Mercury's Surface Reveals a Surprisingly Fiery Past

Researchers from the Max Planck Institute for Solar System Research and the Universities of Münster and Göttingen in Germany have produced the most precise estimate yet of the amount of silicon dioxide — a compound made of silicon and oxygen that is common in Earth's volcanic rocks — on Mercury's surface, and found a surprising result: it makes up about 37% of the surface material by mass, up to 25% less than previous estimates suggested. The findings are published in the journal Planetary Research.

Mercury cooled relatively quickly after it formed, with volcanic activity largely ending roughly one billion years later, leaving the planet covered by a solid, continuous rocky crust — unlike Earth, whose crust remains geologically active. Studying the chemical makeup of Mercury's surface is one of the best ways to understand how that early volcanic history unfolded.

"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 study and head of the Experimental Laboratory Magma Ocean research group at the Max Planck Institute. As a planet's molten mantle cools, the first rocks to solidify contain relatively little silicon dioxide, while lava reaching the surface later in the process tends to contain more — so the unexpectedly low levels on Mercury point to volcanic material that came from deeper regions of the mantle, or to a crust that gradually lost oxygen over time.

Because no lander has ever collected a rock sample from Mercury, the researchers had to rely on remote infrared observations. To interpret these reliably, they first created tiny glass beads in the laboratory, each about half a millimeter across and containing a precisely controlled amount of silicon dioxide, and measured their infrared properties. They tested the method on the Moon first — comparing infrared-based estimates with actual lunar rock samples returned by astronaut and robotic missions — before applying it to infrared observations of Mercury collected with the Bok Telescope at Steward Observatory in Arizona.

The researchers hope that ESA's BepiColombo mission, made up of two probes provided by ESA and the Japan Aerospace Exploration Agency (JAXA), will provide an even more precise test of their findings.