Glaciers or ice sheets are natural accumulations of ice possessing in trinsic motion, which have appeared on the Earth's land surface as a result of the accumulation and transformation of precipitation . Only a very small portion of the surface of the land is now covered by glaciers, and at low latitudes they are found justat high elevations, on mountain slopes. However, glaciers are known to play very important roles in shaping the topography of the Earth, in determiningits past, present, and future climate, and in creating the state of the world Ocean. Recently there has also been a marked increase in the practical value of our knowledge about glaciers, as a part of the human habitat and as a factor affecting the economy. Interest in glaciers on other planets is also growing. Voyages of spacecraft to Jupiter, for instance, have shown that some Jovian satellites possess ice sheets tens of kilometers in thickness.