Science1 publisher2 min readPublished
Half-millimeter glass beads lower the silica estimate for Mercury's surface to 37 percent
A Max Planck team calibrated infrared readings against laboratory glasses of known composition and came out with less silicon dioxide on Mercury's surface than earlier estimates. How much less depends on how the release's 25 percent is read.
The Scientist · Science desk
What happened
- Researchers at the Max Planck Institute for Solar System Research and the universities of Muenster and Goettingen put silicon dioxide at about 37 percent of Mercury's surface material by mass.
- The institute says that is up to 25 percent below previous estimates of the planet's surface silica content.
- To read composition out of infrared measurements, the team first made glass beads about half a millimeter across, each with a controlled amount of silicon dioxide, and measured their infrared properties.
- The release gives two readings of the low silica: lava drawn from deeper mantle that melted more extensively at very high temperatures, or a crust that began richer and lost oxygen over time.
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Why it matters
- contradiction Two different Mercuries fit the same measurement: one whose interior stayed hot enough to melt deep mantle, and one whose crust merely shed oxygen. A mass fraction of silica cannot choose between them.
- constraint No lander has worked on Mercury and no sample has been returned, so the accuracy of any surface composition rests on how closely laboratory analogues match the real material.
- decision Thermal-history models built on a silica-richer Mercury now have a different input to fit, and that fit sets how much heat the early interior has to supply.
The reasoning runs through the order in which a cooling mantle freezes. The first rocks to crystallize out of molten mantle hold relatively little silicon dioxide, so the compound builds up in the melt left behind, and lava reaching the surface late in that sequence carries more of it [5]. Read backwards, a silica-poor crust points to melt from deeper mantle that melted more extensively, at very high temperatures [6].
"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 group at the Max Planck Institute for Solar System Research [7].
For scale, basalt, andesite and granite can contain as much as 75 percent silicon dioxide [9]. Mercury's surface, on the new estimate, sits at about half that ceiling [1]. How large a correction this is depends on how the 25 percent is meant: taken as a relative cut, earlier work sat near 49 percent silicon dioxide by mass [2], and the September 11 release from the institute does not say whether the figure is relative or in percentage points [15].
Everything here is inferred at a distance. What the team estimated is a mass fraction, and the hot early interior is a reading of that chemistry [1][6]. Infrared radiation from the surface carries information about which minerals and compounds are present, and the hard part is turning those signals into a reliable composition [11]. The laboratory glasses address that step. "The glass beads serve a similar function to calibration weights on a scale," said Iris Weber of the University of Münster [13].
Mercury cooled quickly by Earth's standards, and volcanic activity may have largely stopped roughly a billion years after the planet formed [14]. The new number describes the chemistry that volcanism left behind, and the release calls it the most precise estimate of Mercury's surface silicon dioxide so far [3]. The paper is published in Planetary Research, a Diamond Open Access journal [4].
What to watch
- Any constraint on the surface's oxidation state would separate the deep-melting reading from the oxygen-loss one.
- Whether other groups reproduce the 37 percent figure from the same infrared data now that the paper is Diamond Open Access.
- Whether the bead calibration library is extended to compounds other than silicon dioxide.