Remember those Earth Science lessons about the layers of the Earth? The planet boasts a blazing hot, solid inner core, encased by a molten metal outer core, and a mantle that is solid yet flows like a thick liquid due to its molten rock composition. Above these layers lies the thin, solid crust, forming the surface on which all life exists.
Our current understanding of Earth’s inner structure dates back only about 120 years. The ability to monitor earthquakes with seismometers allowed geologists to detect Primary or P waves, which travel through both solids and liquids, and Secondary or S waves, which move only through solids. The variations in P waves as they traveled through the Earth revealed different materials at various depths, and the inability of S waves to traverse the entire planet suggested they encountered a liquid layer.
Using data from underground nuclear tests conducted over 30 years ago, researchers have gained deeper insight into Earth’s internal composition.
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In a recent study, Ying Zhou, a geophysicist from Virginia Tech, examined seismic waves from nuclear tests at Mururoa, a French Polynesian atoll, conducted between 1977 and 1995. These waves traveled through Earth and were recorded at a seismic station in Kazakhstan, providing Zhou with a series of “snapshots” of Earth’s interior over time.
Since the nuclear tests were conducted in close proximity, the seismic waves took nearly identical paths through the Earth. Zhou discovered that the waves passing through the outer core did not consistently take the same time to travel. Compared to 1977, the waves moved 0.1 seconds faster in 1982 and 1983, and 0.15 seconds faster between 1988 and 1990. By 1995, however, they slowed by approximately 0.15 to 0.2 seconds.
Though these time variations might seem minor, they are significant for waves traveling thousands of kilometers. Similar to the early seismic tests that unveiled P- and S waves, these findings suggest the waves are moving through varied materials, which shifted over the three decades observed.
The research also uncovered a previously unknown seismic wave type, termed PKrKP by Zhou. Unlike typical seismic waves that penetrate or reflect off the inner core, PKrKP waves reflect within the outer core’s middle, aiding Zhou in identifying sections of the seismic journey that reveal liquid outer core changes.
Researchers compared these outer-core waves to another seismic wave, named PP, which traverses the mantle instead of the outer core. The study analyzed 112 pairs of nuclear tests, with each pair’s explosions occurring less than 0.05 degrees apart.
What could cause these waves in the outer core to fluctuate in speed over a few years? The scale and rate of these changes imply vigorous mixing within the outer core. Zhou suggests in the study that a broad anomaly, “with a lateral extent over 700 kilometers and a thickness of about 100 kilometers in the low-latitude southern Pacific,” could account for the measured 0.1- to 0.2-second PKP travel time anomalies.
Zhou hypothesizes that this anomaly likely consists of suspended solid material moving through the liquid outer core, akin to large croutons floating in soup. This suggests the outer core may not merely be a vast pool of molten metal.
Understanding these significant anomalies in the outer core could also shed light on the processes driving Earth’s magnetic field.
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Earlier research from the University of Southern California indicated that changes in the outer core might influence the inner core’s shape, potentially affecting Earth’s magnetic field.
Related: Earth’s Core May Be Wrapped in an Ancient, Unexpected Structure
This study stands out for its timescale. While previous investigations into outer core changes often relied on seismic events separated by decades and faced challenges in distinguishing signals from uncertainties in earthquake locations, this research benefited from more controlled and time-limited seismic sources provided by the nuclear tests.
These decades-old nuclear explosions, though damaging to pristine ecosystems, have at least offered scientists a way to explore Earth’s deepest workings.
The full study is available in the Journal of Geophysical Research.
This article was fact-checked by Michael Irving and edited by Michael Irving. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
