Why aren't Deimos or Phobos volcanically active?

Deimos and Phobos, the two moons of Mars, are not volcanically active primarily because they are too small to retain internal heat. The size of an astronomical body plays a crucial role in its ability to maintain geological activity. Smaller bodies have a higher surface area to volume ratio, which means they can lose internal heat more quickly than larger bodies.

In the case of Deimos and Phobos, their small size (with diameters of about 12 km and 22 km, respectively) likely means that they have not had enough mass to generate or sustain significant internal heat over geological timescales. Without sufficient internal heat, the geological processes that lead to volcanism cannot occur, thus resulting in a lack of volcanic activity.

The other factors mentioned, such as whether they had volcanic activity in the past or their composition being primarily ice, do not directly account for their current lack of activity compared to their size. Additionally, their distance from the Sun does not influence their volcanic activity, as it is more related to their internal heat and subsequent geological processes.