School kids are taught that Earth is geologically “alive” while the moon is “dead.” Certainly, the moon appears static compared to the dynamic and ever-changing Earth. But there may be life in the moon yet. global volcanism
A body’s interior heat comes from radioactivity, heat of formation, and external sources. Radioactive elements emit heat as they decay, which accounts for the lion’s share of Earth’s interior heat. Since, as the leading theory of the moon and Earth’s origins suggests, they comprise similar materials, the moon — with 81 times less mass — should have roughly 81 times less heat from radioactivity.
Planets are born when smaller proto-planets fall together into larger agglomerations, whereupon the energy of their infall gets converted to heat. The larger the body, the more infall it takes to build it, and the more residual heat. Furthermore, heat escapes more slowly from a large object than a small one, just as a larger potato stays hot longer than a small one after they’ve been removed from the oven.
External sources include the sun (which only heats surfaces), major impacts (which ceased about a billion years ago), and tidal heating by neighboring bodies. Since the slightly egg-shaped moon keeps its long axis perpetually turned our way, Earth’s tidal forces don’t vary much (versus, say, Jupiter’s moon Io, which is tidally heated to a state of continuous, global volcanism by its neighbors).
So the moon should have long ago solidified through and through. And yet, slight sloshing in its rotation hints that it may still possess a molten layer surrounding its solid core.
Recent research suggests that the moon’s radioactive elements may have long ago sunk to the core with dense, titanium-rich minerals, which could explain the lack of volcanism at the “living” moon’s surface.
Next column: The phases of the planets.
Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or canderson@csi.edu.
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