fav science cartoon

Unstrange Phenomena

09 November 2011

2 Ideas: What Happened to Our Moons Magnetic Personality?

One Giant Leap for Mankind
Apollo 11 mission in July 1969.




From Denny:  Apollo missions astronauts landed on the moon back in the 60's and 70's, gifting Earth's scientists with loads of moon rocks when they returned from their harrowing journeys.

Scientists were shocked to discover those rocks displayed magnetic properties.  How could that be?

They also figured out that some of those moon rocks could possess magnetic north and south as well as individual magnetic fields.  That sure started some serious scientific head scratching as to how this could be a fact.

They knew magnetism was possible if the rocks were comprised of the right minerals and, under the right conditions, like the rock cooling while present in a magnetic field.  Yes, it was possible the rocks could be magnetic.

How does a magnetic field occur?  Get the fluid motion of any conducting material like liquid iron and it creates a dynamo which in turn generates the magnetic field.  For planet Earth's magnetic field to occur, the motion is created in the outer core which is generated by heat convection.

So, back to the moon.  Turns out that the moon is not a large enough body to generate a magnetic field by convection heat like Earth.  Which is why, at the time, over 40 years ago, scientists were unaware there had been a magnetic field on the moon.

Scientists being scientists they are compelled to question and offer ideas as to plausible explanations.  Here are the latest two ideas as to how there was once a magnetic field on the moon:

1) The moon's mantle (the middle layer comprised of solid rock) may stir up the liquid iron core.

How can this happen?  Christina Dwyer, University of California at Santa Cruz, and her colleagues think this happens for two reasons (reported in science journal Nature, Nov 2011):

one, it turns out that the moon's core and the moon's mantle each rotate around slightly different axis.
two, the boundary between the core and the mantle is not quite spherical, causing the fluid to mix around from their relative motion.

What determines how strong is this stirring motion between the moon's core and the mantle?  

1) the angle between the core and the mantle
2) the distance between the Earth and the moon.  It turns out the moon's mantle rotates differently than its core because of the strong tidal gravitational tug from the Earth.

How does Dwyer's new idea explain why the moon once possessed a magnetic field but now no longer does?

1) Over time the angle between the moon's mantle and its core have narrowed.
2) The distance between the Earth and the moon has widened, decreasing significantly the pull from the tidal forces.  Those tidal forces were once strong enough to create the dynamo that generated the magnetic field yet now does not.

Dwyer's calculations estimate the moon may have enjoyed a magnetic field for about a billion years.  They estimate that time period was from about 2.7 billion to 4.2 billion years ago.

From Dwyer:  "Based on what we know about stirring, and everything we know about fluid motion, we can find no reason that this would not work.  All the flags are go, and now this needs to be taken to the next level to get tested."

While Dwyer and her research team have studied the basic scenario they are waiting for other scientists to take their investigation to the next level to validate or disprove it.  There are scientists who study the complex physics of dynamos and create models to prove or disprove current theories.

Idea Number Two for why the moon might once have enjoyed a magnetic field:

The French had to get in on the act, offering up their explanation for a lunar magnetic field.  Michael Le Bars, French National Center for Scientific Research and the Universite Aix-Marseille, also agree the moon's mantle was responsible for stirring up the liquid core.

Le Bars and his research team postulate there is a different impetus to explain the stirring.  They dismiss the notion of the tidal interactions between the Earth and the moon.  They offer up the idea of  the slamming from violent impacts of space rocks changed the rotation rate.  Changing the rotation rate caused a differential motion between the mantle and the core.

Are the two ideas just different sides of the same theory?

Dwyer's idea explains a steady stirring while the Earth and moon were the correct distance apart.  Le Bars' idea offers a different view of where the core could experience especially strong stirring for brief periods.  That scenario could create spikes of the moon experiencing a magnetic field.

Of course, if you really think about it, both of these ideas could easily have occurred during billions of years. Both may be correct, just at different times.

From Dominique Jault, researcher at ETH Zurich, Switzerland and the Universite Joseph-Fourier (not involved in either study, writing in science journal Nature), stated, "The two studies are thought-provoking and may be complementary.  Future palaeomagnetic experiments on samples from very old lunar rocks will enable their theories to be tested."

While the scientists test their billion years' gone moon magnetic field theories the rest of us will smile and wonder: So, why do we need to know this?

Check out this graphic about what's inside the moon from Space.com:

The moon is about 1/4 the diameter of Earth. Learn more about Earth’s natural satellite at SPACE.com.
Source Space.com: All about our solar system, outer space and exploration




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