 Solution

A primary planetary atmosphere is gas captured from the protoplanetary nebula, typically H/He-rich. A secondary planetary atmosphere is assembled or substantially replenished after formation by outgassing, volatile delivery and processing, rather than retaining the original nebular envelope.

Jupiter, Saturn, Uranus and Neptune retain primary H/He envelopes, although their heavy-element enrichment and deep interiors differ. Venus, Earth and Mars have predominantly secondary present-day atmospheres, shaped by volcanic outgassing, impacts, escape and, on Earth, biology. Mercury has a tenuous surface-derived exosphere rather than a substantial retained primary atmosphere. The classification describes origin and evolution, not simply whether a current molecule is heavy or light.