Although geologists and seismologists have known and understood the basics of plate tectonics since the concept was put forth and proved, they are still baffled by many aspects, including the nature of the “boundary” between Earth’s two outermost layers: the lithosphere and asthenosphere.

Just this week, however, seismologist and NASA Postdoctoral Program Fellow Dr. Nicholas Schmerr, who is stationed at NASA Goddard Spaceflight Center in Greenbelt, Maryland, has found an actual layer between the two aforementioned layers that effects the movement of plate tectonics and seismic waves.

Plate tectonics describes the movement of the seven broken plates of the Earth’s crust. These movements are a result of the magma churning in the mantle and cause continental drifts (i.e. the continents move toward or away from one another), earthquakes, and volcanic activity.

The lithosphere (the crust) is the outermost layer of the Earth. The asthenosphere lies between the lithosphere and the mantle and contains viscous magma. This thin layer – thinner than the lithosphere – acts as the transition from cold, solid rock to hot, liquid rock.

The imaginary boundary between the lithosphere and asthenosphere is called the LAB, where the abrupt change in temperature occurs. For many years, there has been a mystery as to what causes the continents to slide over the asthenosphere. Recently, it has been speculated that there is a layer at the LAB that is designated the ‘Gutenberg discontinuity,’ which provides lubricant for the plates to move with ease. The Gutenberg discontinuity is composed of partially molten rock.

“This melt-rich layer is actually quite spotty under the Pacific Ocean basin and surrounding areas, as revealed by my analysis of seismometer data,” Dr. Schmerr said in the NASA press release. He hypothesized that the existence of the Gutenberg discontinuity is the result of decompressed hot rock, or hot mantle, plumes that cause the lower portion of the lithosphere to melt.

“Most of the melt layers are where you would expect to find them, like under volcanic regions like Hawaii and various active undersea volcanoes, or around subduction zones – areas at the edge of a continental plate where the oceanic plate is sinking into the deep interior and producing melt,” he continued. Essentially, the Gutenberg discontinuity is located in only certain areas, mainly in parts as deep as the LAB and where there has been recent volcanic activity.

“However, the interesting result is that this layer does not exist everywhere, suggesting something other than melt is needed to explain the properties of the asthenosphere.”

To find an answer, Dr. Schmerr analyzed shear waves (S-waves) with a seismometer. S-waves, which are a type of wave produced by earthquakes, bounce off different interfaces inside the Earth and arrive at certain locations and times depending on the type of interface. Dr. Schmerr measured their arrival times, heights, and shapes.

From his data, he determined that S-waves having longer paths travel all the way up to the surface without being reflected on any interface. Meanwhile, S-waves with shorter paths are reflected from the melt layers right at the LAB, causing them to travel faster. After comparing the arrival times, he was then able to ascertain the seismic properties and depth of the layers under the Pacific Ocean basin.

Dr. Schmerr will continue his study to see if he can find a presence of the melt layers in other oceans. If he and others are able to determine the exact nature of plate tectonics, the discovery would allow scientists to understand the evolution of the Earth and those of other rocky planets in the solar system.

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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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