The current Martian atmosphere is about five times more enriched in deuterium than Earth’s, providing direct testimony that Mars hosted vastly more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. Isotopic fractionation of D/H between the lower and the upper atmospheres of Mars has been assumed to be controlled by water condensation and photolysis, although their respective roles in influencing the proportions of atomic D and H populations have remained speculative. Here we report HDO and H₂O profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal the prevalence of the perihelion season for the formation of atomic H and D at altitudes relevant for escape. In addition, while condensation-induced fractionation is the main driver of variations of D/H in water vapour, the differential photolysis of HDO and H₂O is a more important factor in determining the isotopic composition of the dissociation products.

目前火星大气中的氘含量大约是地球大气的五倍，这直接证明了火星在其早期青年时期所拥有的水比现在要多得多。从当前的平均 D/H 值估计损失到太空的总水量取决于对氘化水和非氘化水之间的相对逃逸的严格评估。火星低层和高层大气之间的 D/H 同位素分馏被认为是由水凝结和光解控制的，尽管它们各自在影响原子 D 和 H 种群比例方面的作用仍然是推测性的。这里我们报告 HDO 和 H ₂大气化学套件（ExoMars Trace Gas Orbiter）在火星轨道上观察到的 O 剖面，一旦与预期的光解速率相结合，揭示了在与逃逸相关的高度形成原子 H 和 D 的近日点季节的普遍性。此外，虽然冷凝引起的分馏是水蒸气中 D/H 变化的主要驱动因素，但 HDO 和 H ₂ O 的差异光解是决定解离产物同位素组成的更重要因素。