars' core cooled so much that it congealed 3.5 billion years ago, but that enough heat was retained in the mantle to drive remarkably extensive volcanic activity for almost the entire remainder (to date) of the history of the planet?

Now, perhaps I am simply uninformed about the process of planetary cooling; it would make sense to me that a planet would cool from the outside in, not from the inside out. If that "common-sense" perception of planetary cooling is wrong, please, someone explain it to me...

In absence of better data about planetary cooling, though, it occurs to me that perhaps what needs to be questioned is not how Mars could be so volcanically active with a cold, congealed core. The appropriate question is whether or not a planet can spin rapidly (one turn in only a few tens of hours), have a molten core, and *not* generate a magnetic field.

After all, we only *theorize* that Earth's magnetic field is generated solely by the rapid rotation of its molten nickel-iron core. We have precious little data about the core/mantle boundary -- it seems possible to me that it is the rotation of the Earth's core/mantle *boundary* layer, and not the rotation of the core itself, which generates the magnetic field.

That would open up the possibility that Mars could *still* have a small molten core which is still driving mantle convection of some form or another. If a change in state, composition or other nature of Mars' core/mantle boundary is what killed its magnetic field, *not* the solidification of its core, that would mean the same thing could possibly happen some day on Earth.

And an Earth without a magnetic field is, in the long run, pretty much an uninhabitable planet.

I'll also toss into this discussion a morsel I read in the past year. Based on motion measurements using the Apollo laser retro-reflectors, one peer-reviewd paper insists that the Moon's observed motions can only be explained if its core is in fact still molten. Not only molten, but rotating at a slightly different rate, and around a slightly offset axis, from the rest of the Moon. (There does seem to be a thick layer of undifferentiated chondritic material overlying the core and a pretty thin layer of mantle; all of the volcanism we see on the lunar surface, it would seem, was the result of both immediate and stored accretion heating.)

And, of course, we all know the Moon has no intrinsic, global magnetic field.

So, once again, the question is begged: how sure are we that a fast-spinning, rocky planet which lacks a global magnetic field *must* have a cold, congealed core?

-the other Doug