Whether called "dirty snowballs" or "icy dirtballs," comets are rich in water ice. However, until today, little is known about the nature of cometary water ice, such as its origin, formation, and evolution process. For example, sufficient water gas can be found in the coma, yet all observed comet nuclei appear dark and have low reflectivity. Only a limited amount of water ice has been detected on some cometary surfaces. Why is that? Why are we able to detect water...