A close-up view of Mercury: Researchers find the planet may have had a dynamic past

March 22, 2012 by Jennifer Chu
Perspective view of ancient volcanic plains in the northern high-latitudes of Mercury revealed by instruments on board the MESSENGER spacecraft. Image: NASA/JHUAPL/CIW-DTM/GSFC/MIT/Brown Univ/; Rendering by James Dickson

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.

The observations were taken by a probe called MESSENGER (short for Surface, Space Environment, Geochemistry and Ranging), 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, indicating that Mercury was apparently active for a long time.”

Mercurial mission

Getting into orbit about Mercury was no easy feat, mostly because of its proximity to the sun. Any spacecraft heading toward the planet speeds up, drawn in by the sun’s powerful gravitational field. To counteract the sun’s pull and slow MESSENGER down, the MESSENGER team programmed the probe to fly by Venus twice, and Mercury three times, before slowing down enough to be captured in Mercury’s orbit with the help of a main engine burn. 

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Video: Watch a color movie of Mercury’s surface captured by MESSENGER

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. Throughout the one-year mission, the MESSENGER spacecraft battled tides from the sun, which tugged the probe out of its optimal orbit, as well as what Zuber calls “sunlight pressure” — photons or packets of light from the sun that exerted pressure on the . The team periodically adjusted the probe’s orbit and made precise corrections to its measurements to account for the sun’s effects, mapping out the gravity field as well as the elevation of the surface of Mercury’s northern hemisphere.

Inside and out

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.) 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.

“If the iron and sulfur model is correct, it would have implications for how the dynamo inside Mercury produces the planet's magnetic field,” says Gerald Schubert, professor of earth and space sciences at the University of California at Los Angeles, who did not participate in the research. “The dynamo generation process might work differently in Mercury compared with Earth.”

Co-author Dave Smith, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says the scientific process that led to the team’s results was a journey in itself.

“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. This is how science is supposed to work, and it’s a nice result.”

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. Zuber explains that a process called polar wander can cause geological features to shift around on a planet’s surface due to the redistribution of mass on or within a planet by geodynamical processes.

One such process of transporting mass in a planet’s interior is convection within the mantle. Viscous material within the mantle circulates and can push fragments of crust up and out, shifting terrain around the globe. Given Mercury’s extremely thin mantle, as revealed by MESSENGER, Zuber says it’s challenging to understand how convection operated to raise broad expanses of terrain to the elevations observed.

“It’s interesting to think what might be causing the observed deformation,” Zuber says. “It appears there are some very unusual dynamics going on inside Mercury.”

Explore further: MESSENGER Returns Images from Oct. 6 Mercury Fly-By

More information: www.sciencemag.org/content/early/2012/03/20/science.1218805

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1.1 / 5 (8) Mar 22, 2012
From http://www.thunde...rcury-2/

What is most remarkable about Mercury and other members of the Solar System are the numerous crater chains that abound throughout the population. From Phobos to Phoebe; from Mars to Miranda, planets and moons are pocked with holes that run in long lines, sometimes for hundreds of kilometers. The common explanation for them is that a string of meteoroids impacted one after another, one behind the other. The necessary coincidence for that effect notwithstanding, the absence of distortion in adjoining crater walls calls the theory into question. Add to that the twists, turns, loops, and braids that can be seen in many of them and the idea that rocks falling from space caused these features falls apart.
1 / 5 (4) Mar 22, 2012
Anyone who has made an electric arc device called a Jacobs Ladder knows how the line of craters could have formed. A Jacobs ladder is constructed by placing a stiff copper wire on each standoff of a neon sign transformer and then bending them in toward each other until they form an ever-widening V from bottom to top. When the current is turned on, an electric arc begins at the lowest level of the V and then rises up to the top, growing longer across the widening gap until it disconnects with a snap, only to immediately begin again. If a piece of paper is held between the two limbs of the V while the electric arc travels upward, a row of pinholes will be found burned lengthwise into the paper.
1 / 5 (4) Mar 22, 2012
Electric arcs traveling across a conductive medium vary in strength from millisecond to millisecond, so they burn chains of craters instead of smooth channels. In fact, the smooth channels seen on many objects are actually crater chains that are packed so close together that they can no longer be distinguished.


For a picture of two intersecting crater chains, see this:

5 / 5 (1) Mar 23, 2012
From Phobos to Phoebe; from Mars to Miranda, planets and moons are pocked with holes that run in long lines, sometimes for hundreds of kilometers. The common explanation for them is that a string of meteoroids impacted one after another, one behind the other.

Your argument falls apart right there. That is not the common explanation for it. And I don't know how you could've missed it since the common explanation is right there in the first line of that second link you posted:
"When an asteroid or comet impacts a planet, the explosion ejects huge amounts of material, sending it flying in all directions. But there are also plumes of material, long fingers of rock and dust that stream out as well. The boulders and such inside this plume then fall back to the ground, making liner chains of secondary craters."

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