May the continuous trapping of atmospheric water on the permanent South polar cap affect the global atmospheric D/H ratio on Mars? - Archive ouverte HAL Accéder directement au contenu
Communication Dans Un Congrès Année : 2004

May the continuous trapping of atmospheric water on the permanent South polar cap affect the global atmospheric D/H ratio on Mars?

Résumé

It is generally admitted that thermal escape is the only process responsible for hydrogen isotopic fractionation in Mars atmosphere (enrichment by a factor of 5.5 relative to SMOW value). The aim of the present paper is to show that thermodynamic processes (condensation/ sublimation) may potentially play a significant role in fractionating atmospheric H2O. The cycle of Martian H2O is complex. During northern summer, the North permanent water ice cap is assumed to release a few precipitable microns (pr. μ m), globally averaged, to the atmosphere. Part of this water vapor finally returns to North cap during northern winter, whereas a small fraction (<10%) is trapped in the South permanent carbonic ice cap, which acts as a permanent sink. Water vapor is probably cycled through regolith by adsorption/deposition and further sublimation/desorption. It is believed that the ultimate fate of water molecules released by the North water ice cap, and/or by the permafrost, is to stick to the South carbonic ice cap, with a time scale longer than one seasonal cycle. If so, a molecule released by the North cap must undergo a number of adsorption/desorption and deposition/sublimation cycles, through regolith and polar caps, before being trapped irreversibly in South cap carbonic ice. Recent IR observations of pure water ice on South polar cap by Mars-Express during southern spring show that water, migrating southward together with the edge of the retreating seasonal south CO2 cap, is finally deposited near cap edges in substantial amount. The fraction of this deposited water not released back to the atmosphere at later times (southern summer), therefore incorporated to carbonic ice, is not well known at present time. Because, during southern winter, water vapor above the South permanent cap is much depleted in deuterium, by at least a factor of 3 (Montmessin et al, AGU meeting, 2003), the polar cap tends to be depleted in deuterium (similarly to Earth's case), with a related deuterium enrichment of atmosphere. But, according to the calculations of Montmessin et al, most of water condensation on South cap occurs at the end of seasonal cap retreating, during southern spring (as observed by Mars-Express), and this water is not highly fractionated, contrary to water deposited at earlier times, during winter. The relevant parameter in our case is the fractionation ratio of water irreversibly trapped into the South cap, without further release to the atmosphere, which remains quite uncertain.
Fichier non déposé

Dates et versions

hal-03802565 , version 1 (06-10-2022)

Identifiants

Citer

Eric Chassefière, Franck Montmessin, Thierry Fouchet, François Forget. May the continuous trapping of atmospheric water on the permanent South polar cap affect the global atmospheric D/H ratio on Mars?. 35th COSPAR Scientific Assembly, Jul 2004, Paris, France. pp.573. ⟨hal-03802565⟩
16 Consultations
0 Téléchargements

Partager

Gmail Facebook X LinkedIn More