Large comets may have cores of liquid water that could potentially harbor life

Recent research suggests that impacts of comets containing organic compounds could generate peptides, the building blocks of proteins. The Solar System's most cataclysmic storm could very well have been a drizzle of life.
Even more astounding, some of the comets that struck Earth could have already contained life. The chances are remote, but it is possible. According to recent research published to the journal Astrobiology, large comets with a radius of over 10 kilometers could contain liquid water at their cores. The decay of radioactive isotopes of aluminum or iron could supply the heat necessary to melt the inner ice. Katharina Bosiek, along with her colleagues Michael Hausmann and Georg Hildenbrand, suggest that a thick layer of dust could protect the core's liquid environment from solar radiation, echoing learned speculations found in prior research. Their findings make the hopeful words of Nalin Chandra Wickramasinghe, the Cardiff University astrobiologist who was one of the earliest proponents of panspermia, believable.

http://www.realclearscience.com/blog/2016/04/could_comets_contain_life.html