The Martian D/H Ratio In Escaping Atoms and Evidence for Primordial Water
Résumé
A key indicator of the amount of martian water lost into space is the ratio of the isotopes deuterium to hydrogen in the present-day atmosphere. The constituent atoms of water are known to escape into space, with H escaping rapidly from the Jeans process of thermal evaporation and O escaping by different superthermal processes. The lighter isotope H escapes faster than D leading to a gradual increase in the D/H ratio of the remaining water. The present D/H ratio is proportional to the total amount of water that has escaped, but deriving the depth of water on a warmer and wetter primordial Mars requires understanding the factors that control escape processes today. Observations of UV line emissions from H and D atoms has been carried out with an echelle channel in the IUVS instrument on the MAVEN mission, providing direct measurements of the H and D densities and escape rates from the upper martian atmosphere. Both H and D are highly variable over the course of a martian year, with strong seasonal changes in the densities and the D/H ratio of escaping atoms. In addition the ratio D/H in the escaping atoms varies over the year, i.e. the fractionation factor is not constant. The consequences for the history of H and D and water escape will be discussed.