As we all know, 343i released All Hail and The Cost, two Halo 5 trailers detailing conflicting stories (differing between a wounded Master Chief and Agent Locke with the other standing over them with pistol in hand), a few months ago. However, what no one has seemed to figure out (as far as I can see) is that the trailers seem to take place on I know this? I have a few bits of evidence.

First off, on the Halo 5: Guardians page of this wiki, there is a picture of the background of the two trailers, with a large planet and several moons in the background. If you have not seen it, it looks something like this:

Second, at approximately 12 seconds into Agent Locke's trailer, All Hail, we see two moons in the background: an orange one, and one that looks quite a bit like our own. I presume these are Io and Callisto. Their real pictures are included here:

Finally, my last bit of evidence, is the ground upon which the Master Chief and Agent Locke walk. Ganymede is a rather black moon with rocky grooves. The ground they walk on is black like obsidian, and rocky. The images of Ganymede and its surface are included here:

So? What do you think of my hypothesis on this matter? I would enjoy the feedback, so long as it's positive. Thank you!