And the answer is…

MERCURY
.
THE MOON

How the voting went:

Picture 1 is Mercury      15.6%
Picture 2 is the Moon    15.6%

Picture 1 is the Moon      34.4%
Picture 2 is Mercury         34.4%

So how did you do? As you can see most people got this wrong! I managed to find a region of the Moon that didn’t look too obviously Moon-like so don’t feel too bad if you got them the wrong way round. The coordinates are the same for both Mercury and the Moon at lat –45 : long 125. This takes us to the lunar region around Planck and Van der Waals craters on the far side which looks very much like Mercury. At first glance the two bodies do look very similar but there are some differences – though not all obvious from the two pictures.

  • Mercury has more intercrater plains (its oldest surface) than the Moon.
  • Ejecta deposits and secondary craters are less extensive on Mercury due to its higher gravity field. The region of Mercury chosen is atypical in this respect though there are some clues.
  • There are more tectonic features on Mercury than on the Moon (rupes, ridges, troughs etc). The lunar region I chose highlights some lesser-known tectonic features.
  • The Moon (particularly the nearside has expanses of bright highland terrain and dark basaltic maria.  Mercury does not, though it does have splashes of bright relatively fresh craters against the darker terrain and in this respect has more in common with the lunar far side.
  • Mercury has a magnetic field indicating it still has a molten core whereas the Moon has pockets of magnetism but no global magnetic field.

Well done to those who got it right. It wasn’t easy!

References:
mapaplanet
The Geological History of Mercury Spudis 2001
The tectonics of Mercury Watters and Nimmo, 2009
The tectonics of Mercury Melosh and McKinnon, 1988

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