Scientists at the University of Alberta have identified a gravitational interaction deep inside Earth that may help explain tiny changes in day length. By analyzing records from 1964 to 2019, physicists Huifeng Zhang and Mathieu Dumberry found that the slightly irregular shape of Earth's solid inner core creates a gravitational tug with uneven mass in the mantle. This gravitational torque can subtly speed up or slow down the mantle and alter the planet's rotation by a few milliseconds over a roughly 70-year pattern. Their models matched observations best when the inner core was allowed to slowly deform over roughly eight to 10 years rather than remain completely rigid.
Published in Nature, the study found that gravitational forces better explained the observed changes than magnetic or pressure forces alone. The calculations also offer clues about conditions deep within Earth, including a possible thin, iron-rich conducting layer near the bottom of the mantle and warmer, chemically distinct material there. However, the researchers caution that the apparent 70-year pattern may not represent a regularly repeating cycle, and their results depend on models of inner-core rotation and liquid-core flow that still contain significant uncertainty. The findings nevertheless demonstrate how extremely small changes in Earth's rotation can reveal information about the composition, movement, and physical behavior of otherwise inaccessible regions deep inside the planet.