Aside from being smaller, the Moon has a distinctly different bulk composition from Earth--its mantle is proportionately much larger, and its core is proportionately much smaller.

Suppose that Earth had the same primordial composition as the Moon, just scaled up in mass 81 times to match the real Earth's mass. The resulting planet would surely be much more like our real Earth than like our real Moon, if only because it would be able to retain whatever atmosphere outgases from the early mantle, rather than becoming a dry, airless rock.

But would anything necessarily be significantly different from Earth? (Specifically, excluding the sort of accidental differences that you would get from re-running an alternate Earth with identical chemical composition, like incidentally different continent shapes and such.)

  • $\begingroup$ Atmosphere's not the only worry (though a big one). Will this larger moon retain any water (I forget if the Earth had its water by this point, or got it later)? Even if it has enough for life, it may not have the large oceans of Earth and be quite desert-like. Or it might end up as Earth's twin. Finally, the moon's close enough that maybe life can hitch a ride on ejecta, so that both biospheres are biologically related in a true sense. $\endgroup$
    – John O
    May 18, 2022 at 16:18
  • $\begingroup$ @JohnO There's about as much water stored in the mantle as in the oceans, so given a larger mantle... there might be larger oceans. Or maybe the larger mantle would absorb all the oceans, as seems to have happened on Mars! I dunno! Hence, the question. :) $\endgroup$ May 18, 2022 at 16:27
  • $\begingroup$ it would be much more like the moon, with no magnetic field and no tectonics $\endgroup$
    – John
    May 18, 2022 at 20:21
  • $\begingroup$ @John Why no tectonics? Thicker mantle doesn't necessarily mean thicker crust. $\endgroup$ May 18, 2022 at 20:26
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    $\begingroup$ @LoganR.Kearsley the earth has such a super thin crust because of the same event that gave it a giant core, it removed much of the lighter material. also tectonics are driven by the heat from the very large radioactive core. with a much smaller core the planet is too cold internally to run tectonics. If you want earth like surface conditions and earth like size, you really do need an earth like planet. it might end up more like Venus than earth, with no moon forming impact its atmosphere will be around 50 times thicker, if not more. $\endgroup$
    – John
    May 18, 2022 at 21:18

1 Answer 1


Less dense = shorter lived magnetic field.

Earth has a density of 5.51 g/cc. The moon has a density of 3.34 g/cc. Mars has a density of 3.93 g/cc. The moon is closer to Mars than to Earth as regards composition.

Mars lost its magnetic field and then without it, the solar wind stripped its atmosphere. Maybe it lost its field because it cooled off faster than the Earth because is smaller and farther from the sun.

Or maybe it cooled off faster because the lighter elements comprising it had less ability to retain heat as compared to the denser elements comprising Earth.


Scientists attribute this to Mars’ lower mass and density (compared to Earth) which resulted in its interior cooling more rapidly. This caused the planet’s outer core to become solid, thus arresting the Martian dynamo effect...

One could think of Mars as much like your moon-composition Earth. A larger Mars would probably keep the magnetic field longer than Mars did. Closer proximity to the sun could mean a stronger effect from the solar wind, stripping the atmosphere faster once the magnetic field is gone.


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