On March 17th, NASA’s spacecraft MESSENGER revealed surprising details about Mercury’s interior and topography, changing astronomers’ understanding of the small planet and how it was formed.

MESSENGER (MErcury Space Surface ENvironment, GEochemistry, and Ranging) is the first spacecraft sent to orbit and study Mercury, which orbits the Sun a mere 36 million miles away. It’s the innermost and hottest planet in our solar system. MESSENGER was launched in August 2004. Before traveling to Mercury, it made a series of flybys around the Earth (once) and Venus (twice).

MESSENGER finally arrived at Mercury on March 18, 2011 and went around three times. Using radio signals, the spacecraft studied Mercury’s gravitational field, magnetic field, topography, internal geological structure, and chemical composition. Because the results of MESSENGER’S flybys around Mercury were so valuable, its mission was extended to last for another year in November 2011.

Mercury’s topography has changed many times since Mercury was fully formed, meaning that there has been a considerable amount of geological activity. For that reason, before studying any of the planet’s internal structure and history, MESSENGER first produced an accurate map of Mercury’s gravitational field using information derived from the planet’s topography and spin state.

Thereafter, two studies were conducted simultaneously, examining Mercury’s internal structure and geography. In one study, the researchers involved with MESSENGER discovered that the planet’s core was much larger than previously thought: it takes up 85 percent of the planet’s radius. Furthermore, it is liquid instead of solid. Previously, scientists assumed that Mercury would have been cooled enough by now for the core to be solid.

Above the core lies an unusual layer that is composed of solid sulphur and iron – a layer not found in the other rocky planets in the Solar System. The outer layers of the internal structure consist of a solid silicate crust and mantle. It is thought that inside the larger liquid core lies a smaller solid core composed of sulphur and iron.

The other study of Mercury’s topography produced other surprising discoveries. When MESSENGER’s Mercury Laser Altimeter (MLA) produced a topographic model of the northern hemisphere and areas in the mid-latitude range, researchers learned that the elevation spread is smaller than similar regions on the Moon and Mars. The area that sticks out the most is lowlands that contain the northern volcanic plains.

Moreover, according to the Carnegie Institute for Science’s press release,

“… the interior plains of Caloris impact basin — 1,550 kilometers (960 miles) in diameter — have been modified so that part of the basin floor now stands higher than the rim. The elevated portion appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. These features imply that large-scale changes to Mercury’s topography occurred after the era of impact basin formation and large-scale emplacement of volcanic plains had ended.”

This new knowledge of Mercury’s internal structure and topography gives insight as to how Mercury formed thermally and how the planet’s magnetic field is generated. Details of the findings of each study from MESSENGER’s mission will appear in two separate papers, which will appear on March 23 in the journal Science.

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About the author

Sarah Hansen
Sarah is currently earning her M.F.A. in Creative Nonfiction at Sarah Lawrence College. She has an avid interest in the sciences, particularly astronomy, and hopes to one day publish works of fiction, poetry, and creative nonfiction.

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