A broad field encompassing all aspects of the study of ice. While many glaciologists focus their attention on glaciers, the largest ice masses on Earth, glaciology also includes the study of ice that forms on rivers, lakes, and the sea; ice in the ground, including both permafrost and seasonal ice such as that which disrupts roads in the spring; and ice that crystallizes directly from the air on structures such as airplanes and antennas. All forms of snow research, including snow hydrology and avalanche forecasting, fall under the broad rubric of glaciology. Even planetary geologists are involved, as two of the moons of Jupiter, Ganymede and Callisto, are believed to be composed largely of ice. This article, however, will be restricted to discussion of glaciers.