Great question! The moon does still give off
heat, though it gives off much less than a
geologically “active” planet like Earth.
Astronauts in the Apollo missions (Apollo 15 and
17) have actually measured heat flow at two
different locations on the moon. The heat flow
describes how fast an area on the surface of the
moon is losing its heat.
The average heat flow
of the surface of the moon is 18mWm-
to 87 mWm-2 on Earth (Francis &
Oppenheimer, 2004). This means that Earth is
losing heat almost five times faster than the
moon. Most of the heat is probably lost as
infrared radiation, a form of light that the
human eye can’t see but that we can measure with
Most of the Earth’s internal heat comes from
the decay of radioactive isotopes of the
elements potassium, uranium, and thorium.
Radioactive decay produces a lot of energy
(heat); that’s why we use radioactive material
in electricity producing reactors. The budget of
these elements is still high in Earth, so the
planet is relatively hot. This is why we have
plate tectonics and volcanoes. The moon has a
similar concentration of these elements to the
Earth’s mantle, but it is smaller, so the total
amount of these elements is less. The moon was
also hot early in its life, but it cooled much
faster than the Earth because it is smaller. As
spheres (the moon and earth are roughly sphere-
shaped) get smaller, their surface area/volume
ratio gets bigger and bigger and this makes them
cool faster. You should calculated the surface
area and volume of a few spheres of different
size and see how the ratio s.a./v changes as
they get larger and smaller.
Francis, P., Oppenheimer, C. (2004).
Volcanoes: 2nd edition. New York: Oxford
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