The term is the frost line or snow line : the distance from the Sun beyond which temperatures are low enough for water ice and other volatiles to condense, so large moons tend to be more icy than rocky.

Quick Scoop

  • In the inner Solar System, moons and planets are mostly rocky because it is too warm for ice to remain stable.
  • Farther out, icy materials can survive and build larger bodies, so moons become increasingly ice-rich.
  • In planetary science, snow line is the most common term, while frost line is also widely used.

Why it matters

As you move outward from the Sun, the available building blocks change. That shift is what explains why moons around the giant planets often contain much more ice than the Moon or Io.

TL;DR

The name you want is the snow line — also called the frost line.