NASA Balloon Detects Strange Signals Coming From Ice in Antarctica

Unusual radio signals from beneath Antarctica’s ice continue to defy explanation.

Several years ago, scientists using a cosmic particle detector in Antarctica recorded a series of puzzling radio signals, according to an international research collaboration that includes experts from Penn State. Between 2016 and 2018, NASA’s Antarctic Impulsive Transient Antenna (ANITA), a collection of instruments carried on high-altitude balloons over the Antarctic ice, picked up strange bursts of radio energy unlike anything seen before. Now, a new study has added fresh context to these nearly decade-old findings.

The ANITA mission was designed to study distant cosmic phenomena by detecting radio waves produced when cosmic rays strike Earth’s atmosphere. However, instead of bouncing off the ice as expected, some of the detected signals appeared to originate from below the horizon. This upward direction defied current models of particle physics and, at the time, raised the possibility that ANITA had uncovered new types of particles or interactions previously unknown to science, the researchers explained.

To investigate further, scientists turned to the Pierre Auger Observatory in Argentina. By examining 15 years of data on cosmic rays, the international team, including researchers from Penn State, sought to better understand the mysterious ANITA detections. Their results were recently published in the journal Physical Review Letters.

Searching for Answers

“The radio waves that we detected nearly a decade ago were at really steep angles, like 30 degrees below the surface of the ice,” said Stephanie Wissel, associate professor of physics, astronomy and astrophysics who worked on the ANITA team searching for signals from elusive particles called neutrinos. “While the origin of these events is still unclear, our new study indicates that such events have not been seen by an experiment with a long exposure like the Pierre Auger Observatory. So, it does not indicate that there is new physics, but rather more information to add to the story.”

She explained that calculations showed the unusual signal would have needed to travel through thousands of kilometers of solid rock before reaching the detector. Under normal circumstances, that journey should have completely absorbed the radio waves, making the signal impossible to detect.

“It’s an interesting problem because we still don’t actually have an explanation for what those anomalies are, but what we do know is that they’re most likely not representing neutrinos,” Wissel said.

Neutrinos, a type of particle with no charge and the smallest mass of all subatomic particles, are abundant in the universe. Usually emitted by high-energy sources like the sun or major cosmic events like supernovas or even the Big Bang, there are neutrino signals everywhere. The problem with these particles, though, is that they are notoriously difficult to detect, Wissel explained.

“You have a billion neutrinos passing through your thumbnail at any moment, but neutrinos don’t really interact,” she said. “So, this is the double-edged sword problem. If we detect them, it means they have traveled all this way without interacting with anything else. We could be detecting a neutrino coming from the edge of the observable universe.”

A Window Into Distant Cosmic Events

Once detected and traced to their source, these particles can reveal more about cosmic events than even the most high-powered telescopes, Wissel added, as the particles can travel undisturbed and almost as fast as the speed of light, giving clues about cosmic events that happened light-years away.

Wissel and teams of researchers around the world have been working to design and build special detectors to capture sensitive neutrino signals, even in relatively small amounts. Even one small signal from a neutrino holds a treasure trove of information, so all data has significance, she said.

“We use radio detectors to try to build really, really large neutrino telescopes so that we can go after a pretty low expected event rate,” said Wissel, who has designed experiments to spot neutrinos in Antarctica and South America.

ANITA is one of these detectors, and it was placed in Antarctica because there is little chance of interference from other signals. To capture the emission signals, the balloon-borne radio detector is sent to fly over stretches of ice, capturing what are called ice showers.

“We have these radio antennas on a balloon that flies 40 kilometers above the ice in Antarctica,” Wissel said. “We point our antennas down at the ice and look for neutrinos that interact in the ice, producing radio emissions that we can then sense on our detectors.”

These special ice-interacting neutrinos, called tau neutrinos, produce a secondary particle called a tau lepton that is released out of the ice and decays, the physics term referring to how the particle loses energy as it travels over space and breaks down into its constituents. This produces emissions known as air showers.

If they were visible to the naked eye, air showers might look like a sparkler waved in one direction, with sparks trailing it, Wissel explained. The researchers can distinguish between the two signals — ice and air showers — to determine attributes about the particle that created the signal.

These signals can then be traced back to their origin, similar to how a ball thrown at an angle will predictably bounce back at the same angle, Wissel said. The anomalous findings, though, cannot be traced back in such a manner as the angle is much sharper than existing models predict.

Anomalies Without a Clear Source

By analyzing data collected from multiple ANITA flights and comparing it with mathematical models and extensive simulations of both regular cosmic rays and upward-going air showers, the researchers were able to filter out background noise and eliminate the possibility of other known particle-based signals.

The researchers then cross-referenced signals from other independent detectors like the IceCube Experiment and the Pierre Auger Observatory to see if data from upward-going air showers, similar to those found by ANITA, were captured by other experiments.

Analysis revealed the other detectors did not register anything that could have explained what ANITA detected, which led the researchers to describe the signal as “anomalous,” meaning that the particles causing the signal are not neutrinos, Wissel explained. Back when they were first detected, theories swirled that signals did not fit within the standard picture of particle physics, and others suggested that it may be a hint of dark matter, but the recent lack of observations with IceCube and Auger really narrow the possibilities, Wissel said.

The teams have been working on balloon projects for over a decade, Wissel explained, and added that her team is currently working on designing and building the next big detector. The new detector, called PUEO, will be larger and better at detecting neutrino signals, Wissel said, and it will hopefully shed light on what exactly the anomalous signal is.

“My guess is that some interesting radio propagation effect occurs near ice and also near the horizon that I don’t fully understand, but we certainly explored several of those, and we haven’t been able to find any of those yet either,” Wissel said. “So, right now, it’s one of these long-standing mysteries, and I’m excited that when we fly PUEO, we’ll have better sensitivity. In principle, we should be able to better understand these anomalies which will go a long way to understanding our backgrounds and ultimately detecting neutrinos in the future.”

Reference: “Search for the Anomalous Events Detected by ANITA Using the Pierre Auger Observatory” by A. Abdul Halim, P. Abreu, M. Aglietta, I. Allekotte1, K. Almeida Cheminant, A. Almela, R. Aloisio, J. Alvarez-Muñiz, J. Ammerman Yebra et al. (Pierre Auger Collaboration), 27 March 2025, Physical Review Letters.
DOI: 10.1103/PhysRevLett.134.121003

The research conducted by scientists from Penn State was funded by the U.S. Department of Energy and the U.S. National Science Foundation.

Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.