On the basis of isotopes studies, the terrestrial planets appear to have lost volatiles at some stage. Mars is enriched in ¹⁵N, probably implying that nonthermal escape allowed ¹⁴N to preferentially escape. The enrichment of noble gases in Venus relative to Earth and Earth relative to Mars ()is a mystery. The D/H ratio for Venus in , compared to for Earth, implying that a great deal of water has been lost from Venus's atmosphere.
N₂ and CO₂ have approximately the same abundances in Venus and Earth, but are much more abundant in Mars.For ⁴⁰Ar/³⁶Ar, Venus Earth Mars. For the three planets, the following are similar:²⁰Ne/³⁶Ar,³⁸Ar/³⁶Ar, ¹⁸O/¹⁶O, and ¹³C/¹²C.Similar N, Ar, and CO₂abundances between Venus and the Earth suggest a similar degree of outgassing, but higher ⁴⁰Ar/³⁶Ar for the Earth suggests that later outgassing was more efficient.