On March 17, NASA’s MESSENGER spacecraft completed its primary mission, orbiting Mercury, capturing nearly 100,000 images, and recording data that shed light on the planet’s core and topography.
In two papers, published online on March 21 in Science Express, a large international team of researchers analyzes recent data from the spacecraft and provides new insights on topography of the northern hemisphere of Mercury, its gravity field and internal structure.
“Mercury is the last unexplored planet,” said Stanton Peale, a co-author of both papers and a physics professor emeritus at the University of California in Santa Barbara. “The way Mercury was formed may show some constraints on the formation of the Solar system.”
The findings show that the Mercury’s core is larger than expected – almost 85 percent of the planetary radius. The Earth’s core, in contrast, is just over half of the planet’s radius. Additionally, Mercury appears to have a more complex core than Earth’s – a solid iron sulfide layer that is now part of the mantle, which encases a liquid core, which may float on a solid inner core.
The researchers also found that Mercury’s solid outer core and liquid inner core contain more iron than Earth, relative to the whole planet, which influences the way Mercury’s magnetic field was generated.
“We didn’t expect so much sulfur,” Dr. Peale said, “There was almost no iron found on the surface of the planet. With no iron, the volcanic surface rocks are too light to have come from a mantle with the large average density derived for the internal structure. This led to the concept of a two-layer mantle – with a light upper silicate layer, which could provide the low-density surface material – over a dense iron sulfide layer.”
According to Dr. Peale, the presence of inordinately large amounts of sulfur on the surface indicates that Mercury’s formation was not as orderly as some scientists had previously assumed. “Mercury is composed of material that had condensed over a wide range from the Sun,” he said.
Other findings include a precise topographic model of the Mercury’s northern hemisphere, which reveals a smaller spread in elevations compared to those of Mars or the Moon. Radar-bright features at the poles located in areas of permanent shadow have been found to be consistent with a water-ice hypothesis. However, the notion has yet to be tested further with MESSENGER’s neutron spectrometer.
NASA has announced that it will extend the MESSENGER mission for an additional year of orbital operations at Mercury beyond the planned end of the primary mission on March 17, 2012.
“We are still ironing out the funding details, but we are pleased to be able to support the continued exploration of Mercury,” said NASA MESSENGER Program Scientist Ed Grayzeck.
“During the extended mission we will spend more time close to the planet than during the primary mission, we’ll have a broader range of scientific objectives, and we’ll be able to make many more targeted observations with our imaging system and other instruments,” explained MESSENGER Principal investigator Sean Solomon, of the Carnegie Institution in Washington.
“MESSENGER will also be able to view the innermost planet as solar activity continues to increase toward the next maximum in the solar cycle. Mercury’s responses to the changes in its environment over that period promise to yield new surprises.”