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Here\u2019s what you\u2019ll learn when you read this story:<\/p>\n
When the rotation of Earth\u2019s axis speeds up or slows down, which happens over decades, a day might end up being milliseconds longer or shorter than 24 hours.<\/p>\n
Surface forces like hurricane winds and rising seas don\u2019t explain it all, so researchers looked deeper.<\/p>\n
The likely culprit is that the inner core spins at its own rate, clashing with the layers above.<\/p>\n
Though daylight hours shrink and stretch throughout the year, a day on Earth\u2014the time it takes for the planet to fully rotate on its axis\u2014is still 24 hours. But it isn\u2019t always the exact same 24 hours. The true length of days can vary over time<\/a>, give or take as little as a few milliseconds. While we barely realize that these shifts are happening, scientists are constantly trying to find out what\u2019s causing them.<\/p>\n Several forces can either lengthen or shorten Earth\u2019s days by speeding up or slowing down the planet\u2019s rotation. The most intense earthquakes redistribute mass by shifting entire landmasses, as do glaciers and continental ice that melt and raise ocean levels. Atmospheric influences such as hurricane winds<\/a> can have an impact. Even the flow of ocean tides can exert a gravitational torque that tugs on Earth\u2019s spin. But these influences aren\u2019t limited to the surface. They reach far below, down to the hot, ever-flowing liquid metal of the outer core. When forces disturb the flow of that molten metal enough, the rate at which the planet rotates can change.<\/p>\n What has remained a mystery about Earth\u2019s inconsistent rotation is how the torque between the core and mantle contributes to these shifts. Viscous drag from core flows is too weak to exert much stress on the core-mantle boundary<\/a> (CMB). But observations of how Earth\u2019s axis shifts position suggest that the lowermost mantle can conduct electricity, which would let core flows exert electromagnetic stress on it, coupling the two layers. To gauge these forces\u2019 effect on day length, geophysicists Huifeng Zhang and Mathieu Dumberry of the University of Alberta combined seismic estimates of the inner core\u2019s rotation with models of outer-core flow, reconstructing changes from 1964 to 2019.<\/p>\n \u201cWe have known for more than 30\u2009years … that the decadal changes in day length] are caused by core-mantle interactions,\u201d the researchers said in a study recently published in Nature<\/a>. \u201cYet we still lack a successful [length of day] prediction based on a modeling of the torque on the mantle.\u201d<\/p>\n Zhang and Dumberry realized that changes in the length of a day, however imperceptible, might actually be the result of angular momentum being exchanged between the inner core and the outer core (along with the bottom of the mantle<\/a>). The inner core can also deform within just years and affect the layers above as it flattens one way or the other.<\/p>\n When the flow of material at the CMB heads west, it causes a westward torque on the mantle that might also involve electromagnetic or topographic forces. Eastward and westward torques on the mantle fluctuate and compete. Because the inner core rotates slightly faster than the rest of the planet, it drifts east of the alignment that gravity<\/a> would otherwise hold it in relative to the mantle. Gravity pulls to restore that alignment, exerting a persistent eastward torque on the mantle that opposes the westward torque at the CMB. Any imbalance between the two shows up as a change in day length.<\/p>\n The models point to a small but persistent eastward misalignment of the inner core, sustained by its steady eastward drift relative to the mantle. Questions remain, though. The researchers could only predict rotation changes on timescales of two or three decades or longer, because seismic estimates<\/a> of the inner core\u2019s rotation aren\u2019t precise enough to resolve shorter intervals. And while the electromagnetic and topographic torques at the CMB showed no significant decadal fluctuations, that may say less about Earth than about the models. The core-flow model\u2019s resolution is limited, and some forces that shift day length may be missing from the calculations altogether.<\/p>\n \u201cAs our study illustrates, better explaining the nature of the core\u2013mantle torque driving the [length of day] contributes to sharpening our understanding of the structures, material properties and dynamics in the deep interior of Earth,\u201d Dumberry and Zhang said.<\/p>\n