Astronomers all over the world have scrambled to catch up to data that suggests Earthlings are the cosmic kind of luckiest bunch. Now a new study lays out the science behind an ancient, vital, but poorly understood factor in Earth’s defense against deadly galactic cosmic rays—one that’s revealed itself as iron dust, or “rust.”

A decade-long, exhaustive analysis of NASA’s Adirondack 10 survey data of rising galactic cosmic ray heights since the mid-20th century has finally uncovered what metals are most widely exposed to galactic cosmic rays.

“I consider it to be a major breakthrough,” said the study’s lead author, John Hood, professor of astronomy and astrophysics at the University of Michigan at Ann Arbor. “We’ve been systematically looking for these iron elements,” among the more abundant metal elements, such as gold, platinum, and silver, Hood said. “It turned out to be a whole new area.”

The discovery came while Hood and other scientists were trialling a technique in which they could compare the evolutionary trajectories of iron, lead, aluminum, and zinc over time—through, for example, statistical trends in the way they reflect cosmic rays. “We’ve been studying signals for nine years,” Hood said. “We only did this about nine months ago.”

As research has focused on how to counter the most dangerous galactic cosmic rays—the high-energy particles from the Sun or other stars that are a constant threat to human health and the physical systems of life on Earth—studies have also been revealing that the Earth’s defenses against other cosmic rays might not be so terrible after all. Some studies have pointed to a global long-term reduction in the levels of galactic cosmic rays. The benefits of this cutback have been felt by human bodies in the form of a longer life and our ability to breathe. That is why, in some ways, it’s ironic the discovery by Hood and colleagues has served to help against cosmic rays.

Some of these galactic cosmic rays can be harmful to cells, making them reactive, Hood said. The mechanisms that take effect on other metals over time include filtration, or possibly exposure to stronger radiation over an extended period, he said. Hood and colleagues weren’t sure whether these mechanisms play any role on the tinier, lower-energy cosmic rays, which tend to leave comparatively fewer ecological casualties.

That’s where the tiny magnetite iron dust particles come in. These are “hot iron waves” created as iron atoms and positive-charge particles whiz around in an electric charge field that drives them from their solid source to a liquid form, Hood said. The magnetic field causes only a temporary cool-down. As temperatures fall, the waves turn back into gas. “They get so condensed they start to dry out, and that may be where these iron waves occur,” Hood said.

The iron atoms lose their absorption charge as they warm up, he said. But that needn’t be a problem: “It’s not the case that you can get rid of iron atoms just because they are ionized and they don’t absorb any more of the electric field,” Hood said.

What’s more, there are positive-charge iron waves created by the same electric field—but these are slower and the result is a much longer period of cooling. These “core” iron waves were also released by particles moving from inner solar system to outer solar system, as well as particles moving from the outer Solar System towards the inner Oort Cloud. Some have even been found moving from Earth to Mars and back.

The problem is we don’t know exactly where those iron waves come from. “We’re trying to develop a direct mechanism for what the iron is doing and how it’s getting there.”

The results of the study were published in Astrophysical Journal Letters and are available online.