Mercury and Earth followed very different paths after they formed. Mercury, the smallest planet and the one closest to the Sun, appears to have cooled relatively quickly. Roughly one billion years after its formation, volcanic activity may already have largely stopped, leaving the planet covered by a solid, continuous rocky crust.
Earth never reached that same state. Its crust remains geologically active, with volcanoes and plate tectonics continuously reshaping the planet. Mercury, by contrast, has been largely frozen in place for billions of years. Scientists are still working to understand exactly how its volcanic history unfolded and how those early processes produced the world we see today.
One of the best ways to investigate that history is by studying the chemical makeup of Mercury’s surface.
Mercury Has Less Silicon Dioxide Than Expected
Researchers from the Max Planck Institute for Solar System Research in Germany and the Universities of Münster and Göttingen, also Germany, have now produced the most precise estimate yet of the amount of silicon dioxide on Mercury’s surface.
The findings, published in the journal Planetary Research, reveal an unexpected result. Silicon dioxide makes up about 37 percent of Mercury’s surface material by mass, which is up to 25 percent less than previous estimates suggested. Planetary Research is a Diamond Open Access journal, meaning its research is freely available to interested readers.
Silicon dioxide is extremely common on Earth. The compound, made from one silicon atom and two oxygen atoms, occurs in pure form in materials such as sand and is also a major component of volcanic rocks. Basalt, andesite, and granite can contain as much as 75 percent silicon dioxide.
The unexpectedly low amount on Mercury could provide an important clue about how the planet’s crust formed.
“Our findings suggest that the volcanic rocks on Mercury formed from more deeply melted mantle material than previously assumed,” said Christian Renggli, lead author of the new study and head of the “Experimental Laboratory Magma Ocean” research group at the MPS.
A Clue to Mercury’s Hot Interior
The mantle is the layer beneath a planet’s solid crust. Early in a planet’s history, much of this region can remain molten.
As that molten mantle gradually cools, the first rocks to solidify contain relatively little silicon dioxide. As more material crystallizes, silicon dioxide becomes increasingly concentrated in the remaining melt. Lava reaching the surface later in this process therefore tends to contain larger amounts of the compound.
That means unusually low silicon dioxide levels at the surface can point to a different history. In Mercury’s case, the finding suggests that volcanic material may have come from deeper regions of the mantle that experienced more extensive melting at very high temperatures.
Another possibility is that Mercury’s crust originally contained more silicon dioxide but gradually lost oxygen over time.
Tiny Glass Beads Help Decode Another Planet
Determining exactly what Mercury is made of is difficult because scientists have never collected a rock sample from its surface. No lander has ever operated there, so researchers must rely on remote sensing observations from telescopes and spacecraft.
Infrared radiation emitted by the surface can provide clues about which minerals and chemical compounds are present. The challenge is learning how to translate those infrared signals into reliable estimates of composition.
To improve that process, the researchers first created tiny glass beads in the laboratory. Each bead measured only about half a millimeter across and contained a carefully controlled amount of silicon dioxide. The team then measured the precise infrared properties of each sample.
“The glass beads serve a similar function to calibration weights on a scale,” explains Iris Weber from the University of Münster. “Their weight is known precisely. They therefore allow us to correctly interpret the scale’s balance. Similarly, the glass beads allow us to draw the correct conclusions from the properties of the infrared radiation.”
Testing the Technique on the Moon
Before applying the method to Mercury, the scientists tested it on a much larger natural target: the Moon.
The Moon offers an unusually useful test because scientists have both detailed remote sensing observations and actual lunar rock samples. NASA’s Lunar Reconnaissance Orbiter has been circling the Moon since 2009, collecting high-resolution measurements of infrared radiation coming from its surface.
Using those measurements and their new calibration method, the researchers produced the first complete map showing the silicon dioxide content across the Moon’s surface.
They could then compare those results with lunar rocks returned to Earth from different locations by astronaut missions and unmanned spacecraft. That comparison allowed them to check whether their infrared-based estimates matched the known composition of real samples.
“The Moon is a kind of touchstone for us – and an important conceptual stepping stone on our way to Mercury.”
Christian Renggli, first author and head of the “Experimental Laboratory Magma Oceans” research group at MPS
Turning the Method Toward Mercury
Only after the method successfully passed this “Moon test” did the researchers apply it to Mercury.
For the final stage of the study, they analyzed infrared observations of Mercury collected from Earth, including measurements made with the Bok Telescope at Steward Observatory in the U.S. state of Arizona.
Those data led to the estimate that Mercury’s surface contains about 37 percent silicon dioxide, substantially less than scientists had previously believed.
The researchers now hope that ESA’s BepiColombo mission will provide an even more precise test of their findings.
BepiColombo Could Confirm Mercury’s Chemistry
BepiColombo, which consists of two separable probes provided by ESA and JAXA respectively, is scheduled to enter orbit around Mercury in November of this year. The first step toward that phase of the mission occurred on September 3, 2026, when both probes were separated from the transport module.
Once in position, BepiColombo’s MERTIS instrument will collect much more precise infrared measurements of Mercury at substantially higher resolution. MERTIS was developed and built under the leadership of the DLR together with the Institute for Planetology at the University of Münster.
Those observations could confirm whether Mercury truly has such an unusually low concentration of silicon dioxide and provide scientists with a clearer picture of 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,” said Christian Renggli.
