The Mars gaiden is an extra volume which includes three stories: 1)"Namae no nai Uma" (A Horse with No Name), 2)"Nemuru Raion" (Sleeping Lion), and 3)"1 Karatto no Kajitsu" (1-Karat Gem). [Hmm..hopefully those are okay translations.] The first story, "Namae no nai Uma," is actually the only one that is Mars-related (ie the only one that has any of the characters or circumstances described in the first 15 tankoubon.). Which is why I think I'm going to directly translate the first story but only summarize the other two. So watch this space! :D

Namae no nai Uma - A Horse with No Name
Nemuru Raion - Sleeping Lion
1 Karatto no Kajitsu - 1-Karat Gem