Mercury Hollows Explained by Hydrogen Geysers

NASA’s Project MESSENGER spacecraft returned photos of Mercury’s surface revealing “… an unusual landform on Mercury, characterized by irregular shaped, shallow, rimless depressions, commonly in clusters and in association with high-reflectance material” (image right). Mercury is the only rocky planet where such surface-features have been observed. NASA scientists recognized that the “pits” appear to have formed from volatile-related activity, but were unable to explain the origin and composition of the volatile substance, or the composition of the associated “high-reflectance material” on Mercury’s surface.

In a scientific communication published in the August 25, 2012 issue of Current Science, J. Marvin Herndon describes “a basis of planetary formation, not considered by the authors and unlike their cited models, and shows that primordial condensation from an atmosphere of solar composition at pressures of one atmosphere or above leads to the incorporation of copious amounts of hydrogen in Mercury’s core, much of which is released as the core solidifies.” Further, Herndon posits “release of hydrogen escaping at the surface … is responsible for the formation ... of pits, and for the formation of the associated ‘high-reflectance material’ … iron metal reduced from an iron compound, probably iron sulfide, by the escaping hydrogen.”

In the article, Herndon describes his calculations that show that when Mercury’s matter first condensed from primordial gas, if hydrogen pressures exceeded about one atmosphere, iron would condense as a liquid raining out to form Mercury’s core before mantle silicates began to condense. As he points out, “hydrogen is readily soluble in molten iron …” and he calculated that one or more Mercury volumes of hydrogen (at STP) could dissolve in Mercury’s iron core. In the figure at left, the solid line is the beginning of iron condensation; the dashed line the volume of hydrogen that could dissolve in Mercury's core forming under those conditions. For detail description, see paper cited below.

As Herndon reveals in the article, “The release of dissolved hydrogen during Mercury’s core solidification is … certainly sufficient in amount to account for the ‘unusual landform’ on Mercury’s surface and is possibly involved in the exhalation of iron sulfide, which is abundant on the planet’s surface, and some of which may have been reduced to iron metal thus accounting for the associated ‘high-reflectance material’, bright spots.”

The article further describes a test with potentially profound implications on planetary formation. “Verifying that the ‘high-reflectance material’ is indeed metallic iron will not only provide strong evidence for Mercury’s hydrogen geysers, but more generally will provide evidence that planetary interiors ‘rained out’ by condensing at high pressures within giant-gaseous protoplanets” rather than by the NASA planetesimal version that begins with the assumption of dust condensing at low pressures and then gathering progressively into larger objects, ultimately forming planets that would require whole-planet melting to form cores.

Important Paper: Herndon, J. M., Hydrogen geysers: explanation for observed evidence of geologically recent volatile-related activity on Mercury's surface. (click here) Current Science, 2012, 103, 361-362.

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