A balloon drifting 37 kilometers above Antarctica once caught a whisper from the ice that should never have been heard. In 2016, NASA’s Antarctic Impulsive Transient Antenna (ANITA) detected radio pulses rising from deep within the frozen continent-signals that, according to the Standard Model of particle physics, should have been impossible. For such a pulse to emerge upward, the originating particle would have had to traverse nearly 4,000 kilometers of dense rock and ice, a journey no known particle could survive without losing all detectable energy.
ANITA was designed for a conceptually simple yet technically challenging purpose: to scan for radio waves produced when ultrahigh-energy cosmic rays or neutrinos strike Antarctic ice. Neutrinos, sometimes called “ghost particles,” interact so rarely with matter that they carry precious messages from cosmic events of extreme power. At energies suggested by ANITA’s detections, these ghostly particles cannot even traverse the entire Earth. The upward-traveling signals arrived from directions as far as 30° below the horizon, well outside the narrow window in which neutrinos could conceivably appear.
The anomaly drew immediate scrutiny. Data from the Pierre Auger Observatory in Argentina and the IceCube Neutrino Observatory at the South Pole-both designed to capture extensive cosmic ray and neutrino events-showed no matching signals. “Such events have not been seen by an experiment with a long exposure like the Pierre Auger Observatory,” said Stephanie Wissel, associate professor of physics, astronomy and astrophysics at Penn State. So, it does not indicate that there is new physics, but rather more information to add to the story. This lack of corroborating information ruled out the most obvious astrophysical explanations, limiting the options to rare atmospheric phenomena, detector systematics, or physics beyond the Standard Model.
The engineering behind ANITA’s balloon-borne array forms part of a rich tradition of high-altitude particle detection. Similar recent missions, such as SuperTIGER and HELIX, also exploit the thin, dry polar atmosphere to reduce background noise and extend observation times. Above 99.5% of Earth’s atmosphere, instruments can capture these rare, high-energy events with relatively little interference. ANITA’s antennas scanned vast swaths of ice, using the flat, radiotransparent surface of Antarctica to detect faint radio bursts from particle showers. These showers, whether downward from cosmic rays or upward from tau neutrino decay, produce nanosecond-scale pulses that can be reconstructed to infer particle origin and energy.
Neutrino detection remains a formidable challenge. The cubic-kilometer lattice of optical sensors in IceCube collects Cherenkov light from neutrino interactions, whereas ANITA links on radio emission from related processes. Tau neutrinos can produce upward-going “air showers” from secondary tau leptons decaying in the atmosphere, but the particular geometry of ANITA’s anomalous events places them well below the horizon for such an improbable mechanism; energy loss through thousands of kilometers of rock would ensure the signal was undetectable.
The mystery has spurred the design of a successor: the Payload for Ultrahigh Energy Observations, or PUEO. Led by Abigail Vieregg at the University of Chicago, PUEO will fly a phased-array radio detector on a long-duration balloon, offering tenfold sensitivity over ANITA. “PUEO will have a factor of 10 better sensitivity than all previous flights of ANITA combined,” Vieregg said. The interferometric phased array, tested in ground experiments in Antarctica and Greenland, will allow rapid discrimination between real neutrino signals and noise sources such as satellite transmissions or wind-generated static electricity. Operating at 120,000 feet, PUEO will scan millions of square kilometers of ice, with onboard processing to sift terabytes of data in real time.
Underpinning this effort is global collaboration. Hardware, software, and analytical expertise are being provided by institutions across the United States, Japan, Europe, and South America. The Pierre Auger team’s simulations of upward-going showers, IceCube’s null detections, and ANITA’s original data are being cross-referenced in refining search parameters. Conserving the pristine Antarctic environment has been a parallel priority in this scientific activity. .
If PUEO or future detectors prove the anomalous signals real and not artifacts, the implications could be profound, pointing to new particles, forces, or even exotic phenomena like dark matter interactions. If they prove mundane, the lessons learned will nevertheless further advance the engineering and methodology of ultrahigh-energy particle detection. For now, the upward whispers from Antarctic ice remain one of physics’ most tantalizing enigmas, waiting for the next flight to speak again.
