Friday, January 27, 2023

Spinning cores

You probably saw the report that the Earth's inner core changed rotation direction. Color me dubious.

I'm not dubious because there's too much rotational energy involved: it is next to the outer core, which is "liquid" and has its own rotations and vortices, which could conceivably push the inner core one way or another. That has a lot more angular momentum and is, if a cursory read is any guide, fiendishly complicated.

(And the moon apparently makes the inner core's rotation precess--the axis of revolution changes.)

In fact, some calculations suggest that chemical plumes can play a major role in convection--not just thermal ones. The chemical changes come when iron crystallizes out on the inner core, releasing oxygen which gets taken up in other, lighter compounds which (the link suggests) can burst out of the interaction zone when enough accumulate. We don't know the chemistry, or the "burping rate" well at all.

In order to "see" the core, your sound waves have to traverse the crust (easy), the mantle (we sort of understand), and the outer core--and its dynamics are actively debated. Can things change on the time-scale of decades? Unfortunately, papers don't always give the units in forms familiar to me (that one is on convection in the inner core! and Figure 13 shows convection times on the order of at least 10 million years).

One estimate has large-scale flow at the top of the core at about 10km/year. If that's the same at the bottom and all in the same direction (!), and with good coupling to the inner core, it would seem to suggest a "flipping" time of order a few hundred years.

I spent more time than I expected in this really deep rabbit hole, and came up strongly suspecting that the measurement the team made has gigantic error bars, and that the story got publicity for its novelty value.

I don't know how long this visualization of turbulence in the outer core will be up. It's a simulation, of course, salt to taste.

1 comment:

SJBC said...

I read the article and puzzled out what was really meant (it was poorly written). It's not that the earth's core starting rotating backwards, it's that it starting rotating slower than the outer layers.

It is specifically stated later in the article that it would seem like a backwards rotation only from the viewpoint of someone standing on the surface of the earth; not from the viewpoint of an outside observer.

it's often the case that science journalists do not fully understand the science they are trying to explain.

The best example of this is almost any popular article dealing with magnetic pole reversals; all of then assume that if one occurs then we will have to swap our description of north and south. Actually compass directions have nothing to do with magnetic polarity, they are determined by the direction of rotation. That's why we can refer to the north & south poles of celestial objects that have no global magnetic field.