- Cimmerian Sea (See Mare Cimmerium)
- Clerk-Maxwell, determination of molecular speed
- Climate, of Mars
- Cloudlessness, of Martian skies
- Clouds
- Color
- Comet-tail peninsulas
- Cooling
- Cracks, theory of the canals as
- Critical velocity
- Cyane Fons, relative visibility of
- Dardanus, the, double
- Dark areas, depressions over
- Day, of Mars
- Density, of Mars
- Denudation