How the lunar dynamo might have formed
Full Moon by Luc Viatour (via wikimedia commons)
Here’s my article about how the moon could have gained, then lost its magnetism.
Apollo astronauts in the 1960s and 70s found a mystery on the moon. Some of the rocks they picked up were magnetized, a strange discovery given that the moon has no magnetic field.
Now, researchers at UC Santa Cruz have taken a step toward solving that mystery. In this week’s issue of Nature, they proposed that the moon had a long-lasting magnetic field, billions of years ago, created by a stirring of the lunar core as the moon orbited around the earth.
The cooling of the earth’s liquid core created the magnetic field. But the moon is too small to sustain the necessary heat and generate the lava lamp-like motion, or dynamo, in the liquid core.
“People have been scratching their heads for 40 years, ever since Apollo,” said Christina Dwyer, a graduate student in planetary sciences at UCSC and lead author of the study. The moon’s magnetic field must have formed a different way, Dwyer said.
“The other way is stirring,” Dwyer said. “Just like you stick your spoon in a pot of water and stir to move the water. We have a way of stirring the moon.”
The UCSC team’s calculations show that when the moon orbited closer to the earth than it does today, gravitational tugs from our planet stirred up the core like a giant spoon in a bowl.
There’s more to the article if you follow the link.
Christina Dwyer, the graduate student and lead author on the paper was great to talk to. Here is a story she told me that wouldn’t fit in the paper:
“I love geology, there are so many interesting clues buried in rocks,” Dwyer said.
For example, rocks can tell us how day length has changed over time. The moon’s proximity does have an effect on how the earth spins, so as the moon moves farther away from the earth (at at rate of about 1.5 inches per year) the day gets incrementally longer.
Not so you’d notice it on your wristwatch, but it can be measured in the fossil record.
Dwyer explained that you can have plants growing in tidal regions that later become fossils. Researchers can then count the number of tides by the rings of fossilized tidal mud, divide by two and get the number of days the plant stood there. Dating the fossil gives you an idea of the tidal patterns how ever long ago that plant grew.
“If our theory is right and if we can experimentally measure a really detailed history of the intensity of the lunar magnetic field over time, we can take the known lunar magnetic field and our theory and go backwards.”I have to agree, geology is pretty cool. This paper is even cooler.
But wait there’s more! If anyone has access to Nature:
And published at the same time, a paper about how the dynamo could have come from repeated bombardment by asteroids that tweaked the core out of alignment and created the stirring action. I didn’t cover this paper, but a lot of other articles did. I was going with the “local researchers” angle. Also, since I’m not on any embargo lists– I didn’t know about the other paper!
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