Using a novel laboratory calibration, planetary researchers in Germany have produced the most reliable map yet of silica (SiO2) concentrations across the surface of Earth’s Moon, and a first silica estimate for Mercury. Their study is built around the Christiansen Feature (CF), a spectral signature in the mid-infrared that shifts predictably with a material’s silica content.
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER’s primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet’s surface. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
“The silica concentration in planetary surface materials is a fundamental geochemical indicator used to infer the nature and evolution of planetary crusts,” said Dr. Christian Renggli, a researcher at the Max Planck Institute for Solar System Research, and colleagues.
“It is a first-order measure of rock composition and the definition of lithologies, correlates with key mineral assemblages, and serves as a proxy for mineralogy and the degree of magmatic differentiation.”
“However, the determination of the silica concentration on the Moon and Mercury by remote sensing observations has proven challenging.”
For their new study, the researchers synthesized seven glasses spanning an extreme compositional range, from 0.5% to 97.6% silica.
They then used these glasses to refine the mathematical relationship between CF position and silica abundance.
“The glass beads serve a similar function to calibration weights on a scale,” said Dr. Iris Weber, a researcher at 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.”
Applying this calibration to global data from NASA’s Lunar Reconnaissance Orbiter Diviner instrument, the scientists generated a silica map confirming the Moon’s well-known split between silica-poor mare basalts (averaging under 46%) and silica-richer highlands (around 51%).
Their approach detected unusually high concentrations — up to 76% silica — at unusual volcanic features including the Gruithuisen Domes, Hansteen Alpha, and the Lassell Massif, sites long suspected to resemble terrestrial rhyolite domes but never before quantified this precisely from orbit.
The authors validated their results against soil and rock samples from the Apollo, Luna, and Chang’e missions, finding generally strong agreement.
According to the team, the calibration’s usefulness extends beyond the Moon.
Applying it to a decades-old Earth-based infrared measurement of Mercury, the researchers estimate the innermost planet’s surface holds only about 37% silica — notably lower than earlier estimates derived from the MESSENGER spacecraft data.
