The properties of the interior of the Earth remain one of science’s greatest unsolved mysteries. Despite information from indirect methods, such as seismic and gravitational measurements, used to investigate the Earth’s internal structure, many questions remain unanswered.
Atmospheric neutrinos, which are produced in cosmic-ray-induced air showers in the Earth’s atmosphere, are detected by the IceCube Neutrino Observatory at the South Pole. As they traverse through the Earth, these neutrinos can transform or oscillate into other types. Because neutrino oscillations are affected by the density of the Earth’s matter, they can serve as probes for the Earth’s hidden interior.
The IceCube Collaboration predicted the sensitivity of the successfully deployed IceCube Upgrade to the Earth’s internal structure. Using simulated atmospheric neutrino oscillations in the presence of matter inside the Earth, they predicted that statistically significant data from the Upgrade would establish the modification of neutrino oscillations due to interactions between neutrinos and matter inside the Earth. By exploiting these interactions, they predict that the data from the IceCube Upgrade would rule out a uniform Earth density profile with respect to the standard layered-density profile and measure the mass of the Earth and the density of various layers. Their results are discussed in a paper submitted to Physical Review D.
“Neutrinos interact only weakly with matter and can traverse the entire Earth almost unhindered,” explains Krishnamoorthi Jayakumar, a PhD student at the Institute of Physics (IOP) in India and study colead. “Their oscillations carry unique information about the electron density encountered along their path, making them powerful messengers for studying regions that are inaccessible by any direct means.”
The study was also coled by IOP PhD students Anuj Kumar Upadhyay and Sharmistha Chattopadhyay, DESY postdoctoral researcher Anil Kumar, and IOP professor Sanjib Kumar Agarwalla, who all conducted the work while at the University of Wisconsin–Madison.
Researchers performed computer simulations by incorporating the expected improvements from the Upgrade, including better energy and directional reconstruction of atmospheric neutrinos. The simulations looked at the number of atmospheric neutrinos passing through different regions of the Earth and measured changes in oscillation patterns as the neutrinos encountered varying matter densities. The simulated neutrinos were then passed through a realistic model of the Upgrade in order to measure its expected response, event reconstruction capabilities, and experimental uncertainties.
“The IceCube Upgrade will significantly improve our ability to reconstruct low-energy atmospheric neutrinos, precisely where matter effects inside the Earth are strongest,” says Kumar. “These improvements provide an exciting opportunity to explore whether neutrino oscillations can reveal subtle features of the Earth’s internal density structure.”
Using advanced statistical methods, the team compared the expected observations for different Earth models to determine the Upgrade’s sensitivity to density variations in the Earth’s interior, one of its most important features.
“Our analysis shows that the improved statistics and reconstruction capabilities of the IceCube Upgrade will substantially enhance sensitivity to density variations inside the Earth,” says Chattopadhyay. “This lays the foundation for increasingly precise neutrino-based measurements as larger data sets become available.”
The work is a stepping stone toward using neutrinos to probe the Earth’s interior, complementing traditional methods that use seismology and gravitational measurements.
“One of the most exciting aspects of this study is that it demonstrates how a detector built primarily for fundamental particle physics can also address important questions in geophysics,” says Upadhyay. “This highlights the remarkable versatility of neutrino observatories.”
Future predictions will be tested with real Upgrade data and refined using larger data sets. More importantly, this study establishes neutrino Earth tomography as a promising new interdisciplinary field, motivating future studies with next-generation detectors such as the proposed expansion, IceCube-Gen2.
“For more than a century, our understanding of the Earth’s deep interior has relied primarily on seismology,” says Agarwalla. “Our work demonstrates that atmospheric neutrinos offer an entirely independent and complementary probe by directly measuring the Earth’s electron-density distribution through neutrino oscillations.”
+ info “Estimating the sensitivity of the IceCube Upgrade to probe the interior of the Earth using atmospheric neutrino oscillations,” IceCube Collaboration: R. Abbasi et al. Submitted to Physical Review D. arxiv.org/abs/2608.06543