Mercury may have built its crust in a hotter interior

Observation — The researchers made synthetic glasses containing 0.5% to 97.6% silicon dioxide and measured their mid-infrared emission. The position of the so-called Christiansen feature changed strongly with SiO2 abundance; a quadratic fit reached an R² of 0.957, with an estimated uncertainty of ±3.4 percentage points by mass.

Observation — Before applying the relationship to Mercury, the team tested it on data from the Diviner instrument orbiting the Moon and compared the results with lunar samples returned to Earth. The calibration recovered the expected split between maria, averaging 45.8% SiO2, and highlands, at 50.9%, as well as local silica-rich regions reaching roughly 76%.

Observation — When the calibration was applied to the only published disk-wide Christiansen-feature value for Mercury, near 8.5 micrometers, the authors estimated 37.4% SiO2. That is below earlier estimates of 49% to 60%, but it is neither a sample analysis nor a global map: it is a conversion of one integrated telescopic measurement.

Inference — If the estimate is correct, a more silica-poor surface is consistent with more extensive melting at greater depth in the mantle, and therefore a hotter interior while the crust formed. That interpretation comes from models of melting and differentiation; it is not a directly measured temperature and cannot by itself identify the volcanic process involved.

Hypothesis and limit — The authors also consider another explanation: Mercury may have lost oxygen, leaving some silicon in metal or carbide phases rather than detectable silicates. Mineral composition, grain size, and the thermal environment can shift the spectral feature. The MERTIS imaging spectrometer aboard BepiColombo is expected to make global observations that can test the extrapolation.

Responsible speculation — If future maps confirm the low abundance, Mercury's thermal and volcanic history will have to accommodate more extreme conditions than earlier estimates implied. The calibration could also aid remote studies of other airless rocky worlds, but the present result opens a test; it does not close the scientific question of Mercury's crustal composition.