Roughly one billion years after Mercury formed, volcanic activity may already have largely ceased, leaving a solid, continuous rocky crust. Earth, by contrast, remains geologically active, with volcanoes and plate tectonics constantly reshaping its surface. An alternative explanation, the researchers note, is that Mercury's crust once held more silicon dioxide but gradually lost oxygen over time.

How do you measure a planet you've never touched?

No lander has ever operated on Mercury, and scientists have never collected a rock sample from its surface. Everything must be inferred from infrared radiation — heat radiation — emitted by the surface, which carries clues about which minerals and compounds are present. The hard part is translating those infrared signals into reliable chemistry estimates.

The team solved this with tiny glass beads, each about half a millimeter across and containing a precisely controlled amount of silicon dioxide. After measuring each bead's exact infrared properties, they used the beads as reference standards.

"The glass beads serve a similar function to calibration weights on a scale," explains Iris Weber of the University of Münster. "Their weight is known precisely. They therefore allow us to correctly interpret the scale's balance."

Why test the method on the Moon first?

Before pointing the technique at Mercury, the researchers validated it on the Moon, where they could check infrared estimates against real rocks returned by astronaut missions and unmanned spacecraft. Using measurements from NASA's Lunar Reconnaissance Orbiter, which has been circling the Moon since 2009, they produced the first complete map of silicon dioxide content across the lunar surface.

"The Moon is a kind of touchstone for us — and an important conceptual stepping stone on our way to Mercury," Renggli said.

Only after passing this "Moon test" did the team analyze infrared observations of Mercury taken from Earth, including data from the Bok Telescope at Steward Observatory in Arizona. Those observations produced the 37 percent estimate.

Can BepiColombo confirm the result?

A stronger test is coming soon. ESA's BepiColombo mission, built from two separable probes provided by ESA and JAXA, is scheduled to enter orbit around Mercury in November 2026. On September 3, 2026, both probes separated from their transport module, the first step toward that phase.

Once in position, the mission's MERTIS instrument — developed under the leadership of DLR with the Institute for Planetology at the University of Münster — will collect infrared measurements at substantially higher resolution. Those data could confirm whether Mercury truly carries so little silicon dioxide, and clarify the extreme conditions that shaped its crust billions of years ago.

"Our study lays the groundwork for deriving the most accurate information possible about the silicon dioxide content of Mercury's surface from BepiColombo's measurements," Renggli said.