Mercury's Crust Has Far Less Silica Than Anyone Thought. That Means Its Insides Were Hotter.

Mercury's surface contains about 37% silicon dioxide, not the figure as high as 62% that earlier work suggested, according to researchers at the Max Planck Institute for Solar System Research and two German universities. The difference is not a rounding error. It changes where the rock came from.

Silica content is one of the most informative numbers in planetary geology because it tracks the depth at which mantle rock melted. Melting deep in a planet's interior, under high pressure, produces magma poor in silica. Melting shallow produces magma rich in it. Reading 37% instead of 62% moves Mercury's volcanic source rock substantially downward — and a deeper melting zone means a hotter interior than models of the planet have assumed.

"Volcanic rocks on Mercury formed from mantle material that had melted at greater depths than previously assumed," said Christian Renggli, who leads the Experimental Laboratory Magma Ocean group at the Max Planck institute and headed the study, published in the journal Planetary Research. The team, which included Iris Weber of the University of Münster and colleagues at the University of Göttingen, offers a second possible reading: that Mercury's crust has lost oxygen over time, stripped by the solar wind and micrometeorite bombardment at the closest orbit in the solar system.

The measurement problem is that nobody has a rock from Mercury. Everything known about its surface composition comes from remote sensing — reading the infrared light the surface emits and inferring what minerals produce that pattern. That inference needs a calibration curve, and the curve has to come from the lab.

So the team made one. They produced glass beads with precisely controlled proportions of silicon dioxide, measured their infrared signatures, and built a relationship between spectrum and silica content from scratch. They then tested it against a body where the answer is independently known: the Moon, where returned samples from the Apollo, Luna and Chang'e missions provide ground truth. When the method reproduced the lunar values, they applied it to Mercury.

Confirmation is close. The European Space Agency's BepiColombo spacecraft, which has been flying past Mercury on a series of gravity-assist maneuvers since 2021, is scheduled to enter orbit in November 2026, with its two component probes separating on Sept. 3. One of them carries MERTIS, a thermal infrared spectrometer designed specifically for this measurement — the same physics the German team calibrated, at far higher spatial resolution and from orbit rather than from flyby data.

Mercury has been the awkward planet for decades. It is unusually dense, with an oversized iron core, and its surface is unexpectedly rich in sulfur and poor in iron oxide, a combination pointing to formation under conditions with very little available oxygen. A hotter, deeper-melting interior is another piece of the same puzzle: whatever built Mercury, it did not build the other rocky planets.