Mercury's Surface Has Far Less Silica Than Thought, Hinting at a Hotter Interior

Mercury's surface appears to contain substantially less silicon dioxide, a compound of silicon and oxygen that is common on Earth, than researchers previously estimated. A new analysis puts the figure at about 37 per cent by mass, up to 25 per cent below earlier estimates, suggesting the planet's volcanic rocks formed from mantle material that melted more deeply than once assumed. The findings, from researchers at the Max Planck Institute for Solar System Research (MPS) and the Universities of Munster and Gottingen in Germany, are published in the journal Planetary Research.

Mercury, the smallest planet and the closest to the Sun, likely cooled early, and its volcanic activity had largely ceased about a billion years after the planet formed. Because no lander has reached Mercury's surface and no rock samples have been returned to Earth, researchers must infer its composition from remote sensing rather than direct measurement.

The team built its method in three steps. First, they made tiny glass beads with a precisely controlled amount of silicon dioxide and measured their infrared properties in the lab, establishing a relationship between infrared radiation and silicon dioxide content. Second, they tested that relationship on the Moon, using NASA's Lunar Reconnaissance Orbiter data to map silicon dioxide across the lunar surface and checking it against real lunar rock samples. Third, once the method passed that test, they applied it to infrared observations of Mercury, including measurements from the Bok Telescope at Steward Observatory in Arizona.

Lead author Christian Renggli of the Max Planck Institute said the findings suggest Mercury's volcanic rocks formed from mantle material that melted more deeply than previously assumed, since a young planet's earliest rocks to solidify remove relatively little silicon dioxide from the remaining molten material, so lava reaching the surface later can carry more of it; a low abundance at the surface therefore points to high interior temperatures. The researchers note an alternative explanation too: Mercury may once have had more silicon dioxide in its crust and gradually lost oxygen over time.

The European Space Agency's BepiColombo spacecraft, launched toward Mercury and consisting of two separable probes, is set to enter orbit around the planet in November this year, and the researchers hope it will provide an independent test of Mercury's unexpectedly low silicon dioxide abundance.