From: Editor, ENN
Published March 22, 2012 01:04 PM

The Dynamic Mercury

Mercury is the innermost and smallest planet in the Solar System. It orbits the Sun once every 87.97 Earth days, completing three rotations about its axis for every two orbits. Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. Unlike the Moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.  It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about −183 °C to 427 °C.   New observations from a spacecraft orbiting Mercury have revealed that the tiny, pockmarked planet harbors a highly unusual interior — and the craft’s glimpse of Mercury’s surface topography suggests the planet has had a very dynamic history.

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The latest observations were taken by a probe called MESSENGER, the first ever to enter orbit around Mercury. MESSENGER reached Mercury’s orbit in March 2011, and has since circled the planet twice a day, collecting nearly 100,000 images and more than four million measurements of Mercury’s surface.

A team of scientists from institutions including MIT, the Carnegie Institution of Washington, Johns Hopkins University’s Applied Physics Laboratory and NASA’s Goddard Space Flight Center have analyzed the data and precisely mapped the planet’s topography and gravitational fields. From the gravity estimates, the team found that Mercury likely has a highly unusual interior structure — an exceptionally large iron core overlain by a solid layer of iron sulfide and a thin outer shell of silicate mantle and crust.

From topographic measurements, the team mapped out a large number of craters on the planet’s surface, making a surprising finding: Many of these have tilted over time, suggesting that processes within the planet have deformed the terrain after the craters formed.

The researchers detail their findings in two papers published this week in the journal Science.“

Prior to MESSENGER’s comprehensive observations, many scientists believed that Mercury was much like the moon — that it cooled off very early in solar system history, and has been a dead planet throughout most of its evolution,” says co-author Maria Zuber, the E.A. Griswold Professor of Geophysics at MIT. “Now we’re finding compelling evidence for unusual dynamics within the planet."

After entering Mercury’s orbit, the spacecraft began measuring the planet’s surface elevations via laser altimetry. Through radio tracking, the probe estimated the planet’s gravity field.   

The team’s measurements revealed surprising findings both in the planet’s interior and on its surface. From the probe’s gravity estimates, the group inferred that Mercury likely has a huge iron core comprising approximately 85 percent of the planet’s radius. (Earth’s core, by comparison, is about half the planet’s radius in size.) Earlier estimates had the Mercury core as only 42%. 

This means that Mercury’s mantle and crust occupy only the outer 15 percent or so of the planet’s radius — about as thin as the peel on an orange, Zuber says.

The researchers also reasoned, given Mercury’s gravity field, that just above the outer molten layer of the planet’s core may be a solid layer of iron and sulfur — a type of layered structure not known to exist on any other planet.

Co-author Dave Smith said:  "We had an idea of the internal structure of Mercury, [but] the initial observations did not fit the theory so we doubted the observations," Smith says. "We did more work and concluded the observations were correct, and then reworked the theory for the interior of Mercury that fit the observations."

Through laser measurements of the planet’s surface, researchers mapped out multiple geologic features in Mercury’s northern hemisphere, finding the range of elevations to be smaller than that of Mars or the moon. They also observed something unexpected in Mercury’s Caloris basin, the largest impact feature on Mercury: Portions of the floor of the crater actually stand higher than its rim, suggesting that forces within the interior pushed the crater up after the initial impact that created it.

Zuber and her team also identified an area of lowlands approximately centered on Mercury’s north pole that could conceivably have migrated there over the course of the planet’s evolution. 

For further information:  http://web.mit.edu/newsoffice/2012/mercury-close-up-view-0322.html

Photo:  NASA

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