Mars

Mars is a known planet in the object universe, also where Marshmallow was sent in Marsh on Mars, thus making the challenge to rescue her from the planet.

In The Series
After Cherries knocks Marshmallow into space for her defamatory and rude remarks, MePhone4 decides to make the challenge to rescue her from Mars in order to keep her alive and avoid any lawsuits. Mars is interpereted to be identical to the real Mars, as it appears red, abundant in craters, and lifeless, excluding an Light Shimmers Egg found by Apple, and "adopted" by Fan.

Trivia

 * This is the first time a location in Inanimate Insanity identically resembles a real life location.
 * Coincidentally, Marshmallow was sent to Mars, when the word "Mars" is in her name.
 * Although there is no gravity on Mars (at least in I.I.), Marshmallow and Microphone can be seen grounded.
 * It is, however, possible that Marshmallow is simply defying gravity as usual.