Is the centre of the Moon hot or cold?

Question posed by Walter
Buzz Aldrin deploys a seismometer in the Sea of Tranquility [NASA]

The Apollo 11, 12, 14, 15 and 16 missions* all placed seismometers on the Moon, which were used until 1977 as part of the Passive Seismic Experiment.

As on Earth, vibrations caused by Moonquakes or meteoroid impacts** can be detected by seismometers. If three or more seismometers detect waves from the same event the location and time of the event can be calculated. Waves that are detected by more distant seismometers have necessarily travelled deeper into the Moon, and this can be used to make inferences about its internal structure.

The experiments confirmed that, like the Earth, the Moon is made up of a crust, mantle and a core which is relatively small (only 25% of the radius of the Moon***). The collected data along with other observations**** also suggested that the interior of the Moon was relatively cold and dry with a mostly solid core.

The temperature of the Moon's core is likely to be below 1000°C with a number of estimates giving a maximum of around 830°C. In human terms, this is still quite hot, but when comparing it with the core temperatures of other bodies, the Moon is classed as being quite cool.

* July 1969, November 1969, January 1971, July 1971, April 1972, respectively.
** On some occasions a variety of space-borne jetsam, including some Saturn V rocket third stages and lunar module ascent stages, were deliberately crashed into the surface of the Moon in order to create seismic events of a known time and location which helped to calibrate observations of natural seismic events.
*** In comparison, the Earth's core radius is roughly 54% of the Earth's radius.
**** Including the Moon's weak magnetic field - planetary magnetic fields such as that of the Earth are generated by the motions of a hot, fluid metallic core.


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