Mercenary: The Second City, released in 1986, is an expansion pack for the original game. Initially its major distinction is the new colour scheme - a red planet with a pink sky instead of a green planet with a blue sky, representing the other side of the planet. However, after a similar objective, the differences reveal themselves subtly: there is a more intricate underground complex, a new set of puzzles to overcome and several significant changes with game objects. It was later repackaged with the original game under the title Mercenary Compendium
Source: Wikipedia, "Mercenary (computer game)", available under the CC-BY-SA License.