MadSci Network: Astronomy |
The answer lies in the different compositions of the Earth and Moon. Our home planet is made up of four different layer: crust, mantle, liquid outer core, and solid inner core. The Earth's magnetic field is produced by convection currents in the outer core. The moon, from what we can tell by seismic recorders left there by astronauts, has a very different structure. Although it also has a crust, a solid iron core, and a mantle-analogue called the lithosphere, the moon's outer core is not fully liquid. It is described as "plastic". This isn't to say that there are huge deposits of Hefty bags in the moon's interior - just that the matierial there has a higher viscosity (resistance to flow) than a regular liquid. So the moon's outer core is halfway between a liquid and a solid and it doesn't move around like the Earth's outer core does, so It can't keep up a strong magnetic field. Although the moon did have a magnetic field to begin with (we know this because we've examined the crystal structure in moon rocks), magnetic fields tend to fade over time unless there is something replenishing them. That process doesn't work on the moon so these days it has no magnetic field.
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