A beautiful view of Mercury’s horizon. In this scene, which was acquired looking from the shadows toward the sunlit side of the planet, a 120-km (75 mi.) impact crater stands out near the center. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
NASA researchers have found that several volcanic deposits on Mercury’s surface require mantle melting to have started close to the planet’s core-mantle boundary, which lies only 400km below the planets surface and making it unique in the solar system. This is reported at the Goldschmidt conference in Yokohama, Japan. The recent MESSENGER mission to Mercury has shown that the surface of the planet is very heterogeneous, but it can be classified into 2 main types of regions. One is an area of relatively young Northern Volcanic Plains (NVP), 3.7- 3.8 B years old. The other area is older (4 to 4.2 Ga) and consists of intercrater plains and heavily-cratered terrains (IcP-HCT), 4 -4.2 B years old.
The older regions contain several previously unexplained features, including a large magnesium-rich spot, which is ~10,000 000 km square – around the size of Canada although because Mercury is much smaller than the Earth this spot takes up around 15% of the planet’s surface.
Until now, there has been no satisfactory explanation of how the formation and history of the planet would have allowed these heterogeneous areas to develop without invoking melting of a heterogeneous mantle. But now a group of NASA scientists have performed a series of experiments which explain most of the chemical compositions of Mercury’s surface.
They simulated early conditions on Mercury which is believed to have formed under highly reduced conditions. Enstatite chondrites are similarly reduced and may be a good proxy to the chemical building blocks. So the researchers took the same chemistry as found in enstatite chondrites, and began to subject them to the sort of pressures (5GPa = 50 000X Earth’s atm P) and temps found in the deep mantle of Mercury. Unlike the Earth, it has a large core and a comparatively shallow mantle, so the mantle-core boundary is only around 400 km below the crust.
These findings indicate the older terrains are formed by material melting at high pressures up to the core-mantle boundary, while the younger terrains are formed closer to the surface. Mercury likely formed with enstatite chondrites. The particularity of Mercury and these types of meteorites is their high sulfur content. The role of sulfur on magma composition was difficult to predict as Mercury is the only terrestrial planet with such high sulfur concentrations (1 to 4 weight %). For comparison, the mantles of Earth, Mars and the Moon have <0.1 weight % sulfur. Both effects of pressure and sulfur explain overall heterogeneous surface composition of Mercury. http://www.eurekalert.org/pub_releases/2016-06/gc-rfs062816.php
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