= 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.
\b[Jupiter, Saturn, Uranus and Neptune retain primary H/He envelopes], although their heavy-element enrichment and deep interiors differ. \b[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.
Back to article page
