Laboratory Study Finds Mercury's Crust Far Poorer in Silica Than Assumed

Mercury's crust contains far less silica than planetary scientists had assumed, according to a new laboratory-based analysis that reinterprets decades of spacecraft data and suggests the solar system's innermost planet was shaped by hotter, deeper volcanism than previously modeled. The study, led by researchers at the Max Planck Institute for Solar System Research in Göttingen, Germany, appears in the inaugural issue of the journal Planetary Research.

Using a laboratory calibration technique rather than a spacecraft in orbit, the team determined that silicon dioxide, or silica, makes up roughly 37 percent of Mercury's surface by mass — as much as a quarter less than earlier estimates drawn from NASA's MESSENGER mission. Because silica content is one of the clearest chemical fingerprints of how and where a planet's rock formed, the revised figure points to volcanic material that welled up from far deeper inside Mercury, and at higher temperatures, than scientists had modeled.

What the numbers show

Silica abundance is a standard yardstick in planetary geochemistry because it tracks directly with how magma forms and evolves. Earth's volcanic rocks, shaped by plate tectonics and repeated melting, can run as high as 75 percent silica. The Moon's dark mare basalts average under 46 percent, its brighter highlands around 51 percent, and a handful of unusual lunar features — the Gruithuisen Domes, Hansteen Alpha and Lassell Massif — reach as high as 76 percent, a signature of more evolved, silica-rich magma.

Mercury's newly calculated 37 percent places it well below all of those benchmarks. The low figure, the researchers say, is best explained by mantle material that melted at much greater depth and higher temperature than assumed, producing magma poorer in silica than the crust it built. A second possibility raised in the paper is that Mercury's crust has lost oxygen over billions of years of exposure to solar wind and radiation, leaving some of its silicon in reduced metallic or carbide-like forms that standard infrared measurements would not register as silica at all.

A calibration built on lunar rock

The team did not use a new spacecraft instrument. Instead, lead author Christian Renggli, who heads the institute's Experimental Laboratory Magma Ocean research group, and colleagues including Iris Weber of the University of Münster's Institute for Planetology built a laboratory reference scale from scratch. They synthesized seven glass beads, each about half a millimeter across, spanning silica concentrations from 0.5 to 97.6 percent, and measured how each one behaved at a specific point in the mid-infrared spectrum known as the Christiansen Feature, where a mineral's emission and absorption cross over in a way that shifts predictably with silica content.

That calibration was then tested against the Moon, for which decades of returned samples from the Apollo, Luna and Chang'e missions offer ground truth. Combining the lab data with infrared measurements from NASA's Lunar Reconnaissance Orbiter, the team produced what they describe as the first comprehensive silica map of the lunar surface, and confirmed it matched known sample compositions closely. Only after that validation did the researchers apply the same method to Mercury, using existing infrared observations of the planet gathered from the Bok Telescope at Arizona's Steward Observatory.

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

Why the discrepancy matters

Mercury has long puzzled planetary scientists because its geological record ends abruptly. The MESSENGER spacecraft, which orbited the planet from 2011 to 2015, found evidence that widespread volcanism resurfaced much of Mercury between roughly 4.1 and 3.5 billion years ago before shutting down entirely — a stark contrast with Earth, where plate tectonics keeps recycling crust and fueling eruptions today. Explaining why Mercury's volcanic engine died so early, and what its rock chemistry looked like while it was still running, has depended heavily on how accurately scientists can read the planet's surface composition from a distance, since no lander or sample-return mission has ever visited Mercury.

Researchers who study rocky planet formation broadly are likely to take note. A lower, more accurately calibrated silica figure changes assumptions that feed into models of Mercury's interior structure, its unusually large iron core relative to its size, and comparisons between how the four inner planets differentiated after formation. Because Mercury is in many ways the most extreme of the rocky planets — smallest, closest to the sun, with a crust that stopped evolving billions of years ago — it serves as something of a boundary case for theories of terrestrial planet chemistry more broadly. The result has drawn attention from planetary science outlets including Phys.org and Universe Today, both of which flagged the timing as notable given how soon an actual spacecraft will be positioned to check the result.

"Our findings suggest that the volcanic rocks on Mercury formed from more deeply melted mantle material than previously assumed," Renggli said.

A test arrives within months

The paper's authors frame their work explicitly as groundwork for an instrument that is about to start returning real data. The joint European-Japanese BepiColombo mission, after a journey of more than seven years, is scheduled to jettison its transport module this week and settle into orbit around Mercury in November. Once there, its MERTIS infrared spectrometer, built by the German Aerospace Center with the University of Münster's planetology institute, will measure the Christiansen Feature directly and at far higher resolution than any Earth-based telescope can manage.

"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. Weber, describing the calibration approach, compared the glass beads to a familiar laboratory tool: "The glass beads serve a similar function to calibration weights on a scale."

If MERTIS confirms the low silica reading once it begins science operations, planetary scientists say it would firm up the case for a hotter, more deeply sourced Mercurian mantle and force a reassessment of how the planet's crust — and its record of early solar system volcanism — actually formed. If the spacecraft instead finds silica levels closer to the earlier MESSENGER-based estimates, that would point investigators back toward the alternative explanation involving long-term oxygen loss from the crust's surface layers. Either outcome, researchers involved in the work say, will sharpen a chemical record that has so far been built entirely from a distance, without a single rock in hand.