From a JPL news release, February 10, 2015:
Astronomers tinkering with ice and organics in the lab may have discovered why comets are encased in a hard, outer crust. Using an icebox-like instrument nicknamed Himalaya, the researchers show that fluffy ice on the surface of a comet would crystalize and harden as the comet heads toward the Sun and warms up. As the water-ice crystals form, becoming denser and more ordered, other molecules containing carbon would be expelled to the comet’s surface. The result is a crunchy comet crust sprinkled with organic dust, like a deep-fried ice cream: the crust is made of crystalline ice, while the interior is colder and more porous. The organics are like a final layer of chocolate on top.
The composition of comets is important to understanding how they might have delivered water and organics to our nascent, bubbling-hot Earth. New results from the Rosetta mission show that asteroids may have been the primary carriers of life’s ingredients; however, the debate is ongoing and comets may have played a role.