18
$\begingroup$

Recent news articles related to this paper report the claim that Earth's solid inner core sometimes rotates backwards. The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum. The matter is discussed in this SE question, but there is also the matter of multi-decadal cycles. What is the actual assertion? Is the inner core thought to be cycling between prograde and retrograde rotation, or is it something more subtle, like a wobble, sometimes slightly leading and sometimes slightly lagging, and at what amplitude? Is it a matter of "gaining" or "losing" a few degrees over some number of years (so would only be rotating "backwards" in the frame of reference of the crust, not non-rotating space)?

$\endgroup$
17
  • 10
    $\begingroup$ A/B headline testing has shown over and over that a more catchy headline attracts more viewers to read an article. More views means more money in the bank for the publisher. What's catchier: That the Earth's inner core is reversing direction, or that its rotation rate perhaps slowly varies by about a thousandth of a percent? The StackExchange network uses a form of this in its list of Hot Network Questions.More eyeballs = more money. $\endgroup$ Commented Jan 25, 2023 at 3:56
  • 3
    $\begingroup$ @RedSonja It would be better to say rotates slightly slower or slightly faster, and by slightly I mean less than a hundredth of a percent. This phraseology caused confusion when it first came out about eight years ago. But it sure did get eyeballs on the articles! $\endgroup$ Commented Jan 25, 2023 at 15:32
  • 3
    $\begingroup$ Actual current CNN headline: "Earth’s inner core may have stopped turning and could go into reverse, study suggests" $\endgroup$
    – Anthony X
    Commented Jan 25, 2023 at 16:29
  • 7
    $\begingroup$ Great question. I saw this and just thought "huh?" It's like saying that cars you pass on a divided highway are driving backwards. So much sensationalist nonsense out there in the media. It's exhausting. $\endgroup$
    – JimmyJames
    Commented Jan 25, 2023 at 17:03
  • 3
    $\begingroup$ @AnthonyX Those stupid headlines are everywhere. $\endgroup$ Commented Jan 25, 2023 at 18:38

2 Answers 2

26
$\begingroup$

The mantle rotates about 131850 degrees per year. The actual assertion is that the inner core cycles between rotating about 131851 degrees per year versus 131849 degrees per year over the course of 70 year cycle. The paper was only published yesterday, so the scientific consensus is not there yet. The scientific consensus is not there yet on work done by the same authors eight years ago.

The inner core rotates in the same direction as do the mantle and crust, and rotates at almost at the same rate as the mantle and crust. The claim is that this rate varies by a tiny fraction compared to the mantle's rotation rate, plus or minus a degree or so per year compared to the mantle (but keep in mind that is one degree out of 131850 degrees). It does not switch directions.

$\endgroup$
13
  • 8
    $\begingroup$ So "backwards" here means it's either slightly slower or slightly faster compared to the mantle? $\endgroup$
    – gerrit
    Commented Jan 25, 2023 at 10:14
  • 11
    $\begingroup$ @gerrit Exactly. With respect to the rotating mantle, the inner core appears to move forward or backward by up to a degree per year. (This remains to be confirmed.) With respect to the "fixed stars" (one revolution per sidereal day) or with respect to the Sun (one revolution per mean solar day), the various parts of the Earth all rotate at almost the same rate. $\endgroup$ Commented Jan 25, 2023 at 10:33
  • 6
    $\begingroup$ Sure, go ahead, bust the bubble of this headline-reader. See if I care! ;-) $\endgroup$ Commented Jan 25, 2023 at 15:15
  • 4
    $\begingroup$ Wow, this is being wildly misreported in the popular press and media than. $\endgroup$ Commented Jan 26, 2023 at 14:09
  • 3
    $\begingroup$ @DavidHammen I am actually quite familiar with that, but this is a particularly egregious instance. $\endgroup$ Commented Jan 26, 2023 at 14:20
10
$\begingroup$

David Hammen's excellent answer covers the key pieces, but the original question also has this part:

The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum.

This isn't all that simple, actually. What is being conserved is the angular momentum of the Earth as a whole, not of the Earth core in isolation. As a consequence, the Earth's inner core can slow down a little bit if the rest of the Earth accelerates a little bit, and the other way around. Of course there needs to be a torque between the two, but that is not hard to fathom: We know that the outer core is moving rapidly (on the order of centimeters per second), due to thermal and/or chemical convection, and so it is not hard to imagine that there is friction between inner and outer core.

This is not so different from the fact that the solid Earth does not rotate at a fixed rate, but that days are fractions of seconds longer or shorter because the average east-west speed of the atmosphere changes from day to day; because the overall angular momentum must stay the same, an accelerating atmosphere implies a slowing down solid Earth, and the other way around. The same kind of thing happens (on slightly longer time scales) with ocean currents that take on more or less angular momentum and, as a consequence, have an influence on the rotation rate of the solid earth.

$\endgroup$
2
  • 3
    $\begingroup$ There are seven rotating parts of the Earth: The inner core, outer core, mantle, crust, oceans, cryosphere, and atmosphere. The mantle and crust rotate together over the span of hundreds of thousands of years. All of these parts collectively more or less conserve angular momentum over the span of a few centuries. The gradual transfer of angular momentum to the Moon's orbit is very, very gradual; it takes hundreds to thousands of years to observe this transfer. In the short term (a few centuries is short term), the parts exchange angular momentum with one another. $\endgroup$ Commented Jan 26, 2023 at 14:55
  • 3
    $\begingroup$ Underlying my statement was the recognition of a few facts and principles of physics, admittedly distilled down somewhat. Bottom line: what would seem in fact to be a subtle phenomenon (mundane physics, but interesting geologically) has been exaggerated to an absurd and nonsensically impossible extreme in the popular media. $\endgroup$
    – Anthony X
    Commented Jan 26, 2023 at 17:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.