Эрдэмтэд Дэлхийн эргэлтийн хурдны өөрчлөлт нь гаригийн дотоод бүтэц дэх хүчний солилцоотой холбоотой болохыг тогтоожээ.
Альбертагийн их сургуулийн геофизикч Хүйфэн Жан, Матью Дүмбери нар 1964-2019 оны хоорондох Дэлхийн дотоод цөмийн эргэлтийн сейсмик тооцоолол болон гадаад цөмийн урсгалын загварыг нэгтгэн судалжээ. Судалгаагаар өдрийн үргэлжлэх хугацаа миллисекундээр хэлбэлзэх нь гаригийн гүн дэх цөм болон мантийн хоорондох хүчний шилжилтээс хамаардаг болохыг баталсан байна.
Гадаад цөм дэх шингэн металлын урсгал болон дотоод цөмийн эргэлт нь мантид нөлөөлөх өөр өөр чиглэлийн эргүүлэх хүчийг үүсгэдэг ажээ. Дотоод цөм нь мантитай харьцуулахад бага зэрэг зүүн тийш хазайсан хурдаар эргэдэг бөгөөд таталцлын хүч түүнийг тогтворжуулах гэж оролддог байна. Эдгээр эсрэг чиглэлийн хүчний тэнцвэр алдагдах үед Дэлхийн эргэлтийн хурд өөрчлөгдөж, улмаар өдрийн уртад нөлөөлдөг болохыг судлаачид тайлбарлав.
Энэхүү судалгааны үр дүн нь 30 гаруй жилийн турш судлаачдын таамаглаж байсан цөм-мантийн харилцан үйлчлэлийн механизмыг илүү тодорхой болгож байна. Гэсэн хэдий ч сейсмик хэмжилтийн нарийвчлалын хязгаарлагдмал байдлаас шалтгаалан богино хугацааны өөрчлөлтийг бүрэн тайлбарлахад бэрхшээлтэй хэвээр байна. Судлаачид цаашид энэхүү үйл явцыг нарийвчлан судлах нь Дэлхийн гүн дэх материалын шинж чанар болон динамикийг ойлгоход чухал ач холбогдолтой гэж дүгнэжээ.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
Earth’s days are not always exactly 24 hours long. Tiny changes in the planet’s rotation can add or shave off a few milliseconds, and recent research links part of that variation to a hidden exchange of forces between the inner core, outer core and mantle.
Most of us would never notice the difference. But for scientists, those tiny shifts matter because they offer clues about what is happening thousands of miles below the surface, in places that cannot be observed directly.
Earth’s rotation is influenced by many things. Large earthquakes can redistribute mass, melting ice changes how water is spread across the planet, hurricane winds affect the atmosphere, and ocean tides can exert gravitational torque. Even with all that, some decade-scale changes in day length still point much deeper.
Deep Earth Holds The Answer
The focus of the new work is the core-mantle boundary, the region where the molten outer core meets the mantle above it. Popular Mechanics reports that geophysicists Huifeng Zhang and Mathieu Dumberry of the University of Alberta combined seismic estimates of inner-core rotation with models of outer-core flow. Their reconstruction covered the period from 1964 to 2019.
Researchers have known for more than 30 years that changes in the length of a day over decades are connected to interactions between the core and mantle. The tricky part has been figuring out exactly how the forces are transferred.
Viscous drag from flowing liquid metal appears too weak to explain the effect on its own. Observations of changes in Earth’s rotational axis point to another possibility: the lowermost mantle may conduct electricity, allowing moving metal in the outer core to exert electromagnetic stress on it.
“We have known for more than 30 years … that the decadal changes in day length are caused by core-mantle interactions,” Zhang and Dumberry wrote in their study published in Nature.
A Tug-of-war Inside Earth
The research centers on the way angular momentum moves between the inner core, outer core and the lower mantle. When material near the core-mantle boundary flows westward, it can create a westward torque on the mantle. That effect may involve electromagnetic forces, topographic forces, or a combination of the two. The inner core is doing something different.
The team found that it rotates at a slightly different rate from the rest of the planet and maintains a small eastward offset relative to the mantle. Gravity then pulls toward restoring the alignment between the two. That creates a persistent eastward torque on the mantle.
One force is acting westward while another is acting eastward. When the balance between those two shifts, Earth’s rotation shifts with it, producing a small change in the length of day. The inner core may also deform over timescales of just a few years. Changes in its shape can influence the layers above, adding another piece to an already complicated system.
The Models Still Have Limits
The researchers were able to reproduce changes mainly over periods of two or three decades or longer. The main limitation comes from seismic measurements. Estimates of inner-core rotation are not precise enough to resolve shorter intervals, which makes it difficult to track every small change in Earth’s spin.
The study also found no significant decade-scale variation in electromagnetic and topographic torques at the core-mantle boundary. That result does not necessarily mean those variations are absent. The models used to reconstruct outer-core flow still have limited resolution, and some forces affecting day length may not yet be included. As Zhang and Dumberry put it:
“As our study illustrates, better explaining the nature of the core–mantle torque driving the [length of day] contributes to sharpening our understanding of the structures, material properties and dynamics in the deep interior of Earth.”
For everyday life, the changes are almost meaningless. A few milliseconds will not alter anyone’s routine. But for geophysicists, those tiny changes are a signal from deep inside the planet, showing that Earth’s rotation is shaped by a quiet mechanical struggle far below our feet.
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