How does amorphous ice changing into crystalline ice affect a comet?

When water ice in a comet is formed at temperatures below 100 K, it exists in an amorphous state—a random arrangement of water molecules. As the comet approaches the Sun and warms above 110 K, this amorphous ice undergoes a phase transition into crystalline ice, a process that completes at approximately 153 K.1

This transition significantly affects the comet in the following ways:

• Energy Release: The transformation is an exothermic reaction, meaning it releases energy. This heat can further sustain the transition by warming surrounding ice, though the process is eventually limited by the need to also heat the nonvolatile dust components within the nucleus.1
• Cometary Outbursts: The energy released during this phase change is considered a likely source of cometary outbursts, which are sudden, sharp, and often random increases in a comet's activity and brightness.1
• Observational Signatures: This transition helps explain the unusual brightness observed in dynamically new comets as they approach the Sun for the first time. These comets often experience this phase change at distances between 5 and 7 astronomical units (AU), which is where they are frequently discovered.1