It’s pronounced DAY-moss. I always feel I need to point that out when someone plays the card in Terraforming Mars that sends Mars’ smaller, outer moon crashing to the ground purely to just raise the temperature by a couple degrees C.
It’s actually the closer moon, Phobos (FOE-buss), that’s in danger of crashing to Mars at some point. Phobos orbits so close to Mars that it actually appears to be moving backward in the sky.
You probably wonder, as I do, what it would be like to take a vacation on Deimos. Mars would appear much larger from either of its moons than Earth does from our moon, and it would certainly be exciting to watch the red desert cycle endlessly past beneath. Unfortunately, Deimos, like its brother Phobos, is likely just a collection of huge boulders cemented together with layers of dust, and has no really solid surface.
Remember, next time someone tries to crash Deimos into its planet, just remind them of how it’s pronounced.
