NASA scientists at Johnson Space Center have solved a longstanding mystery about why some of Mercury’s surface looks new, but some appears to be old. The scientists in the Astromaterials Research and Exploration Science Division are working with data from NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015. This unprecedented feat provided continuous, up-close observation of our solar system's innermost planet and revealed that the planet is extremely diverse.
One area of the planet, the Northern Volcanic Plains, is very young while the other is older and consists of intercrater plains and heavily cratered terrains. Until now, there has been no good explanation for how such heterogenerous compositions could develop.
“We think that planets start hot and almost completely melt,” said Dr. Asmaa Boujibar, NASA postdoctoral fellow and lead author of the study. “As they cool, they crystallize various minerals. In some cases, minerals can separate to form different layers inside the planets.”
Earth’s Moon is a good example of this as shown in samples brought back during the Apollo missions. In contrast, Earth does not seem to have these layers, either because the minerals never separated, or because the movement of its surface plates, called tectonics, has mixed everything up again, Boujibar said. So the research team at Johnson set out to answer the big question about Mercury, which is whether its interior would be chemically layered like the Moon, or homogenous like the Earth. Previous studies suggested that the surface of Mercury is so heterogeneous that the mantle had to be compositionally layered like the Moon.
The team performed its research in the experimental petrology laboratory at Johnson, where planetary interior conditions are simulated, allowing scientists to study materials at high pressures and temperatures. Mercury is the least oxidized planet of our solar system, where most of its iron is bound to metal or sulfide rather than an oxide.
Chondritic meteorites have compositions similar to the sun and are thought to be the building blocks of planets. The metal-rich and reduced (or least oxidized) enstatite chondrites represent the best candidates as Mercury’s building blocks. The researchers took enstatite chondrite compositions and subjected them to the high temperatures and pressures found deep inside the interior of Mercury.
The team’s study shows that a layered mantle is not needed. With a homogeneous interior of Mercury, a large variety of melts can be brought to its surface from a wide range of depths, which can explain the heterogeneous composition of the surface.
“The key finding is that by varying pressure and temperature on only one type of composition, we could produce the variety of material found on the planet’s surface,” Boujibar said.
In particular, the study shows that older terrains on Mercury have formed by material melting deep in the boundary between the core and mantle, while younger terrains formed closer to the surface. At reduced conditions, sulfur dissolves into the silicate mantle and also influences the melting and melt compositions. The combined effects of pressure and sulfur explain the overall heterogeneous surface composition of Mercury.
The findings have fundamental implications for our understanding of how the solar system formed. They show that Mercury could have formed from materials like enstatite chondrites. This means that three large bodies – Earth, Moon and Mercury – could have formed from similar material and suggests that much of the inner solar system may have been made from the same material rather than from diverse materials as traditionally believed.
Going forward, the researchers will seek to understand if the mantle of Mercury homogenized through convection early in its history, or was it never layered, why Mercury has a larger core than any other planet, if a giant impact stripped out some of the mantle, and if asteroids already have large cores.