A new study confirms that Earth’s inner core has been rotating more slowly than usual since 2010. This “backtracking” could slightly alter the planet’s overall rotation, potentially lengthening our days. The inner core, a roughly moon-sized chunk of solid iron and nickel located over 3,000 miles (4,800 kilometers) beneath our feet, has been spinning unusually slowly for the past 14 years. If this trend continues, it could lengthen Earth’s days, though the effects would likely be imperceptible to us.
The inner core is surrounded by the outer core, a superhot layer of molten metals, which in turn is surrounded by the mantle and the crust. While the entire planet rotates, the inner core can spin at a different speed due to the viscosity of the outer core.
Historically, the inner core has rotated slightly faster than the mantle and crust. However, a new study published on June 12 in the journal Nature reveals that since 2010, the inner core has been slowing down and is now rotating more slowly than the planet’s outer layers.
If the inner core’s rotation continues to decelerate, its gravitational pull could cause the outer layers to spin more slowly, potentially altering the length of our days by thousandths of a second, which would be imperceptible to us.
This phenomenon, known as “backtracking,” has been debated for about a decade but has been difficult to prove. In the new study, researchers analyzed data from over 100 repeating earthquakes along a tectonic plate boundary in the South Sandwich Islands between 1991 and 2023. By comparing measurements of the core’s position relative to the mantle, the team confirmed the inner core’s rotation rate changes over time.
The inner core’s backtracking is likely caused by either the churning of the liquid iron outer core that surrounds it or gravitational tugs from dense regions of the overlying rocky mantle. It remains unclear how frequently backtracking occurs, suggesting that the inner core’s spin may constantly accelerate and decelerate over decades or longer.
Recent advancements in technology are helping researchers uncover more about the inner core, such as its slightly lopsided shape, its unexpected softness, potential wobbling off Earth’s axis, and the existence of a separate innermost core. The study authors plan to continue analyzing seismic data to further understand the inner core’s changes over time.
