Phobos and Deimos are probably captured comet nuclei.

Question

Quizplus · Accepted Answer

Phobos and Deimos are actually believed to be captured asteroids, not comet nuclei. While the origins of Phobos and Deimos are not fully understood, it is thought that they are most likely asteroids captured by Mars' gravity during its formation. Some theories suggest that they may have formed from debris ejected from Mars after a massive impact. There is no evidence to suggest that they are captured comet nuclei.

Phobos and Deimos Are Probably Captured Comet Nuclei