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Mercury's core is similar to Earth's, Messenger shows

By Bill Scanlon, Rocky Mountain News (Contact)

Published July 7, 2008 at 12:05 a.m.

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A vast plain on the planet Mercury is covered with lava to a depth that would obscure the Washington Monument. The lava is proof that the planet had a volcanic past, scientists who analyzed the Messenger Spacecraft's January fly-by of the orb closest to the sun said last week.

They also found that Mercury is shrinking at a faster rate than thought, that it has a magnetic field like Earth's and has such a tenuous atmosphere that its molecules are more likely to bump into the planet's surface than each other.

"The Earth's atmosphere is billions of times more dense than any claims made of Mercury's," Bill McClintock, senior researcher at the Laboratory for Atmospheric and Space Physics, at the University of Colorado, said last week at a NASA teleconference.

Imagine a few ping-pong balls lobbing over a table, much more likely dropping to the table than hitting each other, he said.

That's what's happening in Mercury's exosphere, which is a thin band of near-nothingness between the surface and outer space, McClintock said.

Solar factor

On Earth, the atmosphere is more akin to billions of ping-pong balls forming a near-solid arc over the table.

Mercury's thin exosphere contains sodium ejected from the surface and is accelerated by radiation pressure to form a 25,000-mile sodium tail flowing away from the sun, said McClintock, the principal investigator for CU's Mercury Atmospheric and Surface Composition Spectrometer, one of seven instruments aboard Messenger.

Images from the fly-by showed that about 10 percent of the sodium atoms liberated from the surface during Mercury's day were accelerated into that tail and lost from the planet, McClintock said.

"Atoms of hydrogen, helium, sodium, potassium and calcium have been seen in the exosphere," and it's almost certain that many other elements exist there, he said.

When the elements escape the surface, they are accelerated by pressure from solar radiation to form that long tail. The abundance of the elements depends on whether it is day or night and on the influence of Mercury's magnetic field and the solar wind, he said.

"Now, it appears clear to us that the solar wind plasma plays an important role populating the tail," he said.

That was a big surprise because scientists assumed that the solar wind didn't play much of a role in the mysterious tail.

"These are very exciting findings," he said.

It's now possible to find out whether daily variations in the solar wind can actually change the route of the tail each day.

The scientists found that Mercury's magnetic field is produced by a dynamo in the fluid center of the planet, much like on Earth.

"It's like you had a giant bar magnet in the middle of the planet," said Sean Solomon of the Carnegie Institution of Washington, D.C.

"We know that the outer core on Mercury is fluid," Solomon said. "It must, then, have lighter elements to lower the melting temperature; otherwise it would have a pure iron core that would have solidified by now."

A great source of energy on Earth is the cooling of its core, and the slow and relentless growth of a solid inner core, which releases heat and gravitational energy, Solomon said.

The finding that Mercury has a similar dynamo-type magnetic field in its core is consistent with theories on why Mercury seems to be shrinking, he said.

"Its core is 60 percent of the mass of the planet, 75 percent of its radius," Solomon said.

Planet shrinking

Because solid iron is denser than molten iron, and because Mercury is cooling relatively rapidly, a shrinking is inevitable.

"We imaged huge cliffs, faults that point to cutting of the geological terrain," he said.

"The density of the faults is greater than anticipated," he said, and the scientists now think the shrinkage has been at least one-third greater than they had predicted. The best calculation says that Mercury's radius has shrunk by 1 or 2 kilometers - a reduction of 1/10 of 1 percent.

A mystery lingers, though. Planetary scientists had always assumed that the less shrinkage of a planet, the easier it is for lava to seep out.

So why, the scientists still ask, does Mercury have so much shrinkage and also so much lava on its surface?

It's one of the questions that might be answered on the next fly-by, scheduled for October.

scanlon@RockyMountainNews.com or 303-954-2897

About Mercury

* Size: 3,032 miles in diameter (about two-fifths of Earth's diameter)

* Orbit: Mercury travels around the sun in an elliptical (oval-shaped) orbit. The planet is about 28.6 million miles from the sun at its closest point and about 43.4 million miles from the sun at its farthest point.

* Distance from Earth: About 48 million miles at its closest approach

Comments

  • July 7, 2008

    8:35 a.m.

    Suggest removal

    intothelens writes:

    As for the last question:
    Because the lava has been squeezed out, perhaps. Much like an orange with slices in the surface would be enveloped by its juices it were uniformally squeezed. Lava may have fewer 'leaking' points because of the surface's increased pressure, but once it does find a way out, its volume will be that much more.

  • July 7, 2008

    1:31 p.m.

    Suggest removal

    farhad0 writes:

    I found this article about the Earth's core and this scientist who is making models of the Earth to find out more. interesting stuff.

    http://blackandwhiteprogram.com/inter...

  • July 7, 2008

    5:27 p.m.

    Suggest removal

    Darwin writes:

    I understand that man has an inherent desire to seek knowledge. Having said that, I would like articles like this to discuss the ways the new-found knowledge will benefit mankind. All the article presents is some of the finding and how excited and giddy the scientist are. For someone unschooled in this particular area, such as me, my reaction is; "so, what's the bottom line?"

  • July 7, 2008

    5:29 p.m.

    Suggest removal

    HolierThanThou writes:

    Darwin,

    The bottom line is that the aliens that have been peering into your windows at night don't come from Mercury.