In general, amorphous ice can form below ~130 K. At this temperature, water molecules are unable to form the crystalline structure commonly found on Earth. Amorphous ice may also form in the coldest region of the Earth's atmosphere, the summer polar mesosphere, where noctilucent clouds exist. These low temperatures are readily achieved in astrophysical environments such as molecular clouds, circumstellar disks, and the surfaces of objects in the outer solar system. In the laboratory, amorphous ice transforms into crystalline ice if it is heated above 130 K, although the exact temperature of this conversion is dependent on the environment and ice growth conditions. The reaction is irreversible and exothermic, releasing 1.26–1.6 kJ/mol.