Abstract Apatite preserves a record of the halogen and water fugacities that existed during the waning stages of crystallization of planetary magmas, when they became saturated in phosphates. We develop a thermodynamic formalism based on apatite–merrillite equilibria that makes it possible to compare the relative values of halogen and water fugacities in Martian, lunar and terrestrial basalts, accounting for possible differences in pressure, temperature and oxygen fugacities among the planets. We show that each of these planetary bodies has distinctive ratios among volatile fugacities at apatite saturation and that these fugacities are in some cases related in a consistent way to volatile fugacities in the mantle magma sources. Our analysis shows that the Martian mantle parental to basaltic SNC meteorites was dry and poor in both fluorine and chlorine compared to the terrestrial mantle. The limited data available from Mars show no secular variation in mantle halogen and water fugacities from ∼4 Ga to ∼180 Ma. The water and halogens found in present-day Martian surface rocks have thus resided in the planet’s surficial systems since at least 4 Ga, and may have been degassed from the planet’s interior during a primordial crust-forming event. In comparison to the Earth and Mars, the Moon, and possibly the eucrite parent body too, appear to be strongly depleted not only in H 2 O but also in Cl 2 relative to H 2 O. Chlorine depletion is strongest in mare basalts, perhaps reflecting an eruptive process characteristic of large-scale lunar magmatism.