- Subatomic particles called muons are born from cosmic rays that bombard the atmosphere.
- Because muons can penetrate objects, scientists use them to look into volcanoes.
- This technique – called muography – can help experts predict volcanic eruptions, research suggests.
Muons are everywhere. Unbeknownst to you, hundreds hit your head every second. These subatomic particles – created when cosmic rays penetrate the Earth’s atmosphere – are harmless and quickly decay into clusters of lighter particles.
The particles penetrate objects such as X-rays, making them useful to scientists who used muons to uncover a hidden chamber in Egypt’s Great Pyramid four years ago.
Scientists are also using ghostly muons to map the inner structure of volcanoes, which could one day help predict dangerous eruptions, according to an article published last week in the Proceedings of the Royal Society.
To create these maps, scientists measure how efficiently particles pass through magma flowing through caves, chambers and rock passages in volcanoes, and then use this information to create geological drawings, according to Giovanni Leone, a geophysicist at the University of Atacama in Chile and leader. author of the study.
The technique, known as muography, could one day become the “ultimate magma detection system,” Leone told The New York Times, adding that the technique makes it possible to track magma movements that may precede an eruption.
X-rays of the inside of a volcano
Muons are like fat, fast electrons: they have a negative charge, but are 207 times heavier than electrons and move at almost the speed of light. This weight and velocity allow particles to penetrate dense materials such as volcanic rock. The closer the object is, the faster muons lose speed and decay.
Many muons can hit the side of a volcano and travel straight through. But if the volcano is close enough – for example because a passage is filled with magma – a muon will not come out of the other side of the volcano.
To spot which muons survived the journey, scientists set up muon detectors on the flanks of a volcano. These detectors create an image of the entrails of the volcano by capturing the intrepid muons that did not decay as they passed through the volcano, noting holes where muons did not survive intact. Some scientists make this mapping from the air by placing muon detectors inside helicopters and flying near the flanks of the volcano.
Think of it as having your leg x-rayed. During an x-ray, radiation passes through your leg and is captured on camera. If the radiation passes unimpeded, the image appears black. But because your bone bones absorb some of the X-rays as they pass through, less radiation comes to the camera, which means your bones look brighter in the picture.
In volcanic muography, scientists are looking for the same contrast: Muons that pass through completely cast dark shadows on the muon detector. But when muons hit dense parts of the volcano and decay faster, they leave lighter silhouettes. In short, the closer the object is, the brighter the silhouette.
The more myon detectors that surround a volcano – some can be almost as big as a tennis court – the better the picture.
A detector provides a 2D image, according to David Mahon, a muography researcher at the University of Glasgow who was not involved in the study.
“By using multiple detectors placed around the object, it is possible to build a raw 3D image,” he told The New York Times.
Muons can help predict volcanic eruptions
Scientists have used muography to catch a glimpse of Japan’s Sakurajima and Mount Asama volcanoes, as well as three volcanoes in Italy – including Mount Vesuvius – and a Caribbean volcano in Guadeloupe.
In addition to helping scientists map volcanic interiors, the new article suggests that muography can be used to spot magma reservoirs inside volcanoes that are ready to erupt, and to track magma movements in real time.
Eruptions are often preceded by magma rising towards the volcano summit, and using muons to detect magma flow in the summit area can help scientists detect impending eruptions – allowing people to safely evacuate prior to an eruption.
“Knowing these issues as early as possible buys critically important time for those responsible for local alarm and evacuation protocols,” the study authors wrote, adding, “predicting violent volcanic eruptions is the holy grail of applied volcanology.”