5 Mercury Secrets Revealed by MESSENGER

by Elizabeth Howell on March 18, 2013

Artist's concept of MESSENGER in orbit around Mercury. Courtesy of NASA

Artist’s concept of MESSENGER in orbit around Mercury. Courtesy of NASA

After two years of doing the loop-the-loop around Mercury, MESSENGER has unveiled a bunch of surprises from Mercury — the closest planet to the Sun.

The spacecraft launched in 2004 and made three flybys of the planet before settling into orbit two years ago today. Incredibly, MESSENGER is only the second NASA probe to visit Mercury; the first one, Mariner 10, only flew by a few times in the 1970s. It was an incredible feat for the time, but we didn’t even have a complete map of Mercury before MESSENGER arrived at the planet.

So, what have scientists found in MESSENGER’s two years in orbit? Tales of sulfur, organic materials and iron, it turns out.

Mercury’s south pole has a weak spot

Magnetic field lines differ at Mercury's north and south poles As a result of the north-south asymmetry in Mercury's internal magnetic field, the geometry of magnetic field lines is different in Mercury's north and south polar regions. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Magnetic field lines differ at Mercury’s north and south poles As a result of the north-south asymmetry in Mercury’s internal magnetic field, the geometry of magnetic field lines is different in Mercury’s north and south polar regions. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The magnetic field lines converge differently at the north and south poles of Mercury. What does this mean? There’s a larger “hole” at the south pole for charged particles to do their thing to the surface of Mercury. At the time this information was released, NASA said it’s possible that space weathering or erosion would be different at the north and south poles because of this. Charged particles on the surface would also add to Mercury’s wispy atmosphere.

How the atmosphere changes according to distance from the sun

Comparison of neutral sodium observed during MESSENGER’s second and third Mercury flybys

Comparison of neutral sodium observed during MESSENGER’s second and third Mercury flybys. Credit: NASA

Wondering about the atmosphere on Mercury? It depends on the season, and also the element. The scientists found striking changes in calcium, magnesium and sodium when the planet was closer to and further from the sun.

“A striking illustration of what we call ‘seasonal’ effects in Mercury’s exosphere is that the neutral sodium tail, so prominent in the first two flybys, is 10 to 20 times less intense in emission and significantly reduced in extent,” said participating scientist Ron Vervack, of the Johns Hopkins University Applied Physics Laboratory in 2009. “This difference is related to expected variations in solar radiation pressure as Mercury moves in its orbit and demonstrates why Mercury’s exosphere is one of the most dynamic in the solar system.”

Discovery of water ice and organics

A radar image of Mercury’s north polar region is shown superposed on a mosaic of MESSENGER images of the same area. All of the larger polar deposits are located on the floors or walls of impact craters. Deposits farther from the pole are seen to be concentrated on the north-facing sides of craters. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

A radar image of Mercury’s north polar region is shown superposed on a mosaic of MESSENGER images of the same area. All of the larger polar deposits are located on the floors or walls of impact craters. Deposits farther from the pole are seen to be concentrated on the north-facing sides of craters. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

Late in 2012, NASA finally was able to corroborate some science results from about 20 years ago. Scientists on Earth saw “radar bright” images from Mercury in the 1990s, implying that there was ice and organic materials at the poles. MESSENGER finally confirmed that through three separate lines of investigation that were published in Science in 2012. Scientists estimated the planet holds between 100 billion and 1 trillion tons of water ice, perhaps as deep as 20 meters in some places. “Water ice passed three challenging tests and we know of no other compound that matches the characteristics we have measured with the MESSENGER spacecraft,” said MESSENGER principal investigator Sean Solomon in a NASA briefing.

Mercury has a big iron core

The internal structure of Mercury is very different from that of the Earth. The core is a much larger part of the whole planet in Mercury and it also has a solid iron-sulfur cover. As a result, the mantle and crust on Mercury are much thinner than on the Earth.  Credit: Case Western Reserve University

The internal structure of Mercury is very different from that of the Earth. The core is a much larger part of the whole planet in Mercury and it also has a solid iron-sulfur cover. As a result, the mantle and crust on Mercury are much thinner than on the Earth.
Credit: Case Western Reserve University

While scientists knew before that Mercury has an iron core, the sheer size of it surprised scientists. At 85%, the proportion of the core to the rest of the planet dwarfs its rocky solar system companions. Further, scientists measured Mercury’s gravity. From that, they were surprised to see that the planet had a partially liquid core. “The planet is sufficiently small that at one time many scientists thought the interior should have cooled to the point that the core would be solid,” stated Case Western Reserve University’s Steven A. Hauck II, a co-author of a paper on the topic that appeared in Science Express.

The surface is sulfur-rich

A global view of Mercury, as seen by MESSENGER. Credit: NASA

A global view of Mercury, as seen by MESSENGER. Credit: NASA

At some point in Mercury’s history, it’s possible that it could have had lavas erupt and sprinkle the surface with sulfur, magnesium and similar materials. At any rate, what is known for sure is there is quite a bit of sulfur on Mercury’s surface. “None of the other terrestrial planets have such high levels of sulfur. We are seeing about ten times the amount of sulfur than on Earth and Mars,” said paper author Shoshana Weider of the Carnegie Institution of Washington.

About 

Elizabeth Howell is the senior writer at Universe Today. She also works for Space.com, Space Exploration Network, the NASA Lunar Science Institute, NASA Astrobiology Magazine and LiveScience, among others. Career highlights include watching three shuttle launches, and going on a two-week simulated Mars expedition in rural Utah. You can follow her on Twitter @howellspace or contact her at her website.

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