Final answer:
Mickey attributes Darrow's exceptionally strong heart to his emotional resilience, as depicted in Pierce Brown's 'Red Rising' series.
Step-by-step explanation:
According to Mickey, Darrow's heart is exceptionally strong due to emotional resilience. This is a theme explored in the science fiction series Red Rising by Pierce Brown, where Darrow's journey showcases the power of human spirit and emotion.
Despite the physical alterations and challenges Darrow faces, it is his capacity for love, grief, rage, and ultimately, resilience, that makes his heart strong, rather than genetic modification, physical training, or adaptation to the Martian atmosphere.