“Two of the most surprising elements we saw in these particles were niobium and hafnium,” NOAA research chemist Daniel Murphy, who led the research, said in a news release. “These are both rare elements that are not expected in the stratosphere. It was a mystery as to where these metals are coming from and how they’re ending up there.”

Although unusual, such elements are common in spacecraft manufacturing and can be found in semiconductors, rocket chambers and other applications. The discovery sheds light on the environmental aftermath of a growing number of rockets, satellites and other human-made spacecraft that give off metal vapors as they reenter the atmosphere.

The researchers also identified aluminum, lithium, copper and lead in the stratosphere — all of which are linked to alloys used by the aerospace industry.

They estimate about 10 percent of the sulfuric acid particles studied contain trace metals from spacecraft reentry, but the researchers warn that number could grow to 50 percent or more given an expected surge in satellite launches. One day, they write, the number of aerosols linked to space debris will outnumber the particles produced when meteors vaporize while entering Earth’s atmosphere.

It’s still unclear how the existence of these rare metals and other elements in the stratosphere might influence the climate. Although the researchers write that “direct health or environmental impacts at ground level are unlikely,” they say the metals could affect cloud formation, cause light scattering and change the makeup of the stratosphere over time.