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The suggestion was made that the Earth and the Moon are two parts of a once whole object. If that is true then they must share the same basic composition. They also receive fairly similar amounts of sunlight. Why are they so dissimilar in the present day then?

How come there is no life on the Moon?

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they are different because the earth has a hot molten core which drives convection between the surface and the core. This convective movement breaks the crust up into slabs which crunch into one another in slow motion, driving the creation and continuous renewal of mountain ranges and valleys via a process called plate tectonics. No plate tectonic processes are active on the moon.

In addition, the moon is too small for its gravity to hold onto gas and water vapor, so the moon has no atmosphere, no clouds, no rivers, no oceans and no lakes. This in turn means that the flowing water processes that shape the landscape on earth are absent on the moon, and with no water or oxygen, terrestrial life cannot exist on the moon.

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    $\begingroup$ Also the moon didn't split off the Earth "cleanly". The body that is believed to have impacted the young Earth caused more superficial material to be thrown into orbit than deep material like metals, and because of this the Moon has a lower density than the Earth (part of the reason why there's no molten core, although the size is probably the most important reason) $\endgroup$
    – Kristoffer Sjöö
    Jan 25, 2021 at 21:39
  • $\begingroup$ Thank you so much Niels (and indeed Kristoffer for the point about what material composited the Moon). I had no idea a third body was part of the separation theory. Now I see that it probably came from outside the Solar System and added much of our water supply. Thank goodness the convective movement is so slow, Niels, although it’s still much too fast for complacency. $\endgroup$
    – Wookie
    Jan 27, 2021 at 17:28

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